Determination Of Stigmasterol Oxides And Study On Quality Of Soybean Oil With High Phytosterol

Many oil factories improve the nutritional value of vegetable oil by adding phytosterols,but phytosterols have tendencies to oxidize under heating, illumination and radiation, whichproduce phytosterol oxiditions and influence the quality of vegetable oil. Establishingdetection methods and study the stability of high phytosterol contents soybean oil understorage and cooking have important significance. The main research contents are as follows:First, selecting representative sterol standard-stigmasterol. Prepare phytosterol oxiditionsunder the simulated oxidation condition, detect phytosterol oxidations by solid phaseextraction(SPE) technology and GC-MS. The simulated oxidation condition: heating3h inthe oven. The optimized solvent elution conditions of SPE: SPE cartridge is conditioned with5mL of hexane, load sample, elute with5mL of hexane/diethyl ether(V:V=9:1) and5mL ofhexane/diethyl ether(V:V=1:1), finally the phytosterol oxiditions are eluted out of thecartridge with5mL of acetone, the collected phytosterol oxiditions are dried under nitrogen,the phytosterol oxiditions are derivatized by BSTFA+TMCS, then the oxiditions are detectedby GC-MS. Focuse on the characteristic fragment ion peaks, the identified stigmasteroloxiditions are5α,6α-epoxystigmasterol,5β,6β-epoxystigmasterol, dihydroxystigmasterol,7α-hydrostigmasterol,7β-hydrostigmasterol,7-ketostigmasterol.Secondly, establish the detection method of sterol oxides in high stigmasterol contentssoybean oil, chemical method coupled with SPE method is applied to the pre-enrichment ofsterol oxides.19-hydroxycholesterol is selected as internal standard. Stigmasterol oxidesstandards can not be obtained,5α,6α-epoxycholesterol and7-ketocholesterol are selected asrepresentatives, recoveries of5α,6α-epoxycholesterol and7-ketocholesterol are92.54%,91.76%, relative standard derivation are3.96%,2.37%, the lowest detectable limits ofGC-MS are0.5μg/mL,0.6μg/mL, the datas show that the method has good reproducibilityand accuracy.Finally, simulate commercially available high sterol contents vegetable oil, prepare highsterol contents soybean oil(the contents of stigmasterols are4‰,8‰,12‰), study thechanges of sterol, sterol oxides, peroxide value, acid value, tocopherol, trans-fatty acids andthe relationship between them in high sterol contents vegetable oil:(1) During the process of high-temperature oxidation, the peroxide value increasepreviously, then decrease, the acid value is increasing. The higher the sterol contents ofsoybean oil, the lower peroxide value and the acid value. During the first6h, the sterolcontents of soybean oil and high stigmasterol contents soybean oil decrease slowly, the totalloss rate of sterol is about10%, during the last3h, the total loss rate of sterol is about25%;the tocopherol loss rates are decreasing(the rates are90%,73.4%,73.6%,61.9%); thecontents of trans-fatty acids are decreasing(the contents are0.271%,0.189%,0.156%,0.123%); the stigmasterol oxides increase previously,then decreased, the stigmasterol oxidesare679μg/g,827.44μg/g,1109.44μg/g, stigmasterol oxides have good linear relationshipwith the corresponding peroxide value. (2) During the process of frying, the peroxide value of soybean oil increase previously,then decrease, the peroxide value of high stigmasterol contents soybean oil is increasing, theacid value is increasing; during the first3h, the total loss rate of sterol is below10%, after9hours the total loss rate of sterol is below25%; the tocopherol loss rates are increasing(therates are75.04%,84.93%,86.66%,91.6%); the contents of trans-fatty acids are decreasing(the contents are0.178%,0.161%,0.15%,0.117%); the stigmasterol oxides are161.72μg/g,338.36μg/g,498.48μg/g; during the first3h, stigmasterol oxides have good linearrelationship with the corresponding peroxide value, during the last6h, with the increase ofperoxide value, the stigmasterol oxides reduce gradually.(3)During the process of storage, with the increase of phytosterol, the peroxide value,acid value and trans-fatty acids increase gradually, during the storage of10d, the total lossrate of sterol is about10%; the tocopherol loss rates are70.7%,77.8%,79.8%,86.5%; thestigmasterol oxides are20.8μg/g,66.52μg/g,131.8μg/g.