THERMAL PASTEURIZATION FOR MINIMALLY PROCESSED GRAPE JUICE:

MICROBIAL STABILITY DURING SHELF LIFE AND SENSORY EVALUATION

A Project Paper

Presented to the Faculty of the Graduate School

of Cornell University

In Partial Fulfillment of the Requirements for the Degree of

Master of Professional Studies in Agriculture and Life Sciences

Field of Food Science and Technology

by

Michelle Maldonado

May 2015

© 2015 Michelle Maldonado

ABSTRACT

Minimally processed juices are preferred by consumers because they perceive

them to be closer to the organoleptic and nutritional characteristics of fresh juices. The

study was conducted to investigate the effect of mild thermal pasteurization on the quality

and shelf life of grape juice and to conduct sensory evaluation to compare minimally

thermal processed with UV treated grape juice. Two varieties of white grapes (Niagara

and Riesling) and one variety of red grapes (Concord) were heated to 71°C for 6 seconds

and UV treated (at 14 mJ/cm2). For the shelf life study, pH, °Brix, total plate and mold

and yeast counts were conducted biweekly. The findings of the study indicate that the

shelf-life of refrigerated, minimally thermal processed grape juice ranged from six weeks

(Concord) to eleven weeks (Niagara and Riesling). The sensory evaluation of the grape

juices demonstrated that participants were able to differentiate between the UV treated

and thermally pasteurized juice in all the varieties studied.

Keywords: Grape, Juice, Sensory Evaluation, Minimally processed juice, shelf life

iii

BIOGRAPHICAL SKETCH

Michelle Maldonado was born in Pomona, California in 1988. She attended

University of San Francisco de Quito in Ecuador to earn her bachelor’s degree in Food

Engineering. She worked with her classmate in developing a passion fruit carbonated

beverage for their final project. She graduated with Magna cum laude in 2013. After

graduation, she worked in a juice production company where she collaborated in

implementing good manufacturing practices in the company and worked in new product

development. She later worked in the Ecuadorian Service for Standardization by

developing national standards for the food and beverage sector.

In 2014, she received a Universities of Excellence - Ecuadorian Government

Scholarship allowing her to pursue a Masters in Professional Studies degree in Food

Science and Technology at Cornell University. She worked under the guidance of Dr.

Olga Padilla-Zakour. During her time at Cornell, she worked as a graduate teaching

assistant for FDSC 4100: Sensory Evaluation of Food. She also was a member of the

Ocean Spray product development team that won first place in the competition. She

plans to return to Ecuador to work for the government.

iv

This project is dedicated to my parents Miguel and Beatriz who with their

unconditional love and support have encouraged me every step of the way

v

ACKNOWLEDGMENTS

I would like to express my gratitude to my faculty advisor Dr. Olga Padilla-Zakour

for her support during my stay in Cornell and the writing of this project. She is very

knowledgeable about her field of study and she cares deeply for all the students under

her guidance. I could not imagine having a better advisor. I would like to thank Jessie

Usaga for being so patient when you were teaching me about analytical methods I

needed to use for my project. My sincere thanks also goes to Herb Cooley and Tom

Gibson for their help in the pilot plant and John Churey for counting the plates when I

could not travel to Geneva. My sincere thanks goes to my friend Vanessa Moncayo, I

could not have imagined a better person to have worked side by side in this project. I

thank my fellow lab mates Marcela Patiño, Marcela Villareal and Elizabeth Buerman for

making the short time I spent in the lab very memorable.

This work could have not been completed without the encouragement of my family

members. Thanks mom, dad and my sister Stephanie for supporting my decision to go

into grad school.

In addition, I would like to thank my friends I met in Ithaca and back home with a

special mention to my friends at INEN for wishing me luck when I summited my

application for the scholarship and for asking me to keep them updated during every step

of the process. I am very grateful to all of you for becoming an important part of my life.

Financial support for this project was provided by the Secretaria de Educacion

Superior, Ciencia, Technologia e Inovacion. Thank you for awarding me with the

scholarship.

vi

TABLE OF CONTENTS

Biographical Sketch iii

Dedication iv

Acknowledgements v

Table of Contents vi

List of Figures vii

List of Tables viii

Chapter 1: Introduction 1

Chapter 2: Materials and Methods 12

Chapter 3: Results and Discussion 16

Chapter 4: Conclusions and Recommendations for Future Work 26

References 27

Appendix 31

vii

LIST OF FIGURES

Figure 1.1. Total US retail sales and forecast of juice, juice drinks and smoothies at

current prices 2009-19 .................................................................................................... 1

Figure 3.1. Changes in total plate counts in thermally processed grape juice (71°C for 6

s) over storage period (at 7°C) ...................................................................................... 20

Figure 3.2: Changes in yeast and mold counts in thermally processed grape juice (71°C

for 6 s) over storage period (at 7°C) .............................................................................. 21

Figure A.1. Sample test ballot for grape juice sensory triangle test ............................... 37

viii

LIST OF TABLES

Table 3.1. Physicochemical characteristics of Concord, Niagara and Riesling grape on

day 0 ............................................................................................................................. 18

Table 3.2. Chemical changes during storage (at 7°C) of thermally processed (71°C for 6

s) grape juice until end of shelf life ................................................................................ 23

Table 3.3. Results from triangle test for UV treated (at 14 mJ/cm2) and thermally

pasteurized (71°C for 6 s) juice for the three different grape varieties .......................... 24

Table A.1. Total Plate Count results for thermally processed Concord grape juice ....... 31

Table A.2. Mold and Yeast Count results for thermally processed Concord grape juice ...

