Diversity in NaCl tolerance of Lactococcus lactis strains from dl-starter cultures for production of semi-hard cheeses

Abstract

Mesophilic starter cultures, such as Lactococcus lactis, are exposed to NaCl stress during brining of cheeses influencing their viability. We investigated the effect of NaCl water solutions, used at concentrations of 3.4%, 8.0% and 15% (w/v), on viability of Lc. lactis subsp. cremoris, Lc. lactis subsp. lactis biovar. diacetylactis and Lc. lactis subsp. lactis isolated from commercial dl-starter cultures. Viability was assessed by plate counts and flow cytometry. Bacterial viability decreased with increased concentration of NaCl in a strain-dependent manner (between 0.02% and 62% cfu at 15% (w/v) NaCl, compared with controls at 0.85% (w/v) NaCl), whereas no significant differences were detected between the subspecies. Viability levels determined by plate counts were generally lower than by flow cytometry, indicating the presence of viable but not culturable cells. Knowledge of salt tolerance of Lc. lactis strains would enable rational selection of starter cultures targeting production of cheeses with different NaCl content.