Kinetics of Chemical Reactions in Foods

Abstract

Chemical reactions occur in foods during processing and storage. Some reactions result in a quality loss and must be minimized, whereas others result in the formation of a desired flavor or color and must be optimized to obtain the best product quality. Kinetics is a science that involves the study of chemical reaction rates and mechanisms. An understanding of reaction mechanisms coupled with quantification of rate constants will facilitate the selection of the best conditions of a process or storage, in order that the desired characteristics will be present in the product.

Problems

• 7.1. Nagy and Smoot (J. Agr. Food Chem 25:135, 1977) reported the degradation of ascorbic acid in canned orange juice to be first order, and the following first-order rate constants can be calculated from their data. At T = 29.4 °C, 37.8 °C, and 46.1 °C,k in day⁻¹ was 0.00112, 0.0026, 0.0087, respectively. Calculate the activation energy, Q₁₀, D value, and half-life at 30 °C.

• 7.2. The following data were collected for the sensory change in beef stored while exposed directly to air at −23 °C (From: Gokalp et al. J. Food Sci. 44:146, 1979). Sensory scores were 8.4, 6.2, 5.5, and 5.1 at 0, 3, 6, and 9 months in storage. Plot the data and determine an appropriate form of an equation to which the data can be fitted to obtain the reaction rate constant.

• 7.3. Accelerated shelf life testing is often done to predict how food products would behave in the retail network. If a food product is expected to maintain acceptable quality in the retail network for 6 months at 30 °C, how long should this product be stored at 40 °C prior to testing in order that the results will be equivalent to 6 months at 30 °C? Assume that the temperature dependence of the sensory changes in the product is similar to that for the nonenzymatic browning reaction in Table 7.1.

• 7.4. Ascorbic acid degradation in sweet potatoes at a water activity of 0.11 is first order with a rate constant of 0.001500 h⁻¹ at 25 °C. If the Q₁₀ for this reaction is 1.8, calculate the amount of ascorbic acid remaining in dried sweet potato stored at 30 °C after 3 months in storage if the initial ascorbic acid content was 33 mg/100 g.

• 7.5. In the example problem on B-galactosidase action on lactose in acid whey, calculate the lactose conversion that can be expected using the same level of enzyme addition as in the example, if the whey is preconcentrated prior to treatment to have a lactose content of 12.5%, after a treatment time of 60 min.

• 7.6. Pectin methyl esterase in orange juice has a D value at 85 °C of 8.3 min and the z value is 14 °C. Calculate the target juice temperature for pasteurization such that at least 99% of the enzyme will be inactivated after a 1-min hold time followed by immediate cooling.