...................................................................................................................................... 32

Table A.3. Total Plate Count results for thermally processed Niagara grape juice ........ 33

Table A.4. Mold and Yeast Count results for thermally processed Niagara grape juice ....

...................................................................................................................................... 34

Table A.5. Total Plate Count results for thermally processed Riesling grape juice ....... 35

Table A.6. Mold and Yeast Count results for thermally processed Riesling grape juice ....

...................................................................................................................................... 36

1

CHAPTER 1

INTRODUCTION

Fruit Juice Market

Sales of fruit juice (juice, juice drinks and smoothies) accounted for $16 billion in

2014, a number that has remained relatively unchanged since 2009. Issues such as,

health concerns and competition from other drink categories (flavored water, ready-to-

drink tea and coffee) are the causes for lack of growth in this market (Mintel 2014).

Figure 1.1. Total US retail sales and forecast of juice, juice drinks and smoothies at

current prices, 2009-19

Source: Based on Information Resources INC, Infoscan reviews, US census bureau, Economic census,

USDA economic research/Mintel

2

One hundred percent juice accounts for approximately half the sales (49.6%), with

the category juice drinks coming in second place (45.5%). Even though 100% juice takes

first place in the category, the segment is negatively perceived by customers as having a

high calorie and sugar content. Recommendations to boost market growth is to

emphasize the nutritional benefits (vitamins and minerals) they deliver and to develop

new flavor combinations (Mintel 2014).

Grape Juice Industry

During 2014, 7.9 million tons of grapes were grown in the United States, most of

the grapes were produced in the states of California, New York and Washington. Of those

grapes, only 6.8% (approximately 550 tons) were destined to grape juice production. The

major producers of domestic grape used for juice were the states of Washington, New

York, Pennsylvania and Michigan (NASS 2015).

The most common grape varieties grown are Concord grapes and Niagara grapes.

Concord grapes represent most of the grapes harvested with more than 505,000 tons

processed, followed by more than 70,000 tons of processed Niagara grapes (NASS

2015).

The New York State Department of Agriculture and Markets estimates that the

crop value for grapes was $52.3 million in 2012. Most of the grapes grown in the state

are destined for grape juice production (62%), followed by wine production (36%) and

fresh market (2%). The majority of grapes are produced in the Lake Erie area, the Finger

Lakes, the Hudson Valley and the eastern end of Long Island. (New York State, 2015).

3

Grape Varieties

Concord

This Native American (Vitis labrusca) grape variety is known as being the most

commonly used for grape juice processing. Typically Concords are used for the

traditional purple grape products and in New York State Concord accounts for 88 percent

of the juice grape utilization in the most recent 5 years (MKF 2005). Consumers can

recognize Concord for its balance of sweetness, acidity and astringent characteristics

even if the juice is highly diluted and sweetened (Bates 2001).

Riesling

Riesling is a variety of Vitis vinifera (wine grapes) originating in Germany no later

than 1350. Riesling vines grow small berries and compact and round leaves. Riesling

grapes have a low pH (2.9-3.2) and they are harvested late in the season so they have

the opportunity to produce high sugar levels to balance the acidity. This variety is cold

resistant and is grown in continental climates in Germany, Austria, New York, Washington

and Oregon (Sechrist 2012).

Niagara

Niagara grapes are members of the Vitis labrusca (native to United States) grapes.

This variety was first created when Concord grapes were cross-bread with white Cassady

grape in 1868 in Niagara County, New York. Niagara grapes are the second most

4

cultivated variety and they are used principally in the production of white grape juices

(MFK 2005).

Juice Safety

In 2001, the United States Food and Drug administration (FDA) issued a new

regulation “Part 120: Hazard Analysis and Critical Control Point (HACCP) Systems;

Procedures for the Safe and Sanitary Processing and Importing of Juice” (21CFR Part

120) due to the number of outbreaks of Salmonella spp., Escherichia coli O157:H7 and

Cryptosporidium parvum associated with the consumption of fresh apple and orange

juices. The regulation states that all processors shall develop a HACCP plan to determine

which biological, chemical and physical hazards more likely to occur in fruit juices. This

regulation requires processors to implement measures that will produce at a minimum a

5 log reduction for the most heat resistant microorganism of public health significance

under normal to moderate abuse storage conditions (FDA 2014).

Quality of Juices

A food quality program is designed to ensure the suitability of a product during all

stages of handling, processing, preparation, packaging, storage and distribution for an

intended application. In order to meet product specifications, processing should be

5

carried out in the correct manner using fruit of an optimum level of maturity, and that the

product should be stored under suitable conditions to limit effects of degradation during

shelf-life. There are some common parameters used to measure the quality of juices

(Taylor 2005).

