Trans-Isomerization Of ?-Linolenic Acid In Linseed Oil During Heating And Frying

Linseed oil is an edible vegetable oil rich in?-linolenic acid(ALA).Due to its high unsaturation characteristics,trans isomerization is easy to occur in high temperature processing such as deodorization,solvent recovery and cooking,during which time trans isomers of?-linolenic acid(TALA)will be produced.ALA contains three double bonds,leading to seven different configurations of TALA.Studies shown that the TALA with different configurations owned different toxicological/physiological effects.Therefore,it is necessary to carry out a systematic research on TALA with different configurations in the high-temperature thermally promoted trans isomerization of linseed oil.Based on the improvement of the TALA determination method,the paper studied the changes of the content and composition of TALA and the influence of related factors during the heating process of linseed oil.And under the complicated actual frying system,the trans isomerization of ALA in linseed oil was verified.The details are as follows:Firstly,the gas chromatography method for quantitative detection of TALA using SLB-IL111 ionic liquid strong polar column was optimized and evaluated.The results showed that the optimized method achieved effective separation of C18:3-9c,12t,15t and C18:3-9t,12c,15t which were difficult to separate,with a separation degree of 0.9.The separation of the remaining TALA ranged from 1.6 to 143.3,which showed that the new method could be used for quantitative analysis of 8 different configurations of ALA.The relative standard deviation(RSD)of inter-daily repetition of each isomer in heated linssed oil was 3.16%?5.65%,and the RSD value of intra-day repetition of each isomer in heated linssed oil was 2.00%?4.13%.The detection limit was 0.6782?0.8790 mg/L,and the limit of quantification was 2.2606?2.9301mg/L.The linear correlation coefficients(R~2)of peak area and concentration were greater than0.999.Secondly,the trans isomerization of ALA in heated linseed oil was studied by heating experiments at 140?,170?and 200?,and the effects of heating temperature,heating time,air pumping and deterioration of the linseed oil(except for trans isomerization)on the formation of TALA were discussed.The results showed that 140?might be the critical temperature for ALA to start trans-isomerization.The increase of heating temperature and heating time could significantly promote the formation of TALA.The content of TALA increased significantly at 170?200?.After heating at 200?for 14 h,the increase of TALA was as high as 5.540 g/100 g,which was about 154 times of 140?and about 17 times of 170?.With different heating temperature and time,5 to 6 different configurations of TALA could be measured.There were differences in the degree of difficulty in the forming of trans double bonds at different positions,in which the trans-15 of ALA was the easiest to form,and the trans-12 was the most difficult to form.TALA with different configurations also showed different difficulty in formation.C18:3-9t,12t,15t was the most difficult to generate and had never been detected.C18:3-9t,12t,15c was difficult to produce contributing that it could not be detected unless at 170?for 6 h or at 200?.In other word,there would not be any target product occur below this heating temperature or time.In terms of the composition of TALA(proportion of each TALA)in the product,mono-TALA took up a proportion about 93%?94%,in which C18:3-9c,12c,15t owned the largest proportion of mono-TALA,varing between53%?68%with different heating temperature and time.As the heating temperature increasing,the composition of TALA changed.For example,when the heating temperature increased from140°C to 200°C for 14 h,the proportion of C18:3-9c,12c,15t decreased by about 16%,and the proportion of C18:3-9t,12c,15c increased by about 22%.During heating at different temperatures,the composition of TALA(the proportion of each TALA)varied,among which the change was most dramatic at 170?.At this temperature,the rate constant of the proportion of each TALA changed with heating time was-0.970?0.949.As the air pumped in,the content of TALA in linseed oil decreased in different degrees(16%?56%)at different heating temperatures,in which C18:3-9c,12c,15t with relatively weak oxidation stability decreased the most.The content of TALA decreased significantly at 200?.Under different heating conditions,other degradation reactions such as hydrolysis,oxidation,and polymerization of linseed oil had different effects on the trans isomerization of ALA.Overall,there was a significant positive correlation among the production of TALA,total polar component(TPC),acid value(AV),diglyceride(DG),oxidized triglyceride oligomers(TGO)and dimers(TGD)(r=0.394?0.813,p<0.05).The correlation between TALA and DG was the strongest(r=0.744?0.813,p<0.01),followed by TGO(r=0.512?0.758,p<0.01).During the heating process of linseed oil,the hydrolysis reaction had the largest effect on the trans-isomerization of ALA,followed by the polymerization reaction.Finally,the effect of temperature,time and other deterioration of oil on the trans isomerization of ALA during the frying process were studied by frying French fries in linseed oil at 170?and 200?to further verify the results and conclusions obtained from the above heating experiments.The results showed that heating temperature and time promoted the formation of TALA significantly,while the effect was more complicated.The increments of TALA caused by frying temperature were less than heating experiment.However,some di-TALA which was more difficult to generate under the heating experiment was easier to form under the frying system.For example,C18:3-9t,12t,15c could be detected by frying at 170?for 4 h.Polymerization and hydrolysis reactions also affected the trans isomerization of ALA during frying,where polymerization reactions impact more.In addition,the distribution of TALA in frying oil and French fries was analyzed.The results showed that TALA was more likely to distribute in frying oil with stronger polarity compared with French fries.The content of TALA in frying oil at different frying time points was 1.11?1.57 times as that of corresponding French fries at 170?.With frying temperature increasing,this distribution difference decreased.The content of TALA in frying oil at different frying time points at 200?was 1.01?1.14 times as that of the corresponding French fries.In summary,the hydrolysis and polymerization of linseed oil promotes the formation of TALA,and TALA tends to distribute in frying oil.This paper provides important basic data for inhibiting the formation of TALA in linseed oil,and has practical significance in guiding the reasonable processing and utilization of linseed oil and other edible oils.