GROWTH OF PSYCHROTROPHIC FOODBORNE PATHOGENS IN A SOLID-SURFACE MODELSYSTEM UNDER THE INFLUENCE OF CARBON-DIOXIDE AND OXYGEN

A solid surface model system was developed to study the effect of gas atmosphere composition on the growth of Aeromonas hydrophila, Yersinia enterocolitica, Listeria monocytogenes, and a cold tolerant strain of Bacillus cereus. The organisms were incubated on an agar surface at 8 degrees C under either 1.5 or 21% O-2, combined with 0, 5, 20 or 50% CO2. The remainder of each atmosphere was made up to 100% with N-2. Gr owth was evaluated on the basis of three parameters, namely maximum sp ecific growth rate, maximum population density and lag time. These par ameters were derived from growth data by fitting with a modified Gompe rtz equation. In all instances the maximum specific growth rate decrea sed significantly with increasing CO2 concentration. A strong reductio n in the maximum population density was noted only for B. cereus at th e highest level of CO2. The O-2 concentrations tested did not signific antly affect maximum specific growth rates nor maximum population dens ities in any case. Prolonged lag times were observed only for Y. enter ocolitica under 50% Co-2/21% O-2/29% N-2. The results indicate that th e model system may be a suitable means of estimating the growth of bac teria on minimally processed produce, packaged under modified atmosphe res. Extrapolation of our results to modified atmosphere packaged (MAP ) fruits and vegetables using typical O-2 concentrations of 1-5% and C O2 concentrations of 5-10%, suggest that growth of the above pathogens may occur at 8 degrees C, thereby imposing a safety hazard for these products. (C) 1995 Academic Press Limited