| Iron walnut oil is a characteristic woody oil native to China with excellent antioxidant properties.At present,there are few systematic studies on the compositional properties of iron walnut oil from different origins in China;the composition of lipid concomitants in iron walnut oil,especially the types of polyphenols,is unclear;the influence of lipid concomitants in iron walnut oil on its auto-oxidation process is unclear;how these lipid concomitants function in the oxidation process and what kind of interaction relationship exists is not known.Therefore,in this thesis,samples of iron walnut oil from different origins and processes were collected to compare their compositional characteristics;to identify and analyze the types of polyphenols in iron walnut oil;to investigate the mechanism of antioxidant interactions of lipid concomitants in iron walnut oil in different media;to explore their effects on the oxidation process of oils and fats,and to provide a theoretical basis for the antioxidant effect of iron walnut oil.The main research results are as follows:First,the basic physicochemical indexes,fatty acid composition,lipid concomitants content and antioxidant capacity of iron walnut oil prepared from different origins and processing processes in China were systematically investigated.The experimental results showed that the acid value(0.39～0.63 mg/g)and peroxide value(0.06～0.09 g/100 g)of commercially available finished iron walnut oil met the requirements of the national standard for first grade walnut oil.Iron walnut oil mainly consisted of linoleic acid(51.97～68.81%),oleic acid(15.80～22.43%)and α-linolenic acid(4.16～17.23%),with the highest polyunsaturated fatty acid content(79.68%)in Benxi,Liaoning Province and the highest monounsaturated fatty acid content(22.19%)in Liangshan,Sichuan Province.The highest content of squalene(27.16 mg/kg)was found in iron walnut oil from Chengkou,Chongqing;the highest content of α-tocopherol(78.58 mg/kg),γ-tocopherol(440.27mg/kg),total tocopherol(525.05 mg/kg)and total sterols(6256.42 mg/kg)was found in Liangshan,Sichuan,and the highest content of polyphenols(62.91 mg GAE/kg)had the highest content.From the different processing processes,the low-temperature screw pressing method could retain more fatty acid species and trace concomitants,and the ambient cold pressing could retain more γ-tocopherol,while the processing process had little effect on the extraction of squalene.Meanwhile,by comparing the antioxidant capacity of 12 varieties of iron walnut oil,we found that iron walnut oil from Linyi,Shandong Province had the strongest antioxidant capacity,and the antioxidant capacity of iron walnut oil was significantly and positively correlated with the contents of polyphenols,γ-tocopherol and squalene,and negatively correlated with linoleic acid and α-linolenic acid.Secondly,the effects of polyphenols on the scavenging ability of DPPH,ABTS,and FRAP free radicals were verified.The qualitative and quantitative analysis of phenolic extracts from iron walnut oil was performed by ultra performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry(UPLC-QTOF-MS).The results showed that 13 phenolic acids,2 lignans,2 flavonoids and 2 sesquiterpene compounds of19 phenolic substances were initially identified in iron walnut oil.Among them,ellagic acid of the main polyphenolic substances.Combined with the lipid concomitant content of walnut oil and antioxidant theory,γ-tocopherol,ellagic acid and β-sitosterol were found to be the main antioxidant substances of iron walnut oil.The antioxidant interaction mechanism of γ-tocopherol,ellagic acid and β-sitosterol was investigated by electron spin resonance spectroscopy(ESR)using two different polar media,anhydrous ethanol and ethyl acetate,to monitor the scavenging ability of DPPH radicals.The results showed that the combination of γ-tocopherol and ellagic acid showed synergistic effects in both anhydrous ethanol and ethyl acetate,γ-tocopherol andβ-sitosterol showed antagonistic effects in different media,while ellagic acid andβ-sitosterol showed additive effects in anhydrous ethanol and synergistic effects in ethyl acetate.Kinetic analysis and Fourier transform infrared(FTIR)detection of the hydrogen bonding of the lipid concomitants revealed that the polymerization of the lipid concomitants by hydrogen bonding led to a decrease in their antioxidant activity and thus inhibited the regeneration of the compounds,leading to the antagonistic effect.In contrast,the hydrogen bonding position of ellagic acid and β-sitosterol in the anhydrous ethanol system,which showed an additive effect,was not shifted,indicating that there was no reaction.The oxidative coupling of ellagic acid and β-sitosterol in the ethyl acetate system may have occurred,and a stable intermolecular complex was formed between the substances,with higher antioxidant activity than the ellagic acid monomeric compound.Finally,the antioxidant interactions of three lipid concomitants,γ-tocopherol,ellagic acid and β-sitosterol,in the scavenging rate and oxidative stability of DPPH radicals in iron walnut oil with the removal of lipid concomitants were monitored in combination with ESR electron spin resonance spectroscopy and their effects on the oxidation process of the oil.The results showed that all three lipid concomitants showed antagonistic effects in iron walnut oil,but all of them could slow down the oxidation process of iron walnut oil.γ-tocopherol,ellagic acid and β-sitosterol could act as hydrogen donors and reduce the formation of lipid radicals.In the preoxidation stage of oil,ellagic acid has a higher retarding effect on oil oxidation than γ-tocopherol due to the presence of more phenolic hydroxyl groups and easier binding to free radicals,although its content is lower than that of γ-tocopherol.In conclusion,this paper systematically investigated the composition characteristics of iron walnut oil in China,clarified the types of lipid concomitants in iron walnut oil,and explored the interaction mechanism of lipid concomitants,which provided a new theoretical basis for improving the antioxidant capacity of iron walnut oil and provided strong support for the development of iron walnut in China. |
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