Proteolysis of milk proteins by AprX, an extracellular protease identified in [i]Pseudomonas[/i] 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. Sedimentationof UHT milk was observed; zeta potential of casein micelles decreased, while contents of noncaseinnitrogen and non-protein nitrogen increased. Pseudomonas LBSA1 produced an extracellularprotease that hydrolysed caseins but not whey proteins; this was identified as AprX, a thermoresistantprotease belonging to the serralysin family. This protease showed a broad range of pH activity (pH 6 topH 10) and an optimal temperature of activity of 40 C. Peptides released from purified aS1-, b- and kcaseinswere determined by tandem mass spectrometry. The identified cleavage sites did not reveal astrong specificity of the extracellular protease. However, the presence of basic or aromatic amino acidresidues in the P1 position had a positive influence on cleavage in comparison with acidic amino acidresidues or proline