ABSTRACT
The time course of processes occurring in foods contains information about the underlying mechanism causing these processes to happen. To extract this information, the use of a mathematical model describing these mechanisms and their kinetics is essential. Solution of the resulting equations shows whether the hypothesized mechanisms are consistent with the data. This chapter deals with basic principles and applications of reaction kinetics and the mathematical models describing the processes. Whenever possible, the theory will be illustrated with examples of reactions in foods. Kinetics is the study of the rate of a reaction, usually taken as the change in concentration c over time t
(mathematically expressed as dc=dt) and its dependence on the concentration of reactants, temperature, possibly catalysts, and different environmental conditions. Why is it important to know rates of reactions? Several answers are possible, depending on the level at which one operates. At the fundamental level it provides understanding of how individual molecular transformations occur. This we consider mainly the domain of basic organic and physical chemistry. At the next level one attempts to deduce molecular descriptions of chemical reactions from rate measurements (applied chemistry), and at the third level one uses knowledge about rates of reactions on how to produce substances (technology). The second and third levels are, with regard to foods, the domain of food science and technology, and they are to a large extent dependent on each other. The mechanism of a reaction is a hypothesis about the sequence of molecular events in a certain reaction; each of such an event is called an elementary reaction. This hypothesis, even though it may not represent the actual events, needs to be consistent with the available experimental data. It is very well possible that more mechanisms are consistent with the experimental data. Sometimes, it is possible to discriminate between mechanisms by designing clever experiments, while the process of discrimination may be helped also by statistical techniques, to be discussed in Chapter 7. It is easier to derive a rate expression from a postulated set of elementary reactions than to determine the mechanism of a reaction. In other words, experimental rate expressions can be used to test reaction mechanisms. The reaction mechanism, the frequency of encounters, and the fraction of reactive encounters
ultimately determine the dependence of the rate of a reaction on concentration. It is at this stage perhaps useful to point to the, in principle, stochastic nature of chemical reactions, which are however in (almost) all cases observed as deterministic events. The reason for this apparent discrepancy is as follows. At the level of molecules, ions, atoms, and radicals, the events are discrete (a molecule is reacting or it is not).
