Antagonism between Listeria monocytogenes and lactococci during fermentation of products from ultrafiltered skim milk.

Tyndallized samples of unfiltered skim milk and retentate (concentrated fivefold or twofold by volume) and permeate from UF skim milk were inoculated with 5.5 x 10(3) to 1.5 x 10(5) cfu/ml of Listeria monocytogenes strains California or V7 together with 4 x 10(7) to 2.3 x 10(8) cfu/ml of mesophilic lactic acid bacteria. Numbers of L. monocytogenes (McBride Listeria agar) and lactic acid bacteria (all purpose Tween agar) were determined after 0, 6, 12, 24, 30, and 36 h of incubation at 30 degrees C. Lactic acid bacteria significantly inhibited or inactivated L. monocytogenes in all three products. Inactivation was greater in permeate (6.77 orders of magnitude) than in unfiltered skim milk (3.67 orders of magnitude) or in retentate (4.21 orders of magnitude). Degree of inactivation in retentate was related to the extent of concentration. Inactivation was not complete, and L. monocytogenes survived in these products during fermentation for up to 36 h. When fermented products were refrigerated (4 degrees C), L. monocytogenes survived for 4 to 6 wk in skim milk, 3 to 5 wk in retentate, and 1 wk in permeate. At refrigeration temperature, length of survival was dependent on type of product and strain of the pathogen.