- Email: [email protected]
WATER ACTIVITY | Effect on Food Stability
6094 WATER ACTIVITY/Effect on Food Stability
behavior, and Raoult’s law cannot be used. Then, either sorption isotherms or other water activity calculation methods must be followed to estimate water activity in food product development. Several equations are available for predicting water activity of a food product based on the product composition. These include the Norrish equation, which extends the use of mole fractions of food components with additional coefficients to calculate water activity, and the Grover equation, which uses different empirical constants for the various components in a polynomial. The Ross equation (eqn (15)) gives probably the most accurate water activity predictions for complex solutions. The equation is relatively simple and does not require the use of empirical constants. aw ¼ aw1 aw2 . . . awi :
ð15Þ
Mixtures of two or more ingredients or food components with different water activities in a sealed package will exchange water until the water activity of the components becomes the same. The water content of the components initially and after equilibration may differ significantly, owing to differences in sorption properties. There are several approaches, which can be applied to estimate the final water activity of a food mixture in a sealed package. These methods are based on knowledge of the sorption isotherms and their use to predict the equilibrium water activity. A simple approach is to establish a sorption isotherm for the mixture and predict water activity using the total water content of the ingredients. Another approach assumes that the sorption isotherms of the components over the applied water activity range follow a straight line. This leads to eqn (16), which can be used to predict the equilibrium water activity, aw. The method is also useful in estimating an appropriate ratio of food components to keep the final, equilibrium water activity at an acceptable level. aw ¼
0015
aw1 b1 ww þ aw2 b2 w2 . . . þ awi bi wi , b1 w1 þ b2 w2 . . . þ bi wi
See also: Water: Structures, Properties, and Determination; Physiology; Water Activity: Effect on Food Stability
Further Reading Barbosa-Ca´ novas GV and Welti-Chanes J (eds) (1995) Food Preservation by Moisture Control: Fundamentals and Applications. Lancaster, PA: Technomic. Bell LN and Labuza TP (2000) Moisture Sorption. Practical Aspects of Isotherm Measurement and Use. St. Paul, MN: American Association of Cereal Chemists. Fontana AJ (2000) Understanding the importance of water activity in food. Cereal Foods World 45: 7–10. Iglesias HA and Chirife J (1982) Handbook of Food Isotherms. New York: Academic Press. Jouppila K and Roos YH (1997) Water sorption isotherms of dehydrated milk products: Applicability of linear and nonlinear regression analysis in modeling. International Journal of Food Science and Technology 32: 459–471. Labuza TP (1968) Sorption phenomena in foods. Food Technology 22: 263–265, 268, 270, 272. Labuza TP (1980) The effect of water activity on reaction kinetics of food deterioration. Food Technology 34(4): 36–41, 59. Rockland LB and Stewart GF (eds) (1981) Water Activity: Influences on Food Quality. New York: Academic Press. Roos YH (1995) Phase Transitions in Foods. San Diego, CA: Academic Press. Roos YH (2000) Water activity and plasticization. In: Eskin NAM and Robinson DS (eds) Food Shelf Life Stability. Boca Raton, FL: CRC Press. Roos YH, Leslie RB and Lillford PJ (eds) (1999) Water Management in the Design and Distribution of Quality Foods. Lancaster, PA: Technomic. Seow CC, Teng TT and Quah CH (eds) (1988) Food Preservation by Moisture Control. London: Elsevier. Simatos D and Multon JL (eds) (1985) Properties of Water in Foods. Dordrecht: Martinus Nijhoff. Slade L and Levine H (1991) Beyond water activity: Recent advances based on an alternative approach to the assessment of food quality and safety. Critical Reviews in Food Science and Nutrition 30: 115.
ð16Þ
where aw1, aw2 . . . awi, b1, b2 . . . bi, and w1, w2 . . . wi refer to the initial component 1, 2 . . . i water activity, slope of the sorption isotherm, and weight of dry solids, respectively. The role of water activity as a food stability parameter has been well recognized. Water activities can be measured with commercially available equipment, and sorption isotherms are available for most food components. Furthermore, computer software for water activity calculations and establishment of sorption isotherms has been developed and used widely in the food industry.
Effect on Food Stability Y H Roos, University College Cork, Cork, Ireland Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
Introduction Water activity, together with pH and temperature, is often an important factor in controlling food stability. These three parameters all, but not alone, influence microbial activity, rates of chemical and enzymatic reactions, and changes in food texture. Water activity,
0001
WATER ACTIVITY/Effect on Food Stability
0002
0003
however, has a significant role in controlling texture formation and textural changes in intermediatemoisture foods (IMF) and low-moisture foods. In such foods, removal or sometimes sorption of water results in concentration changes of food solids and thereby has an effect on reaction rates. Enhanced reaction rates resulting from water removal may be desired, as in baking to provide color and flavor, or they may be detrimental as in production of many food powders, such as dairy powders. That makes the time–temperature–water or water activity control in food production and storage extremely important. In some cases, the maximum food stability at low water contents is achieved at a water activity corresponding to the Brunauer–Emmett–Teller (BET) monolayer water content. Furthermore, water affects the physical state and water plasticization, i.e., softening of water miscible food solids. As a plasticizer, water controls the glass transition of food components, such as carbohydrates and proteins. The glass transition is a well-known property of all inorganic and organic amorphous materials. The glass transition occurs over a temperature range, which is often referred to with the glass transition temperature, Tg. The physico-chemical properties of amorphous materials change considerably over the glass transition, as the transformation includes a change between the liquid-like and solid-like material states. There are also established and possible relationships between the physical state, water plasticization, and rates of diffusion-controlled reactions, such as nonenzymatic browning, enzymatic reactions, and oxidative changes. In general, water activity provides valuable information of the effects of water content on water availability and the physical state of food solids. Such information can be described using sorption isotherms and state diagrams. They provide critical values for water content, water activity, and temperature, which are important in the characterization of food behavior in processing and in establishing criteria for packaging requirements and appropriate storage conditions. These areas and the role of water activity in controlling food stability are discussed in the present article.
Water Activity and Food Stability 0004
Most fresh foods are high-moisture materials, and their shelf-life is reduced by enzymatic changes, the growth of microorganisms, and mechanical damage. High-moisture foods have an aw of 0.90–0.999, and they often contain more than 50% (w/w) water (Table 1). These foods include fresh meats and seafood, various dairy products, fruits, vegetables, and
6095
Table 1 Examples of water activity ranges of various foods Food systems
Water activity range
Fresh meat, fish, vegetables, <40% (w/w) sucrose, <7% (w/w) salt Bread, cooked sausages, medium aged cheese Salami, old cheese, >65% sucrose, >15% salt Dried beef, sweet condensed milk, cereals with 15% (w/w) water Jam, marmalade, old salami, >26% (w/w) salt Flour, cereals, nuts Caramels, honey, toffee Breakfast cereals, snack foods, food powders
>0.95
tbl0001
0.90–0.95 0.87–0.90 <0.86 0.80–0.87 0.75–0.80 0.60–0.75 0.20–0.60
beverages. Most bacteria, molds, and yeasts are likely to grow in high-moisture foods. However, the types of spoilage microorganisms and the growth of various species are highly dependent on pH, temperature, and water activity. IMF have a water activity within the range of 0.60– 0.90aw, and their water contents normally vary between 10 and 50% (w/w) (Table 1). These foods include many traditional low-moisture foods, such as grains, nuts, and dehydrated fruits, but also a number of processed foods or foods designed and manufactured to have a known composition to provide stability. Such foods may have particular applications and requirements for stability, e.g., when used as fillings in bakery products or confectionery. Although microbial spoilage is prevented below 0.60aw, and many microbes do not grow in IMF, their stability and shelflife are reduced by deleterious changes, such as structural transformations, enzymatic changes, browning reactions, and oxidation, depending on aw, pH, and temperature. The rates of such changes are often at least to some extent affected by the physical state of the materials and the extent of water plasticization of water-miscible solids. Low-moisture foods obviously have the lowest water contents, often below 10% (w/w), and their water activity is lower than 0.6aw (Table 1). Such foods are not subject to microbial spoilage. Their shelf-life, however, is often limited by chemical and textural changes, particularly browning and other changes in color and flavor as well as oxidation. These foods may be exceptionally hygroscopic and exhibit water sorption from their surroundings. Water sorption is reduced by the use of protective packaging, but sorption occurs during storage as a result of permeation or damage of protective packaging. Many low-moisture foods have a solid appearance, or they have a crispy, solid texture. Water sorption in such foods may lead to stickiness, structural changes, such as loss of crispness and sogginess,
0005
0006
6096 WATER ACTIVITY/Effect on Food Stability
and coincident increases in the rates of browning and enzymatic changes. A number of products contain amorphous components, for example, lactose in dairy powders and sucrose in many bakery products and confectionery. Such amorphous components may crystallize during water sorption, as water activity increases. It is important to notice that low-moisture foods are often glassy materials with Tg values above their normal storage temperature. Water sorption and associated plasticization may decrease the Tg, and the glass transition may occur over the range of the concurrent increase in water activity.