Soluble solids

The soluble solid content relates primarily to sugars and fruit acids present in the

juice because they are the main contributors; pectins, glycosidic materials and the salts

of metals (sodium, potassium, magnesium, calcium etc.), are also present in some juices

but their quantity is very small. The amount of soluble solids is measured in Brix value

by using an optical refractometer. Refractometer readings can be affected by the

presence of other dissolved solids like fruit acids when there are appreciable levels of

acid in the juice, for example, in lemon and lime juices (Taylor 2005).

Titratable acidity

The acid content of a juice is determined using a pH meter by direct titration against

standardized alkali solution (e.g. 0.1 M sodium hydroxide) to an end-point at pH 8.1. As

a general rule, the acidity of juices will decrease with increasing maturity of the fruit

source, or with increasing levels of sugars in the resulting juice. Hence the ratio of soluble

solids (e.g. Brix values) to acidity is an important value in the assessment of juice quality.

The Brix/acid ratio is frequently used to establish standard sensory, or taste, qualities for

6

incoming juice supplies and to minimize the effect of seasonal variation. The higher the

Brix value in relation to the acid content of the juice, the higher the ratio and the ‘sweeter’

the taste (Taylor 2005).

Color measurements

When using the tristimulus method, the absorbance or reflectance of a product is

measured at a range of wavelengths in the red, green and blue areas of the spectrum.

This three-dimensional space can be represented by the L, a, b system, used by Hunter

Lab. L represents the degree of whiteness or blackness (from 0 to 100). The chromatic

portion of the color space is based on rectangular Cartesian coordinates (a, b) with red

represented by +a, green represented by –a, yellow represented by +b, and blue

represented by –b (Lawless 2010).

Microbiological

Because of their low pH, fruit juices will present less than ideal conditions for

pathogenic bacteria species, and these therefore are generally of no major concern for

the juice producer who is operating under good manufacturing practices. There are acid

tolerant bacteria, however, whose presence can give rise to off-flavors, and this effect

can be encountered with citrus juices (Taylor 2005). Yeasts are the most significant group

of micro-organisms associated with spoilage of soft drinks and fruit juices. Spoilage will

be seen as the growth and production of metabolic byproducts, for example, CO2, acid,

7

and tainting compounds. As noted above, most spoilage is therefore by yeasts and mold

species, with yeasts most important, and some spoilage is by acid tolerant bacteria

(Hocking & Jensen, 2001; Jay & Anderson, 2001).

Juice Processing

Thermal Treatment Processing

Normally for self-stable juices, pasteurization is achieved by hot-fill and hold or in

bottle pasteurization. The hot fill and hold process is used with high acid juices that must

pass through a heat exchanger to elevate the temperature to 88 to 95 ºC, then the

container is immediately filled with the juice and set aside to wait at least 3 minutes before

cooling (McLellan & Padilla-Zakour 2005).

For refrigerated juices, the time-temperature combination of 70 to 90 ºC for a few

seconds is used to destroy pathogenic bacteria that may be present in the juice. Even

though the shelf-life is shorter, many consumers prefer refrigerated juices because of the

retention of nutrients and organoleptic properties in contrast to shelf-stable juice

(McLellan & Padilla-Zakour 2005).

Non-thermal Treatment Processing

UV irradiation is a FDA approved method that is used to eliminate pathogens from

fruit juice products. UV light works by damaging the DNA of bacterial cells, protozoa and

viruses so they are not capable of reproduction and therefore they die. UV irradiation,

8

however, does not have the same effect on molds and yeasts (Tandon et. al 2003). The

advantages of using UV irradiation is that UV treatment has no effect on the organoleptic

properties of the juice, the product can be marketed towards consumers who want less

processed products, and the equipment has a much lower cost compared to the

equipment used for pasteurization (McLellan & Padilla-Zakour 2005).

Sensory Evaluation

Sensory evaluation is defined as a method used to quantify, analyze and interpret

responses to a product perceived though the senses. There are three basic types of tests

used in sensory analysis, discriminatory, descriptive and affective tests. Discriminatory

tests are used to answer if there is a difference between two products that can be

perceived by participants. A significant difference between the products would be

declared if the number of correct choices was above the level expected by chance

(Lawless 2010).

There are a large variety of tests based on this principle that can be used: Paired

comparison, triangle, duo–trio, n-alternative forced choice, A-not-A, ABX discrimination,

etc. Although all these tests are used to investigate whether a difference can be

perceived, each test varies their ability to do so in an accurate and efficient manner. For

example, in triangle and duo-trio tests, panelists are not given the nature of the attribute

they are supposed to concentrate on, they just have to identity which sample is different.

One disadvantage of the triangle and duo-trio test is that their low statistical power can

9

result in not detecting sensory differences that can potentially be detected by consumers

and lead to the rejection of the reformulated product (Ishii 2014).

Discrimination tests are usually used by product developers when they reformulate

a product, when they change ingredients they do not want consumers to perceive a

difference. Discrimination tests are also used to identify if a change in a process changes

the sensory characteristics of the product. Difference tests are very popular due to the

simplicity of data analysis. Statistical tables derived from the binomial distribution give

the minimum number of correct responses needed to conclude statistical significance as

a function of the number of participants, so results can be reported quickly (Lawless

2010).

