Effect of Ultra-High-Temperature Steam Injection Processing on Sulfur-Containing Amino Acids in Milk

Abstract

Raw skim milk was processed by a modified No-Bac Unitherm IV System (Cherry-Burrell Corp.) at 143.degree. C for 8 s, vacuum cooled to 71.degree. C, collected and cooled to 4.degree. C. Raw and ultra-high-temperature processed skim milks were oxidized with performic acid, hydrolyzed with hydrochloric acid and analyzed for cysteine and cystine (as cysteic acid) and methionine (as methionine sulfone) on a Beckman Automatic Amino Acid Analyzer. A loss of .apprx. 34% of these amino acids was observed in ultra-high-temperature processed skim milk. Sulfhydryl and disulfide groups determined with Ellman''s reagent [5,5''-dithiobis(2-nitrobenzoic acid)] indicated no free sulfhydryls in raw skim milk and .07 mmol/l in ultra-high-temperature processed skim milk. Loss in total sulfhydryl and disulfide groups was .apprx. 16% in ultra-high-temperature processed milk. Volatile compounds were detected by odor in the vapor collected from the vacuum chamber, but they could not be identified. Amino acid analysis of milk deposit collected from the injection section revealed .09 .mu.g of cysteine and cystine (as cysteic acid)/g of deposit and no detectable methionine.