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Author: Drager, Kurtis L. Title: Shelf Life Evaluation/Prediction of a High Fat/Sugar and a Low Fat/Sugar
Ready to Eat Breakfast Cereal in Standard Packaging and Various Size Airtight Plastic Containers using the Guggenheim-Anderson-de Boer (GAB) Model
The accompanying research report is submitted to the University of Wisconsin-Stout, Graduate School in partial
completion of the requirements for the
Graduate Degree/ Major: MS Food and Nutritional Sciences
Research Advisor: Karunnanithy Chinnadurai, Ph.D.
Submission Term/Year: Winterm, 2014
Number of Pages: 94
Style Manual Used: American Psychological Association, 6th edition
I understand that this research report must be officially approved by the Graduate School and that an electronic copy of the approved version will be made available through the University Library website
I attest that the research report is my original work (that any copyrightable materials have been used with the permission of the original authors), and as such, it is automatically protected by the laws, rules, and regulations of the U.S. Copyright Office.
My research advisor has approved the content and quality of this paper. STUDENT:
NAME Kurtis L. Drager DATE: 01/17/2014
ADVISOR: (Committee Chair if MS Plan A or EdS Thesis or Field Project/Problem):
NAME Karunnanithy Chinnadurai, Ph.D. DATE: 01/17/2014
---------------------------------------------------------------------------------------------------------------------------------
This section for MS Plan A Thesis or EdS Thesis/Field Project papers only Committee members (other than your advisor who is listed in the section above) 1. CMTE MEMBER’S NAME: Eun Joo Lee, Ph.D. DATE: 01/17/2014
2. CMTE MEMBER’S NAME: Ajay Kathuria, Ph.D. DATE: 01/17/2014
--------------------------------------------------------------------------------------------------------------------------------- This section to be completed by the Graduate School This final research report has been approved by the Graduate School.
Director, Office of Graduate Studies: DATE:
2
Drager, Kurtis, L. Shelf Life Evaluation/Prediction of a High Fat/Sugar and a Low
Fat/Sugar Ready to Eat Breakfast Cereal in Standard Packaging and Various Size Airtight
Plastic Containers using the Guggenheim-Anderson-de Boer (GAB) Model
Abstract
Packaging is an important component for storage and extension of shelf life of a food product.
The objective of this study was to evaluate/predict the shelf life of a high fat/sugar (A) and a low
fat/sugar (B) RTE breakfast cereal in standard packaging and various size polypropylene airtight
plastic containers. Moisture sorption isotherms of the two cereals were determined at 10, 23, and
38°C over a humidity range of 11-98.2% using accelerated shelf life testing. Both cereals
exhibited a Type II moisture sorption isotherm. GAB model provided good fits for both cereals
(with R2>0.9524, %RMS<10.2039, E<8.0890, and RMSE<0.0318). Moisture content and water
activity of the cereals decreased as temperature increased. Water activity, breaking strength, and
sensory results determined that the critical moisture content was 5.5% and 6.5% for cereal A and
B, respectively. Water vapor transmission rate (WVTR) increased as size of the container
increased (0.0375 and 0.0407 g/pkg.-day for 4 quart and standard packaging, respectively at
23°C). Using the critical moisture content and WVTR the polypropylene containers extended
the shelf life of both breakfast cereals (A and B), 156 and 59 days and 236 and 89 days in 4 quart
and standard packaging, respectively, at 23°C (80% RH).
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Acknowledgments
I would like to thank my thesis advisor Dr. Karunnanithy Chinnadurai for informing me
of this project and advising me throughout the research process. I would also like to thank the
other two members on my thesis committee Dr. Eun Joo Lee and Dr. Ajay Kathuria for their
support, edits, and suggestions throughout the research process.
I would also like to thank the Packaging/Food and Nutritional Science departments for
allowing me to use their labs and equipment to conduct my research. To the UW Stout
Discovery Center for allowing me to work on this project and purchasing various chemicals and
equipment need for my research. To UW-Stout Research Services for awarding me with a grant
to help fund my research and to Vicki Weber for managing funds and purchasing chemicals
needed for my research.
I am thankful for the support and encouragement that I have received from family and
friends throughout my entire college career, especially that of my mother and grandparents,
without you I would not be where I am today.
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Table of Contents
Abstract ............................................................................................................................................2
List of Tables ...................................................................................................................................8
List of Figures ................................................................................................................................10
Chapter I: Introduction ...................................................................................................................11
Statement of the Problem ...................................................................................................14
Purpose of the Study ..........................................................................................................15
Objectives of the Study ......................................................................................................15
Definition of Terms............................................................................................................15
Assumptions of the Study ..................................................................................................18
Limitation of the Study ......................................................................................................18
Methodology ......................................................................................................................18
Chapter II: Literature Review ........................................................................................................19
Cereal Production...............................................................................................................19
Extrusion. .............................................................................................................. 19
Puffed Products. .................................................................................................... 19
Flaked Products. .................................................................................................... 20
Shredded Products. ............................................................................................... 20
Granulated Products. ............................................................................................. 20
Cereal Coating. ..................................................................................................... 20
Moisture Sorption Isotherms..............................................................................................21
Moisture Sorption Models. ................................................................................... 24
Shelf Life ...........................................................................................................................26
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Accelerated Shelf Life Testing. ............................................................................ 26
Water Activity ....................................................................................................................27
Lipid Oxidation ..................................................................................................................32
Measurement of Lipid Oxidation. ......................................................................... 35
Texture ...............................................................................................................................37
Sensory ...............................................................................................................................38
Packaging ...........................................................................................................................40
Chapter III: Methodology ..............................................................................................................42
Materials ............................................................................................................................42
Chemicals ...........................................................................................................................42
Sample Storage Method .....................................................................................................43
Saturated Salt Solution Preparation ...................................................................................44
Initial Moisture Content .....................................................................................................46
Determination of Moisture Sorption Isotherms .................................................................46
Determination of the GAB Model .....................................................................................47
Determination of Shelf Life Using the Integrated GAB Model ........................................47
Model Validation ...............................................................................................................48
Moisture Content at Equilibrium .......................................................................................49
Determination of Moisture Permeability ...........................................................................49
Thiobarbituric Acid Reactive Substances (TBARS) .........................................................51
Reagent Stock Solution Preparation. .................................................................... 51
Creation Of Malonaldehyde Standard Curve. ....................................................... 52
Determination of Water Activity .......................................................................................53
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Sensory Analysis ................................................................................................................53
Breaking Strength ..............................................................................................................54
Statistical Analysis .............................................................................................................55
Chapter IV: Results and Discussion ..............................................................................................57
Initial Moisture Content .....................................................................................................57
Determination of the GAB Model .....................................................................................57
Determination of Moisture Sorption Isotherms .................................................................58
Moisture Content at Equilibrium .......................................................................................61
Determination of WVTR and Moisture Permeability ........................................................62
Thiobarbituric Acid Reactive Substances (TBARS) .........................................................65
Determination of Water Activity .......................................................................................67
Breaking Strength ..............................................................................................................69
Sensory Analysis ................................................................................................................71
Cereal A. ............................................................................................................... 71
Cereal B. ............................................................................................................... 74
Crispness and Overall Acceptability. .................................................................... 75
Interaction Effects ..............................................................................................................78
Critical Moisture Content ..................................................................................................78
Determination of Shelf Life Using the Integrated GAB Model ........................................79
Chapter V: Conclusions .................................................................................................................82
Recommendations ..............................................................................................................82
References ......................................................................................................................................84
Appendix A: Nutrition Breakdown of Cereals High Fat/Sugar (A) and Low Fat/Sugar (B) ........91
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Appendix B: UW-Stout IRB Approval ..........................................................................................92
Appendix C: Consent Form: Sensory Analysis of Breakfast Cereals ............................................93
Appendix D: Breakfast Cereals Evaluation Form .........................................................................94
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List of Tables
Table 1: 2011 Top Five Global Cereal Companies (%, Retail Value RSP) ..................................12
Table 2: 2012 United States Top 10 RTE Cereal Companies in Sales ($), with Number of Units
Sold ....................................................................................................................................13
Table 3: List of Moisture Sorption Models and Mathematical Expressions .................................25
Table 4: List of Potential Microorganisms at Water Activity Levels ............................................31
Table 5: Rate of Reaction of Fatty Acids Relative to Stearic Acid as the Number of Double
Bonds Increases .................................................................................................................33
Table 6: The Effect of Relative Humidity (%RH) Storage Condition on Moisture Content (%
MC) and Texture of a Breakfast Cereal with an Initial Moisture Content of 2.5% ...........38
Table 7: Solubility of Saturated Salt Solutions ..............................................................................44
Table 8: Relative Humidity (%RH) of Saturated Salt Solutions at 10, 23, and 38°C ...................45
Table 9: Saturation Vapor Pressure (ps) Values at 10, 23, and 38°C .............................................48
Table 10: Standard Solutions for TBARS Analysis ......................................................................52
Table 11: GAB Parameters of Cereal A at 10, 23, and 38°C ........................................................58
Table 12: GAB Parameters of Cereal B at 10, 23, and 38°C .........................................................58
Table 13: Moisture Content (g/100 g dry solids) of Cereal A and B when Stored at 10, 23, and
38°C ...................................................................................................................................61
Table 14: WVTR (g/container-day) of ½ cup, 2 cup, 4 Qt. Containers, and Standard Breakfast
Cereal Packaging at 10, 23, and 38°C ...............................................................................64
Table 15: Surface Area (cm2) and Permeability (g/container-day-mmHG) of ½ cup, 2 cup, 4 Qt.
Containers, and Standard Breakfast Cereal Packaging at 10, 23, and 38°C ......................65
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Table 16: TBARS Value (mg MDA/kg of cereal) of Cereal A and B when Stored at 10, 23, and
38°C ...................................................................................................................................66
Table 17: Water Activity of Cereal A and B upon Equilibrium when Stored at 10, 23, and 38°C68
Table 18: Breaking Strength (kgf) of Cereal A and B when Stored at at 10, 23, and 38°C ..........70
Table 19: Mean Rating Comparisons of Cereal A for Color, Aroma, Sogginess/Crispness, Taste,
Off-flavor/Flavor, and Overall Acceptability ....................................................................73
Table 20: Mean Rating Comparisons of Cereal B for Color, Aroma, Sogginess/Crispness, Taste,
Off-flavor/Flavor, and Overall Acceptability ....................................................................77
Table 21: Statistical Significance (F-statistics/p-value) on Quality Parameters of Ready to Eat
Breakfast Cereals. ..............................................................................................................78
Table 22: Bulk Density (g/cm3) and Holding Capacity (g) of ½ cup, 2 cup, and 4 Qt. Containers.79
Table 23: Shelf Life (Days) of Cereals A and B in ½ cup, 2 cup, 4 Qt., and Standard Breakfast
Cereal Packaging at 80% RH. ............................................................................................80
Table 24: Shelf Life (Days) of Cereals A and B in ½ cup, 2 cup, 4 Qt., and Standard Breakfast
Cereal Packaging at 80% RH with 20 g of Cereal .............................................................81
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List of Figures
Figure 1: The five types of isotherms. ..........................................................................................23
Figure 2: Typical isotherm divided into 3 regions (zone 1, 2, and 3) and showing adsorption and
desorption curves. ..............................................................................................................24
Figure 3: Water activity stability diagram. ...................................................................................29
Figure 4: Lipid oxidation products vs. time. .................................................................................36
Figure 5: Reaction of thiobarbituric acid (TBA) with malonaldehyde (MA) to form the TBA-
MA complex ......................................................................................................................37
Figure 6: Example of moisture sorption chamber for 10 and 38°C samples. ................................43
Figure 7: Moisture sorption chambers for 23°C samples ..............................................................43
Figure 8: Storage for 200 g samples. ............................................................................................46
Figure 9: Universal Testing Machine (UTM). ..............................................................................55
Figure 10: Moisture sorption isotherms of cereal A at 10, 23, and 38°C. ....................................60
Figure 11: Moisture sorption isotherms of cereal B at 10, 23, and 38°C. ....................................60
Figure 12: Net mass gain of ½ cup container at 10, 23, and 38°C ...............................................62
Figure 13: Net mass gain of 2 cup container at 10, 23, and 38°C ................................................63
Figure 14: Net mass gain of 4 quart container at 10, 23, and 38°C. .............................................63
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Chapter I: Introduction
Breakfast cereal has a significant place in an individual’s diet including that of children
due to its appealing flavor, color, taste, nutrition, and the variety of packaging options and
designs. As a matter of fact the packaging constitutes 25% of the processed food cost; hence,
understanding the interactions of food and packaging materials could reduce the cost. Before
introducing a new packaging container, one should know the shelf life of the product to be
stored. Shelf life is defined as the time during which the product will remain safe, be certain to
retain desired sensory, chemical, physical and microbiological characteristics, and comply with
any label declaration on nutritional data when stored under recommended conditions (Kilcast &
Subramaniam, 2004).
Ready to eat (RTE) breakfast cereals are an important aspect for breakfast. In the United
States, cold cereal is still the number one choice with sales topping $9 billion in 2012 (Wells,
2013). In 2012 General Mills Inc. was number one with 29% value share of the RTE breakfast
cereal market, followed by Kellogg Co. with 26% of sales (Euromonitor International, 2013).
Breakfast cereals are prospected to increase slowly by 3% to reach $10.3 billion in 2017
(Euromonitor International, 2013). Many people choose RTE breakfast cereals for a number of
reasons including but not limited to: it is quick, some cereals contain fiber which can curve
hunger, some of these cereals are healthier compared to other breakfast options on the market,
dry cereal is easier to carry to work or school, and cereal can be eaten dry or with milk. RTE
cereal can not only be chosen for breakfast, but also as a snack or any other meal of the day.
In 2011, the global cereal market was valued at $29.1 billion (Culliney, 2012) and is
expected to have a market value of $37,101.3 million by the end of 2016 (Just Food, 2013).
Table 1, displays the top five global cereal companies from 2011, data from Euromonitor
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International, where % shown is retail value RSP (retail selling price) (Culliney, 2012). Number
1, Kellogg’s brands include but limited to: Froot Loops, Fiber Plus, Raisin Bran, and Special K
(Kellogg Co., 2013). General Mill’s, number 2, brands include but not limited to: Cheerios,
Chex, Fiber One, and Lucky Charms. Number 3, Cereal Partners Worldwide (CPW) is a
collaboration between Nestlé and General Mills (General Mills Inc., 2013). CPW brands include
but limited to: Cheerios, Cookie Crisp, Nesquick, and Fitnesse (Cereal Partners UK, 2013).
