Controlled permeabilization of Lactococcus lactis cells as a means to study and influence cheese ripening processes

Cells of Lactococcus lactis were permeabilized by perturbing the membrane structure through the delipidating action of n-butanol to an extent that allowed normally excluded peptidase substrates to enter the cells and be accessible to intracellular enzymes. The degree of permeabilization of cells depended on the concentration of the solvent used, the duration of the treatment, the density of the cell suspension, pH and temperature. This was indicated by the optimum or maximum activities of the intracellular peptidases aminopeptidase N and aminopeptidase X. The possible usefulness of permeabilized cells for cheese-ripening studies was demonstrated with cells that were treated with 5 ml L−1n-butanol at pH 6.5 and 25 °C using a mixture of chymosin-generated primary cheese peptides as substrate. Production of amino acids could be correlated with conversion of peptides that had entered the permeabilized cells and reflected the course of amino acid production in cheese.