APPLICATION OF REVERSED-PHASE HPLC TO THE STUDY OF PROTEOLYSIS IN UHT MILK

Instability of UHT milk as well as the appearance of off flavours are partly attributed to the proteolytic activity of residual or reactivated heat-stable enzymes of both native and bacterial origin (Law, 1979; Mottar et al. 1979; McKellar et al. 1984; de Koning et al. 1985). Among the methods so far employed for assessing the extent of protein degradation in UHT products, PAGE has frequently been used to follow the hydrolysis of individual protein fractions and to detect the appearance of specific casein breakdown bands. This technique is insufficient to resolve low molecular mass peptides and lacks adaptability to automatic procedures. Reversed-phase HPLC (RP-HPLC) has proved to be a reliable method with high resolving power for separating peptides from casein hydrolysates (Gonzalez-Llano et al. 1990). The present paper describes the use of RP-HPLC to investigate the degree of protein breakdown during storage of UHT milk. In addition, RP-HPLC chromatograms of milk samples are compared with chromatograms of plasmin and psychrotrophic bacterial enzyme digests of casein as a contribution to knowledge of the significance of the presence of peptides in UHT milk. © 1993, Proprietors of Journal of Dairy Research. All rights reserved.