Effect of Milk Proteins Contents on Rennet Coagulation Properties

To study milk proteins’ composition, among cattle breeds raised in Estonia, and their effect on rennet coagulation properties, 516 milk samples were taken from 54 cows once a month during one full lactation period. The samples were analysed to determine the contents of major components for αs1-, αs2-, β- and κ- casein and β- lactoglobulin contents as well as the rennet coagulation parameters. The contents of the milk proteins and the relative contents of different caseins in total casein were significantly influenced in the sampling month and the month of lactation. The content of αS2- and κ-Cn, and the relative content of αS1- and κ-Cn in total casein, and also the ratios of κ-Cn/β-Cn and κ-Cn/αS-Cn were significantly higher in milk from Estonian Red cows than that for Estonian Holstein cows.

All the studied milk proteins contents, except αS2-Cn influenced the milk coagulation parameters. Milk rennet coagulation parameters correlated significantly also with other milk compositional characteristics. Rennet coagulation time (RCT) shortened and curds became firmer with increases in the contents of milk fat, lactose, and calcium, and with decreases in milk pH. However, the major milk components correlated significantly with either or both RCT and curd firmness (E30). The concentrations of the major milk components (as indicated by r2) could account for variations, up to a maximum of 12%, in these parameters with two exceptions. pH accounted for a 19% variation in RCT and calcium content accounted for an 18% variation in E30. At the same time all studied the milk proteins contents, except αS2-Cn, accounted for a variation of rennet coagulation time of up to 17% and for a variation of curd firmness even up to 41%.

The increase in milk protein, casein including their different studied fraction contents, and also the casein number leads to a shortening of the milk rennet coagulation time and forming a stronger curd. The milk rennet coagulation time was strongly influenced by casein number, casein, αS1- and β-Cn contents and curd firmness by casein, αS1-, β-, and κ-Cn, and β-Lg contents. The contents of all studied milk proteins were significantly lower for noncoagulated (E30=0 mm) and poorly coagulated (E30=1…20 mm) milks as compared to normally and well coagulated milks (E30>20 mm).

Milk formed a stronger curd when the relative content of αS1- and β -Cn in total casein were lower or the relative content of κ-Cn in total casein was higher. Also the higher value of κ-Cn/β-Cn and κ-Cn/αS-Cn ratios had a positive effect on curd firmness. An increased rate of β-Cn in total casein has a negative effect on milk rennet coagulation properties.