Chemical and Nutritional Changes of Milk Powder Proteins under Various Water Activities

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Milk powders (commercial skim milk and modified milk powders) were stored at 30°C and 40°C for one month under various water activities (Aw), 0.23-0.82. After storage under Aw 0.57 and 0.80, they turned a dark brown color, and the contents of methionine and lysine had significantly decreased: methionine had been oxidized to methionine sulfoxide and lysine had changed to an unavailable form. The contents of arginine, tyrosine and tryptophan had also decreased, as had the contents of leucine and histidine in the skim milk powder. The tryptic digestibility of the milk powders had markedly decreased, and both the chymotryptic and peptic digestibilities of the skim milk powder had also decreased. The samples lost the property of being clotted by chymosin treatment. No significant change was observed with casein under these conditions, nor with milk powders under dry conditions and Aw 0.23.
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