| Walnut(Juglan regia & Juglans sigillata)was an important woody oil species in China.The State Forestry Bureau indicated that in 2015,the domestic production of walnut was 3.3317 million tons,accounting for 59.49 % of the total production of woody oil crops.Walnut kernel contained 60 %~75 % of oils,most of which were unsatured fatty acids.The ratio of ?-6 to ?-3 in walnut oil was good for human health.For now,the bottleneck for walnut oil production and storage was its tendency of oxidation.Researchers have made efforts to improve the stability and shlef life of walnut oil through the use of antioxidants and the control of storage conditions.Based on their results,this study focused on the internal characteristic changes during walnut oil oxidation.Pressed walnut oils were used to explore the generation of free radicals and its influencing factors at the initial oxidation stage.A HS-SPME/GC/MS methodology for volatile analysis was established,and the markers of oxidative levels of walnut oils were selected.For improving oil stability,the effects of Maillard reaction between amino acids and reducing sugars,as well as the independence and interaction effects of tocopherols isomers,were also studied.Main conclusions were as follows:(1)At the initial oxidation stage,the radical formation in walnut oil was effectively inhibited by the total minor components.The inhibition effect of single tocopherol component was not obvious,suggesting synergistic interactions among different components.The formation of radicals was much faster than the rise of peroxide value.The higher the rate of radical formation,the lower the oil storage stability.(2)A HS-SPME-GC/MS methodology was developed to analyze the volatile oxidation products of walnut oils.The key parameters were: DVB-CAR-PDMS fiber,extraction temperature 50 ?,extraction time 30 min,desorption time 1 min.Thirty volatile compounds were identified in oxidized walnut oil,including alcohols,aldehydes,ketones,acids and heterocyclics.Aldehydes were the most important oxidation compounds,accounting for 40.68% of the total volatiles.The content of hexanal was the highest,accounting for 24.46% of the total volatiles.(3)The formation of linoleic-derived 2-heptenal was closely related to the quality deterioration during walnut oil oxidation.The content of 2-heptenal was less affected by variety difference and linearly correlated to peroxide value(R2=0.9146).Therefore,2-heptenal could be used as volatile maker of oxidative level in walnut oils.Meanwhile,at lower oxidative levels(peroxide value?5 meq/kg),the linolenic-derived E-2-pentenal ? E,E-2,4-heptadienal were not detected,indicating that both compounds could be used to determine the walnut oil freshness.(4)Within 0~500 mg/kg,the antioxidant capacity of ?-tocopherol increases with concentration;However,for ?-tocopherol,its antioxidant capacity decreased when concentration >100 mg/kg.In purified walnut oil system,the ?-tocopherol had certain protective effect on ?-tocopherol,since the former showed higher degradation rate when both existed at the same concentration.In cold-pressed oil system,although there was significant difference between the contents of ?-and ?-tocopherol,they showed similar degradation rate.When same concentration(50 mg/kg)of ?-and ?-tocopherol were added into the cold-pressed walnut oil,respectively,the ?-tocopherol was more effective in promoting the oxidation stability of walnut oil.(5)Microwave treatment within 2~4 min could induce Maillard reaction in walnut kernels,while no great changes were occured in fatty acid compositions and peroxide values.After a 2-min treatment,the color of walnut oil darkened while amounts of pyrazines were generated,presenting obvious "roasted" aroma.A 4-min treatment deepened the Maillard reaction in walnut kernels.Although the contents of tocopherols and sterols decreased by 26.62% and 8.98%,respectively,the formation of melanoids recovered the loss of the natural antioxidants and finally improved the stability of walnut oil. |
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