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

The growth of Listeria monocytogenes and Yersinia enterocolitica colonies w as studied on solid media at 4 and 8 degrees C under modified atmospheres ( MAs) of 5% O-2 : 10% CO2 : 85% N-2 (MA(1)), 30% CO2 : 70% N-2 (MA(2)) and a ir (control). Colony radius, determined using computer image analysis, allo wed specific growth rates (mu) and the time taken to detect bacterial colon ies to be estimated, after colonies became visible. At 4 degrees 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 compa red 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 degrees 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 degrees C both MAs decrease d the growth rates of Y. enterocolitica by 1.5- and 2.5-fold under MA(1) (m u = 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 degrees 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 degrees C, aerobic) to 19 d (4 degrees C, MA(2 )). Refrigerated modified atmosphere packaged foods should be maintained at 4 degrees C and below to ensure product safety.