Study of Physical and Chemical Properties of the Adsorption “Membrane” around the Fat Globules in Cream. I. the Effect of the Adsorption “Membrane” of Synthetic Creams on Curd Tension of Cow's Milk

Summary Synthetic creams were made by emulsifying pure butter fat in the following aqueous sols: Dried whey sol, skim milk powder sol, calcium caseinate sol, gelatin sol, and tissue fibrinogen (lecithoprotein material) sol. The resulting synthetic creams were dispersed in normal milk plasma to obtain "remade" whole milks which were separated to give the corresponding "remade" skim milks and "remade" creams. The "remade" buttermilks were prepared by churning the "remade" creams. The fat globule adsorption "membrane" from dried whey cream, when released in buttermilk by churning the "remade" cream, exerted a profound effect on the rennet clot of the buttermilk. This effect was characterized either by the total inhibition of clotting or by the formation of very soft and fragile curd with little or no syneresis. A still more pronounced effect of the fat globule adsorption "membrane" on the rennet clot of "remade" buttermilk was found in the cases of gelatin cream and calcium casemate cream. "Remade" buttermilk from either of these creams invariably had no curd tension whatever and did not show even a visible coagulation on the addition of rennet. Although the addition of CaCl 2 , in rather large concentration, to buttermilks from the above creams restored somewhat their curd tension values, it has been shown by calcium and phosphorus analysis of the papain clot whey from the buttermilk of gelatin cream that the ultimate cause of this phenomenon of low curd tension, or non-coagulation, is more deep seated than that of lack of Ca ++ This being the case the CaCl 2 cannot be regarded as restoring the clotting properties in these experiments simply because it supplies Ca ++ to the system. The fat globule adsorption "membrane" from skim milk powder cream had only a relatively slight curd tension reducing effect on its "remade" buttermilk as compared with the effect exerted by the adsorption "membrane" from dried whey cream. The fat globule adsorption "membrane" derived from tissue fibrinogen (lecithoprotein complex material) had no effect on the curd tension of "remade" buttermilk. The phenomenon of total non-coagulation with rennet, or abnormally low curd tension of "remade" buttermilks, was always accompanied by considerably lower pH and lower surface tension of the "remade" buttermilks as compared with the pH and surface tension of their respective whole and skim milks. The factors involved in the decreased pH and lowered surface tension and their relation to the loss of clotting ability have been the subject of extensive further study by us. 7 7Part of this further study was a post doctorate work and was done by the senior author in the Dairy Chemistry Laboratory of the Dairy Industry Division, University of California, Davis develop as the result of hydrolysis of esters of these fat acids by "natural" esterase in the milk, the normal clotting properties of the milk are inhibited. Other important aspects of these findings will be presented in detail in the papers to follow. It may be stated briefly at this time that the total destruction of the clot in the case of certain "remade" buttermilks was found to be due largely to the liberation of certain fat acids and their adsorption by the calcium caseinate, preventing the clotting of the milk by rennin. It was also found that curd tension is not seriously affected, if affected at all, by butter fat hydrolysis except when an unnatural emulsifying agent is substituted for the natural fat globule adsorption "membrane" of cow's milk. Furthermore, when lauric, myristic or palmitic acid is added directly to skim milk or allowed to develop as the result of hydrolysis of esters of these fat acids by "natural" esterase in the milk, the normal clotting properties of the milk are inhibited. Other important aspects of the se findings will be presented in detail in the papers to fallow.