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Effects of Heat Treatment and Posttreatment Holding Time on Rennet Clotting of Milk¹

Department of Nutrition and Food Sciences, Utah State University, Logan 84322

ABSTRACT

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.

¹ Utah Agricultural Experiment Station Journal Paper No. 2986. Approved by the Director.

² Department of Animal Science, University of Kentucky, Lexington 40546.