Stability Control Microbial Stability 0007
0008
0009
Microbial growth requires a minimum aw, in addition to pH, temperature, and other appropriate conditions that are important for the growth of bacteria, molds, and yeasts. The water activity of high-moisture foods, especially processed foods, can be manipulated to some extent by the addition of salts and sugars or other ingredients, which are known to reduce water activity. Such compositional alterations are highly advantageous in product development and food safety control. For many products, the effects of compositional changes on water activity can be predicted on the basis of composition and confirmed by measuring water activity of the final product. This allows an understanding of storage requirements and estimation of the product shelf-life in various storage conditions. In high-moisture foods, the main role of water activity control is to govern and reduce the risk of the growth of pathogenic and spoilage bacteria. Examples of water activity limits for the growth of selected microorganisms are given in Table 2. The lowest water activity limit for microbial growth of 0.60aw allows the growth of xerophilic yeasts. Above this limiting water activity, IMFs have an increasing possibility for the growth of various microorganisms with increasing water activity. However, the water activities of IMFs are such that pathogenic bacteria are unable to grow, but there is a possibility for the growth of molds and yeast. The growth of these microorganisms must be controlled by careful adjustment of product water activity, use of protective packaging to avoid contamination, and selection of appropriate humectants, pH control, and use of antimicrobial agents. Microbial stability is an obvious, and often the most important, criterion in food preservation. The aw limits for growth of various microorganisms, as shown in Figure 1, are well established and successfully used in food product development and
Table 2 Water activity (aw) limits for the growth of selected pathogenic and spoilage microorganisms Microorganism Bacteria Bacillus cereus Bacillus subtilis Campylobacter jejuni Clostridium botulinum Clostridium perfringens Escherichia coli Halobacterium halobium Lactobacillus plantarum Listeria monocytogenes Salmonella spp. Shigella spp. Staphylococcus aureus Vibrio parahaemolyticus Yersinia enterocolitica Molds Aspergillus candidus Aspergillus flavus Aspergillus niger Erotum echinulatum Penicillium citrinum Penicillium expansum Penicillium patulum Rhizopus nigricans Xeromyces bisporus Yeasts Saccharomyces bailii Saccharomyces cerevisiae Saccharomyces rouxii
tbl0002
Minimum aw 0.930 0.900 0.990 0.940 0.945 0.935 0.750 0.940 0.920 0.940 0.960 0.860 0.936 0.960 0.750 0.780 0.770 0.620 0.800 0.830 0.810 0.930 0.610 0.800 0.900 0.620
manufacturing as well as control of product safety. Furthermore, in high-moisture foods and several IMF products, water activity is relatively constant and dependent on composition, especially solids content, and the type of water-soluble components. Chemical and Enzymatic Stability
The chemical stability of high-moisture foods is not significantly affected by water activity, as the microbial quality is the main determinant of product shelflife. The role of water activity in determining the chemical and enzymatic stability of IMF and low moisture foods is more important. As the water activity and water content decrease, the concentration of reactants in the water phase of foods becomes obviously increased. Therefore, rates of several reactions may increase with decreasing water activity. The relative rate of deteriorative changes in intermediate and low moisture foods is traditionally related to water content and aw, as shown by the ‘Food stability map’ in Figure 2. Early studies applying nuclear magnetic resonance spectroscopy found ‘mobilization points’ (water activity allowing reactant mobility) for solutes in low-moisture food matrices. The mobilization
0010
aw Most bacteria, some yeasts, pathogenic and spoilage organisms >0.95 Most cocci, lactobacilli, some molds, Salmonella, 0.91−0.95 lactic acid bacteria are major spoilage flora Most yeasts, mycotoxin-producing molds, 0.87−0.90 spoilage often by molds and yeasts >0.86
Staphylococcus aureus may grow Most molds, no growth of pathogenic bacteria Most halophilic bacteria
0.80−0.87
G ro w th
Number of growing microbes
WATER ACTIVITY/Effect on Food Stability
Xerophilic molds
0.75−0.80 0.65−0.75
Osmophilic yeasts
0.60−0.65
No growth
0.4
fig0001
<0.65
0.5
0.6
0.9
1.0
Figure 1 Growth of various microorganisms at different water activity conditions.
Relative rate
Oxidation
Growth of: bacteria yeasts molds
Nonenzymatic browning
0 fig0002
0.7 0.8 Water activity
Water activity
1
Figure 2 Food stability map describing the relationship between water activity and food stability.
point was peculiar to the system, and the level of hydration needed to achieve mobility was solutedependent. No solute mobilization occurred below the BET monolayer value, and, for example, experimental data of nonenzymatic browning reaction rates suggested that browning initiated at the mobilization point. An increase in the reaction rate was apparent with increasing aw, and the rate maximum occurred at the water activity corresponding to a hydration level allowing complete mobilization. However, several other factors, including the glass transition, are important in controlling changes occurring in intermediate and low moisture foods.
6097
Enzyme activity in low-moisture foods is often related to hydration. At low water activities enzymatic activity is generally not observed, as water cannot enhance diffusion of substrates to enzyme molecules. Such water activity dependence applies to hydrolases and oxidases, unless the substrates are nonaqueous liquids, allowing changes to occur fairly independently of water activity. It seems that not only water activity but also the ability of water to give a known mobility to enzymes and substrates is important in controlling enzyme activity. The amount of water needed increases with increasing molecular size owing to impaired diffusion, in particular for enzymes, which are active in the water phase of foods. Therefore, for example, lipase activity is not necessarily related to the mobility provided by water. Lipids exist often in a separate phase apart from the water phase. At low water activities and water contents, lipids become more accessible to the atmospheric oxygen, and oxidation rates may increase (Figure 2). The rate of oxidation, however, decreases rapidly with increasing water activity, as the lipids may become protected from atmospheric oxygen owing to the formation of protective aqueous layers in the food microstructure. In several low-moisture foods, lipids, flavors, and other oxygen sensitive compounds are entrapped within the amorphous structure. Such encapsulated compounds also exhibit reduced rates of oxidation as compared with ‘free’ lipids. Traditional shelf-life predictions of low-moisture foods were based on the information of rates of deteriorative changes and loss of nutrients at various temperatures and water contents. An increase in water content above the BET monolayer value at a constant storage temperature often results in rapid deterioration as reaction rates increase at intermediate water contents. The main factors affecting reaction rates in low-moisture food materials, however, can be controlled by a number of factors, which include food composition and the type of the reaction, temperature, pressure, water content, and pH. In concentrated food systems, changes in viscosity and relaxation times in the vicinity of the glass transition may also affect diffusion and thereby contribute to reaction rates and product shelf-life.
