Study On Yak Milk Fat Dryfractionation And Properties Of The Fractions

Tibetan ghee, as one kind of milk fat and obtained from yak milk, it contains many essential fatty acids and more functional fatty acids than any other ordinary milk fat. It is also one of the main food for Tibetan people’s daily life. Due to high production and backward hand-workshop-style production methods, Tibetan ghee has uncentain qualities and is lack of product types. Meanwhile, studies about Tibetan ghee products and deep processing methods of Tibetan ghee are still stay the primary stage. Since fat dry fractionation process has many advantages, such as simple to operate, pollution-free and low cost, the technology has been widely used in the field of oil deep processings. The multi-step dry fractionation procedures of yak milk fat and the properties of fractions were studies in this paper.The fatty acid composition of yak milk fat was measured by gas chromatography. The saturated fatty acid content in yak milk fat accounted for the most part, 60.56%. The content of monounsaturated fatty acid and polyunsaturated fatty acid were 35.05%, 4.39% respectively. Meanwhile, the crystallization behavior and melting behavior of yak milk fat were measured by differential scanning calorimetry and two behaviors were not reversible processes. Crystallization curve had two peaks whose ranges were at 20.77~17.89℃, 15.05~0.00℃ repectively. Three peaks were presented in the melting curve and the temperature ranges were 1.50~15.38℃, 16.00~20.73℃ and 24.32~38.72℃ respectively.The main factor influenced fractionation mostly was the fractionation process. The dry fractionation of yak milk fat was operated by the following steps: equilibrium temperature, 80℃, holding 15 min, then slowly cooled to the crystallization temperature at 35℃, 30℃, 25℃, 20℃, 15℃, respectively and hold 9 h at each crystallization time, using gauze filtration or centrifugation to separate solid fat from liquid oil. Five solid fractions and five liquid fractions were obtained. At 35℃, 30℃, 25℃, 20℃, 15℃, the solid yields were 11.36 g/100 g, 26.80 g/100 g, 14.80 g/100 g, 15.31 g/100 g, 29.36 g/100 g, 2.38 g/100 g repectively and the liquid yields were 88.64 g/100 g, 61.84 g/100, 47.04 g/100 g, 31.73 g/100 g, 2.38 g/100 g respectively.The fatty acid composition was largely changed after multi-step dry fractionations. The saturated fatty acids content in all fractions accounted for the most part, about two third. At different fractionation temperatures, saturated fattys acids were enriched in high temperature fractions and were reduced in low temperature fractions, meanwhile unsaturated fatty appeared the reversely trend. At the same fractionation temperature, unsaturated fattys were enriched in liquid fractions and were reduced in solid fractions. The start temperatures of crystallization processes in ten fractions changed largely. The start crystallization temperatures of all fractions were between 10℃and 30℃, the sample 35 S had the highest start crystallization temperature, 29.58℃, while sample 15 L had the lowest temperature, 10.84 ℃. All fractions component began to melt during the temperature range of 4 ℃ and 10 ℃. But the completed melted temperature changed largely in different fractions, while 35 S had the highest completed melting temperature, 46.82℃, and 15 L had the minimum temperature, about 20.13℃.