Analysis Of Thermally Induced Isomerization Of Corn Oils And Regulation Of The Products

Corn oils are rich in unsaturated fatty acids(UFAs), which mainly exist as cis configuration, and the contents are higher than 80%. UFAs can undergo cis-trans isomerization during heating, leading to the formation of trans fatty acids(TFAs) and conjugated linoleic acids(CLAs). Corn oils contained a variety of fatty acid triglycerides, is a complex mixing oil system, but the formed trans and conjugated isomers and mechanism during heating process are not clear. The aim of this research is to study the kinds of isomers and isomerization mechanism of corn oils during heating, and put forward the effective measures to control the thermally induced isomerization of TFAs and CLAs. The main results are as follows:(1) An effective method of separation the thermally induced isomers was established using Gas chromatographic(GC). The TFAs and CLAs isomers were formed duing corn oil heating at 180~240°C analyzed by infrared spectrometry and UV spectrum. The contents of them increased with the increasing of time and temperature. Nine kinds of TFAs(C18:1-9t, C18:2-9c12 t, C18:2-9t12 c, C18:2-9t12 t, C18:3-9c12t15 t, C18:3-9c12c15 t, C18:3-9c12t15 c and C18:3-9t12t15c/9t12c15t), and eight kinds of CLAs(tt-CLAs, C18:2-9c11 t, C18:2-10t12 c, C18:2-9t11c/10c12 t, C18:2-11c13 t, C18:2-9c11 c and C18:2-10c12c) were found in corn oils which were unheated and heated at 180~240°C. In addition, the formation pathways of thermally induced isomerization products were clearly elucidated.(2) The differences of the fatty acid compositions and the amounts of TFAs, CLAs and ω6/ω3 of various brands of corn oil heated at different time-temperature treatments were investigated. The composition of fatty acids was almost identical among the eleven different brands of corn oil, while the quantities of TFAs, CLAs and ω6/ω3 were differences. The amounts of TFAs, CLAs and the values of ω6/ω3 in corn oil increased with the heating temperature and time. It was suggested that the heating temperature was the main factor to influence the generation of TFAs, CLAs and the ratio of ω6/ω3 compared to that of heating time. And corn oils had good isomerization stability under heating temperature of 180 °C.(3) The dynamics of major thermally induced isomers 180~240 °C by GC were analysed. Thermally induced trilinolein first isomerized to C18:2-9c12 t and C18:2-9t12 c isomers, belonged to first-order reaction and then formed double trans C18:2-9t12 t isomer(zero-order reaction) according to the rate constants. tt-CLAs isomers were the main products of CLAs, and their formation rate was higher than that of C18:2-9c,11 t and C18:2-10t12 c. In addition, Results showed that the formation reaction of C18:1-9t and C18:3-9t12t15c/9t12c15 t isomers belonged to the first-order reaction and zero-order reaction, respectively. Isomerization reaction rate of fatty acids was accelerated in the presence of oxygen, while the presence of oxygen could not change the reaction order. And build the mathematical model of thermally induced isomerization concentration with heating temperature-time in corn oils.(4) Effects of additives(TBHQ, VE, L-ascorbyl palmitate, carnosic acid, tea polyphenol, ferulic acid ethyl ester and oryzanol) on thermally induced isomerizations were studied. Results showed that the suppression or induction of TFAs and CLAs by addition of these additives depended on the heating temperature, heating time and the type of additives. L-ascorbyl palmitate and carnosic acid could be used as isomerization regulation additive for frying oil, reducing the formation of TFAs and no reducing the formation of CLAs during frying. New type additives ferulic acid ethyl ester and oryzanol existed inhibition on thermally induced isomerizations in corn oils only under the heating condition of 180 °C. Also, The synergistic action of compound additives on the isomerizations suppression was not obvious in this research.