Pathogenic Organisms and Shelf Life 19531 Food Safety Risk of MAP Produce

When competitive microflora are eliminated by MAP atmospheres, some pathogens may grow unimpeded. Certain MAP systems can produce anoxic conditions which, while inhibiting growth of spoilage organisms such as aerobic bacteria and molds, can allow growth of obligate anaerobic pathogens such as nonproteolytic C. botulinum even at refrigeration temperatures; temperature abuse conditions that increase product respiration can also result in anaerobiosis. Where high levels of CO2 alone restrict growth of susceptible microorganisms, selection of pathogens that can survive under these conditions may also occur [47]. At 13 or 22° C, CO2 was reported to not inhibit growth of E. coli on shredded lettuce; in fact, atmospheres of 5% O2 and 30% CO2 (balance N2) actually enhanced growth over storage in air. Atmospheres containing 40 to 50% CO2 were only slightly inhibitory towards Yersinia enterocolitica at 4°C, although inhibition increased as storage temperature decreased [9]. Bennik and others [44] observed extended X for this psychrotrophic pathogen under conditions of 50% CO2 and 21% O2 (balance N2), but no effect under decreasing CO2 concentrations of 5 or 20%. Their results suggest that typically employed MAP conditions of 1 to 5% O2 and 5 to 10% CO2 at 8°C may not inhibit growth of the pathogens Aeromonas hydrophila, L. monocytogenes, or cold-tolerant strains of Bacillus cereus.

The behavior of a particular pathogen in a MAP system is influenced by the fruit or vegetable type as well as by the nature of the particular microbial strain. Francis and O'Beirne [48] found that acid-adapted L. monocytogenes grew on mung bean sprouts at 8°C and atmospheres of 2 to 5% O2 and 0 to 15% CO2, while nonacid-adapted strains did not grow. In further work [49] these authors assessed the effects of vegetable type and strain on survival and growth of different pathogens under different modified atmospheres at 4 and 8°C. Different growth responses were observed between strains of E. coli O157:H7 on different RTU vegetables (lettuce, swedes, dry coleslaw, soybean sprouts), while no difference was observed on these same vegetables among multiple strains of L. monocytogenes.

Different methods can be utilized to determine the safety of foods stored under specific MAP systems. Challenge studies, where survival and growth of inoculated pathogens are followed over time, can be performed in isolation or in combination with natural produce microflora. Challenge studies using

C. botulinum can be used to examine toxin production as well as occurrence of spoilage. Predictive models can be generated to determine microbial growth or toxin development in produce. To consider the interactions of fluctuating populations and ratios of spoilage organisms and pathogens, a safety index ratio may be used to indicate relative spoilage and pathogenicity. The ratio of a specific pathogenic organism to a food spoilage organism over MAP storage time can be used as a practical safety index, created for any pathogen of concern where levels required to produce illness may significantly differ. Such an index would not represent an absolute measurement of the safety of a food product: it would quantitatively depict the relationship between spoilage and pathogenicity [47].

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