Proteolytic and Biofilm-forming Capacity of Bacteria Isolated from a UHT Milk Processing Plant

The destabilization of milk during processing, caused by bacteria, represents a significant challenge for the dairy industry. This study aimed to determine the spoilage potential of 31 psychrotrophic, mesophilic, or thermophilic strains isolated from the UHT-milk production chain. This encompassed the biofilm-forming ability on polystyrene and stainless-steel surfaces, along with the strains' caseinolytic activity and heat resistance. The effect of 5 mM MgCl2 on the biofilm-forming capacity was also determined. The results indicated that the spoilage potential is highly strain-dependent. On polystyrene, Pseudomonas brenneri C10.3.4, Staphylococcus pasteuri RE1.9, and B. licheniformis SDP.15 were the strongest biofilm-formers of each group, whereas on stainless-steel, Pseudomonas lactis D1.11, Lacticaseibacillus paracasei P2.20 and Bacillus sonorensis C4.13 exhibited the highest capacity. Staphylococcus pasteuri RE1.9 and Staphylococcus epidermidis E3.6 biofilm cells showed the highest heat resistance. Moreover, biofilm-born cells of the spore-forming B. licheniformis SDP.15 and B. sonorensis C4.13 were significantly more heat-resistant than their free-cell equivalents. Furthermore, 22 of the assessed strains showed proteolytic activity, although only the proteases produced by P. lactis E6.10 maintained their enzymatic activity after being treated at 92°C. These findings may contribute to developing strategies to enhance the dairy industry's long-term economic sustainability.