water activity and Food Stability

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International Journal of Food Properties, 12: 726740, 2009

Copyright Taylor & Francis Group, LLC

ISSN: 1094-2912 print / 1532-2386 online

DOI: 10.1080/10942910802628107

726

FOOD STABILITY BEYOND WATER ACTIVITYAND GLASS TRANSTION: MACRO-MICROREGION CONCEPT IN THE STATE DIAGRAM

Mohammad Shafiur Rahman

Department of Food Science and Nutrition, Sultan Qaboos University, Al Khod,

Oman

The water activity concept proposed that a food product is the most stable at its monolayer

moisture content. Recently, the limitations of the water activity concept were identified and

the glass transition concept was proposed in order to overcome the limitations of wateractivity. Based on the glass-transition concept, a food is the most stable at and below its

glass transition point. Recently it has also become evident that the glass transition concept

is not universally valid for stability determination when foods are stored under different

conditions. The glass transition concept was used to develop the state diagram by drawing

another stability map using freezing curve and glass line. Currently, other components

indicating different characteristics are being included in the state diagram. It is being

emphasized in the literature to combine the water activity and glass transition concepts. In

this paper, an attempt is made to combine these two concepts in the state diagram and to

propose a macro-micro region concept for determining the stability of foods.

Keywords: Isotherm, Food stability, Glass transition, BET monolayer, Food spoilage.

INTRODUCTION

Humans in the Atone Age (10,000 years ago) stored nuts and seeds for winter and

discovered that meat and fish could be preserved by drying in the sun. After the discovery

of fire, cooking made food more appetizing and was an aid to preservation. Modern pro-

cessing developed at the end of the 1700s when the Napoleonic wars raged. As Napoleon

pushed forward into Russia, his army was suffering more casualties from scurvy, malnu-

trition, and starvation. The French government offered 12,000 francs to any one who

could develop a method of preserving food. Nicolas Appert took up the challenge. He had

a theory that if fresh foods were put in airtight containers and sufficient heat applied, then

the food would last longer. Appert packed his foods in bottles, corked them, and sub-

merged them in boiling water, thus preserving them without understanding of bacterial

spoilage. After 14 years of experimentation, in 1809 he won the prize and this was given

to him by Napoleon himself. A theoretical understanding of the benefits of canning did notcome until Louis Pasteur observed the relationships between microorganisms and food

spoilage some fifty years later.[1] Eventually canning, drying, and freezing technology

Received 2 October 2008; accepted 10 February 2009.

Address correspondence to Mohammad Shafiur Rahman, Department of Food Science and Nutrition,

Sultan Qaboos University, PO Box 34, Al Khod 123, Oman. E-mail: [email protected]

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MACRO-MICRO REGION CONCEPT IN THE STATE DIAGRAM 727

evolved. Recently new preservation methods have emerged and been adopted by the food

industry. A comprehensive complete coverage of preservation is available in the Hand-

book of Food Preservation and other references.[24]

Determining food stability from a scientific basis rather than an empiricism is achallenge to food scientists and engineers. Microbial death and growth kinetics, deteriora-

tive physical, chemical and biochemical changes during processing and storage depends

on many factors, such as water content, food composition, preservatives, pH, and environ-

mental or processing factors (temperature, pressure, electro-energy, gases or vapors etc).

In the 1950s, the concept of water activity was proposed to determine the stability of

foods, and in the 1980s, significant data on food stability as a function of water activity

was published. In order to avoid the limitations of water activity, the glass transition con-

cept was extensively proposed in the 1980s, although this concept initially appeared in the

literature in the 1960s. These two concepts provide a strong scientific basis of food stabil-

ity during drying and freezing. The stability of foods is of paramount interest to both food

scientists and engineers; and a better understanding of the factors controlling microbial

stability or reaction rates is clearly needed.[26]In this paper, the concepts of water activity

and glass transition are discussed along with their applications and limitations. In addition,an attempt was made to combine these two concepts in the state diagram by proposing a

macro-micro region concept.

