The Rôle of Surface-Active Constituents Involved in the Foaming of Milk and Certain Milk Products. II. Whey, Skimmed Milk, and Their Counterparts

The foaming properties of some milk products represent a unique colloidal phenomenon; that is, the foam stability decreases to a minimum, as the temperature increases, after which it again increases.The first report on the effect of temperature was made by Siedel (22).He found that the quantity of foam depends on the temperature of the milk and that the nature of the foam at low temperature is different from that at high temperature.Siedel's studies did not go beyond 35 ° C., but he noticed an inflection in the stability of the foam.Leete (11) and Sanmann and Ruehe (18) studied the effect of temperature on milk foam.Separated milk and milk foamed least between 20 and 30 ° C. according to the former, and at 27 ° C. according to the latter.Both investigators agreed on the increase in the foam at temperatures beyond 30 ° C. but disagreed on the temperature at which the maximum foaming appeared.There has been considerable speculation as to the cause of this minimum foaming ability of milk.Rahn and Sharp (15) raised the question whether these kinks in the foam-temperature curve indicate the presence of a specific foam-producing material or of a stabilizing substance in the foam.Ansbacher and co-workers (1) reported that the foam-producing substance eluted from casein showed a critical agglomeration at the temperature of the minimum foaming.They did not present data regarding the foaming stability of this highly foaming constituent of milk.Leviton and Leighton (12) tried to explain this kink on the basis of the work of King (10), who showed that the portion of the surface of milk covered with a film of fat increased with a rise in temperature from 10 to 20 ° C.They postulated that the decrease in the foaming power might be explained on the basis of the increase in the tendency of the fat to spread as the temperature rose.No comment was made on the further increase of foam formation with increasing temperature.Holm (9) explained the minimum foaming of milk as a result of variation in those properties of the solution which evidence themselves in viscosity