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

Abstract

Minced beef was inoculated with low levels (1·2–1·7 log10cfu g−1) of Listeria monocytogenes or Listeria innocua, or a combination of the two strains. Inoculated samples were stored at 0 or 10°C under two packaging atmospheres (aerobic and vacuum) for up to 28 days and surviving organisms recovered on Palcam Agar. The only significant increases in numbers of Listeria spp. occurred in samples held at 10°C under aerobic conditions. In vacuum packs, growth of both strains was inhibited. Under aerobic conditions 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 nature and effects of different background microflora that were observed to develop under each of these packaging conditions.Pseudomonas spp. predominated in aerobically stored beef, whereas in vacuum packed beef lactic acid bacteria predominated. No significant differences were observed between the growth rates of Listeria spp. inoculated into beef mince in pure and mixed culture. This suggests that the more frequent prevalence of Listeria innocua than Listeria monocytogenes in meat and meat products is not due to overgrowth or inhibition of the pathogen (Listeria monocytogenes) by the non-pathogen(Listeria innocua) during low-temperature storage.