Ser2 from Serratia liquefaciens L53: A new heat stable protease able to destabilize UHT milk during its storage
Résumé
The heat-stable protease Ser2 is secreted by the species Serratia liquefaciens, a psychrotrophic bacteria
frequently found in raw milk. To understand the physicochemical modifications of casein micelles
induced by Ser2 and to confirm its implication in UHT milk destabilization, the enzyme was purified
and added to microfiltered raw milk before UHT treatment. UHT milk destabilization was investigated
during 90 days of storage. A visual destabilization appeared after 8 days of storage with the presence
of sediment. Zeta potential increase and formation of aggregates were observed during the storage.
Using tandem mass spectrometry, numerous released peptides from the four caseins were identified at
the end of storage. Caseins were hydrolyzed in the preferential order b- > as1- > j- > as2. No specific peptidic
hydrolysed bond was detected. The present study confirmed that the presence of the protease Ser2
in raw milk can be one of the main causes of UHT milk destabilization.