Competition of Thermally Injured Listeria monocytogenes with a Mesophilic Lactic Acid Starter Culture in Milk for Various Heat Treatments

Abstract

Overnight tryptose broth cultures of three L. monocytogenes strains were combined, centrifuged, suspended in 200 ml of tryptose phosphate broth, and heated at 56°C for 20 min and at 64°C for 2 min to obtain low-heat-injured (LHI) and high-heat-injured (HHI) cells, respectively, showing >99.0% injury. Flasks containing 200 ml of raw, low-heat-treated (56°C for 20 min), high-heat-treated (64°C for 2 min), pasteurized, and ultrahigh-temperature (UHT) milk were tempered to 31.1°C and inoculated to contain 104 to 106 CFU/ml of LHI, HHI, or healthy L. monocytogenes cells and a commercial Lactococcus lactis subsp. lactis–Lactococcus lactis subsp. cremoris starter culture at levels of 0.5, 1.0, and 2.0%. Numbers of healthy and injured L. monocytogenes cells and starter organisms were determined using tryptose phosphate agar with or without 4.0% NaCl at selected intervals during 24 h of incubation at 31.1°C. The presence of L. monocytogenes did not adversely affect the growth of the starter culture at any inoculation level. Overall, L. monocytogenes survived the 24-h fermentation period and grew to some extent. In starter-free controls, 76 to 81% of LHI cells and 59 to 69% of HHI cells were repaired after 8 h of incubation, with the lowest repair rates being observed for raw rather than heat-treated or pasteurized milk. Increased injury was observed for healthy L. monocytogenes cells at the 1.0 and 2.0% starter levels, with less injury seen for LHI and HHI cells. Raw and subpasteurized milk allowed less of a decrease in the percentage of injury and also showed higher numbers of injured cells than did pasteurized and UHT milks. These findings may have important implications for the survival of Listeria spp. in certain cheeses that can be prepared from raw or heat-treated milk.