Monitoring of the coagulation process of soymilk by an integrated electrical sensing and control system

Abstract

Coagulation of soymilk is the most important process in tofu making. The aim of this study is to develop a method which can monitor the coagulation process of soymilk by electrical impedance spectroscopy (EIS) and to control the temperature of soymilk accurately by ohmic heating. Monitoring and heating were operated with the same electrodes in a time-sharing mode. The soymilk coagulation process was investigated by electrical impedance spectroscopy and rheological analysis of tofu breaking stress, breaking strain and Young’s modulus were performed. The impedance amplitude of soymilk decreased at frequencies of 42–200 Hz and increased over 200 Hz with the progress of coagulation. The soymilk coagulation process was considered as two first-order reactions. During the tofu coagulation process, the electrical conductivity at 10 kHz showed a positive linear correlation with the rheological properties. Therefore, impedance measurement can provide a simple, safe and rapid approach for predicting the tofu physical quality. The developed method is capable of controlling the soymilk coagulation process and estimating the tofu textural properties.