0011
0012
0013
Water Plasticization Intermediate and low moisture foods often exist in an amorphous (noncrystalline, no defined structure of molecular arrangement), elastic, ‘rubbery,’ or ‘leathery’ state or in a solid-like, glassy state. The amorphous state is typical of liquids and remains in
0014
6098 WATER ACTIVITY/Effect on Food Stability
0015
tbl0003
Table 3 Examples of melting and glass transition temperatures of food components Food component
Carbohydrates Fructose Galactose Glucose Lactose Maltose Sorbitol Starcha Sucrose Trehalose Xylitol Proteinsa Gelatin Gliadin Glutenin Myoglobin Ovalbumin a
Estimated values.
Melting temperature of crystalline solids (DSC onset, C)
Glass transition temperature (DSC onset, C)
108 163 143 223 160 85 na 173 215 89
5 30 31 101 87 9 250 62 100 29
na na na na na
207 179 189 149 157
100
1.0 Glass transition
Water activity 0.8 Critical water activity
60 40 20 0
0.6 0.4
−20 −40 −60 −80 0
0.2
Water activity (at 25 ⬚C)
80 Glass transition (⬚C)
many biological materials owing to complicated chemical composition, rapid removal of solvent water, or quench cooling of a liquid melt in processes not allowing component crystallization. For example, many sugars typical of foods do not crystallize in dehydration, but remain in a supercooled, liquid state after removal of the solvent water. Glassy, solid-like foods may suffer the glass transition as a result of water sorption, which may be observed from rapid changes in structure and appearance, and enhanced rates of chemical and enzymatic reactions. The glass transitions of food components have a wide range of temperatures depending on the component itself and also on its molecular weight. In general, high-molecular-weight food components, such as carbohydrate polymers and proteins, have glass transition temperatures at very high temperatures well above 100 C. Low-molecularweight components may have very low glass transition temperatures, for example, the glass transition of amorphous water occurs at around 135 C, whereas many anhydrous sugars have a Tg between 0 and 100 C. The Tg is often measured using differential scanning calorimetry, and it can be observed with a number of other thermal and spectroscopic techniques. These include dielectric analysis, dynamic mechanical analysis, electron spin resonance spectroscopy, infra-red and Raman spectroscopy, and nuclear magnetic resonance spectroscopy. Examples of glass transition temperatures of food components are given in Table 3.
Critical water content 5 10 15 20 25 30 35 Water content (g per 100 g of solids)
0.0 40
Figure 3 Relationships between glass transition temperature, water activity, and water content for amorphous lactose with critical values for water content and water activity at room temperature.
Carbohydrates and proteins in foods are watermiscible components, and they are plasticized by water. Water plasticization of the amorphous structure results in a decrease in Tg. The decrease in Tg is substantial even at low water contents, and at high water contents, the Tg approaches that of amorphous water. Therefore, at a constant temperature, e.g., room temperature or a typical storage temperature of intermediate- and low-moisture foods, a water activity or water content range can be established, which corresponds to the glass transition of the material. The relationships between glass transition, water activity, and water content are described in Figure 3. The water activity and water content, which correspond to the Tg depression to the storage temperature can be defined as ‘critical water activity’ and ‘critical water content,’ respectively. Tg ¼
w1 Tg1 þ kw2 Tg2 , w1 þ kw2
fig0003
0016
ð1Þ
where w1 and w2 and Tg1 and Tg2 are the weight fraction and glass transition temperature of solids (1) and water (2), respectively. The relationship between the glass transition temperature depression and water content is often predicted with the Gordon–Taylor relationship (eqn (1)). The model can be fitted to experimental Tg and water content data and used together with water sorption data to establish relationships between the Tg and water sorption properties, as shown for amorphous lactose in Figure 3. Such predictions are useful in food product development, because they allow estimation of food stability in terms of the physical state and water activity.
0017
WATER ACTIVITY/Effect on Food Stability
Water Activity and Material Properties 0018
0019
Several properties, such as dielectric properties, mechanical properties, and viscosity, of amorphous food materials are greatly affected by the glass transition. These changes are related to relaxation times and molecular mobility, which may substantially change and affect food stability over the glass transition. Such changes may contribute to observed changes in textural characteristics, e.g., loss of crispness and sogginess, and reaction rates, e.g., nonenzymatic browning and oxidation. Furthermore, the solid, glassy state supports the amorphous state of food components, but many crystallizing components, e.g., sugars, may crystallize in the supercooled liquid state. Both textural changes and crystallization of amorphous components contribute to a loss of flavor and aroma compounds as well as release and oxidation of encapsulated lipids in numerous dehydrated foods and food powders. In general, the relaxation times of molecular rearrangements in the glassy state are sufficient to provide stability for low-moisture food systems. The changes occurring over the glass transition and in the supercooled liquid state reduce the relaxation times, and the materials lose their long-term stability. For example, amorphous, glassy lactose in food systems is stable, but an increase in water content resulting in glass transition leads to a rapid decrease in viscosity and flow of the material (transformation from solid state to a syrup) accompanied by a loss of structure (stickiness and collapse) and lactose crystallization. Crystallization of amorphous sugars is often observed from water sorption isotherms. When crystallization occurs, sorbed water is not maintained in the crystals, and a decrease in water content is recorded. Such crystallization is time-dependent and detrimental to the quality of amorphous powders. Stiffness
0020
A single model based on the Fermi’s distribution model, as given in eqn (2), allows modelling of the decrease in relaxation times of mechanical properties (stiffness) in the vicinity of the glass transition. According to the model, stiffness can be expressed in terms of a defined and measurable stiffness parameter, Y, which may refer to viscosity, flow, sensory crispness, etc. The stiffness parameter values can be modelled as a function of aw, T, or m relative to its value at a reference state, Ys. A constant, aX, defines the broadness of the transition, e.g., the water activity range over which changes in stiffness occur. The reference value, Xs, obtained from b ¼ Xs/aX, indicates the value for aw, T, or m that decreases Y to 50% below Ys.
Ys 1 1 ¼ b þ X, ln aX Y
6099
ð2Þ
where Y is the stiffness parameter, Ys is the stiffness parameter at a reference state, X is aw, T, or water content, m, and b and aX are constants. The term collapse relates to stiffness and covers various time-dependent structural transformations that may occur in amorphous food and other biological materials at temperatures above the Tg. These changes reflect the effect of changes in relaxation times of mechanical properties on the rates of structural transformations and flow that occur over and above the glass transition temperature range. The rates of these changes increase rapidly above the critical water content or aw during food storage. Stickiness and caking of food powders and collapse of structure are collapse phenomena that are related to water activity and glass transition. The main cause of stickiness is plasticization of particle surfaces by water, which allows interparticle binding and formation of clusters. Collapse of the structure occurs when the decrease in viscosity results in flow, and the material cannot support its own weight. Structural collapse often leads to a reduction in quality of dehydrated, especially freeze-dried, foods, whereas stickiness and caking are detrimental to the quality of spray-dried foods, such as dairy powders.
0021
Crystallization of Food Components
Crystallization of amorphous sugars is known to result in serious quality losses in food powders. For example, crystallization of amorphous lactose in dehydrated milk products has been observed to result in acceleration of the nonenzymatic browning reaction as well as other deteriorative changes and caking. Crystallization of lactose coincides with an increase in free fat, which presumably facilitates lipid oxidation in milk powders. Crystallization in low-moisture carbohydrate matrices, which contain encapsulated volatiles or lipids, results in a complete loss of flavor and release of lipids from the matrix. Water plasticization and depression of Tg to below the ambient temperature are responsible for crystallization of amorphous sugars in foods as a result of increased free volume and molecular mobility, decreased viscosity, and enhanced diffusion. Crystallization seems to initiate at Tg or corresponding aw and proceed with a rate determined by the temperature difference T Tg to a maximum extent also defined by the T Tg. The kinetics of crystallization of sugars and other amorphous food components at a constant temperature above Tg can be related to water content and aw, which define the rate controlling T Tg.
