Abstract: The effects of enzymatic hydrolysis on the sensory quality and free amino acid composition of fresh skim milk were analyzed by a spectrophotometer, an electronic tongue and an amino acid automatic analyzer. The results showed that the degree of hydrolysis of defatted milk was 24.61%. Protease treatment led to changes in its sensory properties, and L* values of all three hydrolysates were significantly decreased (P < 0.05), and a* values (negative value) were significantly increased (P < 0.05) when compared with the untreated control. The effect of flavourzyme treatment on the color of skim milk was greater than that of alkaline protease treatment and protamex treatment. Large differences in taste profile were observed among the different hydrolysates. After enzymatic treatment, the sweet taste of skim milk was decreased significantly. With increasing digestion time, the bitter taste was enhanced accompanied by a decrease in the sweet taste. The taste profile of the alkaline protease hydrolysate was dominated by astringency and astringent aftertaste, the taste profile of the flavourzyme hydrolysate was dominated by saltiness, bitterness and bitter aftertaste, and the taste profile of the protamex hydrolysate was dominated by sourness as well as low and bitter taste. The electronic tongue was able to distinguish the difference in taste among the hydrolysates. The contents of free amino acids and essential amino acids in defatted milk were increased after enzymatic hydrolysis, making bitter amino acids become the dominant taste-active amino acids. In conclusion, enzymatic hydrolysis could change the color, taste and free amino acid composition of skim milk, and the effect of flavourzyme was greater than that of alkaline protease and protamex.

Key words: protease; skim milk; free amino acid; sensory quality; electronic tongue