Technological Performance of Several Lactococcus and Enterococcus Strains of Dairy Origin in Milk

Abstract

Thirty-nine strains (29 Lactococcus strains and 10 Enterococcus strains) isolated from five different artisanal cheeses were subjected to technological characterization. Several strains of lactococci and enterococci produced lactic acid at a rate and final concentration suitable for large-scale cheesemaking. However, extensive phenotypic differences between strains were encountered. Proteolytic activity correlated quite well with acidification for all strains, with the more proteolytic strains being the best acidifiers. The strains were also assayed for the production of organic acids and volatile components in milk. With few exceptions, enterococcus isolates produced more formic acid and acetic acid than did lactococcus isolates. The volatile-compound profiles obtained were rather simple. The main volatile component produced by most strains was ethanol. Since the inclusion of enterococcus strains in food systems is controversial, tests were also performed to detect recognized determinants of virulence (namely, aggregation, gelatinase and hemolysin production, and antibiotic resistance). Aggregation in both liquid and solid media was observed only for two Enterococcus durans isolates. None of the strains studied produced gelatinase under the conditions of the assay. β-Hemolysin activity was clearly detected in two Enterococcus faecalis strains, which also produced the biogenic amine tyramine from tyrosine in a laboratory medium. In general, the enterococcus strains were more resistant to the antibiotics assayed than were the lactococcus strains. Both the minimum inhibitory concentration (MIC) modes and the highest MIC values were consistently higher for the enterococci. Nevertheless, particular strains of lactococci were resistant to antibiotics such as bacitracin, cephalothin, clindamycin, streptomycin, and tetracycline.