Number 4, PepsiCo Inc. brands include but not limited to: Quaker brands (Cap’n Crunch, Life,
Shredded Wheat, and Quisp) (Quaker Oats Company, 2012). Ralcorp Holdings at number 5 is a
private label, store brand specialist. Table 2, displays the top 10 cereal companies in the United
States in 2012 with corresponding sales ($) and number of units sold.
Table 1 2011 Top Five Global Cereal Companies (%, Retail Value RSP)
Cereal Company Retail Value RSP (%)
Kellogg Company 32.1
General Mills 10.6
Cereal Partners Worldwide (CPW) 9.8
PepsiCo Inc. 8.6
Ralcorp Holdings 4.5
(Culliney, 2012).
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Table 2 2012 United States Top 10 RTE Cereal Companies in Sales ($), with Number of Units Sold
Cereal Company Sales ($) Unit Sales
Kellogg Company 6,835,080,704 2,226,265,856
General Mills 2,238,298,624 637,755,840
Kraft Foods Inc. 1,029,982,560 344,343,328,
Quaker Oats Co. 581,412,928 200,987,632
Private Label 554,467,648 240,552,928
MOM Co. 141,202,352 56,953,272
McGee Foods Corp. 16,902,996 8,102,399
Nature’s Path 14,225,237 3,878,437
Barbara’s Bakery 13,527,515 3,703,454
Healthy Valley Natural Foods 10,293,783 2,914,930
(Statistic Brain, 2013, Source: Information Resources Inc.)
In general, the factors influencing shelf life can be categorized into intrinsic and
extrinsic factors. Intrinsic factors are influenced by variables such as type and quality of
raw materials, formulation and structure (water activity, available oxygen, preservatives-
salt/sugar, etc.) whereas, extrinsic factors are those factors that the final product
encounters as it moves through the food chain (temperature, relative humidity, etc.).
Therefore, the shelf life of cereal depends on a variety of factors, such as the composition
(fat, sugar), the preparation method, and how and where the cereal is stored.
Paradiso and coworkers (2008) reported that lipid oxidation is one of the main causes of
the loss of nutritional and organoleptic quality of foods. The auto-oxidation of lipids usually
leads to the formation of hydroperoxides ultimately resulting in nutritional and organoleptic
deterioration of the food product.
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Critical moisture content is the moisture content (dry basis) of a product in which it is no
longer deemed acceptable by the consumer. In terms of dry products, the critical moisture
content is reached when the product loses crispness at a level in which it would be rejected by
the consumer. The critical moisture and lipid oxidation values can be cross verified by consumer
acceptance through sensory evaluation. Crispness is one of the primary attributes for consumer
acceptance that is usually quantified using a texture analyzer. Breakfast cereal would lose its
crispness as it adsorbs moisture at different storage conditions.
The shelf life of breakfast cereals can be established based on the permeability coefficient
of the packaging materials in which the cereal is stored, crispness, and critical moisture
content/lipid oxidation values.
Statement of the Problem
Before introducing new packaging material/container one has to establish shelf life of the
intended product that would determine the sell by data or expiration date. Shelf life can be
determined in two different ways. One way is storing the product in a new container and
drawing samples at certain intervals and testing the critical factor(s). This may take several
months (sometimes more than a year) depending upon the product and storage conditions.
Another way is to create different storage conditions that would accelerate deterioration and test
the critical factor(s). This can be completed between a few weeks to months, considerable
saving in time is an advantage. In this research, a different approach was used i.e., collecting
moisture sorption data at different temperatures and relative humidities, quantifying lipid
oxidation of the breakfast cereals and relating these values with moisture permeability
coefficients. This method would reduce the test time to a few weeks to a few months.
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Purpose of the Study
The purpose of this research was to evaluate/predict the shelf life of a high fat/sugar
cereal and a low fat/sugar cereal when stored in various size airtight polypropylene plastic
containers (1/2 cup, 2 cup, and 4 quart) and standard breakfast cereal packaging under three
different temperatures (10, 23, and 38°C) and seven relative humidities (11 to 97%). More
specifically, it examined moisture sorption data using the GAB (Guggenheim-Anderson-de
Boer) model, lipid oxidation data (thiobarbituric acid number), water activity, breaking strength,
sensory data, and water vapor permeability data.
Objectives of the Study
The objectives of this study was to:
1. Develop moisture sorption isotherms using the GAB model.
2. Determine water activity, lipid oxidation (TBARS value) and breaking strength
upon samples reaching equilibrium.
3. Investigate sensory attributes to determine critical moisture content based on
consumer acceptance in conjunction with water activity, lipid oxidation, and
breaking strength data.
4. Determine water vapor transmission rate (WVTR) of the three different size
airtight polypropylene plastic containers and standard breakfast cereal packaging.
5. Predict the shelf life of selected breakfast cereals using the GAB model.
Definition of Terms
Antioxidant. Antioxidants are used to delay the start or slow the rate of oxidation
reactions (ex. lipid oxidation) in foods that are promoted by oxygen, peroxides, light, or free
radicals.
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Autoxidation. Autoxidation is a natural process that takes place between unsaturated
fatty acids and oxygen via a free radical process involving the basic steps: initiation, propagation,
and termination.
Accelerated shelf life testing (ASLT). “Laboratory studies are undertaken during which
environmental conditions are accelerated by a known factor so that the product deteriorates at a
faster than normal rate…” (Robertson, 2013, p. 331).
Aroma. Aroma is the odor of a food product, from volatile compounds and aromatics
that come into contact with olfactory nerves in the nose.
Crispness. “The force and noise with which a product breaks or fractures (rather than
deforms) when chewed with the molar teeth (first and second chew)” (Meilgaard, Civille, &
Carr, 2007, p. 219).
Free radical. A free radical is an atom or compound that has an unpaired electron
making it very unstable and more likely to react with another substance to form more stable
compound.
Lipid oxidation. Lipid oxidation is associated with oxidative deterioration of fats and
oils leading to the development of unpleasant odors and tastes, in addition to changes in color,
viscosity, and solubility.
Oxidative rancidity. Oxidative rancidity is the uptake of oxygen at an unsaturated site
(double bond) in a fatty acid in a fat to create free radicals and shorter fatty acids resulting in
unpleasant odors and tastes. The process can be facilitated by light, warm temperatures, and/or
certain metals.
Rancidity. Rancidity is the chemical deterioration of fat quality by oxidation or
hydrolytic chemical reactions.
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Relative humidity. Relative humidity is the ratio of the partial pressure of water vapor in
a water-air mixture to the saturated vapor pressure at a certain temperature.
Sensory analysis. Sensory analysis is the subjective analysis by trained or untrained
panelists on taste, smell, sound, feel, and appearance of a food product or beverage.
Shelf life. IFT (Institute of Food Technologists) in the United States defines shelf life as
“the period between the manufacture and the retail purchase of a food product, during which the
time the product is in a state of satisfactory quality in terms of nutritional value, taste, texture,
and appearance” (as cited in Robertson, 2010, p. 10).
Labuza and Schmidt define shelf life as “shelf life is the duration of that period between
the packaging of a product and the end of consumer quality as determined by the percentage of
consumers who are displeased by the product” (as cited in Robertson, 2010, p. 10).
Water activity. Water activity is the partial vapor pressure of water in a food divided by
the vapor pressure of pure water at the same temperature of a food.
Water vapor permeability. “The time rate of water vapor transmission through unit
area of flat material of unit thickness induced by unit vapor pressure difference between two
specific surfaces, under specified temperature and humidity conditions” (ASTM International,
2003, p. 444).
Water vapor permeance. “The time rate of water vapor transmission through unit area
of flat material or construction induced by unit vapor pressure difference between two specific
surfaces, under specified temperature and humidity conditions” (ASTM International, 2003, p.
444).
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Water vapor transmission rate (WVTR). “The steady water vapor flow in unit time
through unit area of a body, normal to specific parallel surfaces, under specific conditions of
temperature and humidity at each surface” (ASTM International, 2003, p. 444).
Assumptions of the Study
The main sources of deterioration in cereal products are moisture gain, lipid oxidation
flavor. These deterioration factors affect texture, flavor, and shelf life of cereal products such as
ready to eat breakfast cereals. This study assumes that moisture content and loss of crispness
will play a critical role in determining the shelf life of two types of ready to eat breakfast cereals.
Limitation of the Study
A validation test through real experimental conditions was not performed due to time
limitations. This would involve drawing samples from each storage container every few days to
weeks depending upon the storage condition.
Methodology
All cereal samples were analyzed for their critical moisture content, water activity, TBAR
value, and breaking strength. Cereal samples that were determined to be safe (based on water
activity) were analyzed for sensory attributes. Airtight polypropylene plastic containers and
standard breakfast cereal packaging were analyzed for water vapor transmission rate (WVTR).
19
Chapter II: Literature Review
Cereal Production
RTE cereals can be made from a variety of grains, be low in fat or sugar, have a high
fiber content or whole grain content. While many cereals are fortified with nutrients those that
are high in fat or sugar, contain flavor ingredients, and are brightly colored with artificial colors.
RTE cereals can also include: nuts, dried fruits, marshmallows, or candies.
RTE cereals are cooked during the manufacturing process while hot cereals are cooked in
the home. Breakfast cereals can be made from a variety of cereal grains: corn, wheat, oats,
barley, rye, or rice. Cereal can be enriched with sugar, honey, or malt extract. Breakfast cereals
can be puffed, flaked, shredded, granulated, extruded, or a variety of the forms. Since all cereals
contain large proportions of insoluble starch, which in the natural form is not fit for human
consumption. In this form, the starch is tasteless and indigestible; once the starch is cooked it
becomes digestible. Cereal processes tend to cause hydrolysis rather than gelatinization of the
starch (Robertson, 2013, p. 547).
Extrusion. The most common technology used in production of RTE breakfast cereal is
extrusion cooking. Extrusion cooking is a process that applies heat, shear, and pressure, to a
mixture and forces it through various dies. Different dies offer a variety of shapes and sizes for
extruded food products. In extrusion, lipids usually do not exceed greater than 6-7% of the raw
material (Paradiso, Summo, Trani, & Caponio, 2008). The lipids help act as lubricants between
the mechanical parts of the extruder, if the lipid level is too high it can interfere with starch
transformation and affect product stability and shelf life.
Puffed products. Puffed cereals are generally made of wheat, rice, and cereal dough
(corn meal or oat flour). In addition, sugar, salt, and oil can be added to the cereal grains or
20
dough. The grains or dough are cooked for 20 minutes (under pressure), dried to 14-16%
moisture content, and then extruded through a die to form pellets (Robertson, 2013, p. 547).
These cereals will expand when the water inside the grain or dough vaporizes (liquid to gas).
Toasting is used to dry the puffed products to 3% moisture content (Robertson, 2013, p. 547).
The puffed products are cooled and then packaged. Puffed cereals and products can be made by
gun-puffing and oven-puffing.
Flaked products. Flaked products are made from wheat, corn, oats, or rice cooked at
high pressure, in an extruder. The cereal is then sent through a flaking machine. Flavorings
(malt, sugar, and honey) are added after cooking. The cereal is then dried to 15-20% moisture
content, conditioned for 1-3 days and then flaked, toasted, cooled, and packaged (Robertson,
2013, p. 547).
Shredded products. Shredded RTE cereals are generally made from whole-wheat
grains, but these products can also be made from rice and corn. The starch in these grains are
gelatinized by cooking, once cooled and conditioned the grain is fed through shredders, hence
the name shredded products. Shredding occurs between two steel rollers, one roller is flat and
the second is corrugated to form the strands. The shreds are collected and baked for 20 minutes
at 260°C, and dried to 1% moisture content (Robertson, 2013, p. 547), once cooled the shreds are
packaged.
Granulated products. Granulated cereal products are made from yeast dough made
with wheat flour and salt, and baked as large loaves (Robertson, 2013, p. 547). Once baked the
loaves are broken into pieces, dried, and ground to a certain degree of fineness.
Cereal coating. Sugar and flavoring can be added to cereals after the initial processing,
by coating drums. Coating drums spray small amounts of coating onto the cereal, with drying
21
steps in between each spraying, until a desired level of coating is reached. Sugar, flavor, and
nutrients can also be added to the base prior to processing. Kent and Evers stated “the sugar
content of cornflakes increases from 7% to 43% as a result of the coating process, and that of
puffed wheat from 2% to 51%” (as cited in Robertson, 2013, p. 548).
Moisture Sorption Isotherms
The theory base that underpins this study is moisture adsorption (moisture gain) kinetics.
Moisture sorption isotherms describe the relationship between moisture content and water
activity of a food product. These relationships are highly dependent on the chemical and
physical composition and structure of the food product. This relationship is also highly
dependent on the temperature condition in which the food product is placed. The moisture gain
affects both the physical and chemical reactivity of the food product and how water is
transported throughout the food product. Moisture isotherms can help determine shelf life and
storage stability of a product, moisture sensitivity of a product, and predict the packaging
requirements based on the sorption properties of a product
Moisture sorption isotherm models can help formulate food products at higher moisture
contents in order to maintain a safe water activity level. Isotherms are also important in
ingredient mixing, packaging prediction, changes in texture, determining the equilibrium
moisture content at a given water activity, determine critical water activity or moisture limits for
texture (crispness, hardness, and rheological properties), and chemical stability. In the food
industry, determination of moisture sorption isotherms of dry products provides vital information
about the product. Moisture sorption isotherms provide information on a food products moisture
content and the humidity and temperature at which the product is exposed to; in addition to the
physical nature of the association between water and the product (Azanha & Faria, 2005).
