A KINETIC-STUDY OF HARDENING AND SOFTENING PROCESSES IN VEGETABLES DURING COOKING

We derived a general expression for the time and temperature dependenc e of both the hardening and the softening rates of vegetables during c ooking on the basis of rate laws. The vegetables studied were such roo t vegetables as Japanese radish, carrot, burdock and potato. Our kinet ic model was based on the following two assumptions: (1) Pectin is tra nsformed into three forms, i.e., the unchanged pectin, the hardening p ectin and the softening pectin. (2) Each pectin contributes additively to the hardness of the vegetables. Considering the causes of the hard ening and the softening as enzymatic deesterification and beta-elimina tion, respectively, first-order rate laws were applied to the formatio n of each pectin and to the inactivation of the enzyme. Rate constants for relevant processes were determined at different temperatures by f itting integrated rate equations to the cooking data by the least-squa res method. The hardness calculated from the derived expression was in good agreement with the experimental values for all samples. Using th e kinetic parameters determined here, we could reproduce not only the simultaneous occurrence of the hardening and softening processes but a lso the desired cooking time for optimum hardness. The hardening rate constant showed Arrhenius behavior between 54 and 63 degrees C, but a non-Arrhenius one at higher temperatures. It was suggested that the ha rdening process was complex in the high temperature region of 70-99.5 degrees C.