| As a high-fat snack food,nuts have a huge market due to their unique flavor and taste,high unsaturated fatty acid content,etc.However,high content of fat and unsaturated fatty acid content of nuts also lead to the problem of excessive oxidation.The oxidation of oil in nuts would not only lead to the decrease of sensory evaluation of nuts,but also produce toxic and harmful substances to consumers' health.Therefore,How to better monitor the oxidation of oil in nuts is crucial to ensure the quality of nuts.This paper took traditional frying and processing of fresh hazelnuts and their storage process as the research object.Electron spin resonance(ESR)and conventional physical and chemical indicators(peroxide value and acid value)were used to evaluate the oxidation degree of hazelnut oil.The results showed that the variation of the lagtime obtained by ESR was more obvious than that obtained by conventional physical and chemical indexes before and after frying or during the process of accelerating storage,indicating that the ESR method was more sensitive to the change of oxidation degree.In the process of accelerating storage,the lagtime had a good correlation with the peroxide value and acid value(p < 0.01).In addition,compared with roasted hazelnut(the highest peroxide value has reached 14 meq/kg,with a high degree of oxidation),the correlation coefficient between raw hazelnut shell(the highest peroxide value was 1.7 meq/kg,with a low degree of oxidation),peroxide value and acid value was better(r =-0.771 and r =-0.921),respectively,indicating that ESR method was more suitable for monitoring samples with a low degree of oxidation.Furthermore,ESR and Ranciamt method were used to evaluate oxidation stability of 14 commercial nut oils.Pearson correlation coefficient and principal component analysis were used to analyze the correlation between oxidative stability,the composition and content of triglycerides and minor oil components in nuts.The results showed that both lagtime and OSI values were significantly negatively correlated with the unsaturation of triglycerides and fatty acids(p < 0.01);There was a significant negative correlation between lagtime and hydroperoxide content(p < 0.01)and free fatty acid content(p < 0.05),while OSI values were not significantly correlated.Therefore,in the evaluation of the oxidation characteristics of nut oil,the ESR method can reflect the oil properties and oxidation products at the same time compared with the Rancimat method.Four representative nut oils including watermelon seed,peanut,hazelnut and macadamia nut were selected as the research objects.The polar and non-polar companions were gradually removed by extraction and column separation,and the effects of different polar oil companions on DPPH scavenging ability and oxidative stability of nut oil were further studied by ESR method.It was found that the ESR method can clearly distinguish the difference in DPPH free radical scavenging ability between polar and non-polar companions present in nut oils,and the ability of non-polar lipid concomitants was greater than polar concomitants.Under the same triglyceride conditions,the order of antioxidant capacity was crude oil > non-polar component > triglyceride.For macadamia nuts,hazelnuts and watermelon seed oil,the three oxidized oils(0.52 meq/kg)had a lagtime order of crude oil > non-polar component > triglyceride,indicating that the oil companions can prolong the oxidative stability of nut oil;for the deeper degree of oxidation of the peanut oil(1.13 meq/kg),the lagtime sequence was triglyceride>crude oil>nonpolar component,the effect of the peanut oil which has its own oxidative stability-promoting oil concomitant was not reflected.In summary,the ESR method had a good applicability in monitoring all aspects of nut oxidation,and could provide a fast and efficient new method for better quality control of nut products during processing and storage. |
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