Behaviour of Listeria monocytogenes in the presence of Listeria innocua during storage of minced beef under vacuum or in air at 0 degrees C and 10 degrees C

Minced beef was inoculated with low levels (1.2-1.7 log(10) cfu g(-1)) of L isteria monocytogenes or Listeria innocua, or a combination of the two stra ins. inoculated samples were stored at 0 or 10 degrees C under two packagin g atmospheres (aerobic and vacuum) for up to 28 days and surviving organism s recovered on Palcam Agar. The only significant increases in numbers of Li steria spp occurred in samples held at 10 degreesC under aerobic conditions . in vacuum packs, growth of both strains was inhibited Under aerobic condi tions meat pH increased from an initial value of pH 5.85 to c. 8.85 within 28 days. The pH of vacuum packaged meat declined to c. 4.95 during the same period. These differences in pH may be related to differences in the natur e and effects of different background microflora that were observed to deve lop cinder each of these packaging conditions. Pseudomonas spp. predominate d in aerobically stored heel: whereas in vacuum packed beef lactic acid bac teria predominated! No significant differences were observed between the gr owth rates of Listeria spp. inoculated into beef mince in pure and mixed cu lture. This suggests that the more frequent prevalence of Listeria innocua than Listeria monocytogenes in meat, and meat products is not due to overgr owth or inhibition of the pathogen (Listeria monocytogenes) by the non-path ogen (Listeria innocua) during low-temperature storage. (C) 2000 Academic P ress.