A Kinetic Study of Hardening and Softening Processes in Vegetables during Cooking.

We derived a general expression for the time and temperature dependence of both the hardening and the softening rates of vegetables during cooking on the basis of rate laws. The vegetables studied were such root vegetables as Japanese radish, carrot, burdock and potato. Our kinetic model was based on the following two assumptions: (1) Pectin is transformed into three forms, i.e., the unchanged pectin, the hardening pectin and the softening pectin. (2) Each pectin contributes additively to the hardness of the vegetables. Considering the causes of the hardening and the softening as enzymatic deesterification and β-elimination, respectively, first-order rate laws were applied to the formation of each pectin and to the inactivation of the enzyme. Rate constants for relevant processes were determined at different temperatures by fitting integrated rate equations to the cooking data by the least-squares method. The hardness calculated from the derived expression was in good agreement with the experimental values for all samples. Using the kinetic parameters determined here, we could reproduce not only the simultaneous occurrence of the hardening and softening processes but also the desired cooking time for optimum hardness. The hardening rate constant showed Arrhenius behavior between 54 and 63°C, but a non-Arrhenius one at higher temperatures. It was suggested that the hardening process was complex in the high temperature region of 70-99.5°C.