Proteolysis of casein micelles by Pseudomonas fluorescens CNRZ 798 contributes to the destabilisation of UHT milk during its storage

Among the incriminating factors in the destabilisation of ultra-high-temperature (UHT) milk during storage, the heat-resistant proteases of Pseudomonas are considered to play a role. The objective of this work was to study the consequences of contamination of raw skim milk with Pseudomonas fluorescens CNRZ 798 on the stability of the corresponding UHT milk during storage. After 92 days, milk destabilisation was determined by the presence of a gelled sediment and low value to phosphate test (2.4 mL for control against 0 mL of phosphate solution for milk contaminated before UHT treatment) and the presence of aggregates. For the UHT control milk and UHT milk manufactured from raw skim milk contaminated with P. fluorescens, an increase in the size of the casein micelles (205 and 332 nm, respectively), a decrease in the zeta potential (−16.6 and −14.0 mV, respectively) and decrease in the level of hydration (2.00 and 1.55 g of water per gramme of dried pellet, respectively) were detected. The increase in pH 4.6-soluble nitrogen and trichloroacetic acid-soluble nitrogen content for UHT milk previously contaminated were ten- and fivefold higher than those for control milk, respectively. The trichloroacetic acid-soluble nitrogen fraction of the milk contaminated before treatment contained 118, 22, 4 and 9 peptides from β-, αs1-, αs2- and κ-caseins in comparison to only 22, 19, 6 and 4 peptides for the control milk. This study showed that destabilisation of UHT milk was due to proteolysis of casein micelles.