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

The growth of Listeria monocytogenes 13-249 in vacuum-packed, minced beef w as 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 degreesC) during a storage period of 30 days. Late logarithmic and late stationary phase cult ures of L. monocytogenes 13-249 showed similar growth in refrigerated, vacu um-packed, raw minced beef with a high pH (6.2). In normal pH (5.6) beef th ere was no growth at 3 degreesC while growth at 10 and 20 degreesC was only observed for logarithmic phase cultures. Heat injured late stationary phas e cultures with 95-99.9% injured cells in the surviving population (as meas ured by differential plating on enriched vs. selective media after sous vid e cooking) did not grow or repair sublethal injuries in sous vide cooked be ef at 3 degreesC while repair and growth took place at 10 as well as at 20 degreesC. In logarithmic phase cultures heat injury occurred very rapidly a nd greater than or equal to 99.9% heat injury was observed in all trials in spite of much lower pasteurization values and fewer log(10) reductions com pared with late stationary phase cultures. Regardless of growth phase, all cultures where a high degree of heat injury (greater than or equal to 99.9% ) was observed, did not subsequently grow in the beef product at 3 or 10 de greesC within 30 days. Growth of heat injured cultures preexposed to heat s hock (46 degreesC, 30 min) or slowly rising temperatures (0.3 degreesC 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 degreesC) except that a longer lag phase was observed in bee f processed at slowly rising temperatures and in normal pH beef at 100C. Al though processing at slowly rising temperatures may slightly increase the s urvival of L. monocytogenes 13-249 in cooked beef, there seem to be no indi cation of an increase in subsequent growth potential of the surviving cells . (C) 2001 Elsevier Science B.V. All rights reserved.