Growth of Listeria monocytogenes and Yersinia enterocolitica colonies under modified atmospheres at 4 and 8 deg C using a model food system

摘要

The growth of Listeria monocytogenes and Yersinia enterocolitica colonies was studied on solid media at 4 and 8 deg C under modified atmospheres (MAs) of 5% O_2:10% CO_2:85% N_2(MA_1), 30% CO_2:70% N_2(MA_2) and air (control). Colony radius, determined using computer image analysis, allowed specific growth rates (#mu#) and the time taken to detect bacterial colonies to be estimated, after colonies became visible. At 4 deg C both MAs decreased the growth rates of L. monocytogenes by 1.5- and 3.0-fold under MA_1 (#mu#= 0.02 h~(-1)) and MA_2 (#mu#= 0.01 h~(-1)), respectively, as compared with the control (#mu#= 0.03 h~(-1)). The time to detection of bacterial colonies was increased from 15 d (control) to 24 (MA_1) and 29 d (MA_2). At 8 deg C MA_2 decreased the growth rate by 1.5-fold (#mu#= 0.04 h~(-1)) as compared with the control (#mu#= 0.06 h~(-1)) and detection of colonies increased from 7 (control) to 9 d (MA_2). At 4 deg C both MAs decreased the growth rates of Y. enterocolitica by 1.5- and 2.5-fold under MA_1 (#mu#= 0.03 h~(-1)) and MA_2 (#mu#= 0.02 h~(-1)), respectively, as compared with the control (#mu#= 0.05 h~(-1). At 8 deg C identical growth rates were obtained under MA_1 and the control (#mu#= 0.07 h~(-1)) whilst a decrease in the growth rate was obtained under MA_2 (#mu#= 0.04 h~(-1)). The detection of colonies varied from 6 (8 deg C, aerobic) to 19 d (4 deg C, MA_2). Refrigerated modified atmosphere packaged foods should be maintained at 4 deg C and below to ensure product safety.