Effect of Calf Chymosin Dose on Protein Degradation in Hard Cheese Made from Yak Milk
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Abstract:
The effect of calf chymosin supplementation on degradation of protein in hard cheese made from yak milk during the three-month ripening process was studied by measuring content of various types of soluble nitrogen. Additionally, urea-polyacrylamide gel electrophoresis and sensory evaluation regarding bitterness in the cheese was conducted. The results showed that chymosin dosage had significant impact on content of soluble nitrogen in pH 4.6 acetate buffer and 12% trichloroacetic acid (p < 0.05). After one month of ripening, there were significant differences in soluble nitrogen content in pH 4.6 acetate buffer between samples with different chymosin dosages, and a strong linear correlation was found between chymosin dosage and soluble nitrogen content. However, chymosin dose had no significant effect on the content of soluble nitrogen in 5% phosphotungstic acid and free amino acid content (p > 0.05). Results of urea-polyacrylamide gel electrophoresis showed that degradation of αs-casein in cheese was dependent on chymosin dosage and was more extensive than that of β-casein. Chymosin dosage had a significant impact on cheese bitterness as well, which increased in intensity with increasing amount of chymosin, with most cheeses ranging from slightly to moderately bitter. The degree of cheese bitterness strongly correlated with soluble nitrogen content in 12% trichloroacetic acid as well as degree of αs-casein and β-casein degradation (Spearman correlation coefficient > 0.7, p < 0.01).
