Factors influencing resuscitation and growth of heat injured Listeria monocytogenes 13-249 in sous vide cooked beef

Abstract

The growth of Listeria monocytogenes 13-249 in vacuum-packed, minced beef was investigated as a function of degree of heat injury (including no injury i.e. uncooked beef), growth phase (logarithmic and late stationary phase), pH (5.6 and 6.2), and storage temperature (3, 10 and 20°C) during a storage period of 30 days. Late logarithmic and late stationary phase cultures of L. monocytogenes 13-249 showed similar growth in refrigerated, vacuum-packed, raw minced beef with a high pH (6.2). In normal pH (5.6) beef there was no growth at 3°C while growth at 10 and 20°C was only observed for logarithmic phase cultures. Heat injured late stationary phase cultures with 95–99.9% injured cells in the surviving population (as measured by differential plating on enriched vs. selective media after sous vide cooking) did not grow or repair sublethal injuries in sous vide cooked beef at 3°C while repair and growth took place at 10 as well as at 20°C. In logarithmic phase cultures heat injury occurred very rapidly and ≥99.9% heat injury was observed in all trials in spite of much lower pasteurization values and fewer log 10 reductions compared with late stationary phase cultures. Regardless of growth phase, all cultures where a high degree of heat injury (≥99.9%) was observed, did not subsequently grow in the beef product at 3 or 10°C within 30 days. Growth of heat injured cultures preexposed to heat shock (46°C, 30 min) or slowly rising temperatures (0.3°C min −1) before heat injury was also investigated. Heat shocked or heat adapted cultures generally responded in the same manner as non-stressed cultures (no growth at 3°C) except that a longer lag phase was observed in beef processed at slowly rising temperatures and in normal pH beef at 10°C. Although processing at slowly rising temperatures may slightly increase the survival of L. monocytogenes 13-249 in cooked beef, there seem to be no indication of an increase in subsequent growth potential of the surviving cells.