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Amperometric Titration of Sulfhydryl and Disulfide Groups in Milk Proteins¹

Department of Food Technology, University of Illinois, Urbana

ABSTRACT

Sulfhydryl (-SH) and disulfide (-SS-) groups in milk proteins were estimated amperometrically with silver nitrate. Cysteine, ß-lactoglobulin, milk, and whey were titrated at different pH levels in sodium acetate, ammonium nitrate, and tris buffer solutions. This was done to study the fate of -SH and -SS- groups in some fluid milk products heated at high temperatures and their possible relationship to flavor and physical defects that appear during storage. Cysteine yielded results 5 to 50% higher than the theoretical values, depending on the titrating conditions. The cysteine content of ß-lactoglobulin in sodium acetate buffer at pH 10.25 was 95.7% of the theoretical amount. Adding 1% sodium lauryl sulfate to the titrating medium made more -SH groups available, whereas 8 M urea decreased them. Decreased titers of -SH groups in heated milk coincided with the denaturation of serum proteins and they were oxidized to -SS- groups. Disulfide was stable at 95 C for 30 min. During sterilization of concentrated milk, titratable -SH decreased and -SS- increased.

¹ This research was supported by U. S. Public Health Grant EF-141.