WATER ACTIVITY CONCEPT

In the 1950s, scientists began to discover the existence of a relationship between the

water contained in a food and its relative tendency to spoil. [7]In the 1980s, Labuza and his

group generated significant data on food stability as a function of water activity. They also

began to realize that the active water could be much more important to the stability of a

food than the total amount of water present. Thus, it is possible to develop generalized

rules or limits for the stability of foods using water activity. For example, there is a critical

water activity below which no microorganisms can grow, this value is about 0.6. Most of

the pathogenic bacteria cannot grow below a water activity of 0.85; whereas most yeasts

and molds are more tolerant to reduced water activity but usually no growth occurs below

a water activity of about 0.62.

Microbial responses to low water activity are shown in Fig. 1. A well-established

response to the temporary loss of turgor pressure after a hyper-osmotic shock (i.e., a

reduction of water activity surrounding the cell) is osmoregulation. Exposure of microorgan-

isms to lower water activity causes an instantaneous loss of water, which is accompanied by

a decrease in the cytoplasmic volume called plasmolysis. This can also cause lysis. Hypo-

osmotic shock generally results in minor changes in cell volume. On the other hand,

hyper-osmotic shock causes considerable shrinkage of the cytoplasmic volume. If the

osmotic shock is not too severe, after an extended lag phase, the cytoplasmic volume

increases as a result of osmotic adjustments made by the cells.[8]After loss of turgor, the

microbial cell raises level of the compatible solutes within the cells.[9]

Compatible solutesincluding glycerol, sorbitol, cyclohexaneterol, erythritol, arabitol, or mannitol in fungal

cells or amino acids such as proline, aminobutyric acid, and glutamic acid in bacteria.

These compatible solutes assist to prevent dehydration, and thus facilitate metabolic activ-

ities necessary for growth. This results in an increase in internal osmotic pressure and

restores turgor pressure. The type and amount of solute accumulated in cells is not only

influenced by water activity, but also other factors in the environment, including those

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728 RAHMAN

associated with preserving hurdles used to control microbial growth, such as pH and preser-

vatives.[10,11]Besides the accumulation of compatible solutes, changes in the membrane lipid

including structural changes in phospholipids and fatty acid were also observed.[12,13]The

compatible solutes produced internally are highly soluble, pH neutral, and are usually end

product metabolites. They can include sugars from the breakdown of carbohydrates, amino

acids from protein degradation, and cations such as K+. Examples include betaine, trehalose,

glycerol, sucrose, proline, chloline, carnitine, mannitol, glucitol, and ectoine. The cell mem-

brane is selectively permeable to them, allowing the cytoplasmic pool to be determined by

the external osmotic pressure.[14,15] The preferred exogenous bacterial-compatible solutes

are glycine and betaine, which are found in higher plants, and the amino acid proline.[4]In

many foods, however, peptides are more readily available than free amino acids and hence

peptides have become an important source of both nutrients and compatible solutes.[16]

A food product is most stable at its monolayer moisture content, which varies with

the chemical composition, structure and environmental conditions, such as temperature.

However, in many instances the critical limit may also be observed at higher moisture than

monolayer moisture content. Figure 2 shows the monolayer and multilayer water on a

solid surface.[17]This principle was the main reason why food scientists started to empha-

size water activity rather than total water content. Since then, the scientific community has

explored the significance of water activity in determining the physical characteristics, pro-

cesses, shelf life, and sensory properties of foods. It is now used to predict the end point of

drying, in process design and control, in ingredient selection, to predict product stability

and to make packaging selection. One of the earlier food stability maps based on the water

activity concepts considers growth of micro-organisms and different types of bio-chemi-cal reactions.[18,19]The updated food stability map is presented in Fig. 3. In this present

map, the trends of microbial growth, bio-chemical reactions and mechanical characteris-

tics are presented in the 3 zones of water activity. In general the rule of water activity con-

cept is: Food products are most stable at its monolayer moisture content or monolayer

water activity and unstable above or below monolayer. A water activity map also provides

the following rules: below monolayer (zone I):(i) stability can be decreased with decreasing

Figure 1 Microbial response at low water activity.