22
There are five types of sorption isotherms, shown in Figure 1. Type 1 isotherms occur
when there is only one adsorption site (monomolecular). First the surface fills randomly and will
eventually become saturated when the surface becomes filled. Type 1 isotherm is also known as
the Langmuir isotherm (Basu, Shivhare, & Mujumdar, 2006). In type 2 isotherms, there is more
than one adsorption site. Initially rapid adsorption occurs, saturation occurs when the first site is
filled, then a second adsorption occurs. This second site can be second monolayer or a second
site on the surface. Type 2 isotherms exhibit a sigmoidal shape and asymptotical trend as the
water activity approaches 1. Type 3 isotherms occur when there are strong attractive interactions
that lead to condensation; initially the isotherm exhibits no adsorption. As pressure increases,
nucleation will occur which will eventually lead to liquids condensing on the surface. Type 3
isotherm is also known as the Flory-Higgins isotherm (Basu et al., 2006). Type 4 isotherms
occur due to multiple phase transitions. These transitions occur from a mixture of attractive and
repulsive interactions. This isotherm can occur in multilayer systems where adsorption occurs
on a second layer before the first layer is saturated. Type 5 is the Brunauer-Emmett-Teller
(BET) multilayer adsorption isotherm and involves attractive interactions. As seen in type 3
isotherms, there is initially no adsorption, next nucleation and growth of liquid drops will occur.
Coverage saturates when there is no more space to hold adsorbates. Type 5 is observed in the
adsorption of water vapor on charcoal, it is also related to type 2 and 3 isotherms (Andrade,
Lemus, & Pérez, 2011). Type 2 and 4 are the two most common types of isotherms found in
food products.
23
Figure 1. The five types of isotherms (Basu et al., 2006).
Moisture sorption isotherms can be divided into three regions (as seen in Figure 2).
Region 1, water is strongly bound to a food product and the enthalpy of vaporization is higher
than that of pure water (Basu et al., 2006). To remove water from its binding sites requires
lowering the water activity to very low levels. In region 2, intermediately bound water occurs;
here water is loosely bound to secondarily binding sites on food molecules. In region 3, unbound
water is easily removed by a very small drop in water activity. In this region, the bulk of water
in most foods is similar to that of free water.
24
Figure 2. Typical isotherm divided into 3 regions (zone 1, 2, and 3) and showing adsorption and
desorption curves (Roudaut & Debeaufort, 2011, p. 68).
Sorption isotherms can be created from an adsorption or desorption process (see Figure
2). Adsorption occurs from the adsorption of water, while desorption occurs due to the loss of
water. Relationships between equilibrium moisture content (EMC) and corresponding relative
humidities at constant temperatures generate moisture sorption isotherms. The EMC equation is
shown below.
In the EMC equation, Me is the EMC (g/g), We is equilibrium or final weight of the sample (g),
Wi is the initial weight of the sample (g),and Mi is the initial moisture content of the product on
dry basis (g/g). The EMC increases with relative humidity but decreases with an increase in
temperature.
Moisture sorption models. A number of moisture sorption isotherms have been
proposed. These moisture sorption isotherms include kinetic models based on multiplayer, semi-
25
empirical, and empirical models. Table 3 lists a few of moisture sorption models used in
research.
Table 3
List of Moisture Sorption Models and Mathematical Expressions
Model Mathematical Expression1 Reference
Langmuir
(Roudant & Debeaufort, 2011, p. 70)
GAB
(Kapsalis, 1987, p. 204)
Peleg (Roudant & Debeaufort, 2011, p. 70)
BET
(Roudant & Debeaufort, 2011, p. 70)
Oswin
(Ertekin & Gedik, 2004)
1It should be noted that variable notation may differ from source to source.
The most common moisture sorption isotherm models used are the BET (Brunauer-
Emmett-Teller) and GAB (Guggenheim-Anderson-de Boer). The BET isotherm model is most
important in interpretation for type 2 isotherm characteristics, while the GAB model is
considered the most versatile (Siripatrawan & Jantawat, 2006). In a study on Jasmine Rice
Crackers, Siripatrawan & Jantawat (2006) found the BET model is the best fitted for water
activity range of less than 0.60 and the GAB model is the best fitted for a water activity range of
0.10 to 0.95. Barbosa-Canovas & Vega-Mercado; Timmermann, Chirife, & Iglesias; Tsami,
Krokida, & Drouzas reported that the GAB model is the best for fitting sorption isotherm data
for a majority of food products to a water activity level of approximately 0.9 (as cited in Prothon
26
& Ahrné, 2004). Water activities above 0.9 were seldom accurately measured or studied. In this
research, the GAB model was used due to its versatility and wide range for water activity.
Shelf Life
A food product reaches the end of its shelf life when after storage under certain
conditions and/or period of time, one or more of the quality attributes makes the product
undesirable, unsuitable, or unappealing for consumption. The three factors that control shelf life
of a food product are: product characteristics (formulation and processing) (intrinsic factors),
environmental (distribution and storage conditions) (extrinsic factors), and properties of the
package in which the product is stored.
Foods can be classified into three categories: shelf stable, perishable, and semi-preserved
(Liu & Gilbert, 1994). Shelf stable foods would not undergo microbial spoilage, only chemical
and physical changes when stored at recommended storage conditions. Perishable foods, such as
fruits, vegetables, milk, and meat have a shelf life of days. In perishable foods, microbial
reactions are the main cause of deterioration or spoilage. Semi-preserved foods, such as cured
meats, pasteurized dairy products, and cooked-chilled products, have a shelf life that is
dependent on moisture content, preservatives, antioxidants, processing, and storage temperature.
Many food products are designed to undergo small changes under normal distribution and
storage condition; testing under these conditions takes time and can be very expensive, especially
when working with dry products with a long shelf life. Accelerated shelf life testing (ASLT) is a
practical method in predicting the shelf life of shelf stable food products.
Accelerated shelf life testing. In ASLT, glass jars (mason) and fish tanks can be used to
create a number of humidity controlled chambers. Fish tanks are generally used due to the large
size, in which a large number of samples can be stored. Whether using glass jars or fish tanks
27
proper care needs to be taken to ensure there is a proper seal, with fish tanks glass or Plexiglas
can be used with vacuum grease to create an airtight seal. Each chamber needs to have a stand in
which the sample dishes are placed in addition to a saturated salt solution to maintain proper
humidity levels. Labuza (1984) and Bell & Labuza (2000) have published excellent information
on humidity control devices (ex. saturated salt solutions) and humidity chambers (jars and fish
tanks) (pg. 64-67, pg. 33-40). Saturated salt solutions must be created only with distilled water
and ACS grade salts and be mixed in clean glass beakers. The salt solutions should be created at
or above the temperature at which the salt solutions will be used and these solutions should be
stirred or agitated to ensure equilibrium (relative humidity).
Water Activity
Water content (moisture content) by itself is not a suitable measurement for predicting
food safety and food quality, whereas water activity is more suitable. Water activity (aW) in
foods is a measurement of the energy status of water. Water activity can be defined as the partial
vapor pressure of water in a food over the vapor pressure of pure water at the same temperature
(Rockland & Nishi, 1980), as seen in equation below.
Where p is the vapor pressure of the water in the food product, and p0 is the vapor pressure of
pure water. Pure distilled water will have a water activity reading of 1.000. Water activity is
influenced by temperature, water content, chemical compounds, concentration of dissolved
solutes (salts and sugars), storage environment, absolute pressure, packaging, and free water.
Water is an important medium and a necessary ingredient in foods, it can act as a heat
transfer medium and it is relatively inexpensive. Water can occur in a one or a combination of
three forms in food: as bound water, as a hydrate, or as free water. Bound water is water that is
28
tied to the structure of larger molecules (starch molecules (carbohydrates) or proteins) by
hydrogen bounds. Bound water does not easily boil or freeze, but will react as part of a molecule
in which it is bound with. A hydrate is any chemical that is loosely bound with water. A
common chemical hydrate is caffeine. Caffeine naturally has one molecule of water attached to
it. Once heated these hydrates give up the water molecule/s attached and are now in the
anhydrous form (free of water). Free water, is the water that is responsible for the growth of
microorganisms that can produce toxins in a food product. Free water can be easily separated
from a food; it can be easily evaporated when foods are dried, and will easily boil and freeze.
Free water is key enzymatic, chemical, and microbial reactions. Water might be the most
dominant factor leading to microbial spoilage in foods (Chirife, 1998, p. 43).
Water activity can help determine the degree of bonding of water in a food product or
material. This degree of bonding can determine its (the water) ability to participate in chemical
and biochemical reactions and biological (microbial) reactions/growth. Figure 3, diagrams the
various reactions that can occur in a food product at various water activities relative to moisture
content or reaction rates (Rockland & Nishi, 1980).
29
Figure 3. Water activity stability diagram. PHF stand for potentially hazardous food (Rockland
& Nishi, 1980).
Foods, such as crackers or cereals that have low water activity readings are usually crispy
and brittle. When these foods absorb water, the water activity of these products increase and the
products will lose crispness and become soggy. Water activity is also important because it
establishes critical limits, when the water activity of a food falls above or below these limits, the
food can become unacceptable on a safety, commercial, or sensory standpoint.
Water activity is an important hurdle in shelf life and microbial growth. Most pathogens
are inhibited by a water activity of 0.95 (Clark, 2009). Knowing how to handle this hurdle is
vital to making sure that food has a water activity below 0.85, which is considered safe (Clark,
2009). Other hurdles used in food preservation include: heat, preserving with salt or sugar,
drying, and reducing pH levels. Water activity can affect taste, color, texture, and preservation
of food. The benefit of having different hurdles in place makes it harder for bacteria to grow and
can provide the consumer with foods that have a better taste and texture and increases shelf life.
30
In order for certain food products to have an increased shelf life, without the need of
refrigeration, it is necessary to control pH and/or water activity. These foods can be made safe to
store by lowering the water activity to a level where dangerous pathogens cannot grow.
At water activities 0.2-0.3 monolayer moisture occurs. This is the most optimal range
where dehydrated foods have maximum shelf life (Leake, 2006). At and below 0.2 reactions
requiring water phase will not occur (Leake, 2006) but the rate of lipid oxidation increases.
Physical changes (ex. loss of crispiness, stickiness of powders and hard) will occur at a aw from
0.35 to 0.45 (Labuza & Altunakar, 2007, p. 129) and from a aw range of 0.4 to 0.5 soft material
will become hard and will eventually dry out (Leake, 2006). At a aw of 0.6 (critical point) the
potential for microbial growth can occur, but a range of 0.6 to 0.75 range, mold growth will
dominate that of bacteria growth. An increase in chemical reactions that require an aqueous
phase will reach a maximum and being to fall at a aw range of 0.6 to 0.8. At aw above 0.85
(critical point) bacterial pathogens begin to grow. Foods that have a finished equilibrium pH
greater than 4.6 and a aw above 0.85 and can support pathogens are defined as potentially
hazardous foods (PHF) by the U.S. Food and Drug Administration (FDA) Food Code (United
States Food and Drug Administration, 2013). Table 4, below, gives the minimum water activity
limit for microorganism’s growth.
31
Table 4
List of Potential Microorganisms at Water Activity Levels*
aw Microorganisms
0.97 Clostridium botulinum, Pseudomonas fluorescens
0.95 E. coli, Salmonella spp., Clostridium perfingens
0.94 Clostridium botulinum, Stachybotrys atra
0.93 Bacillus cereus
0.92 Listeria monocytogenes
0.91 Bacillus subtilis
0.90 Staphylococcus aureus (anaerobic)
0.87 Staphylococcus aureus (aerobic)
0.85 Aspergillus clavatus
0.83 Penicillium expansum, Penicillium islandicum
0.82 Aspergillus parasiticus
0.81 Penicillium patulum
0.75 Aspergillus candidus
0-0.6 No microbial proliferation
*Adapted from Rockland & Nishi (1980) and Decagon (2003).
Water activity can be measured using a hygrometer, by determination of the freezing
point depression of a food product, by thermocouple psychrometer, by dew point hygrometer, by
the chilled mirror dew point method, by determination of the isopiestic equilibrium, or by
resistance or capacitance sensors. Each of these methods has its own advantages and
disadvantages, accuracy and reproducibility, working aw range, and equilibration time. In this
study, a chilled mirror dew point hygrometer was used due to its speed (< 5 minutes), accuracy
32
(±0.003 aw), and precision (0.001 aw) (Decagon Devices, Inc., 2013). The chilled mirror dew
point method is the primary method approved by the AOAC International. Disadvantages of
using this method include a mirror that must be kept clean and difficulty producing accurate
readings in foods that contain propylene glycol and ethanol.
Lipid Oxidation
Lipid oxidation is a major problem of all food products. It is of most concern in food
products that contain high amounts of fat. Lipid oxidation will have an effect on the sensory
attributes of a food product and will affect the shelf life of a food product. Lipid oxidation will
cause unpleasant tastes and odors, as well as cause changes in color, viscosity, and solubility.
Lipid autoxidation is considered a major cause in food deterioration (Robertson, 2010 p. 25).
Fats are composed of one to three fatty acids attached to a glycerol backbone. These
fatty acids can be saturated, unsaturated, or polyunsaturated and can occur in any combination.
Saturated fatty acids are stable due to the absence of double bonds in the fatty acid chain.
Unsaturated fats are fats with double bonds and are either monounsaturated or polyunsaturated.
Monounsaturated fatty acids have one double bond in the fatty acid chain, while polyunsaturated
fatty acids have multiple double bonds. As the number of double bonds increases in a fatty acid
so does the reaction rate with hydrogen, oxygen, light, and enzymes (see Table 5). In relation to
stearic acid, a saturated fat will have an oxidation rate of 1 and oleic acid, a monounsaturated fat
will have an oxidation of 10. As the number of double bonds increases, as seen in linoleic acid, a
polyunsaturated fatty acid with two double bonds and linolenic acid a polyunsaturated fatty acid
with 3 doubles have an oxidation rate of 1200 and 2500, respectively to that that of stearic acid.
33
Table 5
Rate of Reaction of Fatty Acids Relative to Stearic Acid as the Number of Double Bonds
Increases
Type of Fatty Acid Rate of Reaction Relative to Stearic Acid
18:0a 1
18:1∆9b 10
18:2∆9,12c 1200
18:3∆9,12,15d 2500
aStearic acid bOleic acid cLinoleic acid dLinolenic acid
The reaction of a fatty acid with hydrogen (hydrogenation) will eliminate the double bond
(saturates the bond). Hydrogenation is a process that occurs only in industry to produce a variety
of fats/oils and does not occur naturally during storage. Min, Kim, & Han (2010), stated that
“the susceptibility of cereals and snack foods to lipid oxidation is associated with the
concentration and type (quality) of fat used and the number of unsaturated bonds in the fatty
acids” (p. 346).
