Stability of fat globules in UHT milk during proteolysis by the AprX protease from Pseudomonas fluorescens and by plasmin

Fat separation is a limiting factor for the shelf life of UHT milk. It may be promoted by the proteolysis of fat surface-adsorbed proteins (FSAP) by proteases that remain active after UHT treatment. The aim of this research was to explore the relationship between the proteolysis of FSAP and fat destabilization. In this study, we developed a full-fat UHT milk-based model system and added either the major bacterial protease AprX from Pseudomonas fluorescens or the major native milk protease plasmin at high levels to induce fast destabilization of the milk fat globules. We monitored changes in physical properties and FSAP composition, and structural changes in fat globules, over 24 h. Our results showed that AprX-induced sedimentation as a result of the flocculation of fat globules, and plasmin induced cream to float as a result of the coalescence of fat globules. This study confirmed that AprX and plasmin can both lead to fat destabilization in full-fat UHT milk, and it provides insights in the underlying mechanisms.