0022
6100 WATER ACTIVITY/Effect on Food Stability
Water Plasticization and Reaction Rates 0023
0024
0025
0026
It has been well established that water as a plasticizer has a significant effect on molecular mobility and probably rates of quality changes above a critical, temperature-dependent aw or water content. A chemical reaction requires sufficient mobility of reactants and reaction products in addition to the driving force, e.g., temperature or concentration, of the reaction or change in quality. Information on factors affecting rates of various kinetic processes can be used to manipulate and control rates of changes occurring in food processing and to develop food products that are less sensitive to detrimental changes during storage.
reaction rates. Several studies, however, have suggested that the reaction rate increases considerably above the glass transition or the critical water activity. Oxidation
Oxidation is a common reaction in low-moisture foods. Oxidation of lipids directly exposed to atmospheric oxygen at food surfaces or surfaces of porous dehydrated materials may occur freely. It is well known that ‘free fat’ in dairy powders, for example, is highly susceptible to oxidation causing quality defects. The stability of dairy powders is related to the glassy state of lactose, and the lipids are at least partially encapsulated within the lactose–protein matrix in the spray-drying process.
Nonenzymatic Browning
Enzymatic Changes
The nonenzymatic browning reaction is one of the most studied deteriorative reactions affected by water activity. Other reactions include destruction of vitamins, enzymatic changes, and oxidation. The rates of most reactions are affected by water activity and glass transition. However, there are no single determinants of rates of chemical reactions and food stability, although an understanding of both the water activity and glass transition of a food material is important in predicting and controlling food stability. Nonenzymatic browning is often considered as a series of condensations that can be assumed bimolecular. The reaction occurs between a reducing sugar and an amino acid or amino group and produces typical flavors of several foods in processing. When uncontrolled, the reaction produces unpleasant flavors and colors, and decreases dehydrated food quality during storage. The nonenzymatic browning reaction in several foods exhibits increasing rates above the critical aw values or water contents corresponding to the glass transition. However, reactions in amorphous foods are complex and may be controlled by several other factors, including concentration changes, pH, and temperature. Established relationships between Tg and the rate of the nonenzymatic browning reaction have, however, suggested that the reaction may become diffusion-controlled, and the rate may be affected by the Tg. The size of the reactants may also be important in diffusion-controlled reactions. It may be assumed that the rate of diffusion decreases with increasing size of the diffusant. The effect of glass transition on the reaction rate has been related to observed discontinuities in Arrhenius plots suggesting a high activation energy in the vicinity of the transition. Studies of effects of glass transition on reaction kinetics and the nonenzymatic browning reaction in particular are complicated, because of several factors affecting plasticization and
Studies of glass transition effects on enzymatic changes have included, for example, sucrose inverson by invertase. Sucrose inversion by invertase in lowmoisture systems is dependent on water activity. The reaction rate increases at water activities above 0.6, but does not seem to be affected by the glass transition of the main food matrix. It is possible that the enzyme mobility is not sufficient to allow the reaction to occur until the reacting molecules are plasticized by water to an appropriate extent above 0.6aw.
0027
0028
Modern Stability Maps Deteriorative changes affected by water activity include such changes as enzymatic changes, nonenzymatic browning, lipid oxidation, microbial growth, and overall stability. The rate of these changes seems to be related to water activity. However, water activity limits for chemical, enzymatic, and mechanical changes are material-specific, similarly as the ‘mobilization points,’ and they may also be related to the physical state, molecular mobility, water plasticization and glass transition of amorphous food solids. Structural transformations, as well as diffusion-controlled deteriorative reactions and changes affected by crystallization phenomena, occur at increasing rates with increasing water activity above the critical aw of the material. It is likely that water contents lower than the critical water content are needed for maximum stability. Therefore, modern stability maps provide information on both the water activity and material physical state on the relative rates of deteriorative changes (Figure 4). In addition, state diagrams and sorption isotherms are useful as stability maps. Knowledge of material properties is extremely useful in the production of encapsulated flavors, extruded products, confectionery, development of new food products, and avoiding quality
0029
WATER ACTIVITY/Effect on Food Stability
Determination; Physiology; Water Activity: Principles and Measurement; Yeasts
Glass transition range
Relaxation times
6101
Water sorption
0 fig0004
Water content
Relaxation time/reaction rate
Oxidation Nonenzymatic browning
Water activity
1.0
Figure 4 Food stability map taking into account water activity and physical state effects on relaxation times and reaction rates.
changes that may result from mechanical changes, e.g., loss of crispness. The temperature-, water content-, and time-dependent changes that are often problems in manufacturing and storage of food powders and other low-moisture foods can be reduced by avoiding exceeding their critical values of water activity based on the Tg determination or by compositional adjustments that provide sufficiently high values for critical aw and Tg. The most important applications of producing high-quality dehydrated foods include reduced collapse and improved flavor retention in dehydration processes. The kinetics of enzyme activity is important to food quality, and applying the knowledge in food industry may allow the design of improved products with extended shelflives or even improved retention of activity, e.g., in enzyme preparations. Water activity has been successfully used in setting limits for microbial stability in foods and qualitative characterization of relative rates of deteriorative reactions in low-moisture foods. The combined use of aw and glass transition has a high practical applicability as it provides criteria for critical aw and, together with sorption isotherm, can be applied to adjust water content at a constant temperature to achieve the maximum food stability. See also: Browning: Nonenzymatic; Toxicology of Nonenzymatic Browning; Crystallization: Basic Principles; Mycotoxins: Classifications; Oxidation of Food Components; Spoilage: Chemical and Enzymatic Spoilage; Bacterial Spoilage; Molds in Spoilage; Yeasts in Spoilage; Water: Structures, Properties, and
Further Reading Barbosa-Ca´ novas GV and Welti-Chanes J (eds) (1995) Food Preservation by Moisture Control: Fundamentals and Applications. Lancaster, PA: Technomic. Bell LN (1996) Kinetics of non-enzymatic browning in amorphous solid systems: distinguishing the effects of water activity and the glass transition. Food Research International 28: 591–597. Buera MP and Karel M (1995) Effect of physical changes on the rates of nonenzymatic browning and related reactions. Food Chemistry 52: 167–173 Labuza TP (1980) The effect of water activity on reaction kinetics of food deterioration. Food Technology 34: 36–41, 59. Lievonen SM, Laaksonen TJ and Roos YH (1998) Glass transition and reaction rates: nonenzymatic browning in glassy and liquid systems. Journal of Agricultural and Food Chemistry 46: 2778–2784. Matveev YuI, Grinberg VYa and Tolstoguzov VB (2000) The plasticizing effect of water on proteins, polysaccharides and their mixtures. Glassy state of biopolymers, food and seeds. Food Hydrocolloids 14: 425–437. Nelson KA and Labuza TP (1994) Water activity and food polymer science: implications of state on Arrhenius and WLF models in predicting shelf life. Journal of Food Engineering 22: 271–289. Peleg M (1993) Mapping the stiffness–temperature–moisture relationship of solid biomaterials at and around their glass transition. Rheologica Acta 32: 575–580. Roos YH (1993) Melting and glass transitions of low molecular weight carbohydrates. Carbohydrate Research 238: 39–48. Roos YH (1995) Phase Transitions in Foods. San Diego, CA: Academic Press. Roos YH, Karel M and Kokini JL (1996) Glass transitions in low moisture and frozen foods: effects on shelf life and quality. Food Technology 50(11): 95–108. Roos YH (2000) Water activity and plasticization. In: Eskin NAM and Robinson DS (eds) Food Shelf Life Stability. Boca Raton, FL: CRC Press. Roos YH, Leslie RB and Lillford PJ (eds) (1999) Water Management in the Design and Distribution of Quality Foods. Lancaster, PA: Technomic. Shimada Y, Roos Y and Karel M (1991) Oxidation of methyl linoleate encapsulated in amorphous lactosebased food model. Journal of Agricultural and Food Chemistry 39: 637–641. Slade L and Levine H (1991) Beyond water activity: Recent advances based on an alternative approach to the assessment of food quality and safety. Critical Reviews in Food Science and Nutrition 30: 115.
