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The Relationships of Concentration and Time to the Temperature of Coagulation of Evaporated Skim and Whole Milk

Research Laboratories of the Dairy Division, United States Department of Agriculture, Washington, D. C.

ABSTRACT

In order to make the results obtained upon various milks comparable with respect to time and temperature of coagulation it is necessary to calculate all concentrations upon the basis of the proportion of solids not fat to the total weight minus the weight of the fat. While calculations of solids upon the basis of total solids (including fat) will hold for all milks with approximately the same fat content, any variations in the percentage of fat will introduce error into the data of true concentration of proteins and consequently will introduce error into coagulation data. A calculation of results upon the basis suggested will give a true basis for the comparison of the stability of evaporated milks toward heat, and make possible a comparison of milks from day to day, as well as of skim and whole milks, whatever may be their fat content. The presence of a high fat content in a milk will raise its coagulating temperature slightly over that of skim milk from the same sample, due in all probability to the absorption of heat by the fat present in the whole milk. It would seem therefore that in studying the mechanism of coagulation the most reliable results would be obtained with skim milk.

The relation of the coagulation temperature to the concentration of evaporated milk, between the concentrations of 16 and 26 per cent, has been determined. The greater the concentration the greater the variation between skim and whole evaporated milk from the same sample.

In the region of concentration which is of most interest (18 to 20 per cent) the change of coagulation temperature for every 1 per cent change in concentration is approximately 1.5°C. In higher concentrations the change is slightly more.

The relation between concentration and coagulation temperature in a milk of poor quality seems to parallel that in a milk of good quality.

The relation of the temperature of coagulation to the time of coagulation has been determined between ten and sixty minutes and found to approximate very closely a logarithmic relation, with respect to time. Knowing the time and temperature of coagulation of a milk of certain concentration, therefore, one can approximate the temperature of coagulation for any other period of time. The same relationship between time and temperature seems to hold for milks of poorer grade.