Competition of thermally injured Listeria monocytogenes with a mesophilic lactic acid starter culture during milk fermentation

• Main Author: Mathew, Finny P
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2000
• Subjects: Microbiology
• ISBN: 0493087710; 9780493087719

The relationship between heat treatment of milk and the ability of sublethally injured Listeria monocytogenes to survive mesophilic fermentation in milk was investigated. A three-strain cocktail of L. monocytogenes , suspended in 200 ml of tryptose phosphate broth, was heated at 56°C/20 min and 64°C/2 min to obtain low heat-injured (LHI) and high heat-injured (HHI) cells, respectively, showing >99% injury. Flasks containing 200 ml of raw, low heat-treated (56°C/20 min), high heat-treated (64°C/2 min), pasteurized or UHT milk were tempered to 31.1°C, inoculated to contain 104–106 LHI, HHI or healthy L. monocytogenes cells and Lactococcus lactis subsp. lactis/Lactococcus lactis subsp. cremoris starter culture at 0.5, 1.0 or 2.0% levels. Numbers of healthy and injured L. monocytogenes cells were determined using tryptose phosphate agar w/o 4.0% NaCl at selected intervals during the 24h fermentation period along with the numbers of starter organisms. In starter-free controls, ∼76–81% and 59–69% of LHI and HHI cells, respectively, were repaired after 8 hours of incubation, with lowest repair in raw milk. Increased injury was observed for healthy L. monocytogenes cells at 1.0 and 2.0% starter levels, with less injury seen for LHI and HHI cells. The extent of sublethal injury for all L. monocytogenes was inversely related to severity of the milk heat treatment.