behavior, and Raoult’s law cannot be used. Then, either sorption isotherms or other water activity calculation methods must be followed to estimate water activity in food product development. Several equations are available for predicting water activity of a food product based on the product composition. These include the Norrish equation, which extends the use of mole fractions of food components with additional coefficients to calculate water activity, and the Grover equation, which uses different empirical constants for the various components in a polynomial. The Ross equation (eqn (15)) gives probably the most accurate water activity predictions for complex solutions. The equation is relatively simple and does not require the use of empirical constants. aw ¼ aw1 aw2 . . . awi :
ð15Þ
Mixtures of two or more ingredients or food components with different water activities in a sealed package will exchange water until the water activity of the components becomes the same. The water content of the components initially and after equilibration may differ significantly, owing to differences in sorption properties. There are several approaches, which can be applied to estimate the final water activity of a food mixture in a sealed package. These methods are based on knowledge of the sorption isotherms and their use to predict the equilibrium water activity. A simple approach is to establish a sorption isotherm for the mixture and predict water activity using the total water content of the ingredients. Another approach assumes that the sorption isotherms of the components over the applied water activity range follow a straight line. This leads to eqn (16), which can be used to predict the equilibrium water activity, aw. The method is also useful in estimating an appropriate ratio of food components to keep the final, equilibrium water activity at an acceptable level. aw ¼
0015
aw1 b1 ww þ aw2 b2 w2 . . . þ awi bi wi , b1 w1 þ b2 w2 . . . þ bi wi
See also: Water: Structures, Properties, and Determination; Physiology; Water Activity: Effect on Food Stability
Further Reading Barbosa-Ca´ novas GV and Welti-Chanes J (eds) (1995) Food Preservation by Moisture Control: Fundamentals and Applications. Lancaster, PA: Technomic. Bell LN and Labuza TP (2000) Moisture Sorption. Practical Aspects of Isotherm Measurement and Use. St. Paul, MN: American Association of Cereal Chemists. Fontana AJ (2000) Understanding the importance of water activity in food. Cereal Foods World 45: 7–10. Iglesias HA and Chirife J (1982) Handbook of Food Isotherms. New York: Academic Press. Jouppila K and Roos YH (1997) Water sorption isotherms of dehydrated milk products: Applicability of linear and nonlinear regression analysis in modeling. International Journal of Food Science and Technology 32: 459–471. Labuza TP (1968) Sorption phenomena in foods. Food Technology 22: 263–265, 268, 270, 272. Labuza TP (1980) The effect of water activity on reaction kinetics of food deterioration. Food Technology 34(4): 36–41, 59. Rockland LB and Stewart GF (eds) (1981) Water Activity: Influences on Food Quality. New York: Academic Press. Roos YH (1995) Phase Transitions in Foods. San Diego, CA: Academic Press. Roos YH (2000) Water activity and plasticization. In: Eskin NAM and Robinson DS (eds) Food Shelf Life Stability. Boca Raton, FL: CRC Press. Roos YH, Leslie RB and Lillford PJ (eds) (1999) Water Management in the Design and Distribution of Quality Foods. Lancaster, PA: Technomic. Seow CC, Teng TT and Quah CH (eds) (1988) Food Preservation by Moisture Control. London: Elsevier. Simatos D and Multon JL (eds) (1985) Properties of Water in Foods. Dordrecht: Martinus Nijhoff. Slade L and Levine H (1991) Beyond water activity: Recent advances based on an alternative approach to the assessment of food quality and safety. Critical Reviews in Food Science and Nutrition 30: 115.
ð16Þ
where aw1, aw2 . . . awi, b1, b2 . . . bi, and w1, w2 . . . wi refer to the initial component 1, 2 . . . i water activity, slope of the sorption isotherm, and weight of dry solids, respectively. The role of water activity as a food stability parameter has been well recognized. Water activities can be measured with commercially available equipment, and sorption isotherms are available for most food components. Furthermore, computer software for water activity calculations and establishment of sorption isotherms has been developed and used widely in the food industry.
Effect on Food Stability Y H Roos, University College Cork, Cork, Ireland Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
Introduction Water activity, together with pH and temperature, is often an important factor in controlling food stability. These three parameters all, but not alone, influence microbial activity, rates of chemical and enzymatic reactions, and changes in food texture. Water activity,
0001
WATER ACTIVITY/Effect on Food Stability
0002
0003
however, has a significant role in controlling texture formation and textural changes in intermediatemoisture foods (IMF) and low-moisture foods. In such foods, removal or sometimes sorption of water results in concentration changes of food solids and thereby has an effect on reaction rates. Enhanced reaction rates resulting from water removal may be desired, as in baking to provide color and flavor, or they may be detrimental as in production of many food powders, such as dairy powders. That makes the time–temperature–water or water activity control in food production and storage extremely important. In some cases, the maximum food stability at low water contents is achieved at a water activity corresponding to the Brunauer–Emmett–Teller (BET) monolayer water content. Furthermore, water affects the physical state and water plasticization, i.e., softening of water miscible food solids. As a plasticizer, water controls the glass transition of food components, such as carbohydrates and proteins. The glass transition is a well-known property of all inorganic and organic amorphous materials. The glass transition occurs over a temperature range, which is often referred to with the glass transition temperature, Tg. The physico-chemical properties of amorphous materials change considerably over the glass transition, as the transformation includes a change between the liquid-like and solid-like material states. There are also established and possible relationships between the physical state, water plasticization, and rates of diffusion-controlled reactions, such as nonenzymatic browning, enzymatic reactions, and oxidative changes. In general, water activity provides valuable information of the effects of water content on water availability and the physical state of food solids. Such information can be described using sorption isotherms and state diagrams. They provide critical values for water content, water activity, and temperature, which are important in the characterization of food behavior in processing and in establishing criteria for packaging requirements and appropriate storage conditions. These areas and the role of water activity in controlling food stability are discussed in the present article.
Water Activity and Food Stability 0004
Most fresh foods are high-moisture materials, and their shelf-life is reduced by enzymatic changes, the growth of microorganisms, and mechanical damage. High-moisture foods have an aw of 0.90–0.999, and they often contain more than 50% (w/w) water (Table 1). These foods include fresh meats and seafood, various dairy products, fruits, vegetables, and
6095
Table 1 Examples of water activity ranges of various foods Food systems
Water activity range
Fresh meat, fish, vegetables, <40% (w/w) sucrose, <7% (w/w) salt Bread, cooked sausages, medium aged cheese Salami, old cheese, >65% sucrose, >15% salt Dried beef, sweet condensed milk, cereals with 15% (w/w) water Jam, marmalade, old salami, >26% (w/w) salt Flour, cereals, nuts Caramels, honey, toffee Breakfast cereals, snack foods, food powders
>0.95
tbl0001
0.90–0.95 0.87–0.90 <0.86 0.80–0.87 0.75–0.80 0.60–0.75 0.20–0.60
beverages. Most bacteria, molds, and yeasts are likely to grow in high-moisture foods. However, the types of spoilage microorganisms and the growth of various species are highly dependent on pH, temperature, and water activity. IMF have a water activity within the range of 0.60– 0.90aw, and their water contents normally vary between 10 and 50% (w/w) (Table 1). These foods include many traditional low-moisture foods, such as grains, nuts, and dehydrated fruits, but also a number of processed foods or foods designed and manufactured to have a known composition to provide stability. Such foods may have particular applications and requirements for stability, e.g., when used as fillings in bakery products or confectionery. Although microbial spoilage is prevented below 0.60aw, and many microbes do not grow in IMF, their stability and shelflife are reduced by deleterious changes, such as structural transformations, enzymatic changes, browning reactions, and oxidation, depending on aw, pH, and temperature. The rates of such changes are often at least to some extent affected by the physical state of the materials and the extent of water plasticization of water-miscible solids. Low-moisture foods obviously have the lowest water contents, often below 10% (w/w), and their water activity is lower than 0.6aw (Table 1). Such foods are not subject to microbial spoilage. Their shelf-life, however, is often limited by chemical and textural changes, particularly browning and other changes in color and flavor as well as oxidation. These foods may be exceptionally hygroscopic and exhibit water sorption from their surroundings. Water sorption is reduced by the use of protective packaging, but sorption occurs during storage as a result of permeation or damage of protective packaging. Many low-moisture foods have a solid appearance, or they have a crispy, solid texture. Water sorption in such foods may lead to stickiness, structural changes, such as loss of crispness and sogginess,
0005
0006
6096 WATER ACTIVITY/Effect on Food Stability
and coincident increases in the rates of browning and enzymatic changes. A number of products contain amorphous components, for example, lactose in dairy powders and sucrose in many bakery products and confectionery. Such amorphous components may crystallize during water sorption, as water activity increases. It is important to notice that low-moisture foods are often glassy materials with Tg values above their normal storage temperature. Water sorption and associated plasticization may decrease the Tg, and the glass transition may occur over the range of the concurrent increase in water activity.