K+

Turgor Cell

Normal (Turgor Cell)

Low Water Activity

K+

CS: Compatible Solutes

K+

K+

Water (if low aw

environment)

Water

CS

K+K+

CSCS

Low Water Activityaccumulation of compatible

solutes to lesser gradient and

reduce water loss

Plasmolysis

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730 RAHMAN

water activity, for example fat oxidation (line ob); (ii) stability can remain constant (line ab);

and (iii) stability can increase (line nb). In the adsorbed layer (zone II): (i) stability can

increase (line bp); and (ii) stability can decrease (line bc). In multi layer (zone III): stability

can increase, decrease, or remain constant.Recently, the limitations of water activity concept were identified [7,2024] as:

(i) Water activity is defined as at equilibrium, whereas foods may not be in a state of equi-

librium, for example low and intermediate moisture foods. (ii) The critical limits of water

activity may also be shifted to higher or lower levels by other factors, such as pH, salt,

anti-microbial agents, heat treatment, electromagnetic radiation, and temperature.

(iii) Nature of the solute used to reduce water activity also plays an important role, for

example some solutes are more inhibiting than others even at the same water activity

(minimum growth of P. fragi is 0.96 using sodium chloride, while for glycerol water

activity is 0.94.[25](iv) Water activity concept does not indicate the mobility or reactivity

of water and the nature of binding to the substrate. It only provides the information on the

amount of strongly bound and free water without their precise reactivity. (v) Many physical

changes, such as crystallization, caking, stickiness, gelatinization, diffusivity could not be

explained based on the water activity alone. However, these limitations do not completelyinvalidate the concept but rather make it difficult to apply universally. In order to find

other alternatives, the glass transition concept was postulated in the literature.

GLASS TRANSITION CONCEPT

Glassy materials have been known for centuries but the glass transition concept was

first applied to foods with scientific understanding in the 1980s.[26]A low glass transition

means that at room or mouth temperature, the food is soft and relatively plastic, and at

higher temperatures it may even flow. In contrast, a food with a high glass transition

temperature is hard and brittle at ambient temperature. Early attempts to describe glassy

phenomena concluded that glass is a liquid that has lost its ability to flow, thus instead of

taking the shape of its container, glass itself can serve as the container for liquids. Food

materials are in an amorphous or non-crystalline state below the glass transition tempera-

ture and are rigid and brittle. Glasses are not crystalline with a regular structure, but retain

the disorder of the liquid state. Physically it is a solid but resembles closely a thermody-

namic liquid. Molecular mobility increases 100-fold above glass transition. In kinetic

terms, Angell [27]described a glass as any liquid or super-cooled liquid whose viscosity is

between 1012and 1013Pa s, thus effectively behaving like a solid, which is able to support

its own weight against flow due to gravity. To put this viscosity into context, a super-

cooled liquid with a viscosity of 1014Pa s would flow 1014m/s in the glassy state, in con-

trast the flow rate of a typical liquid is in the order of 10 m/s. In other words, a glass is a

liquid that flows about 30 m in a century.[28]This is evidenced by the fact that ancientstained glass windows are thicker at their base due to flow under gravity. [29]

The early papers related to glass transition in food and biological systems appeared

in the literature in the 1960s.[3032]

White and Cakebread[31]

first highlighted the impor-tance of the glassy state of foods in determining its structural stability. They were perhaps

the first food scientists to discuss the importance of the glassy and rubbery states in rela-

tion to the collapse of a number of high solid systems. The significant applications of the

glass transition concept emerged in food processing in the 1980s, when Levine and Slade[33] and Slade and Levine[34] identified its major merits and wide applications. In the

1990s, Roos, Karel, and other groups generated significant data on the glass transition and

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components (i.e., characteristic curves) of state diagrams for a number of food ingredients.

