Impact of chymosin- and plasmin-mediated primary proteolysis on the growth and biochemical activities of lactobacilli in miniature Cheddar-type cheeses

Abstract

Strongly proteolytic starters seem to improve the growth of non-starter lactobacilli during cheese ripening, but no information is available on the impact of the non-microbial proteases usually active in cheese on their development. In the current study, the influence of chymosin- and plasmin-mediated proteolysis on the growth and biochemical activities of lactobacilli during ripening of miniature Cheddar-type cheeses, manufactured under controlled microbiological conditions, was studied. Two experiments were performed; in the first, residual chymosin activity was inhibited by the addition of pepstatin, whereas in the second, plasmin activity was increased by adding more enzyme, obtained in vitro through the activation of plasminogen induced by urokinase. Cheeses with or without a Lactobacillus plantarum I91 adjunct culture and with or without added pepstatin or plasmin solution were manufactured and ripened for 60 days. The addition of the adjunct culture resulted in enhancement of secondary proteolysis, evidenced by an increase in the total content of free amino acids (FAA) and modifications of the individual FAA profiles. Reduction in residual chymosin activity caused a decrease in primary and secondary proteolysis, characterized by the absence of as1-casein hydrolysis and reduced production of peptides and FAA, respectively. The increase in plasmin activity accelerated primary proteolysis, but no enhancement of secondary proteolysis was observed. Chymosin- and plasmin-mediated proteolysis did not influence the growth and biochemical activities of adventitious or adjunct lactobacilli, indicating that it is not a limiting factor for the development and proteolytic-peptidolytic activities of lactobacilli in the cheese model studied. Key words: non-starter lactobacilli, plasmin, chymosin, proteolysis