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Figure 11. Effect of water activity on microbial, enzymic and chemical changes to foods (7). Source: Courtesy of Marcel Dekker.

are inhibited below Aw = 0.7; most yeasts are inhibited below Aw = 0.8; and most bacteria below Aw = 0.9. The interaction of Aw with temperature, pH, oxygen, and carbon dioxide, or chemical preservative has an important effect on the inhibition of microbial growth. When any one of the other environmental conditions is suboptimal for a given microorganism, the effect of reduced Aw is enhanced (Fig. 11). This permits the combination of several mild control mechanisms that result in the preservation of food without substantial loss of nutritional properties or sensory properties (Table 5).

Enzymatic activity virtually ceases at Aw values below the BET monolayer value. This is due to the low substrate mobility and its inability to diffuse to the reactive site on the enzyme. Chemical changes are more complex. The two most important things that occur in foods that have a low Aw are Maillard browning and oxidation of lipids. The Aw that causes the maximum rate of browning varies with different foods. However, in general, a low Aw restricts the mobility of reactants and browning is reduced. At a higher Aw, browning reaches maximum. Water is a product of the condensation reaction in browning, and at higher moisture levels, browning is inhibited by end-product inhibition. At high moisture contents, water dilutes the reactants and the rate of browning falls (Fig. 11).

Oxidation of lipids occurs at low Aw values owing to the action of free radicals. Above the BET monolayer value, antioxidants and chelating agents (which sequester trace metal catalysts) become soluble and reduce the rate of oxidation. At higher Aw values the catalytic activity of metals is reduced by hydration and the formation of insoluble hydroxides but, at high Aw values, metal catalysts become soluble and the structure of the food swells to expose more reactive sites (Fig. 11).

Texture

Changes to the texture of solid foods are an important cause of quality deterioration. The nature and extent of pretreatments (eg, the addition of calcium chloride to blancher water), the type and extent of size reduction, and peeling each affect the texture of rehydrated fruits and vegetables. In foods that are adequately blanched, loss of texture is caused by gelatinization of starch, crystallization of cellulose, and localized variations in the moisture content during dehydration, which set up internal stresses. These rupture, compress, and permanently distort the relatively rigid cells, to give the food a shrunken, shrivelled appearance. On rehydration the product absorbs water more slowly and does not regain the firm texture associated with the fresh material. There are substantial variations in the degree of shrinkage with different foods.

Drying is not commonly applied to meats in many countries owing to the severe changes in texture compared with other methods of preservation. These are caused by aggregation and denaturation of proteins and a loss of waterholding capacity, which leads to toughening of muscle tissue.

The rate and temperature of drying have a substantial effect on the texture of foods. In general, rapid drying and high temperatures cause greater changes than do moderate rates of drying and lower temperatures. As water is removed during dehydration, solutes move from the interior of the food to the surface. The mechanism and rate of movement are specific for each solute and depend on the type of food and the drying conditions used. Evaporation of water causes concentration of solutes at the surface. High air temperatures (particularly with fruits, fish, and meats) cause complex chemical and physical changes to the surface and the formation of a hard, impermeable skin. This is termed case hardening. It reduces the rate of drying and produces a food with a dry surface and a moist interior. It is minimized by controlling the drying conditions to prevent excessively high moisture gradients between the interior and the surface of the food.

In powders, the textural characteristics are related to bulk density and the ease with which they are rehydrated. These properties are determined by the composition of the food, the method of drying, and the particle size of the product. Low-fat foods (eg, fruit juices, potatoes, and coffee) are more easily formed into free-flowing powders than are whole milk or meat extracts. Powders are instantized by

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