The lipid oxidation mechanism occurs in three phases. The first phase, initiation, is the
formation of free radicals. Initiation precursors are air, heat, light, and metals. In initiation, the
abstraction or removal of a hydrogen atom adjacent to a double bond in a fatty acid to form an
alkyl free radical. The second phase, propagation, is the reoccurrence of free radical chain
reactions. During the phase the alkyl free radicals will react with oxygen and form unstable
peroxy free radicals. The peroxy free radicals will further abstract or remove hydrogen atoms
from adjacent double bonds. The third phase, termination, is the formation of peroxides,
34
ketones, and aldehydes. The previous compounds can break down and form secondary
compounds such as heptanal, benzene, and toluene. Shown below, are the steps in initiation,
propagation, and termination in an autoxidation reaction.
Initiation
Propagation
Termination
In the steps above, RH is any unsaturated fatty acid and R· is a free radical that is formed
by removing a liable hydrogen from a carbon atom that is adjacent to a double bond. ROOH is a
hydroperoxide. Hydroperoxides are one of the major initial oxidation products; these peroxides
will decompose and form compounds that are responsible for off-flavors and odors (rancid) in
food. After formation, hydroperoxides may break down via additional mechanisms.
Rancidity falls into two basic types, oxidative and hydrolytic. In oxidative rancidity, also
known as autoxidation, a food or food product will absorb oxygen from the surrounding
environment. Hydrolytic rancidity is also referred to as hydrolysis or enzymatic oxidation
(Koon, 2009), occurs in the presence of moisture via enzymatic peroxidation, but with the
35
absence of air. Enzymes that are naturally found in plant oils (ex. lipoxygenase) and animal fats
(lipase) will help catalyze hydrolytic rancidity (Koon, 2009).
Factors that can speed up lipid oxidation include: temperature, water, catalysts, light, and
time. As the temperature rises, the reaction rate of lipid oxidation will increase. Ultraviolet
(UV) radiation or light (photo oxidation) promotes the degradation and breakdown of
unsaturated fatty acids. Catalysts such as trace metals (transition metals such as copper, nickel,
magnesium and iron), high energy radiation (Sewald & DeVries, 2013), or inorganic salts can
also speed up the rancidity process. Control of light, temperature, oxygen concentration,
presence of catalysts (metal ions), and water activity can reduce the extent of lipid oxidation in
foods. In order to reduce oxidative rancidity, it is important to reduce each of the previously
listed factors.
Measurement of lipid oxidation. Lipid oxidation can be measured either by primary
oxidation products (hydroperoxides) or secondary oxidation products. Primary oxidation
products are both unstable and susceptible to further decomposition. Secondary oxidation
products include aldehydes, alcohols, ketones, hydrocarbons, and epoxy compounds. Additional
tests used on fats and oils to test lipid oxidation can include: iodine value, peroxide value, acid
value, saponification value, color value, smoke point, (Potter & Hotchkiss, 1998, pp. 377-379),
n-hexanal, active oxygen method (Sewald & DeVries, 2013.), thiobarbituric acid (TBA), p-
anisidine value, and carbonyl (Shahidi & Zhong, 2005, p. 372). The primary lipid oxidation tests
include peroxide value and iodine value. Primary lipid oxidation tests are the peroxide value,
hexanal value, and iodine value. Secondary oxidation tests are TBA test, p-anisidine value, and
carbonyl. Secondary oxidation tests are commonly used over primary oxidation tests due to the
ability to determine lipid oxidation throughout storage (for the most part) for longer shelf life for
36
dry food products. When measuring lipid oxidation it is important to know the food product or
oil/fat being tested and the products that each lipid oxidation test is commonly used. Each test
may require special equipment, chemicals, and/or extraction of fats/oils. As shown below in
Figure 4 (Pike, 2003, p. 237) it is important to use the proper primary and/or secondary lipid
oxidation tests during lipid oxidation testing. The figure shows that over time primary
oxidations will increase and eventually decrease, but in the case of the secondary products there
is a continued increase over time (due to the breakdown of primary lipid oxidation).
Figure 4. Lipid oxidation products vs. time (Pike, 2003, p. 237).
Thiobarbituric acid (TBA) test is the most widely used test for measuring the extent of
lipid oxidation in foods and was used in this research. The TBA is widely used due to its
simplicity and high correlation with sensory evaluation scores. In lipid oxidation,
malonaldehyde (MA) forms as a result of the breakdown of polyunsaturated fatty acids. In the
TBA test, one molecule of MA reacts with two molecules of thiobarbituric acid (TBA) (Simic,
Jovanovic, & Niki, 1992, p. 29) to form a red-pink TBA-MA complex; a seen in Figure 5, the
color complex can be quantified spectrophotometrically at 530-535 nm (Shahidi & Zhong, 2005,
37
p. 366). The extent of lipid oxidation is reported as the TBA value and expressed as milligrams
of MA equivalents per kilogram of sample.
Figure 5. Reaction of thiobarbituric acid (TBA) with malonaldehyde (MA) to form the TBA-
MA complex (Simic et al., 1992, p. 29).
The downside to the TBA testing is that it can be insensitive in the detection of low levels
of MA. Additional substances such as reducing sugars, alkenals, alkadienals, and other
aldehydes could interfere with the MA-TBA reaction. It is important to note that the TBA test
should only be used to compare samples of a single material at different states of oxidation.
Texture
Texture of a food product can be determined by texture profile analyses (TPA) on an
instrument known as a Universal Testing Machine (UTM). The UTM consists of two loading
points, upper and lower, and a load cell. The two loading points can be fitted with a variety of
attachments (plates, grips, probes, clamps, etc.) and perform a variety of tests such as tension,
bending, compression, and shearing. Texture is one of the most important sensory characteristics
of dry food products. The downside to sensory evaluation is that it is costly and time consuming.
Therefore, TPA is a cost effective and quick method to predict sensory texture attributes.
TPA can measure a variety of texture attributes such as: hardness, chewiness,
adhesiveness, and springiness (Kim et al., 2009). Instrumental testing can also help calculate
38
fracture stress/strain, fracture energy, cutting force/energy, and puncture force/energy. In this
study, the breaking strength of each sample was reported.
Table 6, shown below, displays the texture of a breakfast cereal at 2.5% initial moisture
content, stored under a various range of relative humidities (Sewald & DeVries, 2013). Chauhan
and Bains, and Katz and Labuza reported the critical water activity values for loss of crispness in
corn curl and extruded rice snacks as 0.36 and 0.43, respectively (as cited in Min et al., 2010, p.
346).
Table 6
The Effect of Relative Humidity (%RH) Storage Condition on Moisture Content (% MC) and
Texture of a Breakfast Cereal with an Initial Moisture Content of 2.5%
RH (%) MC (%) Texture
0.0 1.54 Crisp
11.1 2.27 Crisp
22.9 3.18 Crisp
32.9 4.59 Soft
43.9 6.55 Soggy
53.5 8.27 Soggy
64.8 11.43 Rubbery
75.5 15.88 Moldy
86.5 23.69 Moldy
(Sewald & DeVries, 2013).
Sensory
Sensory is a very important aspect in shelf life testing. Lipid oxidation, water activity,
and texture analyses testing can show that a product stored under various time, temperature, and
39
humidity conditions is still acceptable, but sensory testing may reveal that this is not the case.
Sensory testing results along with other experimental testing results can help determine the
critical moisture content or acceptable limit of a food. This critical moisture content becomes
very important when determining the shelf life of a food product. Sensory results can also
strengthen or confirm texture and lipid oxidation results.
As the water activity of a dry product, such as cereals and crackers, the crispness of these
products will decrease. Katz and Labuza found through sensory testing at a water activity of 0.4-
0.45, saltine crackers will lose the desirable crispness, and is the range where most cereal-based
products will significantly lose crispness (as cited in Labuza, 1984, p. 25). In the case of sugar
coated or sugar glazed ready to eat cereals a 2-3% increase in moisture content causes a loss of
crispness, and causes the cereals to clump (Azanha & Faria, 2005).
Sensory analysis of rancid odor descriptors such as, oxidized, grassy, painty, correlated
with the volatile lipid oxidation products such as hexanal, heptanal, and TBARS correlate better
with sensory scores that that of peroxide value (Eldin, 2010, p. 190). In dry cereal and snack
food products, loss of crispness, caused by moisture gain, and rancidity/off-flavors development
caused by lipid oxidation are the most important modes of destruction. These modes of
destruction cause these products to fail in consumer acceptance.
During sensory evaluation panelists, trained or untrained, evaluate a food product on one
or more various attributes. Quantitative analysis is generally used in shelf life testing of a
product. The data is used to determine preference or liking of a product’s sensory attributes.
Hedonic scales are the most common scales used in quantitative analyses because the collected
data can be further analyzed using a variety of statistics (mean, standard deviation, p-value).
Shelf life sensory analyses involves a control sample (fresh product at initial moisture content)
40
and samples stored under various conditions. Sensory results can be used to determine the
critical moisture content, the point at which a product loses texture at a level in which it is
rejected by sensory panelists.
Packaging
Packaging is key in the food beverage industry to extend shelf life by reducing
environmental (ex. light, temperature, moisture, oxygen, and volatiles), biological (bacteria,
mold, and yeast) factors and protecting foods from breakage (ex. chips, cereals, candies, and
crackers). Without packaging, harvesting, processing, distribution, and storage of food products
would not exist as it does today. Packaging allows for distribution of food products all over the
world and a reduction in waste due to spoilage and damage. Packaging also allows for the
consumer to store left over portions of a food or beverage for future consumption without the
need to purchase additional storage materials.
To develop or choose proper packaging for storage of a food or beverage, it is important
to know the main factors/conditions that will cause deterioration. As reported by Paine, from
most important to least important for the specific deterioration indices for breakfast cereals are:
moisture changes, physical damage, taint; etc., and oxidation changes (as cited in Emblem, 2000,
p. 165). Robertson (2013) reported the indices of failure for breakfast cereals to consider when
selecting suitable packaging materials as; moisture gain, lipid oxidation, loss of vitamins, and
loss of aroma (p. 548).
Requirements for food packaging include but not limited to, must be non-toxic, act as a
moisture barrier, protect from microorganisms, protect from odors, prevent from physical
damage, be low cost, and it needs to be compatible with the food in which it protects (Potter &
Hotchkiss, 1998, p. 478). Traditional cereal packaging of breakfast cereals uses a printed
41
chipboard or paperboard for the outer packaging and a plastic film for the inner packaging. The
chipboard or paperboard is used to display the cereal name, graphics, nutritional information, etc.
The cereal is stored within a plastic film, typically a thin HDPE (high density polyethylene).
HDPE coextruded with a thin layer of EVA (ethylene-vinyl acetate) copolymer is also used for
packaging of cereal products (Robertson, 2013, p. 549).
Experimental testing of packaging and food is generally avoided because it is time
consuming, thus empirical methods are used to test packaging. Packaging conditions can be
determined by mathematical models to predict the shelf life of the packaged food product. In
this research PP (polypropylene) plastic containers were studied to find the effect on shelf life for
two breakfast cereals, a high fat/sugar and a low fat/sugar cereal.
42
Chapter III: Methodology
The objective of this study was to evaluate the shelf life of a high fat/sugar cereal and a
low fat/sugar cereal when stored in various size airtight PP containers (1/2 cup, 2 cup, and 4
quart) at three different temperatures (10, 23, and 38°C) and seven relative humidities ranging
from 11 to 97% determined by the integrated GAB model. Critical moisture content, an
important factor in shelf life, was determined by examining lipid oxidation data (thiobarbituric
acid number), watery activity, breaking strength, and sensory data. Moisture permeability, an
important factor in food packaging containers, was evaluated using the water vapor transmission
rate (WVTR) method. This chapter presents the methodology used to assess shelf life, moisture
content, TBARS value, water activity, sensory, and breaking strength (crispness) of both types of
cereals.
Materials
Two types of dry cereals were obtained from a local grocery store and were assigned as a
high fat/sugar cereal and as a low fat/sugar cereal. The high fat/sugar cereal and low fat/sugar
were referred to as cereal A and cereal B, respectively (nutritional break down Appendix A).
Chemicals
Butylated hydroxyanisole (BHA) (>=98.5%), 2-thiobarbituric acid (TBA)(>=98%),
ethanol (200 proof, HPLC/spectrophotometric grade), and glacial acetic acid (>=99.7%) were
purchased from Sigma-Aldrich, Inc. (Milwaukee, WI). 1,1,3,3-tetraethoxypropane (TEP)(97%)
was purchased from Cole-Parmer. Seven analytical grade salts were used to create specific
relative humidity levels, these salts were lithium chloride, magnesium chloride, potassium
carbonate, sodium bromide, sodium chloride, potassium chloride, and potassium sulfate.
43
Sample Storage Method
For accelerated shelf life testing, all samples were stored at 10, 23, and 38°C at relative
humidities ranging from 11-97%. Temperature control chambers (Lunaire Environmental
Division Lunaire Limited) were used for 10 and 38°C testing. Samples stored at these
temperatures were placed in airtight storage containers, 10.8 cup, as shown in Figure 6.
Saturated salt solutions were placed in petri dishes and two raised perforated platforms were used
to hold cereal samples. Samples stored at 23°C were stored in fish tanks covered with Plexiglas
and sealed with vacuum grease as shown in Figure 7. A platform was used to hold samples and
allow for air circulation. Saturated salt solutions were placed in to beakers.
Figure 6. Example of moisture sorption chamber for 10 and 38°C samples.
Figure 7. Moisture sorption chambers for 23°C samples (note 6 of 7 shown).
44
Saturated Salt Solution Preparation
Seven salts were used to create specific relative humidities. Salt crystals were prepared
according to Table 7 (MitTenGen, n.d.), by dissolving the proper amount of salt in distilled water
(DW) using a using a magnetic stirrer and hot plate.
Table 7
Solubility of Saturated Salt Solutions
Salt Solubility (g/ml)
Lithium chloride (LiCl) 0.80
Magnesium chloride (MgCl) 0.56
Potassium carbonate (K2CO3) 1.15
Sodium bromide (NaBr) 1.16
Sodium chloride (NaCl) 0.37
Potassium chloride (KCl) 0.40
Potassium sulfate (K2SO4) 0.12
(MitTenGen, n.d.)
Excess salt crystals were added to each saturated salt solution to ensure proper humidity levels
throughout the experiment. Saturated salt solutions were placed in each moisture sorption
chamber in order to create equilibrium relative humidities as shown in Table 8.