In 1990s, Chirife, Buera, Bell, Karel, Roos, Labuza, and others started to present data on

food stability based on the glass transition and water activity concepts. It has been men-

tioned in the literature that foods can be considered very stable at the glassy state, since

below glass temperature compounds involved in the deterioration reactions take many

months or even years to diffuse over molecular distances and approach each other to

react.[35]A hypothesis has recently been stated that glass transition greatly influences food

stability, as the water in the concentrated phase becomes kinetically immobilized and

therefore does not support or participate in reactions. Formation of a glassy state results in

a significant arrest of translational molecular motion, and chemical reactions become very

slow[36]. The rules of glass-transition concept are: (i) Foods are most stable at and below

glass transition; and (ii) the higher the T-Tgor T/Tg(i.e above glass transition), the higher the

deterioration or reaction rates. These conditions identify two macro regions (i.e., 3 regions

above and one region below) in the state diagram (Fig. 4). Similarly, mechanical and

transport properties could also be related with glass transition. It is very interesting to see

that this concept has been so widely tested in foods as evident from the literature. In many

instances, glass transition concept does not work alone, thus it is now being recommended

to use both the water activity and glass transition concepts in assessing process-ability,

deterioration, food stability, and shelf-life predictions.[37]

GLASS TRANSITION AND GLASSY STATE

Phase transitions in foods can be divided into two groups: first-order and second-order. At the first-order transition temperature, the physical state of a material changes

isothermally from one state to another (e.g., solid to liquid, liquid to gas) by release or

absorption of latent heat (e.g., melting, crystallization, condensation, evaporation).

Second-order transition occurs (e.g., amorphous state to glassy state) without release or

absorption of latent heat.[38]Glass transition is a second-order, time-temperature depen-

dent transition, which is characterized by a discontinuity or change in slope of physical,

Figure 4 State diagram proposed by Levine and Slade showing 5 macro-regions.

0 1

Solute MassFraction

Temperature

F

G

Xs

A

Tg

Tgw

Tgs

c

Rubber

Glass

Ice

1

2

3

4

GlassL

ine

FreezingCurve

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732 RAHMAN

mechanical, electrical, thermal, and other properties of a material, when plotted as a function

of temperature.

[38]

This transition is most commonly identified as a shift in the thermogramline measured by Differential Scanning Calorimetry (DSC). The process is considered to be

a second order thermodynamic transition in which the material undergoes a change in

state but not in phase. It is more meaningfully defined as analogue as nature to second-

order, since each measurement technique is based on monitoring change in a specific

property, and since change or break in properties are achieved within a certain temperature

range rather than at a specific temperature.[22]A perfect second-order transition occurs at a

specific temperature. During heating, glass transition of food materials does not occur at a

fixed point with the change of specific heat. Instead networks soften or transform over

quite a large temperature range.[29] However, glycerol produces a step change in heat

capacity as a function of temperature at 190 K. [39]Figure 5 shows the ideal second order

transition (A) (i.e., shift at a specific temperature) and non-ideal second order transition

(i.e., shift over a temperature range); (B) as evident by DSC thermogram.

STATE DIAGRAM AND ITS COMPONENTS

State Diagram

A state diagram is another stability map of different states of a food as a function of

water or solids content and temperature.[40]Levine and Slade[33]presented a state diagram

of providone N-vinyl pyrrolidone (PVP) illustrating glass line, freezing curve, and inter-

section of these lines as Tg(in this paper, the author is noted as Tg). The intersection wasidentified by extrapolation of extending the freezing curve by maintaining a similar curva-

ture. This is most probably the first state diagram in the food science literature. Many of

his publications later presented[34]the state diagram of starch by quoting the source refer-

ence of van den Berg.[41] Later he presented another state diagram showing glass line,

freezing curve, melting line, eutectic point, Tgand vapor line.[42]

Starting from the 1990s,Roos, Karel, Kokini, and others presented state diagrams of a number of food components

and food products. The main advantages of drawing a map are to help understanding the

complex changes when the water content and temperature of foods are changed.[37,4349]It

also assists in identifying stability of foods during storage as well as selecting a suitable

condition of temperature and moisture content for processing.[21,5051] The regions of

drying and freezing can be easily visualized in the diagram, and product stability can be

Figure 5 Second order transition in foods identified by DSC thermogram. A: ideal second order transition,

B: non-ideal second order transition.

