The Studies On Enhancing The Oxidation Stability Of The Cold Pressed Sesame Oil

Cold pressed sesame oil(CPs),with a crystal color and a smell of natural fragrance,is much better when compared with hot pressed or water distilling sesame oil for its lower content of PAHs and oxidized triglyceride polymers.However,the shelf life of the CPs is short due to its poorer oxidation stability which severely restricting its development and utilization,ought to be enhanced urgently.This study researched the general rule of the oxidation stability of various oilseeds' cold and hot pressed oil.We researched the relevant factors of microwave pretreatment and acid catalysis sesamolin processes which were used as the possible process measures to enhance the oxidation stability of the CPs.Eventually,we successfully enhanced the oxidation stability and explored the possible reasons.Oven and Rancimat methods were used to compare the oxidation stability of 5 representative oilseeds' cold and hot pressed oil,including sesame,peanut,sunflower,flax and perilla selected basing on the composition of fatty acid.Oven test showed that the AV and the composition of fatty acid had no significant changes,but the PV of the cold pressed oil rose faster than the hot which was also applicable to the wastage rate of Vitamin E.The oxidation induction time of cold pressed oils were also less than the hot.Extraordinary,we predicted the shelf life of the cold and hot pressed sesame oil at the condition of 20?by Rancimat method which were 8022.47 h and 20272.08 h respectively.We found that sesamol,only 3.14 mg/100 g in the hot pressed sesame oil(HPs)and less in the CPs,wasn't the dominant factor which resulting in the high oxidation stability of the HPs through the researching on the different antioxidant contents in the cold and hot pressed sesame oil as well as the promoting ability of pure sesamol on the oxidation stability of the CPs.Sesamol came from the thermal decomposition of sesamolin,if we found another method to convert all of the sesamolin(366.02 mg/100 g in the CPs)to sesamol,there would be 136.51 mg/100 g sesamol in the CPs,resulting in around 70% increase on the oxidation induction time.Sesamolin can be catalyzed into sesamol and sesaminol by phosphotungstic acid(HPW).Thermal treatment has a significant effect on the oxidation stability,we think that a certain extent of microwave pre-treatment could increase the oxidation stability and the antioxidant content of the CPs.Microwave treated the sesame seed for 1-5 min under the power of 480 W,there had produced the roasted fragrant and Yellow Value increasing after being treated 5 min which wouldn't belong to the flavor of the cold pressed sesame oil,however,4 min's treatment could maintain.The oxidation induction time of the CPs had increased from 4.19 h to 5.61 h after 4 min's treatment,meanwhile,the residue oil rate dropped from 32.74% to 14.35%,the content of sesamol and sesamolin in sesame oil increased 15.89% and 17.51% respectively,but there had no significant change in the sesame seed.The DPPH free radical scavenging capacity of sesamol considerably larger than sesamolin,meanwhile,the scavenging capacity increased greatly when sesamolin being catalyzed into sesamol and sesaminol by HPW which proved the feasibility of catalyzing sesamolin to improve the oxidation stability.We optimally choose HPW as catalyst to catalyze the sesamolin in the CPs.Under the optimized conditions,namely,HPW addition 0.8%,reaction temperature 80?,reaction time 120 min,all of the sesamolin converted to sesamol(91.36 mg/100g)and sesaminol(no quantitative)in the CPs,the oxidation induction time increased from 4.19 h to 9.31 h.After being catalyzed,the shelf life of the CPs increased to 13864.36 h,72.82% higher than the 8022.47 h,meanwhile,there had no significant influence on the basic quality index of oil,only the color had a small change,the value of yellow changed from 31.2 to 22.4.We found that there had different acid catalysis types and different unkown substances when sesamolin dissolved in different solvent,sesamol was the only common reaction product.We predicted that the solvents involved into the reaction when the sesamolin was catalyzed by hydrochloric acid in methol or ethanol solution,reacting to form two stereoisomers with methylenedioxy phenoxy substituted by methoxy or ethyoxyl group.