45
Table 8
Relative Humidity (%RH) of Saturated Salt Solution at 10, 23, and 38°C
Saturated Salt Solution Relative Humidity (%RH)
10°C 23°C1 38°C2
LiCl 11.3 11.3 11.3
MgCl 33.5 33 31.9
K2CO3 43.1 43.2 43.2
NaBr 62.2 58.4 53.9
NaCl 75.7 75.4 74.8
KCl 86.8 84.7 82.7
K2SO4 98.2 97.5 96.6
(Fontana, 2007, pg.391-393) 123°C was an average of 20 and 25°C relative humidities 238°C was an average of 35 and 40°C relative humidities
Digital thermometer/hygrometers (HygroSet®, HygroSet® II Adjustable Digital Hygrometer,
Weston, Florida) were used to ensure proper relative humidities were reached and maintained
throughout the experiment.
Additional samples of 200 g were placed into plastic containers under each condition for
breaking strength, thiobarbituric acid reactive substances (TBARS) analysis, and sensory testing
were created by placing 200 g cereal samples into plastic containers. Saturated salt solutions in
petri dishes were used to maintain proper humidity conditions as shown in Figure 8.
46
Figure 8. Storage for 200 g samples.
Initial Moisture Content
Moisture content was determined based on the method of the American Association of
Cereal Chemists (AACC Method 44-15A). Approximately 5 grams cereal samples, weighed to
the nearest 0.0001 g using an analytical balance were placed into aluminum weigh dishes (in
quintuplicate) and dried in a draft oven at 103°C for 72 hours. After drying, the dishes were
weighed (dry weight) and the initial moisture content was calculated as a percentage of dry basis
by
where Wm represents weight of moisture and Wd the weight of dry material
Determination of Moisture Sorption Isotherms
In the determination of moisture sorption isotherms, quintuplicate samples of
approximately 5 grams were placed onto aluminum weigh dishes. The samples were placed into
the proper moisture sorption chambers. Samples were weighed (to the nearest 0.0001 g) every 3-
47
4 days until equilibrium was reached (weight of dishes did not increase). Equilibrium moisture
content (Me) of the cereals were calculated. Moisture sorption isotherms of the cereals were
obtained by plotting Me versus RH.
Determination of the GAB Model
A common method to determine moisture sorption isotherms and calculate shelf life is by
using the GAB model. The GAB model is based on the following equation
where Wm represents the water constant in the mono-layer, Me is the equilibrium moisture
content of the product on dry basis, k is a factor correcting for the properties of the multi-layer
molecules with respect to the bulk liquid, aw is the water activity, and C is the Guggenheim
constant.
Determination of Shelf life using the Integrated GAB Model
The shelf life of both the cereal products was calculated from the experimental data by
using the integrated GAB model (Diosady, Rizvi, Cai, & Jagdeo, 1996). Shelf life was
calculated using the following equations
where t represents shelf life in days and Wm, k, and, C are GAB constants (from the GAB model
equation). Awo represents storage humidity (decimal), Mf is the critical moisture content
48
(maximum moisture content to maintain shelf life), Mi is the initial moisture content of the
cereal, and ps is the saturation vapor pressure (mm Hg), see Table 9. P is the permeability
coefficient of the storage container and WD is product weight (in grams).
Table 9
Saturation Vapor Pressure (ps) Values at 10, 23, and 38°C
Temperature (°C) ps (mm Hg)
10 9.209
23 21.068
38 49.6912
(Kluiber, 1998)
Model Validation
Four forms of validation were used to determine how the experimental data fit the GAB
model. The four forms of validation were: R2 (coefficient of determination), RMS (root mean
square), E (mean relative percentage deviation modulus) and the percentage RMSE (root mean
square error).
The coefficient of determination was used to measure the proportion of variability
attributed to the model. The coefficient of determination was determined from the best fit line of
the experimental data to the GAB model.
RMS was used to evaluate the quality of the fit of the GAB model to the experimental
data. RMS was calculated using the formula below,
where me is experimental value, mp is the predicted value, and N is number of experimental data.
49
E and RMSE values were calculated using the formulas below,
where me is the experimental value, mp is the predicted value, and N is the number of
experimental data.
Moisture Content at Equilibrium
When cereal samples, from the determination of moisture sorption isotherms reached
equilibrium, the samples were transferred into pre-weighed aluminum weigh dishes. Samples
were dried using a vacuum oven (Yamato, ADP-31) at 130°C and 30 inches of Hg for 7 hours.
After drying, samples were allowed to cool in a desiccator dry box. The weight of the samples
was recorded to four decimal places. The moisture content on dry basis of the cereal samples
was calculated. Critical moisture content for each cereal was determined using sensory,
thiobarbituric acid reactive substances, water activity, and breaking strength data.
Determination of Moisture Permeability
Proper packaging can protect foods from environmental factors such as moisture, oxygen,
and microbial growth thus, it is very important in shelf life of food products. Moisture
permeability is determined by finding water vapor transmission rate (WVTR). WVTR is
normally reported as amount of water (in grams) that can pass through a given area (m2) in a
specified amount of time. In this study, WVTR is reported as the grams of moisture (water) per
day, which can pass through a given packaging container. WVTR testing was completed on
various size polypropylene containers, ½ cup, 2 cup, and 4 quart.
50
WTVR was measured according to the ASTM standard method (ASTM E96-00). To
determine WVTR, 5 g, 10 g, and 20 g of desiccant (CaCO4) were added to the ½ cup, 2 cup, and
4 quart container, respectively and placed at 10, 23, and 38°C. Standard breakfast cereal
packaging (paperboard box and HDPE pouch) received 20 g of desiccant. The pouch was rolled
down twice and clipped with a 1.5 inch paper binder clip, placed back into the paperboard box,
and the flaps closed to simulate consumer storage. The boxes were placed at room temperature
(23±1°C and 31±1% RH). Three containers of each size were filled with desiccant and two of
each container without desiccant acted as controls. Containers stored at 10°C and 38°C were
placed in temperature control chambers (Lunaire Environmental Division Lunaire Limited) at
80% RH. Containers stored at 23°C were stored in fish tanks covered with Plexiglas and sealed
with vacuum grease. To maintain a proper humidity of 75.4%, a saturated salt solution of
sodium chloride was placed into each moisture sorption tank. Each container weight was
monitored until constant weight has been reached on consecutive measurements.
Water vapor transmission rate, WVTR, was calculated
where G is weight gain (g) and t is time (days).
Permeability, P, was calculated
where RH, is the relative humidity (decimal) at which the containers were stored and ps,
as stated in the integrated GAB model, represents saturation vapor pressure (mm Hg) at test
temperature (as found in Table 9).
51
Thiobarbituric Acid Reactive Substances (TBARS)
To determine the extent of lipid oxidation a modified TBARS method (Lee & Ahn, 2003)
was used. All samples were tested in triplicate.
Reagent stock solution preparation. A 10% stock solution of BHA was made by
dissolving BHA in ethanol (200 proof, HPLC/spectrophotometric grade). Thiobarbituric acid
(TBA)/glacial acetic acid (GAA) stock solution was created by dissolving 20 mM (2.88 g) of
TBA in warm DW in a 1L volumetric flask. To the flask 150 g of glacial acetic acid was added.
Distilled water was added to the 1L mark. A TEP solution was created by diluting 0.5 ml of TEP
in 499.5 ml of DW.
Five gram samples and 15-mL of deionized DW were added to a 50-mL test tube. The
samples were homogenized using a Politron (Kinematica, Ag Model: PT 10-35 GT) at 30,000
rmp for 15 seconds. In triplicate, 1-ml of the cereal homogenate was transferred to a 13 x 100-
mm glass test tube. Each glass test tube received 50 μL BHA and 2 ml of the TBA stock
solution. The mixture was vortexed and then heated in a boiling water bath (beaker filled with
water and heated on a hot plate (Hot Plate/Magnetic Stirrer, Fisher Scientific, Catalog No. 11-
520-49SH, Pittsburgh)) for 15 minutes, to allow for color development. The mixture was cooled
for 10 minutes in a cold water bath. The mixture was vortexed and then centrifuged for 20
minutes at 4000 rpm. The supernatant was transferred to a 1 cm plastic cuvette. The absorbance
of the supernatant was read at 531 nm using a UV-Visible Spectrophotometer (Varian-Cary 50
Bio UV-Vis Spectrophotometer, Agilent Technologies, Santa Clara, California) against a blank
containing 1 ml of DW and 2 ml TBA/GAA solution. The amounts of TBARS were expressed
as milligrams malondialdehyde (MDA) per kg of cereal.
52
Creation of malonaldehyde standard curve. A TEP standard solution (1 x 10-3 M) was
created by diluting the TEP solution 1:3.0418 (TEP solution: DW) in DW. The standard curve
was created by adding 0, 5, 10, 20, 30, 40, and 50 μL of the TEP standard solution to 13 x 100
mm glass test tubes and DW was added to make a 1 mL solution. Four replications were made
for each concentration. Each test tube received 2 mL of the TBA/GAA solution. The solutions
were vortexed and heated in a boiling water bath for 15 minutes, to allow for color development.
The solutions were allowed to cool for 10 minutes in a cold water bath and then vortexed. The
solutions were transferred to 2 cm plastic cuvettes. The absorbance of the solutions were read at
531 nm using UV-Visible Spectrophotometer against a blank containing DW.
Table 10
Standard Solutions for TBARS Analysis
MDA (ppm) TBA stock solution (μL) d-H2O (μL) TBA/GAA (mL)
0 0 1,000.0 2.0
0.36 5.0 995.0 2.0
0.72 10.0 990.0 2.0
1.44 20.0 980.0 2.0
2.16 30.0 970.0 2.0
2.77 40.0 960.0 2.0
3.6 50.0 950.0 2.0
53
To obtain a standard curve, absorbance was plotted versus ppm malondialdehyde (MDA).
The data was fit with a linear line and MDA was calculated by
where y is the absorbance of the sample, a is the slope of the linear line of the standard
curve, x is the amount of TBARS expressed as milligrams (ppm) of MDA per kilogram of the
cereal sample, and b is the y-intercept of the linear line of the standard curve.
Determination of Water Activity
Water activity testing was conducted with an Aqua Lab Water Activity Meter (Decagon
Devices Inc., Model Series 3 TE, Pullman, Washington, USA). To determine the watery activity
(aw) of each cereal sample, one piece of each sample was placed into a water activity cup. All
cereals were allowed to equilibrate to room temperature (23 - 25°C). The cup was placed into
the water activity meter, the door was closed, and the machine was switched to read. After the
water activity reading was given, the process was repeated (in triplicate) for each cereal storage
condition.
Sensory Analysis
On March 5, 2013, permission to conduct this study was approved through the University
of Wisconsin Stout Institutional Review Board (Appendix B). The sensory analysis was
completed over two days with a total of 24 panelists in Menomonie, WI and at the University of
Wisconsin-Stout Discovery Center (Menomonie, WI). Day one included a total of 9 panelists
and 18 samples. Day two had a total of 15 panelists and 2 samples. The panelists were
comprised of students, faculty, and community members. Panelists were asked to evaluate color,
aroma, sogginess/crispness, taste, off flavor/flavor, and over acceptability.
54
The samples were removed from each moisture sorption chamber and water activity
readings were taken to ensure samples to be tested had a water activity reading under 0.500. The
cereal was then placed into pre-labeled sample cups. All sample cups were coded with a random
three digit number. Each sample cup contained 4 pieces of the high fat/sugar cereal and 6 pieces
of the low fat/sugar cereal. Participants were briefed on the purpose of the sensory evaluation
and the risks involved. Participants signed a consent form (Appendix C) after being briefed.
Participants were asked to first to evaluate a sample on color and aroma. Participants were then
instructed to taste each sample and evaluate the sample on sogginess/crispness, taste, off
flavor/flavor, and overall acceptability. After each sample, participants were instructed to drink
room temperature drinking water. Panelists did not have to eat the entire sample contained in
each sample cup nor did the participants have to swallow the samples. Cereal attributes were
ranked on a nine point hedonic scale where one being “dislike extremely”, five being “neither
like nor dislike”, and nine being “like extremely”. Data was collected using a sensory evaluation
form (Appendix D) and entered in to IBM Statistical Program for Social Sciences version 21
(IBM SPSS Statistic 21, 2012, New York, New York) and Microsoft® Excel 2010 (Microsoft
Corporation, Cambridge, Massachusetts, USA).
Breaking Strength
Breaking strength was conducted on an Instron Universal Testing Machine (Model 3342,
Instron, Norwood, Massachusetts, USA) with a 500 N load cell and compression plates (Figure
9). All cereal samples were brought to room temperature (23°C) before testing.
Cereal A samples had dimensions of length and width of 17.00 mm and a thickness of
2.00mm. Cereal B samples had dimensions of diameter of 10.00 mm and thickness of 5.00 mm.
Samples were compressed between 45 mm plates. The test was initiated at preload of 0.00002
55
kgf (kilograms of force). The crosshead speed was 100 mm min-1. Samples were compressed
until breakage or to 20% of the original height. Compressive load at break was measured in
kilograms of force.
Figure 9. Universal Testing Machine (UTM).
Statistical Analysis
Raw data for sensory results was entered and analyzed using IBM Statistical Package for
the Social Sciences 21 (IBM SPSS Statistic 21) where a one-way ANOVA was used to examine
significant differences among the sample means (p ≤ 0.05), Tukey’s honestly significant
difference (HSD) post hoc test was used to determine where differences existed if a main effect
was found, p ≤ 0.05. Resulting data was entered in to Microsoft Excel 2010 (Microsoft
Corporation, Cambridge, Massachusetts, USA) to generate graphs. Data was analyzed for main
effects of cereal type, temperature, and relative humidity on critical moisture content, water
activity, TBARS value, and breaking strength using a one-way ANOVA (p ≤ 0.05), post hoc
56
Duncan’s multiple range test was used to determine where differences existed if a main effect
was found, p ≤ 0.05.
57
Chapter IV: Results and Discussion
Water activity, breaking strength, and sensory results were evaluated to determine the
critical moisture content for both cereals A and B. Initial and critical moisture contents were
used along with water vapor permeability of various size airtight polypropylene containers to
determine the effect on shelf life compared to standard cereal packaging. The results obtained on
GAB model parameters, moisture sorption isotherms, moisture content, WVTR and
permeability, TBARS value, water activity, breaking strength, sensory evaluation, critical
moisture content, and shelf life are discussed in this chapter.
