D-Amino Acids in Milk as Related to Heat Treatments and Bacterial Activity

The occurrence in milk of free and bound D-amino acids was investigated by chiral phase GC and related to heat treatments and bacterial activity. Significant amounts of free D-alanine (D-Ala), D-aspartic acid (D-Asp), and D-glutamic acid (D-Glu) have been found in raw cow milk but not in human milk. The amount did not increase by pasteurization, UHT treatment, and sterilization. In contrast, the content of D-Ala in raw milk samples increased during cold storage at 4°C, and it was suggested that D-Ala may be considered as an indicator of bacterial milk contamination. The presence of D-Ala, D-Asp and D-Glu detected in milk protein hydrolysates was due to hydrolysis conditions.