Temperature (C)

HeatFlow(

W/g)

HeatFlow(

W/g)A B

Temperature (C)

onset (Tgi)

peak (Tgp)

end (Tge)

onset and end

Tgi= Tge

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assessed based on moisture content and temperature. Figure 6 shows a state diagram indi-

cating the different states as a function of temperature and solute mass fraction (updatedfrom Rahman [22]).

Components of State Diagram

Earlier state diagrams were constructed with only the freezing curve and glass tran-

sition line. Recently, attempts have been made to add other structural changes along with

the glass line, such as solubility line. Numbers of micro-regions and new terminologies

are being included in constructing the state diagram. The presented state diagram shown in

Fig. 6 is updated from Rahman.[22,40]In Fig. 6, the freezing line (ABC) and solubility line

(BDL) are shown in relation to the glass transition line (EQFS). Point F (Xsand Tg)whichis lower than Tm (point C) is a characteristic transition (maximal-freeze-concentration

condition) in the state diagram, defined as the intersection of the vertical line from (Tm)a

to the glass line EQFS.[22]

At this maximal-freeze-concentration, all possible freezablewater is transformed into ice. The water content at point F or C is considered as the

un-freezable water (i.e., 1- Xs). The un-freezable water mass fraction is the amount ofwater remaining unfrozen even at very low temperature. It includes both un-crystallized

free water and bound water attached to the solids matrix. Point Q is defined as TgandXsas the intersection of the freezing curve to the glass line by maintaining the similar curvature

of the freezing curve ABC.

Figure 6 State diagram showing different macro-micro regions (updated from Rahman [22]).

0 1

Solute MassFraction

Temperature

F

E(Tgs)

J

Xs

B

A

Glass

ice + glassat Tg

Crystal

Collapse

Stickiness

Rubber

Softeningzone

Reaction zone

Entangle flow

Solution

Tgw

Tg

Tm

Cice+rubber (solidmatrix + un-freezable water)

G

Tu

H

I

ice + solution (solute+free water)

LiquidL (T

ms)

Solid

DM

Tbw

Water vapor

N

O

BET

-Mono

layer

Q

ice + glass Tg

R

K

Tg

1

24

5

6

7

8

1110

9

12

13

Tg

glass

Xs

ice + solution (solute crystal+free water)

ice + glass Tg

3

S

Highest Molecular Mobility

P

Tgiv

U (Tds)

Xs

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734 RAHMAN

The apparent glass transition (Tg)aof the solids matrix in the frozen sample is usu-ally determined by Differential Scanning Calorimetry (DSC) below (Tm)a(Fig. 7A). This

is due to the formation of a similar solid matrix associated with un-freezable water and

transformation of all free water into ice, although the sample contains different levels of

total water before the start of DSC scanning.[52]The values of apparent (Tg)aand (Tm)adecreased with increasing solids content and reached to a nearly constant value. The inter-

sections of the lines in Fig. 7B show the Tg(point R in Fig. 6) and Tm(point C in Fig. 6).

If the sample containing moisture close to the points R and C, these apparent values

become similar to Tgand Tmas marked in Fig. 7.

