This study was undertaken to achieve more information on the aminopeptidase activities expressed in Propionibacterium freudenreichii strains naturally occurring in milk and dairy products. Fifty four strains belonging to both subspecies freudenreichii and shermanii were analyzed for activity towards different amino acyl β-naphthylamide (βNA) derivatives. The ability to efficiently hydrolyze aminoterminal leucine was common to all the isolates, with notably high activities for five strains. Proline iminopeptidase activity was generally expressed. Accordingly, PCR targeted to the P. freudenreichii ATCC 9617 prolyl iminopeptidase encoding gene (pip) demonstrated its presence in all but three strains. Activity towards Lys- and Arg-βNA was less expressed and strain-dependent. The X-prolyl-dipeptidyl aminopeptidase activity was relatively low in absence of NaCl. It increased in presence of 3% NaCl only toward Phe-Pro-βNa and not toward Gly-Pro-βNA. The substrate Glu-βNA was not cleaved by any strain. Findings suggest that the adventitious P. freudenreichii strains may have a significant and variable effect on secondary proteolysis of hard cheeses, being a possible factor of organoleptic differences.
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