There have been a number of studies done to compare if UV treatment has an

effect on the sensory parameters of treated fruit juices. Tandon et al. (2002) investigated

the storage quality of hot-filled (at 63°C), flash pasteurized (at 71°C x 6 s) and UV

irradiated (at 14000 uW) apple cider. Participants carried out ranking tests on the three

treatments and rated the samples on a 7-point preference scale with 1 being “dislike

extremely” and 7 being “like extremely”. The results indicated no statistical difference

between UV treatment and thermal pasteurization between the preference ratings and

the average ranking scores in the beginning of the study.

Donahue et al. (2004) investigated if UV inactivation of Escherichia coli O157:H7

had an effect on flavor in apple cider. The apple cider used in the study was irradiated at

8.77 mJ/cm2 power intensity. A triangle test was used for the sensory evaluation of the

cider. Participants were given treated and untreated cider and asked to identify the

10

different sample. Results from the participants indicated that a statistical significant

difference could not be found between the sensory qualities between the juices.

Another study conducted in 2010 by Caminiti et al. measured sensory

characteristics of apple juice treated with UV light at energy dosages ranging of 5.31,

10.62, 26.55 and 53.10 J/cm2. Panelists were asked to evaluate color, odor, sweetness,

acidity, flavor and overall acceptability on a 9-point preference scale on the UV treated

samples and an untreated control. Statistical analysis revealed that there was no

significant difference (p≥0.05) between the samples that were treated with 5.31 and 10.62

J/cm2 and the control. Panelists reported lower hedonic scores for juices that were treated

with dosages greater or equal to 26.55 J/cm2 indicating that those dosages caused

adverse changes in odor, flavor and color. In all these studies, low dosages of UV

treatment did not affect the sensory qualities of apple cider.

Pala & Toklucu (2013) have also done research on sensory properties of UV

processed orange juice. Panelists were asked to rate untreated, UV (48.12 kJ/L) and

heat treated (90°C for 2 min) orange juice on flavor and aroma and overall acceptability

using a 7-point hedonic scale. Participants were also given two sets of oranges juices for

a triangle test; one that contained untreated and UV treated and another one that

contained UV treated and heat treated. Hedonic results indicated that UV treated orange

juice samples were more liked than heat treated (P <0.05). Results from the triangle test

indicated that there was no statistical significant difference between the control and the

UV pasteurized orange juice (P <0.01). However, the panelists were able to detect a

11

difference between the UV treated and the thermally processed orange juice that was

statistically significant (P <0.01).

Another study on fruit juices by Guevara et al (2012) evaluated the sensory quality

of guava and passion fruit nectars treated by ultra violet radiation. Guava nectar was

treated at 14.55 and 23.62 kJ/L and passion fruit nectar was treated at 6.19 and 11.03

kJ/L. Panelists participated in triangle tests, each set contained treated and untreated

samples, and they had to identify which sample was different. In all trials, panelists were

able to detect statistically significant differences between the treated and untreated

samples. Panelists expressed that they were able to identify the different sample

because the UV treated samples differed from the control in color, aroma and taste.

12

CHAPTER 2

MATERIALS AND METHODS

Grape Juice

Two varieties of white grapes (Niagara and Riesling) and one variety of red grapes

(Concord) obtained by a grape yard located in Geneva, NY were used. Niagara and

Riesling varieties were stored at 0°C until used and thawed at 7°C for seven days.

Concord grapes were stored at 7°C before processing. Grapes were pressed in batches

using a custom made hydraulic press rack and frame. The Riesling juice had 17.8° Brix

and 3.47 pH value, the Niagara juice had 16.2 °Brix and 3.47 pH value, and the Concord

juice was characterized by a 13.7 °Brix and 3.25 pH value. 125mL Nalgene™ PET Sterile

Square Media Bottles with HDPE Closure (Thermo Scientific, Lima, OH) were used to

pack the juice samples.

Physicochemical measurements

Chemical and physical measurements were made on the grape juice samples in

triplicate. pH, titratable acidity (TA), soluble solids (°Brix), turbidity, absorption coefficient

and color were measured. pH was measured using an Accument Basic AB15 pH meter

(Fisher Scientific, Hampton, New Hampshire). Titratable acidity was measured using G20

Compact Titrator Mettler Toledo, 5 mL of grape juice were taken and diluted with distilled

water until a total volume of 40 mL was reached. The results were indicated as (w/v)

malic acid percentage. Total soluble solids of the samples were measured using an Auto

13

ABBE Refractometer Leica 10504 (Leica Inc., Buffalo, NY) and reported as °Brix.

Turbidity was measured using a Hach 2100P turbidimeter 4500-00 (Hach Co., Loveland,

CO) and expressed as Nephelometric Turbidity Units (NTU). Color was measured using

a Hunter UltraScan VIS spectrophotometer (Hunter Lab Assoc., Reston, VA) results were

expressed as L, a and b values. The absorption coefficient (α) was calculated following

the protocol described by Koutchma et al. (2004) by measuring the sample absorption at

254 nm using a UV-1800 spectrophotometer (Shimadzu Scientific Instruments, Columbia,

MD). Samples were subjected to a 10-fold dilution in distilled water and placed into

demountable fused quartz cuvettes of 0.1, 0.2, 0.5 and 1.0 mm path length (NSG

Precision Cells, INC., Farmingdale, NY).

