Effects Of Energy Inputs

It is generally correct to say that some form of energy input is required to cause a reaction to happen. A classic way of supplying the said energy is by heating up the system. As a rule of thumb, for every increment of 10°C, the reaction rate will double. That is the Qi0 principle. The temperature effect can be visualized by looking at the increased kinetic energy of the molecules that makes the molecules move faster and so increases the probability of colliding with other molecules in the system. Such reasoning is good for explaining temperature effects in gaseous and solution chemistry. For food systems that are solid or semisolid, the resistances of heat and mass transfer and the fact that the mobility of macromolecules are relatively small cause the temperature effect to deviate from the simple Q10 principle. Another idiosyncrasy of biological reactions is that many of them are catalyzed by enzymes that are unstable at high temperatures. This causes nonlinearity of Arrhe-nius plot for biological and biochemical reactions. Arrhe-nius derived the following equation to fit experimental data obtained from studying the effect of temperature on the kinetics of sucrose inversion:

The logarithmic form of the preceding equation is shown as follows:

Equation 22 states that the natural log of reaction rate constant In k varies linearly with the reciprocal of absolute temperature 1/71 with a constant negative slope, — AE/R. The evaluated value of the said slope provides a way to calculate AE, the activation energy of the reaction for the temperature range that the said slope was evaluated on. If 1.987 is used for the value of R, the gas constant in calculating AE, the dimension for the calculated value of AE is cal/g mol. For some reactions involved in food processing, different temperature ranges may result in different calculated activation energy. As shown in Table 3, the thermal activation energy for the conversion of several starches at different temperature ranges varies from about 10 to 230 kcal/mol. These differences can be rationalized by looking at the different granular sizes of the native starches from rice, corn, and potato; the different moisture contents in the samples; and possibly different mechanisms of conversion at different temperature ranges. In dealing with biological or biochemical reactions where activities of the catalysts (enzyme or cells) involved are also affected by temperature, a composite Arrhenius plot can be obtained. The study of the effect of temperature on the

Table 3. Activation Energy for Starch Conversion Due to Thermal Effect


Temperature (°C)

Ea (kcal/mol)


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