Physicochemical properties, proteolysis and storage stability of samples of raw milk, UHT milk, UHT milk with added KIO3 and UHT milk in which the original level of milk plasmin activity had been restored by the addition of exogenous plasmin were compared. UHT treatment of milk resulted in association of denatured beta-lactoglobulin with micelles (as demonstrated by immunogold localisation combined with electron microscopy), and greatly reduced plasmin activity, while this milk was extremely stable over storage. Preserved raw milk was unstable over storage, and showed extensive proteolysis, including 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 storage, showing extensive plasminogen activation, rapid proteolysis and formation of sediments at a similar time, and of similar appearance, to those seen 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 formation of sediments. The results of this study suggest strongly that plasmin activity is a major influence on the storage stability of UHT milk. (C) 1999 Elsevier Science Ltd. All rights reserved.