Thermal Processing

For the thermal pasteurization treatment, a continuous tubular pasteurizer

(UHT/HTST unit, Micro Thermics, Raleigh, NC) was used. The grape juices were heated

to 71°C for 6 seconds, cooled to 20-25°C and manually packed. Juice bottles were

refrigerated at 7°C and three samples of each juice were taken biweekly for conducting

the shelf life study.

Sensory Evaluation

A sensory study was carried out one day after the production of the juice. A triangle

discrimination test was conducted in the New York State Agricultural Experimental Station

(Geneva, NY). The aim of the study was to determine if consumers were able to perceive

differences between the thermal and UV-treated grape juices. Forty volunteers including

14

college students, faculty members and staff from the Experimental Station participated in

the study. Participants were at least 18 years old. Each participant was presented three

different sets of juices, each one of a different grape variety. Samples were kept at 7°C

and served in plastic cups labeled with randomly generated three digit codes. Panelists

were asked to indicate the different sample.

Shelf Life Study

Microbiological counts including total plate count and molds and yeast count were

made every fourteen days. Plate count Agar (PCA) was used to determine total aerobic

microbes, and acidified (3.5 pH) Potato Dextrose Agar (PDA) was used to determine the

presence of yeast and mold. Both media were supplied by Difco, Becton Dickinson

(Sparks, MD). Three bottles of each juice were taken for microbiological analysis. One

mL of sample was subjected to serial dilutions in 1% sterile peptone water and then

placed into Petri dishes. Agar was poured and mixed thoroughly, a duplicate of each

dilution was plated. Petri dishes were incubated for 48 h at 30°C. Colonies were reported

as log10 CFU/mL.

Statistical Analysis

Three independent analytical replicates were used for each physicochemical

measurement and microbiological analysis. Results were reported in mean ± standard

deviation. Results were subjected to analysis of variance (ANOVA) and Tukey’s

significant difference test was used for the comparison of means using JMP 11.0 version

15

statistical software (SAS Institute Inc., Cary, NC). Differences were considered significant

at a P value < 0.05.

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CHAPTER 3

RESULTS AND DISCUSSION

Quality of Raw Grape Juices used in the Study

The PCA counts for the raw Concord, Niagara and Riesling grape juice produced

in the pilot plant were 239883 CFU/mL (5.4 log10), 53703 CFU/mL (4.7 log10) and 54954

CFU/mL (4.7 log10) respectively. For PDA counts, the raw Concord juice had 281838

CFU/mL (5.5 log10), the raw Niagara juice had 14454 CFU/mL (4.2 log10) and raw Riesling

11220 CFU/mL (4.1 log10). Fruit juice produced by undamaged fruit are reported to have

yeast counts between 1,000 to 100,000 CFU/mL (Vasavada and Heperkan 2002).

Splittstoesser and Mattick (1981) cited that high yeast counts between 100,000 to

1,000,000 CFU/mL are common in unpasteurized juice extracted from both handpicked

and mechanically harvested fruit. The higher PCA and PDA counts in the raw Concord

juice than the raw Niagara and Riesling juices could have been caused by the one-week

storage period of the grapes in refrigeration temperatures (7°C).

Physicochemical Characteristics of Grape Juices

Juice was obtained from three different grape varieties (Concord, Niagara and

Riesling). Table 3.1 indicates the pH, soluble solids (°Brix), Titratable acidity (TA),

turbidity, color components and absorbance coefficient values from these varieties.

Juices from the three varieties were not significantly different in titratable acidity and

17

absorbance coefficient. White grape varieties (Niagara and Riesling) were significantly

different from Concord grapes in pH, soluble solids, turbidity and Hunter a values.

Table 3.1. Physicochemical characteristics of Concord, Niagara and Riesling grape juices1 on day 0

Variety pH Soluble

solids (°Brix)

TA (% (w/v)

malic acid)

Turbidity

(NTU)

Color components Absorbance

Coefficient

(Abs 430

nm)

L a B

Concord 3.26 ±

0.06a 13.69 ± 0.60a 0.64 ± 0.27

607.33 ±

75.48a

28.13 ±

0.30b

-0.03 ±

0.01b

0.15 ± 0.07

a

0.175 ±

0.002

Niagara 3.47 ±

0.07b 16.17 ± 1.16b 0.37 ± 0.004

428.00 ±

18.73b

28.44 ±

0.25b

1.32 ±

0.02a

2.44 ±

0.26b

0.152 ±

0.012

Riesling 3.47 ±

0.01b 17.81 ± 0.08 b 0.44 ± 0.04

330.33 ±

27.01b

29.64 ±

0.21a

1.44 ±

0.09a

4.78 ±

0.28c

0.158 ±

0.010

1 Mean ± SD, n=3. Numbers followed by different letters are significantly different (p ≤ 0.05) within columns

18

19

Effect of Minimal Thermal Processing on Stability during Shelf-life Study of Grape

Juice

Comparing the initial PCA counts, the thermal treatment achieved a reduction in

Concord, Niagara and Rieslings of 3.3 to 3.6 log10; 2.2 to 2.8 log10 and 2.0 to 3.0 log10,

respectively. With thermal processing, large reductions in PDA counts were also

observed, ranging from 3.6 to 4.3 log10 in Concord grape juice; 3.8 to 4.4 log10 in Niagara

grape juice and 5.3 to 5.6 log10 in Riesling grape juice.

