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The Coagulation Temperature of Milk as Affected by pH, Salts, Evaporation and Previous Heat Treatment

Department of Dairy Industry, University of Wisconsin

ABSTRACT

1. Sealing the milk in small glass tubes and subjecting them to a rocking motion in an oil bath was found to be a convenient method for studying the heat coagulation of milk. The milk came up to the temperature of the oil bath in less than two minutes.
   
2. The relationship between pH and coagulation temperature of skim milk formed a characteristic coagulation curve. The milks were very sensitive to pH changes within two pH ranges; from approximately pH 6.4 to 6.2 and to a lesser extent between pH 5.4 and 5.2. The maximum coagulation temperature of all the milks studied was above 152° C. The coagulation temperature of milks initially less stable than this was increased in heat stability by adding acid.
   
3. The presence of added salts displaced the coagulation curve; at pH values below 6.4, phosphate displaced the curve into the more stable region whereas calcium had the opposite effect. The effect at pH values above 6.4 depended upon the milk in question. The milks were initially destabilized by adding phosphate and the subseqtient addition of acid restabilized the samples. The addition of small amounts of calcium stabilized the milks but larger additions destabilized the milks.
   
4. Concentrating and preheating displaced the coagulation curve of the milks in the same general manner, but the effect of concentrating was much greater than the effect of heat treatment alone. The presence of added salts displaced the coagulation curve for concentrated and preheated milks in a manner similar to that of the original milk.
   
5. The pH of the milk was decreased at increased temperatures; the presence of added calcium increased the effect of temperature changes while phosphate had the opposite effect. A more detailed study of various physico-chemical equilibria in milk at elevated temperatures would therefore appear to be a definite contribution to the problem of the heat stability of milk.
   
6. These conclusions are based on tests involving definite conditions, such as heating and holding so that coagulation occurred in two minutes, and do not necessarily represent the behavior of milk under other conditions.