714. The relation between the chemical composition of milk and the stability of the caseinate complex: III. Coagulation by rennet

Abstract

1. The variation in the time required for the coagulation of milk by rennet and the relationship between milk composition and renneting time were examined.2. The renneting times of herd bulk milks varied over the relatively narrow range of 3·2–5·4 min., whereas the renneting times of milks from individual cows varied from 1·4 to 12·9 min. and two did not coagulate.3. In general, milk quick to coagulate gave a firm curd (early lactation milk) and milk slow to coagulate gave a soft curd (late lactation and subclinical mastitis milk).4. The renneting time of milk was related to the stage of lactation of the cow. In early lactation, renneting times were short and as lactation advanced there was a progressive increase in renneting time; the increase became more pronounced towards the end of lactation.5. The property of milk most closely related to renneting time was acidity. As pH increased from 6·4 to 7·2, there was a curvilinear increase in renneting time from 1·5 to 13 min. Titratable acidity was related to renneting time in the opposite way to pH.6. Although milk containing a relatively high concentration of ionized calcium coagulated quickly with rennet, the renneting time of milks containing average, or less than average, amounts of ionized calcium was not related to their content of this constituent.7. Weak relationships between the concentrations of other milk constituents and renneting time could be attributed to the interrelations of the concentrations of these constituents and pH.