Initial Moisture Content
Fresh cereal samples had an initial average moisture content (dry basis) of 4.37 ± 0.12
and 3.62 ± 0.12 for cereal A and B, respectively. Due to the high fat/sugar nature of cereal A, it
had higher moisture content and standard deviation compared to the low fat/sugar (cereal B).
Determination of the GAB Model
The GAB parameters at each temperature are shown in Table 12 and 13 for cereal A and
B, respectively. Table 13 shows that only the constant C decreases with a temperature increase.
In general, a good moisture sorption model should have a high R2, low %RSM, E, and RMSE. It
was observed that the GAB model is a good fit and is quantified with the regression coefficient
R2 (Table 12 and 13). The GAB models fits up to a water activity of 0.982, 0.975, and 0.966 for
10, 23, and 38°C, respectively for both cereal A and B. A model is considered suitable if the E
value is less than 10 (Ayranci & Duman, 2004), in this study all E values were less than 10 for
both cereals A and B (Table 11 and 12). The low values of E and RMSE help strengthen the
usefulness of the GAB model when studying adsorption of ready to eat breakfast cereals.
58
Table 11
GAB Parameters of Cereal A at 10, 23, and 38°C
Temperature (°C) Wma Cb kc R2d RMSe Ef RMSEg
10 0.0450 45.0207 0.9142 0.9577 10.2039 8.0890 0.0318
23 0.0426 78.0433 0.8893 0.9524 7.3604 6.3346 0.0017
38 -0.0721 0.9718 26.8949 0.9852 3.5139 2.9036 0.0048
aWm represents that water constant in the mono-layer. bC is the Guggenheim constant. ck is the factor correcting for the properties of the multi-layer molecules with respect to the bulk liquid. dR2 is the coefficient of determination. eRMS is the root mean square. fE is the relative percentage deviation modulus. gRMSE is the root mean square error.
Table 12
GAB Parameters of Cereal B at 10, 23, and 38°C
Temperature (°C) Wma Cb kc R2d RMSe Ef RMSEg
10 0.0583 47.6634 0.8358 0.9639 5.1529 4.5595 0.0061
23 0.0590 17.6245 0.7211 0.9651 3.4811 2.8576 0.0038
38 -0.0564 0.9614 19.5350 0.9899 4.2987 3.7315 0.0049
aWm represents that water constant in the mono-layer. bC is the Guggenheim constant. ck is the factor correcting for the properties of the multi-layer molecules with respect to the bulk liquid. dR2 is the coefficient of determination. eRMS is the root mean square. fE is the relative percentage deviation modulus. gRMSE is the root mean square error.
Determination of Moisture Sorption Isotherms
Figure 10 and 11 display the moisture sorption isotherms for cereal A and B, respectively
upon reaching equilibrium. Both cereals moisture sorption isotherms exhibit a sigmoidal (Type
II) shape, indicating multilayer adsorption. These results are similar to another cereal product,
jasmine rice crackers, where a type II isotherm was exhibited (Siripatrawan & Jantawat, 2006).
59
The same study found that at a constant water activity (relative humidity) that a shift from 30 to
60°C led to a shift of the isotherms to a lower EMC (equilibrium moisture content). This shift
was observed in both cereals when temperature shifted from 10 to 23°C.
Roca, Guillard, Guilbert, and Gontard (2006) found that adding fat (0, 0.11, 0.30 g/g d.b.
fat content) to sponge cake significantly decreased the equilibrium moisture content at higher
water activities (aw > 0.9). Roca et al. (2006) stated that a foods moisture sorption isotherm is
the sum of the hygroscopic properties of a foods individual components and that modifying this
composition can influence the sorption isotherm. For example the addition of fat, a hydrophobic
component, will cause a decrease in moisture equilibrium content.
60
Figure 10. Moisture sorption isotherms of cereal A at 10, 23, and 38°C.
Figure 11. Moisture sorption isotherms of cereal B at 10, 23, and 38°C.
61
Moisture Content at Equilibrium
An ANOVA showed a significant effect of storage temperature and relative humidity on moisture content (p < 0.001and p <
0.0001, respectively) but no significant main effect was found between moisture content (%d.b.) and the type of cereal. The moisture
content of the cereals significantly increased as the storage humidity increased, but as the temperature increased the moisture content
significantly decreased for the corresponding relative humidity (Table 13).
Table 13
Moisture Content (g/100 g dry solids) of Cereal A and B when Stored at 10, 23, and 38°C
10°C Cereal A Cereal B 23°C Cereal A Cereal B 38°C Cereal A Cereal B
Control 4.37 ± 0.12st 3.62 ± 0.12t-v Control 4.37 ± 0.12st 3.62 ± 0.12t-v Control 4.37 ± 0.12st 3.62 ± 0.12t-v
11.3 2.58 ± 0.67vw 2.17 ± 0.31w 11.3 2.54 ± 0.52vw 2.20 ± 0.20w 11.3 1.13 ± 0.36x 2.76 ± 0.29u-w
33.5 4.13 ± 1.18t 4.37 ± 0.49st 33.0 3.91 ± 0.10t 6.05 ± 0.15qr 31.9 3.63 ± 1.03t-v 3.56 ± 0.32t-v
43.1 4.51 ± 0.47st 6.73 ± 0.18pq 43.2 4.13 ± 1.18t 7.48 ± 0.27op 43.2 3.82 ± 0.12tu 5.28 ± 0.50rs
62.2 9.45 ± 0.22m 8.21 ± 0.32no 58.4 9.00 ± 0.23mn 6.61 ± 0.82pq 53.9 8.20 ± 0.36m-o 6.30 ± 1.26qr
75.7 16.69 ± 0.36hi 12.27 ± 1.75k 75.4 15.76 ± 0.29ij 11.25 ± 0.24l 74.8 15.07 ± 0.74j 10.69 ± 0.51l
86.8 21.80 ± 0.67de 18.13 ± 0.85g 84.7 17.32 ± 0.27gh 15.09 ± 1.21j 82.7 15.90 ± 0.34ij 10.79 ± 1.14l
98.2 36.74 ± 1.24b 52.81 ± 1.85g 97.5 24.48 ± 1.16c 22.65 ± 1.70d 96.6 20.01 ± 0.64f 21.40 ± 0.43e
Mean values followed by different letters are significantly different (p < 0.05). (n=5).
62
Determination of WVTR and Moisture Permeability
As the storage temperature increased from 10, to 23, and to 38°C for the ½ cup, 2 cup,
and 4 quart container the net mass gain (slope, g/day) increased along with the water vapor
transmission rate (WVTR) as seen in Figures 12 to 14. As expected, as the size of the container
increased an increase in WVTR was also seen. The WVTR and storage conditions (relative
humidity and temperature) can be used to further calculate the moisture permeability of each
container at the experimental temperature.
Figure 12. Net mass gain of ½ cup container at 10, 23, and 38°C. Testing at 10 and 38°C
occurred under 80% RH and 23°C at 75.4%. Each experimental condition had 5 containers, 3
experimental (w/5 g of desiccant) and 2 controls (no desiccant).
63
Figure 13. Net mass gain of 2 cup container at 10, 23, and 38°C. Testing at 10 and 38°C
occurred under 80% RH and 23°C at 75.4%. Each experimental condition had 5 containers, 3
experimental (w/10 g of desiccant) and 2 controls (no desiccant).
Figure 14. Net mass gain of 4 quart container at 10, 23, and 38°C. Testing at 10 and 38°C
occurred under 80% RH and 23°C at 75.4%. Each experimental condition had 5 containers, 3
experimental (w/20 g of desiccant) and 2 controls (no desiccant).
64
Using the WVTR data (Table 14), the saturation vapor pressure (Table 9), and the
permeability equation, the permeability values were calculated for each container (Table 15). As
the temperature increased the permeability coefficient also increased for the same size container.
As shown in Table 15, as the size of the container increased the surface area increased, but this
increase does not correspond to the size increase of the container. Standard breakfast cereal
packaging had a smaller surface area compared to the 4 quart container, 1698 and 1921 cm2,
respectively, but the standard breakfast cereal packaging had a permeability coefficient over 2.5
times that of the 4 quart container. As expected, as the size of the container increased under the
same temperature the permeability coefficient also increased. Standard breakfast cereal
packaging (paperboard box and HDPE pouch) was tested at room temperature (23°C and a %RH
of 31±1%).
Table 14
WVTR (g/container-day) of ½ Cup, 2 Cup, 4 Qt. Containers, and Standard Breakfast Cereal
Packaging at 10, 23, and 38°C
WVTR (g/container-day)
Temperature (°C) ½ cup 2 cup 4 quart Breakfast Cereal Packaginga
101 0.0052 0.0073 0.0330 n/a
232 0.0059 0.0100 0.0375 0.0407
381 0.0084 0.0151 0.1049 n/a
1WVTR was tested at a relative humidity of 80%. 2WVTR was tested at a relative humidity of 75.4%. aWVTR was tested at a relative humidity of 31(±1)%.
65
Table 15
Surface Area (cm2) and Permeability (g/container-day-mmHg) of ½ Cup, 2 Cup, 4 Qt.
Containers, and Standard Breakfast Cereal Packaging at 10, 23, and 38°C
Permeability (g/container-day-mmHg)
Temperature (°C) ½ cup 2 cup 4 quart Breakfast Cereal Packaginga
Surface Area (cm2) 257 551 1912 1698
101 0.0007058 0.0009909 0.004479 n/a
232 0.0003714 0.0006295 0.002361 0.006232
381 0.0002113 0.0003798 0.002639 n/a
1Permeablitiy was tested at a relative humidity of 80%. 2Permeability was tested at a relative humidity of 75.4%. aPermeability was tested at a relative humidity of 31(±1)%.
Thiobarbituric Acid Reactive Substances (TBARS)
An ANOVA showed a significant effect of storage temperature on TBARS value (p <
0.0001). No significant effect between TBARS value and the type of cereal and relative
humidity was found. Cereal A generally had lower TBARS values compared to cereal B (Table
16) with exception to samples stored at 23°C where cereal A samples had higher TBARS values.
TBARS values of cereal samples stored above 80% could have been affected due to the start and
presence of mold growth. Paradiso et al. (2008) studied the effect of natural mixed tocopherols
on the shelf life of corn flakes. The authors found that the addition of natural tocopherols to corn
flakes reduced the oxidation of hexanal into hexanoic acid (secondary lipid oxidation) from 32%
to 15% via headspace analysis.
66
Table 16
TBARS Value (mg MDA/kg of cereal) of Cereal A and B when Stored at 10, 23, and 38°C
10°C Cereal A Cereal B 23°C Cereal A Cereal B 38°C Cereal A Cereal B
Control 2.06 ± 0.39op 2.72 ± 0.03g-n Control 2.06 ± 0.39op 2.72 ± 0.03g-n Control 2.06 ± 0.39op 2.72 ± 0.03g-n
11.3 2.64 ± 0.01h-o 2.79 ± 0.07g-m 11.3 2.38 ± 0.59k-o 1.42 ± 0.16q 11.3 3.69 ± 0.52c-e 4.41 ± 0.18ab
33.5 2.55 ± 0.09h-o 2.81 ± 0.03g-m 33.0 3.99 ± 1.04bc 2.10 ± 0.26o 31.9 4.04 ± 0.18bc 4.09 ± 0.11a-c
43.1 2.83 ± 0.10g-l 2.70 ± 0.20g-n 43.2 2.28 ± 0.55l-o 1.53 ±0.03q 43.2 3.44 ± 0.16d-f 3.70 ± 0.47c-e
62.2 2.92 ± 0.11f-k 2.98 ± 0.07f-j 58.4 2.23 ± 0.34m-o 2.23 ± 0.08m-o 53.9 3.44 ± 0.13d-f 4.58 ± 0.34a
75.7 2.44 ± 0.03j-o 2.46 ± 0.04i-o 75.4 3.03 ± 0.19f-i 1.34 ± 0.57q 74.8 3.09 ± 0.14f-h 4.14 ± 0.16a-c
86.8 2.19 ± 0.17no 2.53 ± 0.21h-o 84.7 2.16 ± 0.38no 1.46 ± 0.13q 82.7 3.23 ± 0.14e-g 4.05 ± 0.04bc
98.2 2.53 ± 0.36h-o 2.86 ± 0.13g-l 97.5 3.02 ± 0.10f-j 1.60 ± 0.01pq 96.6 2.56 ± 0.21h-o 3.86 ± 0.20cd
Mean values followed by different letters are significantly different (p < 0.05). (n=3).
67
Determination of Water Activity
An ANOVA showed a significant effect of storage temperature and humidity on the
water activity (p < 0.0001) of each cereal. The water activity value of the cereals increased as
the storage humidity increased, but as the temperature increased the water activity values
decreased for the same corresponding relative humidity (Table 17). In general, cereal A had a
higher water activity compared to cereal B, but there was no significant difference in water
activity value among the type of cereals. At a water activity range from 0.6 to 0.75 mold growth
will dominate that of bacteria growth, if samples are left at equilibrium for an extended period of
time mold growth can be seen (Table 6). Only one sample had a water activity above the critical
point of 0.85 (Cereal A, 10°C and 98.2% RH), the point at which bacterial pathogens begin to
grow.
68
Table 17
Water Activity of Cereal A and B upon Equilibrium when Stored at 10, 23, and 38°C
10°C Cereal A Cereal B 23°C Cereal A Cereal B 38°C Cereal A Cereal B
Control 0.362 ± 0.007p 0.318 ± 0.002r Control 0.362 ± 0.007p 0.318 ± 0.002r Control 0.362 ± 0.007p 0.318 ± 0.002r
11.3 0.554 ± 0.010k 0.432 ± 0.025n 11.3 0.366 ± 0.016p 0.341 ± 0.038q 11.3 0.338 ± 0.003q 0.325 ± 0.001rq
33.5 0.598 ± 0.016j 0.496 ± 0.008m 33.0 0.503 ± 0.002lm 0.486 ± 0.006m 31.9 0.343 ± 0.002q 0.391 ± 0.008o
43.1 0.584 ± 0.006j 0.552 ± 0.002k 43.2 0.504 ± 0.003lm 0.517 ± 0.005l 43.2 0.363 ± 0.004p 0.492 ± 0.008m
62.2 0.624 ± 0.002i 0.630 ± 0.003hi 58.4 0.619 ± 0.005i 0.546 ± 0.015k 53.9 0.541 ± 0.006k 0.546 ± 0.015k
75.7 0.726 ± 0.002e 0.709 ± 0.006ef 75.4 0.706 ± 0.004f 0.678 ± 0.005g 74.8 0.643 ± 0.013h 0.618 ± 0.010i
86.8 0.785 ± 0.003d 0.779 ± 0.004d 84.7 0.716 ± 0.002ef 0.726 ± 0.002e 82.7 0.666 ± 0.007g 0.726 ± 0.010e
98.2 0.862 ± 0.015a 0.866 ± 0.006a 97.5 0.822 ± 0.009b 0.806 ± 0.005bc 96.6 0.804 ± 0.013c 0.789 ± 0.003cd
Mean values followed by different letters are significantly different (p < 0.05). (n=3).
