| Milk fat oxidation is an important biochemical reaction that occurs in dairy products during storage.The milk fat content of yak dairy products is higher than that of ordinary milk,and the oxidative stability of milk fat has an important influence on the quality during storage.Yak milk hard cheese and ghee are typical high-fat products.Their main components are fatty acids.The oxidative degradation of fatty acids will not only change the flavor and shelf life of the product,but also have an important impact on human health;the dynamic balance system of oxidation and anti-oxidation of yak ghee during storage affects the oxidation degradation of fatty acids and the oxidation process of milk fat.Therefore,it is necessary to explore the oxidative stability of milk fat and the dynamic changes of fatty acids during the storage of yak dairy products.In this study,the oxidative stability of yak milk hard cheese and yak shortening was studied,and the dynamic changes of fatty acids in yak shortening during storage were determined by GC-MS,so as to provide the theoretical basis and support for the systematic study of milk fat oxidation and fatty acids in yak dairy products.The main findings are as follows:(1)The fat content of yak milk hard cheese is 49.96% ± 2.48%.The yak milk hard cheese had continuous primary oxidation and secondary oxidation during the 0-6 months of maturation.The total increase in primary oxidation and the average oxidation rate were 33.63% and 0.0223 meq/(kg·month),respectively.The total increase in grade oxidation and the average oxidation rate were 330.48% and 0.035 4 mg/(kg·month),respectively.The total increase in primary oxidation and secondary oxidation and the average oxidation rate in 3-6 months were lower than those in 0-3 months;during the whole ripening process,yak milk hard cheese showed the characteristics of high milk fat content and low oxidation rate.(2)During storage,the oxidation stability of yak ghee gradually decreased,the degree of oxidation increased significantly(P<0.05),and the antioxidant level decreased significantly(P<0.05).At the end of the storage period of Haibei and Gannan yak ghee,POV increased by1 time and 0.93 time,respectively;TBA increased by 10.23% and 2.30%,respectively.Primary and secondary oxidation showed rapid oxidation in 0-3 months and slow oxidation in 3-5months;DPPH free radical scavenging ability and reducing power showed a rapid decline in 0-3 months,and a slow decline in 3-5 months;with the extension of storage time,the antioxidant factors in ghee(VA,VE,CAT,SOD and GSH-PX)were significantly consumed(P<0.05).Among them,the VA and VE of Haibei yak ghee decreased by 34.0?g/100 g and 0.350mg/100 g,respectively,and the VA and VE of Gannan yak ghee decreased by 42.0?g/ 100 g and0.392mg/100 g,the activity of the enzyme antioxidant factor decreased significantly during storage(P<0.05).(3)With the extension of storage time,the types and content of fatty acids in yak ghee showed dynamic changes.The palmitic acid,oleic acid and stearic acid in Haibei yak ghee and Gannan yak ghee were the main fatty acids in the whole storage process;The relative PUFA content of Haibei yak ghee and Gannan yak ghee were 5.99%?4.62% and 7.77%?6.60%,respectively,MUFA content was between 28.61%?26.42% and 29.51%?26.95%,Gannan yak ghee OCFA was significantly lower than fresh ghee at 5 months(P<0.05),a decrease of 0.5%;Haibei and Gannan yak ghee trans-10-C18:1,DPA and CLA were detected in all of them,and at the end of the storage period,they were still at a high level,respectively 7.13% ± 0.01%,0.12% ± 0.01%,4.04% ± 0.01% and 8.98% ± 0.08%,0.10% ± 0.00% and 6.66% ± 0.13%;with the extension of the storage period,both n-3 and n-6 fatty acids were significantly reduced(P<0.05),and fresh ghee was closer to the ideal of S: M: P Ratio 1: 1: 1.According to the health index assessment,the AI and TI of the two types of yak ghee were significantly increased during storage(P<0.05),and HPI and HH were significantly reduced(P<0.05).In summary,the yak milk hard cheese and yak ghee reduced their oxidation stability with the extension of storage time,and showed the phenomenon of rapid oxidation in the early stage of storage and slow oxidation in the later stage;as the degree of oxidation intensifies,the content of fat-soluble vitamins and antioxidant enzymes decreased continuously,and the consumption of antioxidants maintained the quality of ghee;at the end of the storage period,the content of functional fatty acids such as DPA and CLA were relatively high,and the pathogenicity index was low,and the health promotion index was relatively high.The results of this study can provide a theoretical basis for the production and storage of yak dairy products and improve the milk fat nutrition mechanism. |
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