| Linolenic acid is an important kind of fatty acid in edible oil,and beneficial for human health.However,the cis configuration of linolenic acid in edible oil will isomerize to trans configuration during the longer heating,leading to the formation of trans linolenic acid isomers.Studies have shown that excessive intake of trans linolenic acids may induce cardiovascular and cerebrovascular diseases.Linolenic acid exists in oils mainly by the form of trilinolenin.Therefore,the aim of this study was to use trilinolenin as a typical target to simulate the thermal processing of edible oil,to establish the effective gas chromatography(GC)method for separating thermally induced trans linolenic acid isomers,conduct the kinetic analysis and quantum chemical calculation of thermally induced trans linolenic acid isomers,and propose the effective control methods for inhibiting the formation of trans linolenic acid isomers.The main results were as follows:(1)A GC method for effective separation of linolenic acid methyl ester cis-trans isomers using the high polarity ionic liquid chromatography column(SLB-IL111,200 m×0.25 mm×0.2μm)was established.The chromatographic conditions were as follows:injection volume was 1μL,inlet temperature:230°C,split ratio was 10:1,column temperature was 60°C and FID temperature was230°C.The initial temperature of the SLB-IL111 capillary column was set at 60°C for 5 min and then raised at 175°C at 20°C/min for 15 min and then at 185°C at a rate of 1°C/min for 70 min.This method has the advantages of high precision and low detection limit,which can meet the qualitative and quantitative requirements of cis and trans linolenic acid isomers.There were 3 kinds of single trans linolenic acid,2 kinds of double trans linolenic acid and 1 kind of tri-trans linolenic acid isomers were formed from trilinolenin heating at 180°C~240°C,and the amount of these linolenic acid isomers increased with the heating time and temperature.(2)The kinetic analysis of the trans linolenic acid isomers from trilinolenin heating from 180°C to240°C were investigated by GC.Result showed that the degradation reaction of trilinolenin was a second-order reaction and the formation of trans linolenic acid isomers were zero-order reaction.According to the analysis of the reaction rate constants,the heated trilinolenin firstly isomerized to single trans C18:3-9t,12c,15c,C18:3-9c,12t,15c,and C18:3-9c,12c,15t isomers,then to double trans C18:3-9c,12t,15t,C18:3-9t,12C,15t,and C18:3-9t,12t,15c isomers,finally to tri-trans C18:3-9t,12t,15t isomers.(3)The thermally induced isomerization pathway of trilinolenin was studied at the level of B3LYP/6-311++G**by density functional theory of quantum chemistry.Firstly,the geometrical configurations of the reactants(trilinolenin),transition state and final product(trans linolenic acid isomers)on the isomerization reaction path were optimized,and the bond length and dihedral angle of each structure were obtained.The correlation between the reactants,the transition state and the final product and the change of the isomerization transition state and the structure of the intermediates were found by the vibration frequency analysis and the intrinsic reaction coordinates(IRC).Studies confirmed that the thermally isomerism reaction pathway of trilinolenin involved six routes and 12transition states.(4)The effects of tea polyphenols,rosemary extract,V_E and tert butyl hydroquinone(TBHQ)on the formation of trans linolenic acid isomers of heated trilinolenin were studied.Result showed that the control effects of additives on thermally induced trans linolenic acid isomers was related to the heating temperature,time and the kinds of additives in oils.0.2%TBHQ had the greatest inhibitory effects on the trans isomers formed from trilinolenin at 240°C,followed by 0.2%tea polyphenol,0.1%VE and0.2%rosemary.The kinetics of L-ascorbyl palmitate(L-AP)on the formation of trans linolenic acid isomers of heated trilinoleninwasstudied. |
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