69
Breaking Strength
An ANOVA showed a significant effect of breaking strength on storage relative humidity
and type of cereal (p < 0.0001). No significant effect was found between breaking strength and
storage temperature. Cereal B had a higher breaking strength compared to cereal A (Table 18).
The significant differences in breaking strength can be contributed to cereal A having a thickness
of 2.00 mm and had similarities with a flaked cereal, while cereal B had a thickness of 5.00 mm
and had similarities with a hard formed cereal. Breaking strength increased as the storage
humidity increased, with the exception of cereal A at 38°C (decreased). Breaking strength
values could not be compiled above 53.9% RH due to the cereals being soggy/rubbery and
causing a max out of 50.00 kgf on the UTM. These samples would compress and not break.
Loss of crispness begins to occur at water activity range of 0.35 to 0.45. Using the water activity
data (Table 17) and breaking strength data (Table18), cereal A and B lost the ability to break at a
water activity above 0.584 and 0.522, respectively at 10°C. At 23°C cereal A and B lost the
ability to break at a water activity above 0.504 and 0.517, respectively. At 38°C cereal A and B
lost the ability to break at a water activity above 0.363 and 0.492, respectively. Cereal samples
above 82.7% RH were not tested due to the presence of mold growth.
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Table 18
Breaking Strength (kgf) of Cereal A and B when Stored at 10, 23, and 38°C
10°C Cereal A Cereal B 23°C Cereal A Cereal B 38°C Cereal A Cereal B
Control 1.22 ± 0.37e 7.10 ± 1.68d Control 1.22 ± 0.37e 7.10 ± 1.68d Control 1.22 ± 0.37e 7.10 ± 1.68d
11.3 0.57 ± 0.36e 7.08 ± 1.83d 11.3 0.44 ± 0.27e 7.30 ± 1.72d 11.3 0.82 ± 0.29e 8.03 ± 2.86cd
33.5 0.68 ± 0.52e 10.73 ± 2.89c 33.0 0.78 ± 0.21e 7.69 ± 1.71cd 31.9 0.72 ± 0.14e 9.10 ± 1.41cd
43.1 0.90 ± 0.42e 17.14 ± 3.75b 43.2 0.75 ± 0.17e 10.11 ± 1.71cd 43.2 0.63 ± 0.09e 38.97 ± 11.89a
62.2 Soggy1 Soggy1 58.4 Soggy1 Soggy1 53.9 Soggy1 Soggy1
75.7 Soggy1 Soggy1 75.4 Soggy1 Soggy1 74.8 Soggy1 Soggy1
86.8 Mold Growth2 Mold Growth2 84.7 Mold Growth2 Mold Growth2 82.7 Mold Growth2 Mold Growth2
98.2 Mold Growth2 Mold Growth2 97.5 Mold Growth2 Mold Growth2 96.6 Mold Growth2 Mold Growth2
Mean values followed by different letters are significantly different (p < 0.05). (n=10). 1Soggy samples were not tested because samples would only compress causing load cell to max out. 2Moldy samples were not tested in order to prevent contact with mold.
71
Sensory Analysis
A total of 24 participants participated in the informal sensory analysis of two types of
cereals, a high fat/sugar and a low fat/sugar. The sensory included questions regarding the liking
of cereal attributes using a 9 point hedonic scale (1= dislike extremely, 2= dislike very much, 3=
dislike moderately, 4= dislike slightly, 5= neither like nor dislike, 6= like slightly, 7= like
moderately, 8= like very much, 9= like extremely). Sensory data were analyzed by a one-way
ANOVA (p < 0.05) and post hoc Tukey’s HSD (IBM SPSS Statistics 22).
Cereal A. Table 19 shows the mean rating of the liking for color, aroma, crispness, taste,
flavor, and overall acceptability for cereal A samples. All samples tested had an average water
activity less than 0.500 at the time of testing. Sensory analysis was not completed on 10°C
samples due to water activity readings above 0.500.
A one-way ANOVA revealed that there was no difference in liking scores for color, F(8,
72) = 2.474, p=0.774. All cereal samples on average scored above the midpoint (5 = neither like
nor dislike).
A one-way ANOVA displayed that there was differences in liking scores for aroma, F(8,
72) = 2.474, p=0.020 noting that all samples stored at 23°C were significantly liked more than
the control sample.
A one-way ANOVA revealed that there was a significant difference in liking scores for
crispness, F(8, 72) = 15.593, p<0.000. Post hoc testing revealed no significant differences
between the control (7.11) and samples stored 23°C and 11.3 (7.44) and 33.0 (7.22) % RH. As
the storage temperature and/or humidity increased a significant decrease in the likeness of
crispness was observed (with exception to samples stored at (38°C and 43.2 and 53.9% RH).
72
A one-way ANOVA declared differences F(8, 72) = 7.851, p<0.000 on taste. Post hoc
testing revealed that samples stored at 23°C and 11.3% RH (7.56) were significantly liked more
for taste than the control sample (7.00) and all other samples. As the storage temperature and
humidity increased the liking scores for taste decreased significantly.
A one-way ANOVA presented existence of significant differences F(8, 72) = 6.828,
p<0.000 on flavor. Post hoc testing revealed that samples stored at 23°C and 11.3% RH (7.22)
were significantly liked more for taste than the control sample (6.11) and all other samples. As
the storage temperature and humidity increased the likeness of flavor of the samples decreased
(with exception to samples stored at 38°C and 31.9% RH).
A one-way ANOVA indicated existence of differences F(8, 72) = 9.469, p<0.000 on
overall acceptability. Post hoc testing revealed that samples stored at 23°C and 11.3% RH (7.33)
were significantly liked more than the control sample (6.33) and all other samples. Post hoc
testing revealed that samples stored at 23°C were significantly liked more than samples stored at
38°C. The overall acceptability of cereal A samples decreased as the storage RH increased.
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Table 19
Mean Rating Comparisons of Cereal A for Color, Aroma, Sogginess/Crispness, Taste, Off-flavor/Flavor, and Overall Acceptability
Color1 Aroma1 Sogginess/Crispness1 Taste1 Off-flavor/Flavor1 Overall Acceptability1
Control 6.11 ± 2.52a 4.67 ± 2.87ab 7.11 ± 2.76c 7.00 ± 1.50de 6.11 ± 2.47cd 6.33 ± 2.06cd
23°C
11.3 6.89 ± 1.45a 7.11 ± 1.62b 7.44 ± 1.67c 7.56 ± 1.24e 7.22 ± 1.56d 7.33 ± 1.12d
33.0 5.78 ± 1.99a 4.67 ± 1.12ab 7.22 ± 2.11c 6.22 ± 1.99c-e 5.89 ± 2.32b-d 6.00 ± 1.87b-d
43.2 6.33 ± 1.50a 6.78 ± 1.48ab 6.89 ± 1.45bc 6.11 ± 2.03b-e 5.56 ± 2.46b-d 6.11 ± 1.76cd
58.4 6.44 ± 1.94a 6.11 ± 2.62ab 2.22 ± 1.09a 3.56 ± 1.94a-c 3.22 ± 1.72a-c 3.33 ± 2.00cd
38°C
11.3 6.11 ± 1.76a 3.78 ± 2.11a 5.56 ± 2.79bc 4.44 ± 2.96a-d 3.56 ± 2.13a-c 4.44 ± 2.88a-c
31.9 6.00 ± 2.00a 5.89 ± 2.03ab 4.22 ± 1.30ab 3.89 ± 1.05a-c 4.22 ± 1.56a-c 3.89 ± 1.27a-c
43.2 5.56 ± 2.46a 4.78 ± 2.22ab 2.67 ± 1.00a 3.33 ± 1.87ab 2.78 ± 1.56ab 3.00 ± 1.66a
53.9 7.11 ± 1.45a 5.22 ± 2.22ab 1.56 ± 0.73a 2.89 ± 1.45a 2.44 ± 1.24a 1.78 ± 0.67a
Mean values with the same attribute followed by different letters in the same column are significantly different (p < 0.05). 1Scale for liking: 1= Dislike extremely, 2= Dislike very much, 3= Dislike moderately, 4= Dislike slightly, 5= Neither like nor dislike, 6= Like slightly, 7= Like moderately, 8= Like very much, 9= Like extremely. (n = 9).
74
Cereal B. Table 20 shows the mean rating of the liking for color, aroma, crispness, taste,
flavor, and overall acceptability for cereal B samples. All samples tested had an average water
activity less than 0.500 at the time of testing. Cereal samples stored at 23°C (43.2 and 58.4%
RH) and 38°C (53.9% RH) had not reached equilibrium upon sensory testing, but testing was
done to reinforce temperature and humidity effects on sensory aspects of a low fat/sugar cereal.
A one-way ANOVA indicated that there was differences in liking scores for color, F(10,
99) = 2.721, p=0.005. Post hoc testing did not reveal the location of the main effect. Three
samples had a mean color score higher than the control (6.11); 10°C samples at 11.3 and 33.5%
RH (6.21 and 6.27) and 23°C at 33.0% RH (6.22). As the storage temperature increased the
likeness of color decreased for corresponding humidities.
A one-way ANOVA declared that there was differences in liking scores for aroma, F(10,
99) = 3.670, p<0.001. The control received a likeness score of 3.89, which is below the midpoint
(5). Aroma values for samples varied from temperature to temperature and from temperature to
humidity. The variances in scores may indicate difficulty in panelists perceiving loss of aroma
or taints in the samples.
A one-way ANOVA displayed that there was a significant difference in liking scores for
crispness, F(10, 99) = 10.411, p<0.000. Post hoc testing revealed a significant difference
between the control (7.11) and samples stored at 10°C and 11.3% RH (7.43). As the storage
humidity increased within the same temperature, a decrease in crispness was observed. As the
temperature increased within the same humidity a significant decrease in crispness was generally
observed.
A one-way ANOVA presented differences F(10, 99) = 5.215, p<0.000 on taste. Post hoc
testing revealed that samples stored at 10°C and 11.3 and 33.5% RH (6.00 and 5.33) were
75
significantly liked more for taste than the control sample (4.78) and all other samples. As the
storage temperature and/or humidity increased the liking scores for taste decreased significantly.
A one-way ANOVA revealed significant differences F(10, 99) = 4.608, p<0.000 on
flavor. Post hoc testing revealed that samples stored at 10°C and 11.3 and 33.5% RH (5.71 and
5.40) were significantly liked more for flavor than the control sample (4.67) and all other
samples. As the storage temperature and/or humidity increased the likeness of flavor of the
samples decreased.
A one-way ANOVA indicated an existence of differences F(10, 99) = 6.523, p<0.000 on
overall acceptability. Post hoc testing revealed that samples stored at 10°C and 11.3 and 33.5%
RH (6.21 and 5.20) were significantly accepted more than the control sample (4.67) and all other
samples. The 10°C samples were the only samples to have an overall acceptability above the
midpoint (5). The overall acceptability of cereal B samples generally decreased as the storage
RH and temperature increased.
Crispness and overall acceptability. Texture acceptance is an important factor in
consumer acceptance of cereal products. Control samples were rated 7.11 for crispness, which
was higher than overall acceptance (6.33) for cereal A. For cereal B control samples were rated
7.11 for crispness and 4.67 for overall acceptability. This result was similar to a sensory
evaluation on consumer acceptance of roasted peanuts stored under various temperature and
humidity conditions (Lee & Resurreccion, 2006), in which texture acceptance received higher
ratings than that of overall acceptance. In the sensory evaluation of cereal A, all samples had
higher likeness scores for crispness than overall acceptability, except samples at 23°C (58.4%
RH) and 38°C (43.2 and 53.9% RH). This indicates that crispness is a driving factor in
acceptance of a high fat/sugar cereal when stored below 58.4% RH at 23°C and below 43.2% RH
76
at 38°C. In the sensory evaluation of cereal B, all samples had higher likeness scores for
crispness than overall acceptability, with exception to samples stored at 23°C and 33.0% RH.
These results indicate that crispness is a driving factor in the acceptance of a low fat/sugar cereal.
77
Table 20
Mean Rating Comparisons of Cereal B for Color, Aroma, Sogginess/Crispness, Taste, Off-flavor/Flavor, and Overall Acceptability
Color1 Aroma1 Sogginess/Crispness1 Taste1 Off-flavor/Flavor1 Overall Acceptability1
Control 6.11 ± 2.15a 3.89 ± 1.45ab 7.11 ± 0.93de 4.78 ± 1.86a-c 4.67 ± 1.66ab 4.67 ± 1.73b-d
10°C2
11.3 6.21 ± 1.37a 5.50 ± 1.51a-c 7.43 ± 1.34e 6.00 ± 2.32a 5.71 ± 2.13b 6.21 ± 2.15d
33.5 6.27 ± 1.16a 5.80 ± 1.42bc 5.80 ± 1.82b-e 5.33 ± 1.54bc 5.40 ± 1.72b 5.20 ± 1.86cd
23°C3
11.3 4.78 ± 2.17a 3.22 ± 1.56a 6.22 ± 2.11c-e 3.67 ± 2.00a-c 3.67 ± 1.87ab 3.56 ± 1.81a-c
33.0 6.22 ± 2.64a 5.56 ± 1.67a-c 3.78 ± 2.22a-c 4.33 ± 2.29a-c 4.56 ± 2.30ab 4.11 ± 2.15a-d
43.2 4.44 ± 2.40a 6.67 ± 1.73c 3.22 ± 1.99ab 3.22 ± 1.64ab 3.78 ± 1.64ab 2.89 ± 1.76a-c
58.4 4.56 ± 2.01a 5.11 ± 2.20a-c 2.67 ± 1.50a 2.67 ± 1.58ab 2.67 ± 1.50a 2.56 ± 1.33ab
38°C3
11.3 4.78 ± 2.11a 5.44 ± 2.07a-c 4.78 ± 2.28a-d 3.33 ± 2.24a-c 3.56 ± 2.35ab 3.22 ± 2.33a-c
31.9 3.89 ± 1.96a 3.89 ± 1.83ab 4.33 ± 1.50a-c 3.00 ± 1.00ab 3.33 ± 1.22ab 3.22 ± 0.97a-c
43.2 4.22 ± 2.39a 4.67 ± 1.94a-c 2.89 ± 1.83a 2.67 ± 1.41ab 2.67 ± 1.50a 2.33 ± 1.22ab
53.9 3.56 ± 1.94a 3.78 ± 1.56ab 1.86 ± 1.36a 2.22 ± 1.09a 2.11 ± 1.27a 1.78 ± 1.09a
Mean values with the same attribute followed by different letters in the same column are significantly different (p < 0.05). 1Scale for liking: 1= Dislike extremely, 2= Dislike very much, 3= Dislike moderately, 4= Dislike slightly, 5= Neither like nor dislike, 6= Like slightly, 7= Like moderately, 8= Like very much, 9= Like extremely. 2(n=15). 3(n = 9).
