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The Effect of Various Degrees of Forewarming upon the Heat Stability of Milks of Different Solids Concentrations

Division of Dairy Research Laboratories, Bureau of Dairy Industry, Agricultural Research Administration, U. S. Department of Agriculture

ABSTRACT

1. Whole milk quickly forewarmed at temperatures between 110° C. (230° F.) and 150° C. (302° F.) and held for the optimum time period for that milk (
   to 5 minutes) exhibited much greater heat stability after concentration to 26 per cent solids content, or higher, than did those samples of the same milk forewarmed to 95° C. (203° F.) for 10 minutes in the conventional manner.
   
2. The heat stability of concentrated milks which before concentration were forewarmed at high temperatures and of those forewarmed at 95° C. for 10 minutes decreased at the same rates as the solids content was increased up to 25 per cent. At higher solids concentrations the heat stabilities of the milks forewarmed quickly at the high temperatures more closely approached those of the control samples forewarmed at 95° C. (203° F.) for 10 minutes. Finally a concentration (32 to 37 per cent) was reached, different for each milk and each heat treatment, at which the heat stabilities of the test and control samples were about the same.
   
3. The general heat stability relationships of milks subjected to different forewarming treatments were not affected by the development of acidity in fresh milk or by the addition of stabilizing salts to concentrated milks; the difference in stability caused by acid or salts was one of degree and it affected the general heat stability level rather than the stability interrelationships of milks forwarmed one way or another.
   
4. From a heat-stability viewpoint it should be commercially possible to manufacture evaporated milk of approximately 32 per cent total solids content, A good grade of milk should be quickly forewarmed to about 120° C. (248° P.) and held 3 to 4 minutes, then drawn into the vacuum pan and handled in the usual way. The optimum forewarming treatment which will give the heat stability and body desired in the finished product can be best determined through experience with high-temperature-forewarming procedures on each individual milk supply.