Proteolysis of casein micelles by heat-stable protease secreted by Serratia liquefaciens leads to the destabilisation of UHT milk during its storage

Serratia liquefaciens is a psychrotrophic species, frequently found in raw milk, which secretes Ser2, a heat-resistant protease. Involvement of this species in UHT milk destabilisation was investigated in the present study. Microfiltered milk was inoculated independently with strains S. liquefaciens L53 or L64. Then, UHT treatment was performed and stability of the corresponding UHT milk was investigated during three months of storage. The residual proteolytic activity of strain L53 led to destabilisation of UHT milk, with sedimentation and formation of aggregates. Hydrolysis of casein micelles was confirmed by the increase in the content of non-casein nitrogen and the identification of numerous peptides coming from the four caseins using mass spectrometry. For strain L64, no visual and biochemical alteration were found. This study showed that Ser2 resists UHT treatment and could be a cause of UHT milk destabilisation; however, this destabilisation by S. liquefaciens was strain-dependent.