Effects of adding potassium iodate to milk before UHT treatment. ii. iodate-induced proteolysis during subsequent aseptic storage.
The addition of potassium iodate to milk at 9.1 mM before UHT treatment resulted in rapid breakdown of alpha s- and beta-casein during subsequent aseptic storage. Maximum rates of proteolysis were observed at storage temperatures of 37-45 degrees C, but the reaction was strongly inhibited by storage at 55 degrees C and by increased holding time at 140 degrees C during the UHT sterilization. Iodate-induced proteolysis of purified alpha s1-and beta-casein was detected only with solutions in the serum phase of raw milk; no proteolysis occurred with solutions in 0.1 M-phosphate buffer (pH 6.7) or in milk ultrafiltrate, irrespective of whether whey proteins and lactose were also added. Thus, it appears that iodate increased the activity of one or more proteolytic components which were present in milk and were unable to pass through an ultrafiltration membrane. However, it is unlikely that iodate acts by increasing the activity of proteinases produced by contaminant bacteria; the presence of iodate did not affect the activity of a proteolytic enzyme isolated from Pseudomonas fluorescens PM-1. Furthermore, iodate promoted protein breakdown during storage of milk drawn aseptically from the cow and subsequently UHT processed. It is suggested that iodate increased the activity of native milk proteinases, other than plasmin which was inactivated by UHT treatment, possibly by preventing thiol-disulphide exchange reactions during the heating process.