Characterization of milk coagulation ability in bulk milk samples

Basic requirements for cheese-making production are milk coagulation properties (MCP), which can be measured as: (i) rennet coagulation time (RCT, min), (ii) curd-firming time (k20, min), and (iii) curd firmness (a30, mm). The aim of this study was to investigate sources of variation of coagulation properties in bulk milk from dairy cattle farms of northern Italy. A total of 1,570 samples were collected from 436 herds that delivered milk to 4 dairy cooperatives, and analyzed for quality traits and MCP. About 4% of total samples did not coagulate within 30 min. Rennet coagulation time, k20, and a30 averaged 18.83 min, 6.85 min, and 26.97 mm, respectively. Milk coagulation properties were analyzed through a general linear model which included the fixed effects of dairy cooperative, herd nested within dairy cooperative, year and season of sampling, and milk quality traits, inserted as class factors in the model. Dairy cooperative and herd effects were the major sources of variation in explaining the variation of MCP (P < 0.001). Season of sampling was statistically significant for RCT and k20 (P < 0.05), showing the best results in summer, whereas year of sampling was statistically significant only for RCT (P < 0.001). Rennet coagulation time was influenced by titratable acidity and bacterial count (P < 0.05), and k20 and a30 by casein content and titratable acidity (P < 0.01).