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

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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 protein deposited on the heating surfaces, indicated that 6-15% of this amino acid was lost during a heat treatment of 100.degree. C for 30 min, presumably by volatilization. A smaller fraction was lost during ultra-high-temperature processing. Comparison of this 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 that a larger fraction was oxidized during storage at room temperature. These concentrations of reactive sulfhydryl groups in sterilized milk were correlated with undesirable cooked flavor and possibly could contribute to instability of milk protein through disulfide interchange reactions.
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