Advances in more efficient and versatile methods of food processing and preservation have been occurring exponentially over the past few decades in order to meet the continually increasing population and consumer's demands for quality foods, with particular focus on their nutritional aspects. Not only does quality of processed foods depend on the initial integrity of the raw materials, but also on the changes occurring during processing and subsequent storage. Emphasis is growing in the area of nutraceuticals and fortified high-energy foods (83-85). Not only nutritional quality is important to the food processor, however, but also the general appearance of the food, its flavor, color, and texture, many factors of which are highly dependent on the target consumer groups of interest, which are based on different cultural, geographical, and sociological backgrounds. It is, therefore, of critical importance to the food industry to minimize losses of quality in food products during processing, as well as subsequent storage. It is through the development of mathematical models to predict behavior of food components under a set of conditions and optimization of processes for maximum product quality that continued advancement can be achieved in processing techniques. To obtain these goals, extensive information is needed on the rates of destruction of quality parameters and their dependence on variables such as temperature, pH, light, oxygen, and moisture content. A food engineer can then develop new processing techniques to achieve optimum product quality based on an understanding of reaction rates and mechanisms of destruction of individual quality factors combined with the knowledge of the application of kinetics. There are three main areas of concern when dealing with reaction kinetics: (1) the stoichiometry, (2) the order and rate of reaction, and (3) the mechanism. For simple reactions, the stoichiometry is probably the first consideration. Once this is clarified or elucidated, the mechanisms involved in the reaction can be determined. It should be mentioned that based on actual kinetic data, our idea of the stoichiometry may change. In highly complex reactions, as is the case of many reactions occurring in food systems, a great deal of overlap exists among the three aforementioned areas. Thus, it is of critical importance to take a close and analytical look at the overall system to be able to characterize reaction pathways.
The first step in developing the basis for the kinetics of degradation of a particular nutrient is the determination of the order of the reaction. Understanding of the mechanisms involved in a reaction is important to properly obtain and report meaningful kinetic information, select reaction conditions leading to a desired end product, and/or minimize the appearance of undesirable compounds. Unfortunately, very seldom has effort been given to clearly understand the mechanisms involved in the reaction in complex systems, as in the cases with food and biological materials. Most information available has been oversimplified. In fact, most investigators have often tried to adapt fairly simple zero- or first-order reaction kinetics to complex situations without trying to understand the actual pathways involved. Although, from a practical point of view it is clear that simplifications may be taken, applicability of the information may be restricted only to the conditions encompassed by the experimental design, and thus, one may incorrectly predict trends by directly using reported information.
The reaction pathway, also called reaction mechanism, may be determined through proper experimentation. A chemical reaction may take place in a single step, as in the case of elementary reactions, or in a sequence of steps, as would be the case of most reactions occurring in food systems. Conditions such as temperature, oxygen availability, pressure, initial concentration, and the overall composition of the system may affect the mechanism of the reaction. For instance, the degradation of folic acid and ascorbic acid can be affected by the presence of oxygen, resulting in modification of the reaction pathway and thus, the type of breakdown products. Moreover, the rate at which these parent compounds disappear may be highly influenced by the presence and concentration of the breakdown products generated. It is true that the level of complexity involved in these reactions may be of such magnitude that a complete understanding of the mechanism of deterioration cannot always be easily determined or identified or, even more, may hinder the development of simple techniques to rapidly evaluate the stability of a given system. Nevertheless, it should be stressed that more reliable information is obtained when understanding of the reaction pathways is achieved. Reaction pathways for vitamin and pigment degradation kinetics have been reviewed (5).
The most common approach for the determination of the reaction order for a simple reaction, taking into consideration its initial rate, is as follows: —dC/dt = k • Cn, where C = concentration, k = reaction rate constant, n = order of the reaction, and t = time. After taking the natural logarithm on both sides of the equation, a plot of the In ( — dC/dt) versus In C will give a straight line, whose slope corresponds to the reaction order (n), as shown in Figure 8. Although the intercept should correspond to the reaction rate constant (k), it is normally considered that, for the sake of accuracy, this would not be the preferred approach for its estimation. Rather, once the reaction order has been determined, the rate constant can be calculated by applying the corresponding equation for that reaction order. Other methods can also be used to determine the order of the reaction with respect to the reactants and products involved in the reaction, depending on the stoi-chiometry (5).
A common approach in reporting reaction rates in food systems is as the change in concentration of a reactant as a function of time. The reaction rate thus provides a measurement of the reactivity and stability of a given component in a particular system. A number of factors that have been observed to influence the reaction rate include (1 ) concentration of reactants, products, and catalysts; (2) environmental factors such as temperature, pressure, and oxygen availability; (3) wavelength and intensity of light; and (4) physicochemical properties such as viscosity, pH, ionic strength, and conductivity. Depending on the type of reaction and the components, other factors will also be influential in controlling reaction kinetics.
Although traditionally one can apply reaction kinetics to monitor chemical changes occurring in a system, other physicochemical changes may also be described using a kinetic approach. For instance, textural and color changes occurring in food systems can be described using reaction rates. It is obvious that the numbers obtained represent the final effect caused by other complex reaction mechanisms leading to an overall result. For instance, color changes in a product containing carotenoids, may be an indication of the stability of the system, and in particular, stability of the carotenoids as related to environmental conditions. Another example is the textural changes in starch-based systems as a function of time, which may be the result of starch rétrogradation mechanisms as well as lipid-amylose interactions as influenced by environmental conditions. Since reactions in food systems are normally complex and a combination of several elementary processes, additional basic information may be necessary to postulate reaction rate expressions. Identification of intermediates and previous knowledge of rate equations to fit data for other systems may provide assistance in properly characterizing a given reaction. The most commonly found reactions in food and biological systems are the zero-, first-, and second-order reactions.
Zero-Order Reactions. In zero-order reactions the rate is independent of the concentration. This may occur in two different situations: (2) when intrinsically the reaction rate is independent of the concentration of reactants and (2) when the concentration of the reacting compound is so large that the overall reaction rate appears to be independent of its concentration. Many catalyzed reactions fall in the category of zero-order reactions with respect to the reactants. On the other hand, the reaction rate may depend on the catalyst concentration or other factors unrelated to the concentration of the compound under investigation. Thus for a zero-order reaction at constant density, the overall expression would be as follows: —dC/dt = k0, where C = concentration, k0 = the zero-order reaction rate constant, and t = time. After integrating, this equation be-
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