Sulfhydryl and Disulfide Groups in Skim Milk as Affected by Direct Ultra-High-Temperature Heating and Subsequent Storage

Abstract

Concentrations of “reactive” and total sulfhydryl groups and disulfide bonds were determined in raw and ultra-high-temperature sterilized skim milk and compared to values for conventional laboratory heat treatments with lower temperatures and longer times. The method of analysis, which represents a modification of previously reported methods, gave results agreeing with theoretical values calculated from the protein composition of milk. Analyses for total half-cystine, which included that protein deposited on the heating surfaces, indicated that 6 to 15% of this amino acid was lost during a heat treatment of 100C for 30min, presumably by volatilization. A smaller fraction was lost during ultra-high-temperature processing. Comparison of our data for direct heating with that previously reported for indirect heating suggests that less whey protein is “denatured” by the direct heating method. Studies of sterilized skim milk stored at refrigeration or room temperature suggested that the “reactive” sulfhydryl groups oxidized more rapidly and also that a larger fraction was oxidized during storage at room temperature. These concentrations of “reactive” sulfhydryl groups in sterilized milk have been correlated with undesirable “cooked” flavor and possibly could contribute to instability of milk protein through disulfide interchange reactions.