Stability Control Microbial Stability 0007
0008
0009
Microbial growth requires a minimum aw, in addition to pH, temperature, and other appropriate conditions that are important for the growth of bacteria, molds, and yeasts. The water activity of high-moisture foods, especially processed foods, can be manipulated to some extent by the addition of salts and sugars or other ingredients, which are known to reduce water activity. Such compositional alterations are highly advantageous in product development and food safety control. For many products, the effects of compositional changes on water activity can be predicted on the basis of composition and confirmed by measuring water activity of the final product. This allows an understanding of storage requirements and estimation of the product shelf-life in various storage conditions. In high-moisture foods, the main role of water activity control is to govern and reduce the risk of the growth of pathogenic and spoilage bacteria. Examples of water activity limits for the growth of selected microorganisms are given in Table 2. The lowest water activity limit for microbial growth of 0.60aw allows the growth of xerophilic yeasts. Above this limiting water activity, IMFs have an increasing possibility for the growth of various microorganisms with increasing water activity. However, the water activities of IMFs are such that pathogenic bacteria are unable to grow, but there is a possibility for the growth of molds and yeast. The growth of these microorganisms must be controlled by careful adjustment of product water activity, use of protective packaging to avoid contamination, and selection of appropriate humectants, pH control, and use of antimicrobial agents. Microbial stability is an obvious, and often the most important, criterion in food preservation. The aw limits for growth of various microorganisms, as shown in Figure 1, are well established and successfully used in food product development and
Table 2 Water activity (aw) limits for the growth of selected pathogenic and spoilage microorganisms Microorganism Bacteria Bacillus cereus Bacillus subtilis Campylobacter jejuni Clostridium botulinum Clostridium perfringens Escherichia coli Halobacterium halobium Lactobacillus plantarum Listeria monocytogenes Salmonella spp. Shigella spp. Staphylococcus aureus Vibrio parahaemolyticus Yersinia enterocolitica Molds Aspergillus candidus Aspergillus flavus Aspergillus niger Erotum echinulatum Penicillium citrinum Penicillium expansum Penicillium patulum Rhizopus nigricans Xeromyces bisporus Yeasts Saccharomyces bailii Saccharomyces cerevisiae Saccharomyces rouxii
tbl0002
Minimum aw 0.930 0.900 0.990 0.940 0.945 0.935 0.750 0.940 0.920 0.940 0.960 0.860 0.936 0.960 0.750 0.780 0.770 0.620 0.800 0.830 0.810 0.930 0.610 0.800 0.900 0.620
manufacturing as well as control of product safety. Furthermore, in high-moisture foods and several IMF products, water activity is relatively constant and dependent on composition, especially solids content, and the type of water-soluble components. Chemical and Enzymatic Stability
The chemical stability of high-moisture foods is not significantly affected by water activity, as the microbial quality is the main determinant of product shelflife. The role of water activity in determining the chemical and enzymatic stability of IMF and low moisture foods is more important. As the water activity and water content decrease, the concentration of reactants in the water phase of foods becomes obviously increased. Therefore, rates of several reactions may increase with decreasing water activity. The relative rate of deteriorative changes in intermediate and low moisture foods is traditionally related to water content and aw, as shown by the ‘Food stability map’ in Figure 2. Early studies applying nuclear magnetic resonance spectroscopy found ‘mobilization points’ (water activity allowing reactant mobility) for solutes in low-moisture food matrices. The mobilization
0010
aw Most bacteria, some yeasts, pathogenic and spoilage organisms >0.95 Most cocci, lactobacilli, some molds, Salmonella, 0.91−0.95 lactic acid bacteria are major spoilage flora Most yeasts, mycotoxin-producing molds, 0.87−0.90 spoilage often by molds and yeasts >0.86
Staphylococcus aureus may grow Most molds, no growth of pathogenic bacteria Most halophilic bacteria
0.80−0.87
G ro w th
Number of growing microbes
WATER ACTIVITY/Effect on Food Stability
Xerophilic molds
0.75−0.80 0.65−0.75
Osmophilic yeasts
0.60−0.65
No growth
0.4
fig0001
<0.65
0.5
0.6
0.9
1.0
Figure 1 Growth of various microorganisms at different water activity conditions.
Relative rate
Oxidation
Growth of: bacteria yeasts molds
Nonenzymatic browning
0 fig0002
0.7 0.8 Water activity
Water activity
1
Figure 2 Food stability map describing the relationship between water activity and food stability.
point was peculiar to the system, and the level of hydration needed to achieve mobility was solutedependent. No solute mobilization occurred below the BET monolayer value, and, for example, experimental data of nonenzymatic browning reaction rates suggested that browning initiated at the mobilization point. An increase in the reaction rate was apparent with increasing aw, and the rate maximum occurred at the water activity corresponding to a hydration level allowing complete mobilization. However, several other factors, including the glass transition, are important in controlling changes occurring in intermediate and low moisture foods.
6097
Enzyme activity in low-moisture foods is often related to hydration. At low water activities enzymatic activity is generally not observed, as water cannot enhance diffusion of substrates to enzyme molecules. Such water activity dependence applies to hydrolases and oxidases, unless the substrates are nonaqueous liquids, allowing changes to occur fairly independently of water activity. It seems that not only water activity but also the ability of water to give a known mobility to enzymes and substrates is important in controlling enzyme activity. The amount of water needed increases with increasing molecular size owing to impaired diffusion, in particular for enzymes, which are active in the water phase of foods. Therefore, for example, lipase activity is not necessarily related to the mobility provided by water. Lipids exist often in a separate phase apart from the water phase. At low water activities and water contents, lipids become more accessible to the atmospheric oxygen, and oxidation rates may increase (Figure 2). The rate of oxidation, however, decreases rapidly with increasing water activity, as the lipids may become protected from atmospheric oxygen owing to the formation of protective aqueous layers in the food microstructure. In several low-moisture foods, lipids, flavors, and other oxygen sensitive compounds are entrapped within the amorphous structure. Such encapsulated compounds also exhibit reduced rates of oxidation as compared with ‘free’ lipids. Traditional shelf-life predictions of low-moisture foods were based on the information of rates of deteriorative changes and loss of nutrients at various temperatures and water contents. An increase in water content above the BET monolayer value at a constant storage temperature often results in rapid deterioration as reaction rates increase at intermediate water contents. The main factors affecting reaction rates in low-moisture food materials, however, can be controlled by a number of factors, which include food composition and the type of the reaction, temperature, pressure, water content, and pH. In concentrated food systems, changes in viscosity and relaxation times in the vicinity of the glass transition may also affect diffusion and thereby contribute to reaction rates and product shelf-life.
