Studies on the Inhibition against Oxidative Deterioration of Deep Frying Oil for Soy-bean Curd

Abstract

In the studies on oxidative deterioration of deep frying oils for soy-bean curd, we investigated the degree of oxidative deterioration of deep frying oils which are used in factories. The effects of coagulant and moisture contained in soy-bean curd on the oxidative deterioration of deep frying oil were also studied. The following results were obtained. (1) Among fifty-one oils tested, fifteen of them showed the acid value (AV) more than 3. (2) After exchanging oil in the fryer to fresh one at five factories, time course of the oxidative deterioration of oils which were used for deep frying for soy-bean curd were monitored by AV and carbonyl value (CV) every day. AV reached 3 within 15±3 days after deep frying, and after that time it increased linearly with respect to frying time. On the other hand, although CV attained approximately 50meq/kg after 7 days in three factories, this value reached nearly 20meq/kg after 2 days in the other two factories. However, after that time, CV of both groups remained almost unchanged. (3) Moisture content of soy-bean curd manufactured with MgCl₂, CaSO₄ or CaCl₂ as coagulant was 81.55, 79.44 and 79.28%, respectively. Soy-bean curd coagulated with MgCl₂ promoted the highest oxidative deterioration of deep frying oil and followed by CaSO₄ and CaCl₂ in this order. (4) By using CaSO₄, two soy-bean curds having different moisture contents of 79.44 and 83.32% were prepared. The latter accelerated oxidative deterioration of frying oil more rapidly than the former.