EFFECT OF THERMAL-PROCESSING ON SOYMILK

Heating conditions are the most important variables in the processing of soymilk. The heat treatments given to soymilk during extraction, cooking, and subsequent pasteurization or sterilization, principally influence (1) the yields and nutritive quality of solids and proteins, (2) destruction of spoilage microorganisms, and (3) the colour and the flavour of the milk. The qualities of other soy products derived from soymilk, e,g. the texture of tofu and the suitability for lactic acid fermentation, may also be affected by the net heat treatment received by the milli. Excess heating generally leads to the destruction of amino acids and vitamins, browning and the development of cooked flavour. Considerable research efforts have been devoted to establish the effects of heat on the elimination of off-flavours, inactivation of antinutritional factors such as trypsin inhibitors, and the recovery of solids and proteins in soymilk. Other heat-induced chemical changes such as loss of vitamins, Maillard reaction and protein denaturation have not been studied as extensively. Published studies show that higher temperature heating and UHT treatment may have beneficial effects on the yields of solids and proteins, retention of the nutrients, and minimizing chemical changes in soymilk, provided that optimum processing conditions are used. Process optimization is only possible if data on the kinetics of various chemical reactions involved are available, However, such data are lacking, especially in the UHT range. The principal objective of this review article is to compile a comprehensive description of heat-induced changes occurring in soymilk. Attention has been given to the analysis of published data relating to the kinetics of various reactions. Such an analysis would permit the evaluation of kinetic parameters (such as rate constants and activation energies), and would form the basis for reaction engineering studies leading to the design of optimum heat exchangers for the processes.