the handbook of food engineering practice crc press chapter 10 ...

proposed (Peleg, 1995). The main applicability of such a model is to account for changes
that take place at a temperature range where transition from growth to lethality occurs.
Finally, temperature can have an additional indirect effect by affecting other
reaction determining factors, which will be discussed in the next section. A temperature
increase, increases the water activity at the same moisture level or enhances the moisture
exchange with the environment in cases of permeable packaging affecting the reaction rate.
Reactions that are pH-dependent can be additionally affected by temperature change, since
for many solute systems pH is a function of temperature (Bates, 1973). Solubility of
gases, especially of oxygen, changes with temperature (25% decrease with every 10°C
increase for O 2 in water) thus affecting oxidation reactions where the oxygen is limiting.
10.2.2.2.Effects of other environmental factors
Moisture content and water activity (a w ) are the most important E j factors besides
temperature that affect the rate of food deterioration reactions. Water activity describes the
degree of boundness of the water contained in the food and its availability to act as a
solvent and participate in chemical reactions (Labuza, 1977).
Critical levels of a w can be recognized above which undesirable deterioration of
food occurs. Controlling the a w is the basis for preservation of dry and intermediate
moisture foods (IMF). Minimum a w values for growth can be defined for different
microbial species. For example, the most tolerant pathogenic bacterium is Staphylococcus
aureus , which can grown down to an a w of 0.85-0.86. This is often used as the critical
level of pathogenicity in foods. Beuchat (1981) gives minimum a w values for a number of
commonly encountered microorganisms of public health significance.
Textural quality is also greatly affected by moisture content and water activity.
Dry, crisp foods (e.g., potato chips, crackers) become texturally unacceptable upon gaining
moisture above the 0.35 to 0.5 a w range (Katz and Labuza, 1981). IMF like dried fruits
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