Dai

Activation energy

Activation energy for reverse reaction

Average energy of products

AH = heat of reaction (overall)

Changes in reaction mechanisms may occur for a large temperature range. For instance, it is highly possible that mechanisms of deterioration may change at conditions below the freezing point due to a concentration effect. On the other hand, at high temperatures, changes in the physical state of some compounds, including fats and sugars, may occur. Lipids may change from a solid to a liquid state, while sugars may change from an amorphous to a crystalline or to a liquid state. Because of the high complexity of food systems, it is also possible that when various mechanisms of deterioration operate simultaneously, the effect of temperature may alter the rate of one, thus causing inhibition or catalysis in the other mechanisms. Finally, irreversible changes such as starch hydrolysis or protein de-naturation may occur due to temperature, thus modifying the reactivity of the system. In fact, although enzyme-catalyzed reactions will have an increasing reaction rate upon an increase in temperature, a decrease will be observed beyond a certain temperature due to enzyme inac-tivation.

Of particular importance to food processors is the determination of any nutritional and overall quality changes that may occur as a result of processing conditions encountered in operations such as dehydration, sterilization, extrusion, and so on. It is not only important to establish the extent of these undesirable changes, but it is also crucial to know the rate at which these undesirable changes take place. It is the ultimate goal of the food manufacturer to be able to optimize the quality of the final product while still maintaining an accurate perspective of the economics of this approach. Although a variety of methodologies have been proposed for the optimization of nutrient or quality retention during processing, only a few authors have actually tested the feasibility of these methods. Due to the complex nature of food materials, complete kinetics models are not easily attainable, resulting in one of the major obstacles in optimization in the food industry. Several researchers, however, have successfully applied optimization techniques to different types of food processes such as thiamin retention during sterilization (93) and ascorbic acid retention during air drying (94) using Pontryagin's maximum principle. Mishkin et al. (95) applied the complex method for optimization of ascorbic acid in air drying and minimizing browning in the dehydration of white potatoes. Saguy (96) published a practical text on computer-aided techniques for food technologists covering model building with applications and implementation of kinetics, simulation, heat transfer, and so on. With the fast-paced advancement in computer software, computer prototyping based on mathematical models rather than actual physical models has greatly aided product and process design. Datta (97) has reviewed some different commercially available computer-aided engineering (CAE) software programs and their sources with examples of computational fluid dynamics (CFD) and heat transfer for processes such as canning of liquid and solid foods, extrusion, and continuous sterilization of liquids.

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