Identification of Pseudomonas jessenii and Pseudomonas gessardii as the most proteolytic Pseudomonas isolates in Iranian raw milk and their impact on stability of sterilized milk during storage

Identification of the most proteolyticPseudomonasstrains that can produce heat-resistant proteases and contribute to the Ultra High Temperature (UHT) milk destabilization is of great interest. In the present study, among the 146Pseudomonasisolates that encoded theaprX gene, five isolates with the highest proteolytic activity were selected and identified based on 16S rRNA,rpoDandgyrBgene sequences data. The identification results were confirmed by phylogenetic analysis based on multilocus sequence analysis and identified the representative isolates asP. jessenii(two isolates) andP. gessardii(three isolates). Casein zymography demonstrated the ability of these species to produce heat-resistant enzymes, AprX, with molecular mass of about 48 kDa during storage at 7 degrees C for 72 h. In sterilized milk samples, the residual activity of AprX caused a considerable enhancement in the degree of protein hydrolysis, non-protein nitrogen and non-casein nitrogen contents of the samples during a two-month storage. This enhancement was slightly higher in samples containing enzyme produced byP. jesseniicompared toP. gessardiiones, resulting in earlier onset of sterilized milk destabilization. Hence, this study revealed thatP. jesseniiandP. gessardiican play a considerable role in deterioration of Iranian commercial long-life milk.