Study On The Relationship Between The Proteolysis And Bitternessof Different Milk-Clotting Enzymes Of Yak Milk Hard Cheese

Fresh yak milk hard cheese is not bitter in taste, but after a period of maturation, the residual rennet and bacterial protease degradation of the protein produce bitterness. In recent years, the production rate increase in yak milk hard cheese of high casein content, but the flavor defects, particularly bitter taste is cared for people.In this paper, Tianzhu yak milk as material, calf rennet, microbial rennet and papain was used as milk-clotting enzymes to produce hard cheese, three kinds of rennet effect on the yak milk hard cheese yield, performance and physicochemical properties of curd. By Kjeldahl method, and studied the milk-clotting enzymes types to effect on yak milk hard cheese TN, pH4.6-SN, 12% TCA-N, 5% PTA-N, FAA, Casein N, Protein N nitrogen and polypeptides during ripening. Explore the milk-clotting enzymes types of yak milk hard cheese protein degradation. Urea polyacrylamide gel electrophoresis of different rennet yak milk hard cheese protein degradation extent of its impact bitterness. Use of anti-performance liquid chromatography, to explore during ripening, rennet species to peptide of yak milk hard cheese produced by protein degradation and its bitter contact. The results showed as:1. Calf rennet, microbial rennet and papain yak milk hard cheese yield was 24.92%, 24.43% and 22.3%, respectively.Calf rennet curd best, followed by microbial rennet, rennet worst effects of papain. The pH of three rennet yak milk hard cheese are first decline, after increased. NaCl content of the gently rising.2. During ripening, papain, calf rennet and microbial rennet total nitrogen content of yak milk hard cheese, respectively, from 25.44%, 16.36% and 17.87 to 34.52%, 32.13% and 30.21%, proteolysis greatly affected by rennet and ripening time, pH4.6 soluble nitrogen content increased significantly, three kinds of rennet yak milk hard cheese free amino acids and 5% phosphotungstic acid nitrogen content increase, rennet its influence significantly.3. Three kinds of yak milk hard cheese casein is significant degradation during ripening, and ?s-casein much more extensive and faster than ?-casein, The Papain show higher intensity bands in the Pre-?s-casein region after urea-PAGE, papain yak milk hard cheese bitterness is significantly higher than the value of cheese microbial rennet and calf rennet yak milk hard cheese(p<0.01).4. The ratio of hydrophobic/hydrophilic(HO/HI) peptides was significantly lower(p<0.05) in three kinds of milk-clotting enzymes yak milk hard cheese. The RP-HPLC peptide profiles of pH4.6 water-soluble fractions.The bitterness and the content of hydr ophobic peptides showed a negative positive correlation(r=0.651), the content of hydro philic peptides negative positive correlation(r=0.545),and the ratio of hydrophobic/hydro philic peptides positive correlation(r= 0.420) in yak milk hard cheeses.