Proteolysis of milk proteins by AprX, an extracellular protease identified in Pseudomonas LBSA1 isolated from bulk raw milk, and implications for the stability of UHT milk

UHT milk made from milk contaminated by Pseudomonas LBSA1 destabilised during storage. Sedimentation of UHT milk was observed; zeta potential of casein micelles decreased, while contents of non-casein nitrogen and non-protein nitrogen increased. Pseudomonas LBSA1 produced an extracellular protease that hydrolysed caseins but not whey proteins; this was identified as AprX, a thermoresistant protease belonging to the serralysin family. This protease showed a broad range of pH activity (pH 6 to pH 10) and an optimal temperature of activity of 40 °C. Peptides released from purified αS1-, β- and κ-caseins were determined by tandem mass spectrometry. The identified cleavage sites did not reveal a strong specificity of the extracellular protease. However, the presence of basic or aromatic amino acid residues in the P1 position had a positive influence on cleavage in comparison with acidic amino acid residues or proline.