The results from the aerobic counts (PCA) and mold and yeast counts (PDA) from

the grape juice samples during storage at 7°C are presented in Figure 3.1 and Figure 3.2

respectively. In all three varieties, the shelf life study ended because of visible mold

growth on the surface of the juice sample. Because the sample had visible mold growth,

it was not tested and noted as “spoiled”, assuming that the microbiological counts were

higher than 106 CFU/mL. The shelf life of the thermally pasteurized Concord juice was 4

weeks (28 days) and the shelf life of both Niagara and Riesling grape juices was 11 weeks

(77 days). Although, the shelf-life of the Concord grape juice was much shorter, it was

still in the shelf life range for mild thermally processed juices reported by Esteve and

Frígola (2007). The shorter shelf for the Concord juice could have resulted from the

higher initial microbiological counts from the raw juice. Also there could have been a

possible contamination from the packaging materials, as the samples that were labeled

as “spoiled” had visible mold growth on the cap and on the neck of the bottle.

20

* Shelf life study was terminated due to visible mold growth on the surface of the sample

Figure 3.1. Changes in total plate counts in thermally processed grape juice (71°C for 6

s) over storage period (at 7°C)

21

* Shelf life study was terminated due to visible mold growth on the surface of the sample

Figure 3.2. Changes in yeast and mold counts in thermally processed grape juice (71°C

for 6 s) over storage period (at 7°C)

22

For chemical analysis, soluble solids and pH were measured during the shelf life

study as indicated in table 3.2. In both Concord and Niagara there was an increase in pH

and in all grape varieties there was an increase in soluble solids throughout the storage

period. These results corroborate with Siricururatana and coworkers (2013) who

indicated that the changes in pH and brix in Concord and Niagara grape juices were

caused by the metabolic activity of the yeast present. Yeasts cause the increase in

soluble solids and pH by converting polysaccharides present in the juice into soluble

sugars (Bal 2014).

Table 3.2. Chemical changes during storage (at 7°C) of thermally processed (71°C for 6 s) grape juice until end of shelf

life1,2

1 Mean ± SD, n=3. Numbers followed by different letters are significantly different (p ≤ 0.05) within columns

2 --- indicates samples where not taken

pH °Brix

Weeks Concord Niagara Riesling Concord Niagara Riesling

0 3.3 ± 0.1 3.5 ± 0.0a 3.5 ± 0 13.7 ± 0.6a 17.8 ± 0.1a 18.0 ± 0.3abc

2 3.4 ± 0 --- --- 15.8 ± 0.1b --- ---

4 3.4 ± 0.1 3.7 ± 0.0c 3.6 ± 0 15.7 ± 0.0b 17.3 ± 0.6ab 17.7 ± 0.3ab

6 3.7 ± 0.0c 3.6 ± 0.1 17.9 ± 0.1b 18.0 ± 0.2abc

8 3.6 ± 0.0bc 3.6 ± 0.1 17.6 ± 0.3ab 17.5 ± 0.4a

10 3.5 ± 0.0ab 3.6 ± 0 18.1 ± 0.1b 18.5 ± 0.0c

11 3.6 ± 0.0bc 3.5 ± 0 17.9 ± 0.1b 18.3 ± 0.0bc

23

24

Sensory Analysis – Discrimination Test

This sensory study investigated if there is a statistically significant difference

between the sensory aspects of UV treated and thermally pasteurized grape juice. Table

3.3 lists the results of the triangle test performed by the untrained panelists comparing

the UV-treated (at 14 mJ/cm2) juice and thermally pasteurized (at 71°C for 6 s) juice.

Table 3.3. Results from triangle test for UV treated (at 14 mJ/cm2) and thermally

pasteurized (71°C for 6 s) juice for the three different grape varieties.

Grape Variety Panelists

(N)

Correct Judgments

reported

Correct Judgments required for

significance (p < 0.05)

Concord

Niagara

Riesling

40

40

40

24*

23*

24*

19

19

19

* indicates statistical difference between treatments at p = 0.05

The untrained panelists were instructed to taste a set of three juice samples and

to identify the odd sample. The minimum number of correct responses to establish

significance at p = 0.05 with 40 panelists is 19 (Lawless and Heymann 2010). Therefore,

with all three grape varieties (Concord, Niagara and Riesling) participants were able to

perceive a statistically significant difference between the UV treated and thermally

pasteurized juice.

