Reaction Kinetics

Authored by: S. Taoukis Petros , N. Tsironi Theofania , C. Giannakourou Maria

Food Engineering Handbook

Print publication date:  December  2014
Online publication date:  December  2014

Print ISBN: 9781482261691
eBook ISBN: 9781482261707
Adobe ISBN:

10.1201/b17843-16

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Abstract

Advances in new, cost-efficient, environmental friendly and versatile methods of food processing and preservation have been presented and implemented over the past years to meet the continually increasing consumer demands for quality foods with particular focus on their nutritional and functional aspects. The quality of processed foods is not a simple property; it depends not only on the initial integrity of the raw materials but also on the changes occurring during processing and subsequent storage that may result in potential losses and decreased bio-availability. Therefore, quality is an attribute of food, on which understandably a lot of consideration is focused. Food quality can be defined as the assemblage of properties that differentiate individual units and influence the degree of acceptability of the food by the consumer or user (Kramer and Twigg 1968). Owing to the nature of foods as physicochemically and biologically active systems, food quality is a dynamic state continuously moving to reduced levels (with the notable exception of the cases of maturation and aging). Therefore, for each particular food, there is a finite length of time after production within which it will retain a required level of quality organoleptically and safetywise, under stated conditions of storage. This period of time can be generally defined as the shelf life of the food product. There is no established, uniformly applicable definition of shelf life. The definition of shelf life and the criteria for the determination of the end of shelf life are dependent on specific commodities and on the definition’s intended use (i.e., for regulatory vs. marketing purposes). Food-related authorities have proposed various definitions that can serve as guidelines. The International Institute of Refrigeration (IIR) recommendations for frozen food (IIR 1972) introduce two different definitions. High quality life (HQL) is the time from freezing of the product for a just noticeable sensory difference to develop (70–80% correct answers in a triangular sensory test). Another type of shelf-life definition that can be extended to other types of food products is the practical storage life (PSL). PSL is the period of proper (frozen) storage after processing (freezing) of an initially high-quality product during which the sensory quality remains suitable for consumption or for the process intended. PSL is usually in the order of two to three times longer than HQL. Time of minimum durability, introduced by the EEC directive on food labeling, is defined as the time during which the foodstuff retains its specific properties when properly stored is different in principle from the aforementioned ones, in that it relates to properties of the product itself and not to considerations of its use. It is a working definition for the food scientist satisfying the often made fundamental assumption that the highest quality product is the freshly processed (or harvested) one. However, since characteristic properties are overlaid, a decision has to be made at what level the change in a certain characteristic or the development of an undesirable one can be detected by the consumer. For example, if having a specific flavor means the absence of off-flavors, it has to be decided at what intensity levels these flavors are detectable by the consumer. Thus, this definition is closely related to the HQL definition. Chemical, microbiological, and physical tests are being used widely in the study of food quality. Characteristics used by the consumer for evaluation of a product, such as flavor, color, and textural properties can be measured instrumentally or chemically. The study of the chemical and biological reactions and physical changes that occur in the food during and after processing allow the recognition of the ones that are most important to its safety, integrity, and overall quality. Physicochemical or microbiological parameters can be used to quantitatively assess quality. The values of these parameters can be correlated to sensory results for the same food and a limit that corresponds to the lowest acceptable sensory quality can be set. However, caution should be drawn to the fact that correlation of values of individual chemical parameters to sensory data is often not straightforward because overall organoleptic quality is a composite of a number of changing factors (Trant et al., 1981). The relative contribution of each factor to the overall quality may vary at different levels of quality or at different storage conditions. Food kinetics is based on the thorough study of the rates at which physicochemical reactions proceed. The area of kinetics in food systems has received a great deal of attention in past years, primarily due to efforts to optimize or at least maximize the quality of food products during processing and storage (Villota and Hawkes 2007). Moreover, a good understanding of reaction kinetics can provide a systematic approach on how to redesign the formulation of food products to better preserve the desirable sensory, nutritional, and functional components system and minimize the evolution of undesirable products. For example, chemical changes as a result of enzymatic action, oxidative reactions, and nonenzymatic browning can lead to quality deterioration. The use of chemical kinetics, the study of the rates, and mechanisms that are involved in the reactions of interest, and the mathematical relationships that best describe the influence of temperature on the reaction rate constants have been used to model changes in food quality. In parallel, significant published work regarding mathematical modeling has focused on microbiological safety and spoilage in a variety of food matrices.

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