Food-grade controlled lysis of Lactococcus lactis for accelerated cheese ripening

An attractive approach to accelerate cheese ripening is to induce lysis of Lactococcus lactis starter strains for facilitated release of intracellular enzymes involved in flavor formation. Controlled expression of the lytic genes lytA and lytH, which encode the lysin and the holin proteins of the lactococcal bacterio-phage ΦUS3, respectively, was accomplished by application of a food-grade nisin-inducible expression system. Simultaneous production of lysin and holin is essential to obtain efficient lysis and concomitant release of intracellular enzymes as exemplified by complete release of the debittering intracellular aminopeptidase N. Production of holin alone leads to partial lysis of the host cells, whereas production of lysin alone does not cause significant lysis. Model cheese experiments in which the inducible holin-lysin overproducing strain was used showed a fourfold increase in release of L-Lactate dehydrogenase activity into the curd relative to the control strain and the holin-overproducing strain, demonstrating the suitability of the system for cheese applications.