The Rôle of Surface-Active Constituents Involved in the Foaming of Milk and Certain Milk products. I. Milk Proteins

Abstract

An apparatus is described for accurate measurements of the foaming properties of milk. Experimental data have revealed that the subsidence of skim milk foams cannot be denned by a single equation at temperatures between 5 and 55° C. For this reason, a unit called a “half-volume time” has been proposed and used to compare the stabilities of static and dynamic foams. The unit can be converted by a given equation to give average life of a foam bubble, if the foam follows a logarithmic subsidence. The major surface-active proteins of milk, including casein, α-β-laet-albumin and lactoglobulin, have been separated, purified and studied for their foaming properties. A new procedure for isolating undenatured casein with a lipid content as low as 0.06 per cent has been introduced. The method is based on super-centrifuging skim milk, precipitating the casein at a low temperature with 0.1 N HC1, washing, freezing, and dehydrating the frozen casein by pervaporation. The foam “half-volume time” values of milk protein solutions in M/30 phosphate buffers at the normal hydrogen ion concentration of milk varied between 2 and 3000 seconds (or over), depending upon the kind of protein, concentration, method of isolating the protein, temperatures, and the amount of milk lipids present in association with the protein molecule or added in the form of an emulsion to the solution. Studies of these variables are reported and some theoretical explanations advanced. It is shown that calcium casemate solutions of a concentration equivalent to that of milk and prepared by the present procedure have high foam stabilities at temperatures below 40° C. Lactalbumin solutions foam well at all temperatures, but lactoglobulin solutions show no appreciable foaming. The foam depressing action of milk fat is shown to be greater at temperatures over 15° C. with calcium caseinate solutions and at temperatures lower than 35° C. with lactalbumin solutions. © 1944, American Dairy Science Association. All rights reserved.