In the region AGB as shown in Fig. 6, the phases present are ice and solution. Below

point B, the first crystallization of solute occurs, transforming the GBCH region to three

states: ice, solution and solute crystal. There is no free water (i.e able to form ice) to the

right side of point C (Tm,end point of freezing with maximal freeze-concentration-condition)and below the very concentrated solution is transformed to the rubber state. The maximal-

freeze-concentration condition can be achieved using optimum conditions by slow cooling

and/or annealing of the samples so that all freezable water can be transformed into ice.

The region HCRI contains ice, rubber, and solute crystal. Point F is the Tg, below this

point all portion of the rubber state is transformed to the glass state, thus the region KFS

contains glass, ice, and solute crystal. The rate of cooling can shift the points B, C, Q and

F. More detailed effects of cooling on the shift are discussed by Rahman.[40]The glass line

should follow the line EQFS if maximum ideal plasticization occurs in the solid matrix

with the addition of water. The line EQF could be shifted upward if water causes different

degrees of plasticization (as shown by line EP) in the sample containing un-freezable

water. The vertical line from F intersects at point P and the temperature is defined as Tgiv.

In this situation, fitting the glass transition data to the Gordon-Taylor equation, consider-

ing the two ends as Tgsand Tgw, may not be valid. Based on the authors hypothesis of dif-

ferent characteristic temperatures Tgiv, Tm, Tg, Tg, and Tg, the modified equation could

be proposed as:

whereXwandXsare the mass fraction of water and solids; Tcand Tgsare the characteristic

temperature and glass transition of solids; and kcis the characteristic of the material, respec-

tively. In the case of the Gordon-Taylor equation, the characteristic temperature is the

glass transition of water instead of Tc(i.e., Tgiv, Tm, Tg, Tg, or Tg).

Figure 7 Identification of (Tm)a, (Tg)a, Tm, Tg, Tg, or Tgin the state diagram.

HeatFlow

(W/g)

Temperature (C)

A B

Temperature(C)

Solute Mass Fraction

(Tg)

a

(Tm

)a

Tg

Tm

Tg

Tg(T

g)a

(Tm

)a

T X T k X TX k

s gs c w c

s c wg

X= +

+(1)

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The region BQEL is important in food processing and preservation, many character-

istics, such as crystallization, stickiness, and collapse are phenomena that are observed in

this region.[37,53]In case of cereal proteins, Kokini et al.[48]determined an entangled poly-

mer flow region and a reaction zone based on the mechanical characteristics. The lineBDL is the melting line which is important when products are exposed to high tempera-

ture during processing, for examples frying, baking, roasting, and extrusion cooking. In

the case of a multi-component mixture such as food a clear melting is difficult to observe

at high temperature due the reactions or interactions between the components. In this case,

Rahman.[40] and others have defined melting as the decomposition temperature. Line

MDL is the boiling/evaporation line for water from the liquid phase (line MD) and solid

matrix with a degree of saturation with water (line DL). It is possible for the melting and

evaporation lines to intersect since water evaporation could happen in a saturated matrix

before melting of the solid matrix mixed with water.

It was identified that further work is required on the relationships between glass

transition, water activity and food stability.[3]It appears that the interrelationships can be

very complex, depending on the complexity of the food system and on the type of stability

being studied. Recently many papers have presented data on both water activity and glasstransition as a function of water content. However the link between them that determines

stability has not been identified. Karel et al.[54]attempted to relate water activity and glass

transition by plotting equilibrium water content and glass transition as a function of water

activity. By drawing a vertical line on the graph, stability criteria could be determined

from the moisture isotherm curve and the glass transition line. At any temperature (say

25oC), stability moisture content determined from the glass transition line was much

higher than the stability moisture from the isotherm. Similarly Sablani et al. [55]predicted

stability criteria (solids content) from an isotherm model (GAB equation), considering

water activity 0.20, and then estimate the solids content from glass model (Gordon-Taylor)

considering storage temperature as Tg. They found a surprising gap between the predic-

tions of the stable solids contents using the two different approaches for a number of food

products. At present it is a challenge to link them in a meaningful way.

