Food Chemistry Temperature, moisture content and water activity are important physical factors in influencing the chemistry and biochemistry properties of food product during drying and storage. Water is not only an important medium for heat transfer and heat storage but also takes part in various biochemical reactions in food. A water molecule provides protons (H+), hydroxide ions (OH-), hydrogen atoms (H), oxygen atoms (O) and radicals (H•, •OH).
Hence, water may act as a solvent, reactant or dispersing agent in the food matrix (Chieh, 2006). As such, condition of the water presents in dehydrated foods is very important as it affects several deterioration reactions in food such as nonenzymatic browning, lipid oxidation, vitamin degradation, enzyme activity, microbial activity and pigment stability (Osuna-Garcia and Wall, 1998).
Moreover, dissolved species in food matrix are concentrated as water is removed during drying. As a general rule of thumb, reaction rate increases with temperature and reactant concentration. Therefore, with the simultaneous concentration of dissolved solutes and elevated temperature during drying, reaction between species can be accelerated and thus increases the destruction rate of nutritional value (Labuza and Tannenbaum, 1972).
Generally, first order reaction is assumed for most food deteriorations (Equation 2.1) unless the rate is too slow and zero order reaction has to be used (Labuza and Tannenbaum, 1972). k[A] dt d[A] = (2.1) where [A] (mol dm-3) is the concentration of the interested quality parameter and k (s-1) is the temperature dependant rate constant which can be described by Arrhenius equation (Equation 2.2).
Details of quality parameters and quality attributes can be referred to Chapter 6. E / RT 0 a k k e− = (2.2) where k0 (s-1) is the absolute rate constant, Ea (J mol-1) is the activation energy, R (8.314 J mol-1 K-1) is the gas constant and T(K) is the absolute temperature. Table 2.1Table 2.1 shows the typical activation energies for some deteriorative reactions occur in food materials compiled by Labuza and Tannenbaum (1972).