Effects of Heat Treatment and Posttreatment Holding Time on Rennet Clotting of Milk

Samples of raw whole milk were heated at 25, 50, and 75°C for 0, 30, 60, 120, and 240 min.After heating, each sample was subdivided into portions that were maintained at 0°C for 0, 30, 60, and 120 min.In a second experiment samples of milk were heated at 25 and 50°C for the same intervals as before but at 75°C for only 5, 10, 15, and 20 min.Samples heated at all three temperatures were kept at 0°C for 30 min after heat treatments.Following heating, pH of each sample was measured, and rennet clotting properties of milk at a constant rennet concentration were determined by the Formagraph instrument.Cold storage at 0°C for up to 2 h did not have a significant effect on pH or on rennet-clotting properties of milk.Increase from 25 to 50°C and prolonged heating reduced pH and clotting time.Curd firming rate and cutting time were affected by the time milk was heated.When milk was heated at 75°C, clotting time, curd firming rate, and cutting time of curd were reduced markedly.Milk heated at 75°C for 30 min or longer did not clot.