As a first attempt, Rahman[22]plotted the BET-monolayer value as the LNO line in

the state diagram shown in Fig. 6. It intersects at point N with the glass line EQFS, which

shows that at least in one location (point N), glass and water activity concepts provide the

same stability criteria. This also justifies the variability of deteriorations observed by

Sablani et al.[55] This approach forms more micro-regions, which could give different

degrees of stability in the state diagram. More studies regarding stability need to be done

on both the left (above and below glass) and right sides (above and below) of the line

LNO. It should be mentioned here that the BET-monolayer could be achieved by mainly

removing water from a system (since the isotherm is relatively unaffected by tempera-

ture), but glassy state could be achieved by removing water through drying, as well as by

decreasing the temperature of the system. A successful combination of water activity and

glass transition could develop a more in depth knowledge on stability criteria. In addition,

how other factors, such as pH, and preservatives act could be linked with these concepts.At present the scientific community far from developing unified theory.

It is evident from the literature that stability below or above glass transition varies

even based on specific cases, indicating that applying only the glass transition temperature

for developing the stability rule is not enough. Samples with freezable water are more

complex and four characteristic temperatures were defined by Rahman et al. [52]In current

paper a total of five temperatures are defined as Tgiv> Tm> Tg> Tg> Tg. In addition,

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736 RAHMAN

the stability below or above Tgiv, Tm, Tg, Tg, and Tgshould also be explored. There are

only a few references available that include all four characteristic temperatures with their

moisture content. It is important to know how these temperatures affect the stability of

foods. It would be interesting to explore the differences in stability in product within thesedifferent ranges.[22]

VALIDITY OF GLASSY STATE

Applications of glassy state concept in food systems are thoroughly reviewed by

Kalichevsky-Dong[3]and Rahman.[22]They grouped the applications of glassy state in the

processes of controlling diffusion process, structure, crystall ization, stickiness, grain

damage, pore formation, microbial stability, seed stability, oxidation, non-enzymatic

browning, enzymatic reaction, protein denaturation, hydrolysis, and enzyme inactivation.

Most of the literature used two criteria to test the validity specifically: (i) whether food is

stable if it is in the glassy state or unstable if it is above the glass transition; and/or

(ii) whether the change in attributes above glass is related to the (T-Tg) or T/Tg. It is clear

from the literature that all experimental results could not be explained by the above rules,thus it is now important to identify when it fails and why. It is also a challenge to combine

other concepts with glass transition.

The water activity concept is based on the binding nature of water molecules in the

matrix. When water is bound (i.e., unavailable to take part in reactions) to the solid matrix

or non-solvent, then no deterioration reactions could be expected. The glass transition

concept is based on the molecular mobility of the reacting components in the matrix, thus

diffusion of the reactants through the system to take part in reactions is very slow and

stability is achieved. Although combining both the concepts could be a powerful tool for

stability determination. A successful combination of water activity and glass transition

could open more precise and unified determination of stability criteria. Attempts should be

made to provide alternative solutions to combine both concepts. In fact there are other

factors which play a role and in many instances it is incorrect to say even combining both

would give complete stability. However, other building blocks could be added. Stability

could be determined from boundary modeling and then kinetics prediction models.

MACRO-MICRO REGION CONCEPT

BET-monolayer line as LNO in the state diagram (shown in Fig. 6) makes four

regions: below BET-monolayer, one above and one below; and above BET-monolayer,

one above and another below.[22]This approach forms more micro-regions, which could

give different stability in the state diagram and could explain the limitations of each

concept. The present hypothesis proposed thirteen micro-regions having highest to lowest

stability based on the locations from glass and BET-monolayer lines. For example, the

region-1 (relatively non-reacting zone, below the BET-monolayer line and glass line) is

the most stable and region-13 (highly reacting zone, far from BET-monolayer line andglass line) is least stable. The stability decreased as the zone number increased. Each

micro-region could be studied for specific deterioration separately, considering different

characteristics of a medium.

