The effect of different technological treatment upon the quality of milk protein

The aim of the present investigation was to study the effect of industrial processing of milk upon the chemical indices, characterizing the biological value of milk protein. Specific processes for milk industry, such as heat treatment, fat reducing and lactic acid-fermentation were traced out. The milk products studied were: raw fresh milk, nonfat milk, full fat dried milk "Vitalact 1" and dietetic curds. The processing technology of "Vitalact 1" was found to be the least affecting with regard to the protein structure. With the exception of the aromatic amino acids, isoleucine and threonine, whose quantities were reduced by not more than 5%, the percentage of all the other essential amino acids was found to be increased as compared with their initial values in raw milk. The tryptophan relative increase attained as high a value as 10.9%. The quantity of the available lysine, which in fresh milk was 1.49 g/16 g N, raised almost twice, while that of sulfur-containing amino acids was slightly reduced (7.5%) as compared with the raw milk values. It was established that fat-reducing had a manifest negative effect both on the spectrum of amino acids and upon the availability of lysine and sulfur-containing amino acids. The lactic acid-fermentation had a positive compensatory effect, although not sufficient to ensure attainment of the initial raw milk values. Fat-reducing, repeated heat-treatment and fermentation provoked substantial changes in electrophoresis mobility of milk proteins.