Abstract:

In view of the bitter taste defect in hard yak milk cheese, calf rennet, microbial rennet and papain were used to
prepare yak milk hard cheese in the present study to explore the degradation degree of pH 4.6 water-insoluble casein of hard
yak milk cheese, as evaluated by urea-polyacrylamide gel electrophoresis (urea-PAGE) and sensory evaluation of bitterness
during the ripening period, as well as explore the effect of degradation degree of pH 4.6 water-insoluble casein on bitterness
of hard yak milk cheese. The results indicated that hard yak milk cheese casein revealed significant degradation during the
ripening period; moreover, αs-casein underwent much more extensive and faster degradation than β-casein. The pH 4.6
water-insoluble casein of hard yak milk cheese made with papain showed higher intensity bands in the Pre-αs-casein region
as shown by urea-PAGE, while the degradation degree of αs-casein and β-casein was significantly higher than that of pH 4.6
water-insoluble casein from hard yak milk cheeses prepared with microbial rennet and calf rennet (P < 0.05). The bitterness
of hard yak milk cheese prepared with papain was significantly higher than that obtained from microbial rennet and calf
rennet (P < 0.01). These results suggested that the contents of residual αs-casein and β-casein showed a highly significant
negative correlation with bitterness for three yak milk hard cheeses by principal component analysis. This investigation can
provided a theoretical basis for the quality control of hard yak milk cheese.

Key words: hard yak milk cheese, casein degradation, urea-polyacrylamide gel electrophoresis (urea-PAGE), bitterness, principal component analysis