25

Most studies that perform sensory evaluation of UV treated juice compare the

samples with untreated juice to see if the process has any effect on the sensory

characteristics of the juices (Donahue et al., 2004; Caminiti et al., 2010; Guevara et al.,

2012). Tandon and coworkers (2002), found there was no statistical significance

difference between thermally processed and UV treated apple cider. However, these

findings are consistent with those reported by Pala & Toklucu (2013) with panelists being

able to differentiate UV treated and the thermally processed orange juice. The difference

in the findings of the studies conducted could be due to the different composition of the

fruit juices used and the UV dose applied.

26

CHAPTER 4

CONCLUSIONS AND RECOMMENDATION FOR FUTURE WORK

The thermal treatment applied (71°C for 6 seconds) to the three varieties of grape

juices produced 2.0 to 3.6 log10 reductions in total aerobic microbe counts and 3.6 to 5.6

log10 reductions in mold and yeast counts. The findings of the study indicate that the

shelf-life of refrigerated, minimally thermal processed grape juice ranged from four weeks

(Concord) to eleven weeks (Niagara and Riesling). The difference could be due to the

varieties used, the initial microbiological counts or contamination of the packaging

material. The shelf life of the three varieties didn’t end due to high microbiological counts

but due to visible mold growth in the samples. In future research, in addition to

microbiological analysis it would be useful to conduct sensory analysis and

physicochemical measurements during the shelf life study to detect if there is a decline in

quality before the end of the shelf-life.

The sensory evaluation of the grape juices demonstrated that participants were

able to differentiate between the UV treated and thermal pasteurized juice in all the

varieties studied. There should be further investigation in order to determine which

sensory characteristics (color, aroma or flavor) of the juices the consumers were able to

perceive as different by using descriptive analysis. Also, preference testing should be

done to determine if there is a difference in consumer preference between thermal

processed grape juice and UV treated grape juice.

27

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During Storage of Guava Nectar Cv. Lalit from Different Pulp and TSS Ratio. Journal of

Food Processing & Technology, 5(5), 1-5.

Bates, R.P., Morris, J.R. & Crandall, P.G. (2001). Principles and Practices of

Small- and Medium-scale Fruit Juice Processing. FAO.

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Caminiti, I.M. Palgan, I. Muñoz, A. Noci, F. Whyte, P. Morgan, D.J. Cronin, D.A.

Lyng, J.G. (2012). The effect of ultraviolet light on microbial inactivation and quality

attributes of apple juice. Food and Bioprocess Technology, 5, 680–686.

Donahue, D., Canitez, N., & Bushway, A. (2004). UV inactivation of E. Coli

O157:H7 in apple cider: quality, sensory and shelf-life analysis. Journal of Food

Processing and Preservation, 28(5), 368–387

Esteve, M.J & Frígola, A. (2007). Refrigerated Fruit Juices: Quality and Safety

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&showFR=1 >

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Guevara, Miguel, Tapia, Maria S. & Gómez-López, Vicente. (2012). Microbial

Inactivation and Quality of Guava and Passion Fruit Nectars Treated by UV-C Light. Food

and Bioprocess Technology, 5(2), 803-807.

Hocking, A.D. and Jensen, N. (2001) Spoilage of various food classes: soft drinks,

cordials, juices, bottled water and related products. Spoilage of Processed Foods:

Causes and Diagnosis (eds C.J. Moir and D.C. Waterloo), Australian Institute of Food

Science and Technology Incorporated Food, Microbiology Group, AIFST Inc., 91–100.

Jay, S. and Anderson, J. (2001) Spoilage of various food classes: fruit juice and

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Food, Microbiology Group, AIFST Inc., 187-198.

Ishii et al., (2014). Triangle and tetrad protocols: Small sensory differences,

resampling and consumer relevance. Food Quality and Preference 31, 49–55

Lawless, H. T, & Heymann, H. (2010). Sensory evaluation of food: principles and

practices. 2nd ed. New York: Springer.

Mintel. (November 2014). Juice, juice drinks and smoothies - US. Retrieved 02/22,

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31

APPENDIX

A. DATA FROM THE SHELF LIFE STUDY

Concord Grape Juice

Table A.1. Total Plate Count results for thermally processed Concord grape juice (71°C

for 6 s) over storage period (at 7°C)

Week Replicates Dilution

Count

1

Count

2 Average Log10

Total

log10

CFU/mL

0

1 0 63 89 76 1.8808 1.88

2 0 57 79 68 1.8325 1.83

3 0 58 75 66.5 1.8228 1.82

2

1 0 58 38 48 1.6812 1.68

2 0 57 47 52 1.716 1.72

3 0 55 31 43 1.6335 1.63

4

1 1 5 9 7 0.8451 1.85

2 1 1 5 3 0.4771 1.48

3 1 12 5 8.5 0.9294 1.93

32

Table A.2 Mold and Yeast Counts for thermally processed Concord grape juice (71°C for

6 s) over storage period (at 7°C)

Week Replicates Dilution Count

1

Count

2 Average Log10

Total

log10

CFU/mL

0

1 0 0 0 0 --------- 0

2 0 0 0 0 ---------- 0

3 0 0 0 0 ---------- 0

2

1 0 0 0 0 ---------- 0

2 0 0 0 0 ---------- 0

3 0 0 0 0 ---------- 0

4

1 0 0 0 0 ---------- 0

2 0 0 0 0 ---------- 0

3 0 0 0 0 ---------- 0

33

Niagara Grape Juice

Table A.3. Total Plate Count for thermally processed Niagara grape juice (71°C for 6 s)

over storage period (at 7°C)