0011
0012
0013
Water Plasticization Intermediate and low moisture foods often exist in an amorphous (noncrystalline, no defined structure of molecular arrangement), elastic, ‘rubbery,’ or ‘leathery’ state or in a solid-like, glassy state. The amorphous state is typical of liquids and remains in
0014
6098 WATER ACTIVITY/Effect on Food Stability
0015
tbl0003
Table 3 Examples of melting and glass transition temperatures of food components Food component
Carbohydrates Fructose Galactose Glucose Lactose Maltose Sorbitol Starcha Sucrose Trehalose Xylitol Proteinsa Gelatin Gliadin Glutenin Myoglobin Ovalbumin a
Estimated values.
Melting temperature of crystalline solids (DSC onset, C)
Glass transition temperature (DSC onset, C)
108 163 143 223 160 85 na 173 215 89
5 30 31 101 87 9 250 62 100 29
na na na na na
207 179 189 149 157
100
1.0 Glass transition
Water activity 0.8 Critical water activity
60 40 20 0
0.6 0.4
−20 −40 −60 −80 0
0.2
Water activity (at 25 ⬚C)
80 Glass transition (⬚C)
many biological materials owing to complicated chemical composition, rapid removal of solvent water, or quench cooling of a liquid melt in processes not allowing component crystallization. For example, many sugars typical of foods do not crystallize in dehydration, but remain in a supercooled, liquid state after removal of the solvent water. Glassy, solid-like foods may suffer the glass transition as a result of water sorption, which may be observed from rapid changes in structure and appearance, and enhanced rates of chemical and enzymatic reactions. The glass transitions of food components have a wide range of temperatures depending on the component itself and also on its molecular weight. In general, high-molecular-weight food components, such as carbohydrate polymers and proteins, have glass transition temperatures at very high temperatures well above 100 C. Low-molecularweight components may have very low glass transition temperatures, for example, the glass transition of amorphous water occurs at around 135 C, whereas many anhydrous sugars have a Tg between 0 and 100 C. The Tg is often measured using differential scanning calorimetry, and it can be observed with a number of other thermal and spectroscopic techniques. These include dielectric analysis, dynamic mechanical analysis, electron spin resonance spectroscopy, infra-red and Raman spectroscopy, and nuclear magnetic resonance spectroscopy. Examples of glass transition temperatures of food components are given in Table 3.
Critical water content 5 10 15 20 25 30 35 Water content (g per 100 g of solids)
0.0 40
Figure 3 Relationships between glass transition temperature, water activity, and water content for amorphous lactose with critical values for water content and water activity at room temperature.
Carbohydrates and proteins in foods are watermiscible components, and they are plasticized by water. Water plasticization of the amorphous structure results in a decrease in Tg. The decrease in Tg is substantial even at low water contents, and at high water contents, the Tg approaches that of amorphous water. Therefore, at a constant temperature, e.g., room temperature or a typical storage temperature of intermediate- and low-moisture foods, a water activity or water content range can be established, which corresponds to the glass transition of the material. The relationships between glass transition, water activity, and water content are described in Figure 3. The water activity and water content, which correspond to the Tg depression to the storage temperature can be defined as ‘critical water activity’ and ‘critical water content,’ respectively. Tg ¼
w1 Tg1 þ kw2 Tg2 , w1 þ kw2
fig0003
0016
ð1Þ
where w1 and w2 and Tg1 and Tg2 are the weight fraction and glass transition temperature of solids (1) and water (2), respectively. The relationship between the glass transition temperature depression and water content is often predicted with the Gordon–Taylor relationship (eqn (1)). The model can be fitted to experimental Tg and water content data and used together with water sorption data to establish relationships between the Tg and water sorption properties, as shown for amorphous lactose in Figure 3. Such predictions are useful in food product development, because they allow estimation of food stability in terms of the physical state and water activity.
0017
WATER ACTIVITY/Effect on Food Stability
Water Activity and Material Properties 0018
0019
Several properties, such as dielectric properties, mechanical properties, and viscosity, of amorphous food materials are greatly affected by the glass transition. These changes are related to relaxation times and molecular mobility, which may substantially change and affect food stability over the glass transition. Such changes may contribute to observed changes in textural characteristics, e.g., loss of crispness and sogginess, and reaction rates, e.g., nonenzymatic browning and oxidation. Furthermore, the solid, glassy state supports the amorphous state of food components, but many crystallizing components, e.g., sugars, may crystallize in the supercooled liquid state. Both textural changes and crystallization of amorphous components contribute to a loss of flavor and aroma compounds as well as release and oxidation of encapsulated lipids in numerous dehydrated foods and food powders. In general, the relaxation times of molecular rearrangements in the glassy state are sufficient to provide stability for low-moisture food systems. The changes occurring over the glass transition and in the supercooled liquid state reduce the relaxation times, and the materials lose their long-term stability. For example, amorphous, glassy lactose in food systems is stable, but an increase in water content resulting in glass transition leads to a rapid decrease in viscosity and flow of the material (transformation from solid state to a syrup) accompanied by a loss of structure (stickiness and collapse) and lactose crystallization. Crystallization of amorphous sugars is often observed from water sorption isotherms. When crystallization occurs, sorbed water is not maintained in the crystals, and a decrease in water content is recorded. Such crystallization is time-dependent and detrimental to the quality of amorphous powders. Stiffness
0020
A single model based on the Fermi’s distribution model, as given in eqn (2), allows modelling of the decrease in relaxation times of mechanical properties (stiffness) in the vicinity of the glass transition. According to the model, stiffness can be expressed in terms of a defined and measurable stiffness parameter, Y, which may refer to viscosity, flow, sensory crispness, etc. The stiffness parameter values can be modelled as a function of aw, T, or m relative to its value at a reference state, Ys. A constant, aX, defines the broadness of the transition, e.g., the water activity range over which changes in stiffness occur. The reference value, Xs, obtained from b ¼ Xs/aX, indicates the value for aw, T, or m that decreases Y to 50% below Ys.
Ys 1 1 ¼ b þ X, ln aX Y
6099
ð2Þ
where Y is the stiffness parameter, Ys is the stiffness parameter at a reference state, X is aw, T, or water content, m, and b and aX are constants. The term collapse relates to stiffness and covers various time-dependent structural transformations that may occur in amorphous food and other biological materials at temperatures above the Tg. These changes reflect the effect of changes in relaxation times of mechanical properties on the rates of structural transformations and flow that occur over and above the glass transition temperature range. The rates of these changes increase rapidly above the critical water content or aw during food storage. Stickiness and caking of food powders and collapse of structure are collapse phenomena that are related to water activity and glass transition. The main cause of stickiness is plasticization of particle surfaces by water, which allows interparticle binding and formation of clusters. Collapse of the structure occurs when the decrease in viscosity results in flow, and the material cannot support its own weight. Structural collapse often leads to a reduction in quality of dehydrated, especially freeze-dried, foods, whereas stickiness and caking are detrimental to the quality of spray-dried foods, such as dairy powders.
0021
Crystallization of Food Components
Crystallization of amorphous sugars is known to result in serious quality losses in food powders. For example, crystallization of amorphous lactose in dehydrated milk products has been observed to result in acceleration of the nonenzymatic browning reaction as well as other deteriorative changes and caking. Crystallization of lactose coincides with an increase in free fat, which presumably facilitates lipid oxidation in milk powders. Crystallization in low-moisture carbohydrate matrices, which contain encapsulated volatiles or lipids, results in a complete loss of flavor and release of lipids from the matrix. Water plasticization and depression of Tg to below the ambient temperature are responsible for crystallization of amorphous sugars in foods as a result of increased free volume and molecular mobility, decreased viscosity, and enhanced diffusion. Crystallization seems to initiate at Tg or corresponding aw and proceed with a rate determined by the temperature difference T Tg to a maximum extent also defined by the T Tg. The kinetics of crystallization of sugars and other amorphous food components at a constant temperature above Tg can be related to water content and aw, which define the rate controlling T Tg.
