Comparison between the growth of Yersinia enterocolitica, Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella spp. in ground beef packed by three commercially used packaging techniques

Growth of the pathogens Yersinia enterocolitica, Listeria monocytogenes, Escherichia coli O157:H7 and strains of Salmonella were compared in ground beef packed in modified atmospheres of 60% CO2/40% N2/0.4% CO (high CO2/low CO mixture), 70% O2/30% CO2 (high O2 mixture) and in chub packs (stuffed in plastic casings). The ground beef was inoculated with rifampicin-resistant or nalidixic acid/streptomycin-resistant strains of the pathogens (final concentration 102–103 bacteria/g) and stored at 4 and 10°C for up to 14 days. At 4°C the shelf life, based on colour stability and background flora development, was prolonged for the high CO2/low CO mixture compared to the two other packaging methods, but at 10°C the shelf life was <8 days for all the packaging methods. Growth of Y. enterocolitica was nearly totally inhibited both at 4 and 10°C in the high CO2/low CO mixture, while the bacterial numbers in the samples packed in the high O2 mixture increased from about 5×102 bacteria/g at day 0 to about 104 at day 5 at 4°C and to 105 at 10°C. Growth in the chub packs was even higher. L. monocytogenes showed very little growth at 4°C in all treatments. At 10°C there was slow growth from about 5×103 bacteria/g to about 104 at day 5 in the high CO2/low CO mixture, while the numbers in the high O2 mixture and the chub packs were about 10 times higher. Growth of E. coli O157:H7 at 10°C in the ground beef was nearly totally inhibited in both the high CO2/low CO mixture and the high O2 mixture. Growth in the chub packs was higher, as the number of bacteria increased 3 log in 5 days. The Salmonella strains (S. typhimurium, S. dublin, S. enteritidis and S. enterica 61:k:1,5,(7)) in the ground beef stored at 10°C for 5 and 7 days grew to a higher number in the high CO2/low CO mixture than in the high O2 mixture. This study shows that the growth of Y. enterocolitica and L. mononcytogenes in ground beef stored in the high CO2/low CO mixture was not increased as a result of prolonging the shelf life. However, the observed growth of strains of Salmonella at 10°C in this mixture and in chub packs does emphasise the importance of temperature control during storage.