Reserarch On Yak’s Milk Hard Cheese Fatty Acid In Ripening Process

In this paper, yak milk in gannan pastoral area was used as raw material, and hard cheeseof yak milk was maded. the dynamic variation rule of fatty acid of hard cheese of yak milkwhich matured for0-6months was determined by GC-MS under5℃、10℃、15℃storageconditions,Studied the degradation of fat in hard cheese of yak milk through the acid value,peroxide value, carbonyl group value and thiobarbituric acid value study.The main resultsare as follows:（1）The yield of hard cheese of yak milk was19.8%±0.14%. the protein content is27.7%±2.0%,Average content of fat is47.5%±1.0%; Moisture content was42.2%±0.33%, theash content is4.77%±0.02%, pH is5.13±0.01.（2）56kinds of fatty acids were detected in hard cheese of yak milk witch has stored at5℃for6months, including short-chain fatty acids (C6-C11), medium-chain fatty acids(C12-C16) and long chain fatty acid (C17-C27). The main fatty acids are C14:0, C16:0, C18:0and C18:1, and they account for over50%of the total amount of fatty acids. Another: twokinds of the aromatic fatty acids: phthalic acid and terephthalic acid;3glucuronic acid: oct-glucuronic acid, uronic acid and nonanoic acid, capric aldehyde; one dicarboxylic acid:azelaic acid;1A oxy fatty acids:8,10-dimethoxy-stearic acid. C27:0fatty acids was onlydetected in the cheese which stored at10℃or15℃.（3）In yak milk hard cheese mature0to3months, the fatty acid species increased, the mainincrease in the short chain and chain fatty acid, then type does not change.Mature temperaturehas little effect on cheese types of fatty acids. The fatty acid content of yak milk cheeseincreases gradually during the0-6month’s mature period, and the fatty acid content at15℃was significantly higher than5℃or10℃. The total fatty acid content in fresh cheesewas6323mg/Kg, and it reached to12720.7mg/Kg,13993.28mg/Kg and14258.74mg/Kg at5℃,10℃and15℃after6month. After4month, The increase range of total fatty acidcontent at10℃and15℃was significantly higher than5℃. It shows that the temperatureand time has a great influence on the degradation of fat in the cheese.（4）The acidity value, carbonyl group value and thiobarbituric acid value of the cheese wasincreased with mature time at3temperatures. The increase range at15℃was significantlyhigher than(P<0.05)5℃and10℃.The trend of peroxide value increased at first thendecreases, and the peroxide value was reached the peak3month later. This shows that lipidoxidation in the process of cheese maturation is ongoing. （5）The acidity value was the same under15℃after2months ripening（1.33mg/Kg） andunder10℃after3months ripening（1.33mg/Kg）,So the mature time of the cheese can beshorten by raising temperature.（6）Yak milk cheese mature period, as the extension of time, the mature fat oxidation anddecomposition, so the content of free fatty acids also continues to increase. At15℃storagecheese lipase vigor is5℃and10℃, so fast, fat oxidation decomposition of free fatty acidcontent is higher, fat oxidation degree is15℃higher than5℃and10℃storage.