THE INFLUENCE OF GENETIC-VARIANTS OF MILK-PROTEINS ON THE COMPOSITIONAL AND TECHNOLOGICAL PROPERTIES OF MILK .2. RENNET COAGULATION TIME AND FIRMNESS OF THE RENNET CURD

Milk samples (total = 870) of different genetic Variants of the milk p roteins were analysed for the rennet coagulation time (RGT) and the cu rd firmness (A(R)-value) using a gelograph. Natural variations in the milk composition and differences in the pH-value and protein content o f the milk were considered. Special interest was given to the rare gen etic variants (alpha(s1)-casein (Cn) C, beta-Cn B or C, kappa-Cn C or E, and beta-lactoglobulin (Lg) C, D or W). There were significant rela tionships between the genotypes of the alpha(s1)-Cn (p<0.01), beta-Cn (p<0.001) and kappa-Cn (p<0.001) locus and the RCT of milk. Milk with the genotype alpha(s1)-Gn CC clotted significantly faster than milk wi th the genotype alpha(s1)-Cn BB. The beta-Cn locus had the greatest in fluence on the RCT, with the RCT increasing from 9.5 min to 12.5 min i n the following order: beta-cn BB<A(1)B<A(1)C<A(2)B<A(1)A(1)<A(1)A(2)/ A(2)A(2). Milk with the kappa-Cn genotype AC clotted significantly slo wer than milk of any other kappa-Cn genotype. The RCT for the kappa-Cn genotypes increased in the following order: kappa-Cn BE<BB<AB<AE/AA<B C<AC. A significant relationship was also observed between the genotyp es of the beta-Cn, kappa-Cn and beta-Lg locus and the curd firmness. F or the beta-Cn genotypes only the differences between the curd firmnes s of milk with the beta-Cn BB-type and all other beta-Cn genotypes wer e significant. The kappa-Cn locus had the greatest effect on curd firm ness. Milk with the rare type kappa-Cn AE and BE was characterized by a very weak curd as was the curd made from kappa-Cn AA-milk, whereas k appa-Cn BB and BC-milk formed a very strong curd. The curd firmness of kappa-Cn AB- and AC-milk took an intermediate position. The beta-Lg g enotype also had a significant influence on the curd firmness. Milk fr om cows with the beta-Lg genotype BB showed a significantly higher A(R )-value than the genotypes beta-Lg AA, AB, AD and ED, whereas the A(R) -values for the other genotypes were very similar.