78
Interaction Effects
Table 21, below, displays the main interactions effects of the type of cereal (A and B),
temperature (10, 23, and 38°C) and relative humidity (11-98.2%) on water activity, moisture
content (% dry basis), TBARS value, and breaking strength.
Table 21
Statistical Significance (F-statistics/p-value) on Quality Parameters of Ready to Eat Breakfast
Cereals
Water Activity MC (%d.b.) TBARS Value
(mg MDA/kg cereal)
Breaking Strength (kgf)
Cereal Type 0.176/0.675 0.029/0.865 0.012/0.914 55.336/<0.0001
Temperature (°C) 10.222/<0.0001 5.237/0.001 52.293/<0.0001 1.158/0.329
Relative Humidity 120.85/<0.0001 106.1/<0.0001 1.350/0.232 8.583/<0.0001
Statistical significance (F-statistics/p-value). Interaction is significant at p < 0.05.
Critical Moisture Content
Determining the critical moisture content before loss of crispness occurs is required in
any shelf life model, for example the integrated GAB shelf life model. Keying in on the critical
moisture content can help achieve the desired shelf life and offer better results for the shelf life
of a food product. Data based on the breaking strength, water activity, and sensory attributes, the
critical moisture content for cereal A and cereal B was 5.5% and 6.5%, respectively. Similar
results were found for a toasted non sugar coated cornflake (6.2%) corresponding to a water
activity of 0.31 (Azanha & Faria, 2005). In commercial samples of cornflakes (6.0%) and rice
crispies (6.9%) corresponding to a water activity between 0 and 0.500 (Sauvageot & Blond,
1991). It is important to note that the effect of moisture gain on crispness depends on the
formulation of the cereal product.
79
Determination of Shelf Life using the Integrated GAB Model
Shelf life determination of dry cereal products is highly dependent on the permeability
characteristics of the packaging material in which it is contained and the product formulation of
the breakfast cereal. To properly determine shelf life when a food product is stored in a
container it is important to know how much of the product the container can hold. With ready to
eat breakfast cereals coming in a variety of small packages upwards to boxes containing 350 to
800 g it can be important to choose the proper packaging for breakfast cereals. Table 22 (below)
displays the holding capacity for cereals A and B. Containers were filled to the fill line for the ½
cup and 2 cup, and an estimated holding capacity was given to the 4 quart container based on
filling height and height of the container. Each container on average could hold 17% more of
cereal B than cereal A. This was due to the higher bulk density of cereal B and its ability to limit
the level of porosity.
Table 22
Bulk Density (g/cm3) and Holding Capacity (g) of ½ Cup, 2 Cup, and 4 Qt. Containers
Bulk Density (g/cm3) ½ cup 2 cup 4 quart
Cereal A 0.18 21 83 688
Cereal B 0.21 25 97 805
Using the integrated GAB model the shelf life of cereals A and B was examined at 10,
23, and 38°C at 80% RH (Table 23) in a ½ cup, 2 cup, 4 quart, and an open standard breakfast
cereal packaging (inner pouch rolled twice and clipped with a paper binder and placed back into
paperboard box). As the temperature increased both cereals A and B had a reduction in shelf life
at all container sizes. As the size of the container increased the shelf life also increased, even
80
though the permeability increased with an increase in container size. This means that the amount
of sample stored in a container has an effect on shelf life. Cereal A had a lower shelf compared
to cereal B under the same storage conditions. The differences in shelf life can be explained by
two key factors needed in the integrated GAB shelf life equation, initial and critical moisture
content. Cereal A had an initial moisture content of 4.37% and a critical moisture content of
5.5%. Cereal B had an initial moisture content of 3.62% and a critical moisture content of 6.5%.
With cereal B having a greater difference between initial moisture content and critical moisture
content allowed for a longer shelf life. Cereal A had ~2 times more fat than cereal B, which can
lead to a lipid oxidation, leading to a lower acceptance of the cereal (low critical moisture
content).
Table 23
Shelf Life (Days) of Cereals A and B Stored in ½ Cup, 2 Cup, 4 Qt., and Standard Breakfast
Cereal Packaging at 80% RH
10°C 23°C 38°C
Package Cereal A Cereal B Cereal A Cereal B Cereal A Cereal B
½ cup1 24 44 22 33 4 13
2 cup2 34 64 26 39 10 14
4 quart3 170 315 156 236 15 45
Standard3 n/a n/a 59 89 n/a n/a
11/2 cup contained 10 g. 22 cup contained 20 g. 34 quart and standard packaging contained 450 g.
When a constant mass of cereal is used for all sizes of packaging the shelf life
dramatically changes (Table 24). The results in this table show the importance of choosing the
proper size packaging for storage of RTE breakfast cereals, as the increase in size of the
81
container (due to higher moisture permeability) will dramatically decrease the shelf life of the
RTE breakfast cereal.
Table 24
Shelf Life (Days) of Cereals A and B Stored in ½ Cup, 2 Cup, 4 Qt., and Standard Breakfast
Cereal Packaging at 80% RH with 20 g of Cereal
10°C 23°C 38°C
Package Cereal A Cereal B Cereal A Cereal B Cereal A Cereal B
½ cup 48 88 44 66 8 25
2 cup 34 63 26 39 4 14
4 quart 7 14 7 10 <1 2
Standard n/a n/a 2 4 n/a n/a
82
Chapter V: Conclusions
The two RTE breakfast cereals studied, a high fat/sugar and a low fat/sugar, exhibited a
type II isotherm, indicating that moisture sorption occurred in the cereal samples was a
multilayer adsorption. The GAB model showed a good fit with experimental adsorption data for
a high fat/sugar and a low fat/sugar RTE breakfast cereal at 10, 23, and 38°C in a water activity
range of 0.113 to 0.982. The GAB model is considered suitable to predict the moisture sorption
isotherm of a RTE breakfast cereal due to the low E (mean relative percentage deviation
modulus) and RSME values.
This research originally study effect of various size airtight polypropylene containers on
shelf life of RTE breakfast cereals compared to standard breakfast cereal packaging. This
research revealed that RTE breakfast cereals stored in polypropylene containers had an increase
in shelf life compared to open standard breakfast cereal packaging. This research provides
evidence that once a consumer opens a RTE breakfast cereal package shelf life can be increased
by storing the cereal in an airtight polypropylene container.
Recommendations
Based on research, data collected, and limitations, further studies are recommended:
1. A validation experiment should be completed to confirm the results found in this
study.
2. Additional secondary lipid oxidation tests should be used to determine lipid
oxidation levels throughout the study.
3. A colorimeter should be included in additional studies to monitor color changes.
Changes in color can help determine consumer acceptance of cereals when stored
at higher temperatures and humidities.
83
4. Sensory analysis should also include panelists rating cereals on intensity for
cereal attributes, for example crispness and stale/rancid/oxidized notes.
5. To conduct ASLT (accelerated shelf life testing) on additional RTE breakfast
cereals (high fat/sugar and low fat/sugar) to determine if these cereals have
similar critical moisture contents and GAB parameters to their counterparts.
84
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Appendix A: Nutrition Breakdown of Cereals High Fat/Sugar (A) and Low Fat/Sugar (B)
Cereal Ingredient1 Cereal A Cereal B
Total fat 9.68 4.55
Saturated fat 1.61 0
Polyunsaturated fat 3.23 1.51
Monounsaturated fat 4.84 1.51
Sodium 0.58 0.26
Total Carbohydrate 80.65 78.79
Sugar 29.03 15.15
Other carbohydrate 45.16 9.09
Protein 3.23 n/a
Other BHT, vitamin A and C, iron Vitamin E acetate, vitamin A and C, iron
1Ingredients are based on g/100g of cereal. Nutritional information was obtained from the nutritional label.
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Appendix B: UW-Stout IRB Approval
March 5, 2013 Kurtis Drager Nutrition and Food Science UW-Stout RE: Evaluation of breakfast cereals for new packaging containers Dear Kurtis, The IRB has determined your project; “Evaluation of breakfast cereals for new packaging containers” is Exempt from review by the Institutional Review Board for the Protection of Human Subjects. The project is exempt under Category # 6 of the Federal Exempt Guidelines and holds for 5 years. Your project is approved from 2/18/2013, through 2/17/2018. Should you need to make modifications to your protocol or informed consent forms that do not fall within the exemption categories, you will need to reapply to the IRB for review of your modified study. If your project involved administration of a survey, please copy and paste the following message to the top of your survey form before dissemination:
If you are conducting an online survey/interview, please copy and paste the following message to the top of the form: “This research has been reviewed by the UW-Stout IRB as required by the Code of Federal Regulations Title 45 Part 46.” Informed Consent: All UW-Stout faculty, staff, and students conducting human subjects research under an approved “exempt” category are still ethically bound to follow the basic ethical principles of the Belmont Report: 1) respect for persons; 2) beneficence; and 3) justice. These three principles are best reflected in the practice of obtaining informed consent from participants. If you have questions, please contact Research Services at 715-232-1126, or [email protected], and your question will be directed to the appropriate person. I wish you well in completing your study. Sincerely,
Susan Foxwell Research Administrator and Human Protections Administrator, UW-Stout Institutional Review Board for the Protection of Human Subjects in Research (IRB)
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Appendix C: Consent Form: Sensory Analysis of Breakfast Cereals
Consent to Participate In UW-Stout Approved Research
Title: Sensory Evaluation of Breakfast Cereals for New Packaging Containers Investigators: Kurtis Drager and Dr. Karunanithy Chinnaduari Description: You will be taking part in the sensory evaluation of breakfast cereal products. If you have any dietary restrictions that would make you unable to eat these food items, then you should not take part in the evaluation. Risks and Benefits: Care has been taken so that all risks associated with food products have been reduced. Water activity has been tested to determine the chance for microbial and mold growth. The cereals have been prepared in a state-inspected processed facility using good manufacturing practices and produced according to USDA safety standards. All ingredients are FDA-approved. Time Commitment and Payment: Each evaluation should require no more than 15 minutes. Confidentiality: Your name will not be included on any documents. We do not believe that you can be identified from any of this information. This informed consent will not be kept with any of the other documents completed with this project. Right to Withdraw: Your participation in this study is entirely voluntary. You may choose not to participate without any adverse consequences to you. Should you choose to participate and later wish to withdraw from the study, you may discontinue your participation at this time without incurring adverse consequences. IRB Approval: This study has been reviewed and approved by The University of Wisconsin-Stout's Institutional Review Board (IRB). The IRB has determined that this study meets the ethical obligations required by federal law and University policies. If you have questions or concerns regarding this study please contact the Investigator or Advisor. If you have any questions, concerns, or reports regarding your rights as a research subject, please contact the IRB Administrator. Investigators: Kurtis Drager IRB Administrator:
715-965-3454 Sue Foxwell, Director, Research Services [email protected] 152 Vocational Rehabilitation Building UW-Stout Dr. Karunanithy Chinnaduari Menomonie, WI 54751 715-232-2519 715-232-2477 [email protected] [email protected]
Statement of Consent: By signing this consent form you agree to participate in the project entitled, “Sensory Evaluation of Breakfast Cereals for New Packaging Containers.” _________________________________________________ _________________________ Signature Date
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Appendix D: Breakfast Cereals Evaluation Form
Sample # ________________ Breakfast Cereals Evaluation
Place a mark (x) in the appropriate box below the statement that best describes your impression. Check only one response per question and be sure that you respond to each question. Please observe the sample. All things considered, which of the statements below best describes your impression of the color of the sample? Dislike Extremely
Dislike Very Much
Dislike Moderately
Dislike Slightly
Neither Like Nor Dislike
Like Slightly Like Moderately
Like Very Much
Like Extremely
□ □ □ □ □ □ □ □ □ Please observe the sample. Which of the statements below best describes the aroma of the sample? Dislike Extremely
Dislike Very Much
Dislike Moderately
Dislike Slightly
Neither Like Nor Dislike
Like Slightly Like Moderately
Like Very Much
Like Extremely
□ □ □ □ □ □ □ □ □ Please taste the sample. Which of the statements below best describes the sogginess/crispness of the sample? Dislike Extremely
Dislike Very Much
Dislike Moderately
Dislike Slightly
Neither Like Nor Dislike
Like Slightly Like Moderately
Like Very Much
Like Extremely
□ □ □ □ □ □ □ □ □ Which of the statements below best describes the taste of the sample? Dislike Extremely
Dislike Very Much
Dislike Moderately
Dislike Slightly
Neither Like Nor Dislike
Like Slightly Like Moderately
Like Very Much
Like Extremely
□ □ □ □ □ □ □ □ □ Which of the statements below best describes the off flavor/flavor of the sample? Dislike Extremely
Dislike Very Much
Dislike Moderately
Dislike Slightly
Neither Like Nor Dislike
Like Slightly Like Moderately
Like Very Much
Like Extremely
□ □ □ □ □ □ □ □ □ Which of the statements below best describes your overall acceptability of the sample? Dislike Extremely
Dislike Very Much
Dislike Moderately
Dislike Slightly
Neither Like Nor Dislike
Like Slightly Like Moderately
Like Very Much
Like Extremely
□ □ □ □ □ □ □ □ □