It is clear that the stability based on macro-regions in the state diagram considering

below and above BET-monolayer or glass transition would not be enough to determine

stability. The literature showed that many systems were stable above glass transition and

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unstable below glass transition. One of the arguments was proposed that mobility of water

could also occur below glass transition. Based on the variability in the stability, micro-

regions were identified within the macro-regions. In the state diagram, 3 factors are

clearly identified or mapped: (i) how far it is from the BET-monolayer line; (ii) how far itis from the glass transition line; and (iii) how low is the temperature. In addition to the

binding nature of water (water activity) and mobility of reactants (glass transition), sample

temperature could be identified as a separate factor in the state diagram, considering

temperature dependent reaction rates. The reactivity does not increase only based on a

function of (T-Tg) or T/Tg and variation in increasing or decreasing behavior could be

observed, for example micro-region 10 is below the BET-monolayer and above the glass

transition and could be very reactive. The micro-region 1 is considered most stable since it

is below the glass transition and BET-monolayer, and low temperature storage. The most

unstable micro-region is region 13 since it is the most reactive. In this case, there is no

point in applying the concepts of glass transition or water activity alone and other preser-

vation hurdles must be used. The micro-region could also be sub-divided further to

explain the variability of stability, for example regions between 3 to 9 based on Tu, Tm,

Tg, Tg, or Tg. This could open another dimension to the food stability map with moremicro regions. Similarly regions 10 and 11 could be divided based on Tgand Tg

ivfor the

samples containing no freezable water. Another important aspect in stability is to study the

types of glass formed in foods along with their characteristics, such as fragile/strong

glass,[56, 57]degree of plasticization,[57]strength and density of hydrogen bonds [58], vari-

ability in molecular mobility and/or relaxation below glass,[59,60]and complex matrix of

crystalline/semi-crystalline/amorphous regions. There is a definite need to clarify the

validity of each concept by identifying the valid and invalid conditions and systems.

COMBINING MULTICONCEPTS

IFT[6] presented the factors that influence the microbial growth in foods. In this

report the multi-factor nature of microbial stability was identified, and how two factors,

pH and water activity interact each other were described. The USDA pathogen-modeling

program was used to identify critical limits of water activity and pH when both factors are

interacting. They also identified that a general model for foods to cover all interactions of

atmospheric gases and/or preservative combinations with pH and water activity does not

currently exist. In reality the problem of stability determination could not be solved by

identifying macro-regions or micro-regions, but it could help in developing scientific,

systematic and rational approaches to determine the stability. The next task is to test the

stability in each micro region and there may be a possibility to explore more generic rules

for stability of individual stability criteria in the micro-region. A knowledge-based

approach could be used to identifying how other factors, such as pH, preservatives, and

types of solutes affect the stability in each micro-region.

KNOWLEDGE BASE OR DATA MINING

The author believes that developing a knowledge base of the stability for each

macro and micro region would be a valued approach for exploring further generic rules for

stability. He is confident that a data mining approach and/or a boundary modeling technique

could be used to explore and to develop the generic rules in the future. This database, if

developed, could be the foundation of new theoretical progress.

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738 RAHMAN

CONCLUSION

The state diagram could be used to combine water activity and glass transition con-

cepts by including BET-monolayer and glass lines. In addition different macro- and

micro-regions could also be drawn to identify food stability. In this paper 13 micro regions

are proposed for determining food stability. A library of knowledge needs to be developed

for each macro- and micro-region for developing further generic rules.

ACKNOWLEDGMENTS

This paper was presented in the 18thInternational Congress of Chemical and Process Engineering

(CHISA 2008), 24-28 August 2008, Prague, Czech Republic as a keynote lecture. The author would

like to acknowledge the support of Sultan Qaboos University towards this research in the area of food

structure and stability. He is grateful to all members of his research group for their continued support

and encouragement. Special thanks to Dr. Ann Mothershaw for checking the clarity of the paper.

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water activity and Food Stability