| Soybean food is a very popular and healthy product all over the world. Soybeanprocessing industry, especially the tofu manufacturing industry has been developed rapidly.There are many problems in tofu making in China: no packaging, poor sanitation, short shelflife, changeable quality, simple equipment and lack of process standard. To improveautomation of tofu process and tofu's quality, the paper mainly studied the conductivity ofsoymilk, dynamic rheological properties and the relationship between them during thecoagulation process by ohmic heating, and explored the feasibility of online automaticdetection of dynamic rheological properties by measuring the electrical conductivity. A finiteelement model was established and temperature distribution inside the heating tank wassimulated and validated. On the basis of those findings, tofu process system that can performohmic heating of soymilk and detect the coagulation the coagulation process of tofu wasdesigned, which laid a foundation for development of tofu manufacturing equipment usingohmic heating. In this paper, the following conclusions were drawn.(1) An equivalent circuit model was used to describe the soymilk-electrode systemconsisting of resistance and capacitance in series.(2) Heating rate during ohmic heating process of the soymilk was increased with theincrease of the frequency and electric field strength. The required times for heating soymilksample to90℃were1400s,360s and170s, respectively, when electric field strengths of6,12and18V/cm were applied.(3) Electrical conductivity of soymilk during ohmic heating process increased with theincrease of frequency. Electrical conductivity of soymilk was correlated linearly withtemperature.(4) During ohmic heating process, the temperature distribution in the center part ofsoymilk was more uniform than the other locations and the temperature close to the heatingtank walls was low. For heating time increasing from50s to300s, temperature differenceinside the soymilk increased from3℃to20.2℃. The main reason was heat loss from theheating tank to the ambient. (5) During the coagulation process of soymilk, elastic modulusG'and viscous modulusG"showed the same trends. In the initial stage, coagulation process was fast and thenbecame low. When the coagulation temperature was about80℃, better texture of tofu gelwas easily obtained.(6) The dynamic temperature scanning curve of soymilk during the coagulation processwas divided into three stages. The first stage was induction stage from70to85℃,G'and G"increased gradually. The second stage was acceleration stage from85to95℃,G'and G"increased rapidly. The third stage was stabilization stage over95℃,G'and G"changed minimally and coagulation reaction finished. In temperature below70℃,tofu gel was not formed. Especially at87℃, the intersection point ofG'andG"wasobserved indicating that phase transformation of tofu gel took place.(7) During the coagulation process of soymilk, resistivity-time,G'-time andG"-timecurves were fitted by a consecutive first-order reaction kinetics. It could be concluded thatcoagulation process finished when the volume resistivity remained unchanged. So electricalmeasurement provided an indirect method to judge the end point of coagulation process.(8) Good agreements betweenG'and resistivity were obtained during soymilkcoagulation process, the same as betweenG"and resistivity. High correlation coefficientswere obtained at different temperatures.(9) On the basis of the above findings, tofu process system by ohmic heating waspreliminarily developed, Thermocouple temperature measurement circuit and impedancemeasurement circuit were mainly designed and online detection of soymilk coagulationprocess was realized.
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