| Oxidized compounds,such as oxidized triglycerides?TAGs?and TAGs polymers,are generated during lipid thermal oxidation.The formation of these compounds lead to oil deterioration and nutrition loss,which could be a potential threat to human health.In our study,we brought forward an electron spin resonance?ESR?spin trapping technique with PBN??-phenyl-N-tert-butylnitrone?and DMPO?Dimethylpyrroline-N-oxide?as spin trap.The spin trapping technique with PBN as spin trap could be used to detect oil oxidative stability,while DMPO was suitable for the detection of lipid oxidation mechanism.The experimental spectrum of DMPO was simulated by three lipid-derived free radicals.The thermal oxidative mechanism of palm acid,oleic acid and linoleic acid were deducted by the simulation of lipid-derived radicals and the detection of volatile compounds.Then,the TAGs degradation of palm oil,rapeseed oil and sunflower oil,which containing large amounts of saturated,monounsaturated and polyunsaturated fatty acid,respectively,was investigated by HPLC.Then,the formation mechanisms of thermal oxidative compounds were deducted based on the separation of polar compounds and detection of lipid-derived free radicals.The present studies provided novel insight into the mechanism of thermal oxidative compounds at high temperatures,and it would be a valuable tool to prevent the generation of toxic compounds during lipid thermal oxidation.Firstly,spin trapping techniques with PBN and DMPO as spin traps were established and their conditions were optimized as follows:PBN and DMPO in toluene was added to oil samples,and the final concentration was 25 mM.The spectrum of PBN was simple,which could be a good method to detect lipid oxidative stability.g value=2.0067,aN=14.7 G,aH=2.5G.?-phenyl-N-tert-butylnitrone?PBN?was used as spin trap to discriminate the adulteration of qualified vegetable oils.All five qualified vegetable oils with higher adulteration proportions gave higher initial signal intensities detected at 2 mins,which could be assigned to thermal oxidation products that were present in adulterated oils.Initial signal intensities and adulteration proportions?<20%?were treated for quantitative analysis by linear regression.The simple linear equations were built and the coefficients of determination?R2?were all above0.9400 except PO?0.8920?.Spin trapping technique with DMPO as spin trap was suitable to detect lipid-derived radicals at high temperatures.The DMPO spectrum of linoleic acid oxidized at 110°C was simulated by three radicals:alkyl radical adducts,aN=14.9 G,aH=22.3 G;alkoxyl radical adducts,aN=13.6 G,a?H=7.3 G,a?H=1.5 G;DMPO oxidized adducts,aN=14.5 G,aH=2.1 G.The alkyl radicals adducts were the main radical adducts.Secondly,thermal oxidation mechanism of fatty acid with different saturation was investigated.The percentage of alkyl radical adducts of linoleic acid oxidized at 110°C was higher than 90%,which indicated that the thermal oxidation rate of linoleic acid at 110°C was really high.In the study of oleic acid,alkoxyl radical adducts were found to be the main radical adducts between 120 to 135°C.At 140°C,the percentage of alkyl radical adducts was 63.89%,and total amount of spin of oleic acid was 10 times higher than that at 135°C.Therefore,the thermal oxidation rate of oleic at 140°C was far higher than that at 135°C.Besides,9-COOH was found to be the main hydroperoxides in the study of volatile compounds,and the formed unsaturated aldehyde would affect the flavor of frying.In the study of palm acid,total amounts of palm acid at 180°C were almost 10 times higher than that at 175°C.The peak intensity of alkyl and alkoxyl radical adducts increased from0.405 to 4.785 and 1.934 with temperatures increasing from 175 to 180°C.besides,amount of n-decane to tetradecane increased with temperature increasing,which indicated that C2-C6 of palm acid could break to radical chain reaction.Thirdly,the TAGs composition of different oils during thermal oxidation were investigated.During thermal oxidation of palm oil with high contents of saturated fatty acids,the oxidation rates of OLL,LLP and OLO?L:linoleic acid,O:oleic acid,P:palmitic acid?were the fastest,due to their high contents of unsaturated fatty acids.The contents of POO and POP were 25.73%and 28.28%,respectively.There were 5.62 g and 6.46 g/100g oil POO and POP being oxidized after 36h thermal oxidation,which were the highest in that of palm oil.During thermal oxidation of rapeseed oil with high contents of oleic acid,5.43 g and 5.22 g/100g oil OLO and OOO were oxidized,because of their highest contents in rapeseed oil.The content of OLnO?Ln:linolenic acid?was 13.60%in rapeseed oil,and linoleate acid in sn-2 was easily to be oxidized.After oxidation of 36h,4.95 g/100g oil OLnO degraded.During thermal oxidation of sunflower oil with high contents of linoleic acid,the contents of LLL and OLL were 26.75%and 27.21%,respectively,which were the highest in sunflower oil.11.09 g/100g oil LLL and9.69 g/100g oil OLL were oxidized after oxidation because of their high degree of unsaturation.The initial contents of LLP and LOP were lower than LLL and OLL,of which 3.41 g and 3.42g/100g oil degraded after thermal oxidation.Lastly,in the study of thermal oxidation of palm oi,rapeseed oil and sunflower oil,DMPO oxidized adducts were the main radical adducts,then alkyl radical adducts became the main radical adducts.therefore,it is concluded that alkoxyl radical was the main lipid-derived radicals in the initial stage,then alkyl radical was the main lipid-derived radical.Therefore,alkoxyl radicals were the main lipid-derived radicals in the initial stage,which could lead the formation of polar TAG polymers linked with-C-O-O-C-.Then,with oxygen in the system being consumed,alkyl radicals were the main radical adducts.polymers linked with different bonds formed in different position of the fryer:in the top:-C-O-O-C-;in the bottom:-C-C-and-C-O-C-.Based on the correlation analysis between the amounts of polar TAG polymers and degradation of TAGs and fatty acid,it was found that there was no correlation between formation of TAG polymers and saturated fatty acids.However,there was a significant correlation between formation of TAG polymers and unsaturated fatty acids.The main structures of polar TAG polymers in palm oil,rapeseed oil and sunflower oil were POL-LOP,POL-OOP,POO-OOP;OLL-LnLO,OLLn-OLnO,OOO-OLO,OLLn-OOO;LLL-LLO,LLL-LLL,OLL-LLO. |
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