Proteolysis and physicochemical changes in milk on storage as affected by UHT treatment, plasmin activity and KIO3 addition

Physicochemical properties, proteolysis and storage stability of samples of raw milk, UHT milk, UHT milk with added KIO3 and UHT milk in which the ori ginal level of milk plasmin activity had been restored by the addition of e xogenous plasmin were compared. UHT treatment of milk resulted in associati on of denatured beta-lactoglobulin with micelles (as demonstrated by immuno gold localisation combined with electron microscopy), and greatly reduced p lasmin activity, while this milk was extremely stable over storage. Preserv ed raw milk was unstable over storage, and showed extensive proteolysis, in cluding clear evidence of non-plasmin proteolytic enzymes. The microscopic appearance of micelles in raw milk changed considerably over storage, with the formation of chains of small submicellar particles, and, after 84 d of storage, cross-linked aggregates of micellar particles were recovered from the milk. UHT milk with added KIO3 behaved somewhat like raw milk during st orage, showing extensive plasminogen activation, rapid proteolysis and form ation of sediments at a similar time, and of similar appearance, to those s een in raw milk. The addition of plasmin to UHT milk after heating reduced the stability of the milk, increased proteolysis, and lead to the early for mation of sediments. The results of this study suggest strongly that plasmi n activity is a major influence on the storage stability of UHT milk. (C) 1 999 Elsevier Science Ltd. All rights reserved.