The Origin of Sulfhydryl Groups in Milk Proteins and their Contributions to “Cooked” Flavor

Abstract

Use of an argentometric-amperometric titration procedure has revealed that the only source of -SH groups in skimmilk is the serum proteins. Casein and protein-free milk serum were found devoid of such groups. Praetionation of the serum protein material into a number of components by (NH4)2 SO4 additions and pH adjustments revealed that β-lactoglobulin can account for practically all the -SH groups present. Study of the contributions of various major serum protein fractions to heat-induced cooked flavor in skimmilk demonstrated βlacto-globulin to be responsible for the flavor. Conversion of -SH groups to H2S as a result of heat treatment may explain, in a general way, the mechanism whereby β-lactoglobulin gives rise to cooked flavor. The AgNO3 titration, when conducted in aqueous medium, appears to measure the same quantity of -SH groups in heated milk as nitroprusside and thiamin disulfide. When conducted in alcoholic medium the total number of -SH groups capable of activation by heat treatment presumably can be determined in unheated skimmilk. The argentometric-amperometric method gives values for -SH content of slightly less than half of those obtained with o-iodosobenzoate for both skimmilk and β-lactoglobulin. © 1952, American Dairy Science Association. All rights reserved.