INCREASE OF FREE SIALIC-ACID AND GELATION IN UHT MILK

The free sialic acid content of 4 pasteurised and 3 UHT-treated commer cial milks has been measured and compared. The sialic acid content of the UHT milks was found to be 50%-215% greater than comparable fresh p asteurised samples. One sample of UHT milk was kept 17 months after th e expiration date. The sialic acid content in this sample was approxim ately 3-fold higher than the equivalent fresh UHT sample. Electrophore tic analysis of pH 4.6 insoluble casein from the UHT milks showed evid ence of proteolysis of the kappa-casein component into para-kappa-case in and the glycomacropeptide. It is suggested that residual plasmin an d proteases from psychrotrophic bacteria slowly cleave kappa-casein in the UHT milks, thus releasing the sialic acid-containing glycomacrope ptide and ultimately resulting in gelation of the milk. The determinat ion of free sialic acid appears to be a useful method to monitor the q uality of UHT milk during storage.