Effect of cell-free milk culture liquid of Enterococcus faecalis subsp. liquefaciens on growth, acid production and proteolysis of some commercial starter cultures in milk

Abstract

Inclusion of 50 μl/ml cell-free culture liquid of Enterococcus faecalis var. liquefaciens in skim milk intended for culturing separately 20 multi- and single-strain starters did not increase the acid production by the starters and neither the cell growth nor the proteolysis was stimulated. On the contrary, both growth and acid production were variably inhibited, with the reduction in acid production not always resulting from growth inhibition. The inhibition ranged from 0 to 99% for growth and 0.5 to 62% for acid production and was maximum by heating the cell-free culture liquid at 100°C for 5 min. Growth of the single-strain starter cultures was affected by the heated but not the unheated culture liquid in general. The crude inhibitor proved to be non-neutralizable, non-dialyzable, sensitive to proteolysis and resistant to heat (100°C for 30 min) at 0.59% acidity, implicating bacteriocin-like agent as a participant in the antimicrobial activity. Enterococcin positive strains of Enterococcus faecalis are, therefore, unsuitable for inclusion in the multi-strain starter cultures in cheese making even though these variants might have a good ability to grow at low temperatures and exhibit excellent fermentative and proteolytic characteristics in addition to high salt-tolerance. Multiplication of these strains in raw milk might also result in failure of the growth and acid production by the starter cultures.