Effects of the CSN1A(G) allele on the clotting time of cow milk and on therheological properties of rennet-curd

The aim of this research was to study the effects of the CSN1A(G) allele on the main rennet coagulation properties of milk. The study was carried out on individual milk samples with low alpha (s1)-casein obtained from 19 Ital ian Brown cows heterozygous for the CSN1A(G) allele (seventeen CSN1A BG and two CSN1A CG) from four herds in the province of Parma (Italy). Control co ws (sixteen CSN1A BE and three CSN1A BC) giving milk with normal alpha (s1) -casein levels were chosen from within the same herds in order to establish pairs of cows with identical environment and management conditions, and co mparable lactation stages and numbers. Individual milk samples from single pairs of cows with somatic cell counts and lactose and chloride levels with in the normal ranges were collected and analysed in parallel. Rennet coagul ation properties of milk were analysed using Formagraph and Gel Tester. Mil k from low alpha (s1)-casein cows was characterized by lower casein content , lower titratable acidity and a higher proportion of kappa -casein in tota l casein. The clotting time of this milk was similar to 23 % lower than tha t obtained with milk from normal alpha (s1)-casein cows. Rennet curd from l ow alpha (s1)-casein milk was obtained more rapidly and had a higher final firmness: curd-firming time was similar to 35 % lower and curd firmness mea sured 30 min after rennet addition was similar to 27 % higher compared with that for normal alpha (s1)-casein milk. In addition, curd from low alpha ( s1)-casein milk had a higher resistance to compression. These results sugge st that, although a role for the CSN2 locus cannot be definitely excluded, the CSN1A(G) allele can considerably affect the main rennet coagulation pro perties of milk.