0022
6100 WATER ACTIVITY/Effect on Food Stability
Water Plasticization and Reaction Rates 0023
0024
0025
0026
It has been well established that water as a plasticizer has a significant effect on molecular mobility and probably rates of quality changes above a critical, temperature-dependent aw or water content. A chemical reaction requires sufficient mobility of reactants and reaction products in addition to the driving force, e.g., temperature or concentration, of the reaction or change in quality. Information on factors affecting rates of various kinetic processes can be used to manipulate and control rates of changes occurring in food processing and to develop food products that are less sensitive to detrimental changes during storage.
reaction rates. Several studies, however, have suggested that the reaction rate increases considerably above the glass transition or the critical water activity. Oxidation
Oxidation is a common reaction in low-moisture foods. Oxidation of lipids directly exposed to atmospheric oxygen at food surfaces or surfaces of porous dehydrated materials may occur freely. It is well known that ‘free fat’ in dairy powders, for example, is highly susceptible to oxidation causing quality defects. The stability of dairy powders is related to the glassy state of lactose, and the lipids are at least partially encapsulated within the lactose–protein matrix in the spray-drying process.
Nonenzymatic Browning
Enzymatic Changes
The nonenzymatic browning reaction is one of the most studied deteriorative reactions affected by water activity. Other reactions include destruction of vitamins, enzymatic changes, and oxidation. The rates of most reactions are affected by water activity and glass transition. However, there are no single determinants of rates of chemical reactions and food stability, although an understanding of both the water activity and glass transition of a food material is important in predicting and controlling food stability. Nonenzymatic browning is often considered as a series of condensations that can be assumed bimolecular. The reaction occurs between a reducing sugar and an amino acid or amino group and produces typical flavors of several foods in processing. When uncontrolled, the reaction produces unpleasant flavors and colors, and decreases dehydrated food quality during storage. The nonenzymatic browning reaction in several foods exhibits increasing rates above the critical aw values or water contents corresponding to the glass transition. However, reactions in amorphous foods are complex and may be controlled by several other factors, including concentration changes, pH, and temperature. Established relationships between Tg and the rate of the nonenzymatic browning reaction have, however, suggested that the reaction may become diffusion-controlled, and the rate may be affected by the Tg. The size of the reactants may also be important in diffusion-controlled reactions. It may be assumed that the rate of diffusion decreases with increasing size of the diffusant. The effect of glass transition on the reaction rate has been related to observed discontinuities in Arrhenius plots suggesting a high activation energy in the vicinity of the transition. Studies of effects of glass transition on reaction kinetics and the nonenzymatic browning reaction in particular are complicated, because of several factors affecting plasticization and
Studies of glass transition effects on enzymatic changes have included, for example, sucrose inverson by invertase. Sucrose inversion by invertase in lowmoisture systems is dependent on water activity. The reaction rate increases at water activities above 0.6, but does not seem to be affected by the glass transition of the main food matrix. It is possible that the enzyme mobility is not sufficient to allow the reaction to occur until the reacting molecules are plasticized by water to an appropriate extent above 0.6aw.
0027
0028
Modern Stability Maps Deteriorative changes affected by water activity include such changes as enzymatic changes, nonenzymatic browning, lipid oxidation, microbial growth, and overall stability. The rate of these changes seems to be related to water activity. However, water activity limits for chemical, enzymatic, and mechanical changes are material-specific, similarly as the ‘mobilization points,’ and they may also be related to the physical state, molecular mobility, water plasticization and glass transition of amorphous food solids. Structural transformations, as well as diffusion-controlled deteriorative reactions and changes affected by crystallization phenomena, occur at increasing rates with increasing water activity above the critical aw of the material. It is likely that water contents lower than the critical water content are needed for maximum stability. Therefore, modern stability maps provide information on both the water activity and material physical state on the relative rates of deteriorative changes (Figure 4). In addition, state diagrams and sorption isotherms are useful as stability maps. Knowledge of material properties is extremely useful in the production of encapsulated flavors, extruded products, confectionery, development of new food products, and avoiding quality
0029
WATER ACTIVITY/Effect on Food Stability
Determination; Physiology; Water Activity: Principles and Measurement; Yeasts
Glass transition range
Relaxation times
6101
Water sorption
0 fig0004
Water content
Relaxation time/reaction rate
Oxidation Nonenzymatic browning
Water activity
1.0
Figure 4 Food stability map taking into account water activity and physical state effects on relaxation times and reaction rates.
changes that may result from mechanical changes, e.g., loss of crispness. The temperature-, water content-, and time-dependent changes that are often problems in manufacturing and storage of food powders and other low-moisture foods can be reduced by avoiding exceeding their critical values of water activity based on the Tg determination or by compositional adjustments that provide sufficiently high values for critical aw and Tg. The most important applications of producing high-quality dehydrated foods include reduced collapse and improved flavor retention in dehydration processes. The kinetics of enzyme activity is important to food quality, and applying the knowledge in food industry may allow the design of improved products with extended shelflives or even improved retention of activity, e.g., in enzyme preparations. Water activity has been successfully used in setting limits for microbial stability in foods and qualitative characterization of relative rates of deteriorative reactions in low-moisture foods. The combined use of aw and glass transition has a high practical applicability as it provides criteria for critical aw and, together with sorption isotherm, can be applied to adjust water content at a constant temperature to achieve the maximum food stability. See also: Browning: Nonenzymatic; Toxicology of Nonenzymatic Browning; Crystallization: Basic Principles; Mycotoxins: Classifications; Oxidation of Food Components; Spoilage: Chemical and Enzymatic Spoilage; Bacterial Spoilage; Molds in Spoilage; Yeasts in Spoilage; Water: Structures, Properties, and
Further Reading Barbosa-Ca´ novas GV and Welti-Chanes J (eds) (1995) Food Preservation by Moisture Control: Fundamentals and Applications. Lancaster, PA: Technomic. Bell LN (1996) Kinetics of non-enzymatic browning in amorphous solid systems: distinguishing the effects of water activity and the glass transition. Food Research International 28: 591–597. Buera MP and Karel M (1995) Effect of physical changes on the rates of nonenzymatic browning and related reactions. Food Chemistry 52: 167–173 Labuza TP (1980) The effect of water activity on reaction kinetics of food deterioration. Food Technology 34: 36–41, 59. Lievonen SM, Laaksonen TJ and Roos YH (1998) Glass transition and reaction rates: nonenzymatic browning in glassy and liquid systems. Journal of Agricultural and Food Chemistry 46: 2778–2784. Matveev YuI, Grinberg VYa and Tolstoguzov VB (2000) The plasticizing effect of water on proteins, polysaccharides and their mixtures. Glassy state of biopolymers, food and seeds. Food Hydrocolloids 14: 425–437. Nelson KA and Labuza TP (1994) Water activity and food polymer science: implications of state on Arrhenius and WLF models in predicting shelf life. Journal of Food Engineering 22: 271–289. Peleg M (1993) Mapping the stiffness–temperature–moisture relationship of solid biomaterials at and around their glass transition. Rheologica Acta 32: 575–580. Roos YH (1993) Melting and glass transitions of low molecular weight carbohydrates. Carbohydrate Research 238: 39–48. Roos YH (1995) Phase Transitions in Foods. San Diego, CA: Academic Press. Roos YH, Karel M and Kokini JL (1996) Glass transitions in low moisture and frozen foods: effects on shelf life and quality. Food Technology 50(11): 95–108. Roos YH (2000) Water activity and plasticization. In: Eskin NAM and Robinson DS (eds) Food Shelf Life Stability. Boca Raton, FL: CRC Press. Roos YH, Leslie RB and Lillford PJ (eds) (1999) Water Management in the Design and Distribution of Quality Foods. Lancaster, PA: Technomic. Shimada Y, Roos Y and Karel M (1991) Oxidation of methyl linoleate encapsulated in amorphous lactosebased food model. Journal of Agricultural and Food Chemistry 39: 637–641. Slade L and Levine H (1991) Beyond water activity: Recent advances based on an alternative approach to the assessment of food quality and safety. Critical Reviews in Food Science and Nutrition 30: 115.