Liquid chromatography-mass spectrometry was employed to determine the change of free amino acid contents in hard yak cheeses with different salt levels during 6 months of ripening. The results showed that a total of 23 free amino acids were detected, the major ones being aspartic acid, glutamic acid, isoleucine, leucine, phenylalanine, threonine and cysteine, together representing 94% to 95% of the total free amino acid content. The content of each free amino acid showed an increasing trend between months 1 and 6. In the late stage of maturation, the contents of threonine, asparaginate, aspartic acid, valine, proline and phenylalanine increased with increasing salt addition, the contents of alanine, histidine, arginine and tyrosine decreased, and the contents of leucine, isoleucine, serine, glycine and tryptophan first increased and then decreased. The contents of of glutamic acid, leucine, isoleucine, aromatic amino acid, cysteine, glycine and serine were significantly higher in long-ripened cheese with 1.3% salt addition than in those with higher salt contents. This study provides theoretical support for the development of low-salt hard yak cheese.

To provide insights into the chemical composition and functional properties of yak whey powder, in this experiment, native yak whey powder (NYW) from the supernatant of acid precipitated yak milk and sweet yak whey powder (SYW) from waste cheese whey (resulting from enzymatic milk coagulation) for measurement of their total protein, lactose and ash contents, pH values and functional properties such as solubility, water-holding capacity, oil-holding property, foaming capacity, emulsifying capacity and thermal stability. The results showed that the difference between the total protein contents of SYW and NYW was 3.433% which was significant (P < 0.05) while the difference in lactose content was not significant (P > 0.05). The ash content of NYW was 11.188%, which was 3.156% higher than that of SYW, with a significant difference being found between them (P < 0.05). The pH values of NYW and SYW solutions were 4.837 and 5.410, respectively, with the difference being significant (P < 0.05). The difference in solubility was significant (P < 0.05), 34.207% for SYW versus 27.079% for NYW. The water-holding capacity of NYW was significantly higher than that of SYW, but the oil-holding capacity was significantly lower than that of SYW (P < 0.05). The foaming ability, foam stability, emulsifying activity and emulsion stability of SYW were significantly higher than those of NYW (P < 0.05). When NYW and SYW were heat treated at 60 ℃, precipitation began to occur. When the temperature exceeded 75 ℃, the amount of precipitation increased significantly (P < 0.05), and the maximum value occurred at 85 ℃, indicating yak whey powder to be the least stable at 85 ℃. On the other hand, yak whey powder was more thermos-stable commercial Holstein whey powder (HW), the most unstable at 80 ℃. The findings from this study show that the main components and functional properties of the yak whey powders obtained by acid precipitation and enzymatic coagulation are different.