Week Replicates Dilution Count 1 Count 2 Average Log10

Total

log10

CFU/mL

0

1 1 7 16 11.5 1.0607 2.06

2 1 15 18 16.5 1.2175 2.22

3 1 22 24 23 1.3617 2.36

2

1 2 8 3 5.5 0.7404 2.74

2 2 1 2 1.5 0.1761 2.18

3 2 3 1 2 0.301 2.3

4

1 2 6 14 10 1 3

2 2 11 13 12 1.0792 3.08

3 2 14 16 15 1.1761 3.18

6

1 2 16 10 13 1.1139 3.11

2 2 9 17 13 1.1139 3.11

3 2 50 82 66 1.8195 3.82

8

1 1 56 15 35.5 1.5502 2.55

2 1 47 6 26.5 1.4232 2.42

3 1 7 6 6.5 0.8129 1.81

10

1 2 86 24 55 1.7404 3.74

2 2 1 2 1.5 0.1761 2.18

3 2 86 1 43.5 1.6385 3.64

11

1 2 1 2 1.5 0.1761 2.18

2 2 1 1 1 0 2

3 2 1 1 1 0 2

34

Table A.4. Mold and Yeast Counts for thermally processed Niagara grape juice (71°C for

6 s) over storage period (at 7°C)

Week Replicates Dilution Count

1

Count

2 Average Log10

Total

log10

CFU/mL

0

1 0 0 0 0 ---------- 0

2 0 0 0 0 ---------- 0

3 0 0 0 0 ---------- 0

2

1 0 0 0 0 ---------- 0

2 0 0 0 0 ---------- 0

3 0 0 0 0 ---------- 0

4

1 0 0 0 0 ---------- 0

2 0 0 0 0 ---------- 0

3 0 0 0 0 ---------- 0

6

1 1 9 14 11.5 1.0607 2.06

2 1 3 5 4 0.6021 1.6

3 1 2 11 6.5 0.8129 1.81

8

1 2 56 32 44 1.6435 3.64

2 1 30 7 18.5 1.2672 2.27

3 1 25 6 15.5 1.1903 2.19

10

1 2 28 14 21 1.3222 3.32

2 0 1 1 1 0 0

3 1 69 42 55.5 1.7443 2.74

11

1 0 1 2 1.5 0.1761 0.18

2 0 1 1 1 0 0

3 0 1 1 1 0 0

35

Riesling grape juice

Table A.5. Total Plate Count for thermally processed Riesling grape juice (71°C for 6 s)

over storage period (at 7°C)

Week Replicates Dilution Count 1 Count 2 Average Log10

Total

log10

CFU/mL

0

1 2 7 2 4.5 0.6532 2.65

2 1 9 6 7.5 0.8751 1.88

3 1 13 6 9.5 0.9777 1.98

2

1 2 2 3 2.5 0.3979 2.4

2 2 3 2 2.5 0.3979 2.4

3 2 6 1 3.5 0.5441 2.54

4

1 2 5 2 3.5 0.5441 2.54

2 2 28 19 23.5 1.3711 3.37

3 2 24 35 29.5 1.4698 3.47

6

1 1 23 90 56.5 1.752 2.75

2 2 11 10 10.5 1.0212 3.02

3 2 8 10 9 0.9542 2.95

8

1 1 19 28 23.5 1.3711 2.37

2 2 9 5 7 0.8451 2.85

3 2 2 6 4 0.6021 2.6

10

1 1 16 12 14 1.1461 2.15

2 1 22 15 18.5 1.2672 2.27

3 2 4 9 6.5 0.8129 2.81

11

1 1 19 12 15.5 1.1903 2.19

2 1 5 8 6.5 0.8129 1.81

3 1 13 17 15 1.1761 2.18

36

Table A.6. Mold and Yeast Count for thermally processed Riesling grape juice (71°C for

6 s) over storage period (at 7°C)

Week Replicates Dilution Count

1

Count

2 Average Log10

Total

log10

CFU/mL

0

1 0 0 0 0 ---------- 0

2 0 0 0 0 ---------- 0

3 0 0 0 0 ---------- 0

2

1 0 0 0 0 ---------- 0

2 0 0 0 0 ---------- 0

3 0 0 0 0 ---------- 0

4

1 0 0 0 0 ---------- 0

2 0 0 0 0 ---------- 0

3 0 0 0 0 ---------- 0

6

1 0 42 59 50.5 1.7033 1.7

2 0 83 66 74.5 1.8722 1.87

3 0 51 43 47 1.6721 1.67

8

1 0 1 1 1 0 0

2 2 17 52 34.5 1.5378 3.54

3 0 1 6 3.5 0.5441 0.54

10

1 1 1 1 1 0 1

2 0 1 1 1 0 0

3 0 1 1 1 0 0

11

1 0 10 5 7.5 0.8751 0.88

2 0 6 1 3.5 0.5441 0.54

3 0 1 1 1 0 0

37

Figure A.1. Sample test ballot for grape juice sensory triangle test