Dissolved CO2 has been found to inhibit microbial growth [3-5], affecting the lag phase (a), maximum growth rate (^max)m and/or maximum population (Nmax) densities reached; levels in excess of 5% in MAP systems have been found to be bacteriostatic . The mode of action, although not yet fully understood, is thought to be due to a number of effects, including changes in intracellular pH, alteration of microbial protein and enzyme structure and function, and alteration of cell membrane function and fluidity. The partial pressure and concentration of CO2, package headspace gas volume, temperature, pH, water activity, specific microorganism and growth phase, and growth medium (produce commodity) all influence the inhibitory effect of CO2. The antimicrobial effect of CO2 is enhanced as temperatures decrease and CO2 becomes increasingly soluble.
Low to moderate levels of CO2 have been shown to inhibit growth of many common aerobic produce spoilage bacteria. Moderate CO2 levels of 20 to 60% have been found to reduce the ^ax and Nmax of Pseudomonas spp. and Moraxella spp., two predominant spoilage bacteria found on produce . Low CO2 levels below 20% were found to primarily increase x, with slight reductions in ^ax and no changes in Nmax. CO2 is not antimicrobial towards all microorganism strains or species, and may in some cases actually promote growth. Lactobacillus spp. are generally unaffected by CO2; however, some levels can enhance growth, and 100% CO2 environments have inhibited growth of some strains. In absence of O2, it has been generally shown that the growth and toxin production of Clostridium botulinum is only minimally affected by CO2 concentrations less than 50%; 100% CO2 has been reported to delay toxin production compared to a 100% N2 atmosphere  and decreases growth at 5 and 10°C . Levels of 10% CO2 have been found to be inhibitory to growth of Yersinia enterocolitica while 40% CO2 increased x, and 100% CO2 both increased X and decreased ^ax . There is no agreement on the effect of CO2 on Listeria monocytogenes; however, generally it has been found that CO2 does not affect or in some cases promotes growth. L. monocytogenes has been found to grow well under atmospheres of both 100% N2 and 3% O2/97% N2; growth was enhanced by increasing levels of CO2 in either atmosphere .
Superatmospheric O2 as an MAP atmosphere is a new concept not yet commercially in use due to an incomplete understanding of its effects on MAP systems and mode of action towards microbial populations. Additionally, O2 in excess of 25% is considered explosive, and practical safety issues that need to be employed in its use may not be feasible. Conventional MAP systems that commonly target an EMA of 3 to 6% O2 may be exposed to fluctuating temperatures or temperature abuse conditions during handling, resulting in complete or near depletion of O2. Under these conditions, growth of some pathogens such as C. botulinum may be enhanced due to anaerobiosis, or unrestricted growth of psychrotrophic facultative anaerobic pathogens such as L. monocytogenes may occur due to removal of competitive aerobic microorganisms. Under certain atmospheric conditions, Staphylococcus aureus, Vibrio spp., E. coli, Bacillus cereus, and Enterococcus faecalis have also been shown to grow with restricted or zero O2 . As an alternative MAP atmosphere strategy, high oxygen atmospheres (typically above 70%) that surpass optimal levels for growth of aerobes (21%) and anaerobes (0 to 2%) could generally result in growth inhibition of both anaerobic and aerobic microorganisms, resolving some of the food safety issues possible with lower O2 EMA.
Few reports exist of the effects on specific microorganisms of superatmos-pheric O2 in MAP systems, and some data are conflicting. Jacxsens and others , in a study on RTE mushrooms, grated celeriac, and shredded chicory endive, found that growth of Pseudomonas fluorescens, Candida lambica, Botrytis cinerea, Aspergillus flavus, and Aeromonas caviae was retarded by high O2 MAP atmospheres (70, 80, or 95% O2, bal. N2), an effect that increased with increasing levels of O2; increasing O2 levels extended X of L. monocytogenes. Using an agar-surface model system, Amanatidou and others  found 90% O2 (bal. N2) extended X of L. monocytogenes and Salmonella typhimurium, reduced p.max of E. coli and S. enteritidis, and significantly increased p.max of P. fluorescens, E. agglomerans, Candida guilliermondii, and C. sake. Combined applications of high O2 and 10 to 20% CO2 generally both increased X and reduced Nmax for all strains tested. On mixed vegetable salad, Allende and others  found that yeast growth was stimulated by MAP atmospheres of 95 kPa O2 while growth of psychrotrophic bacteria and L. monocytogenes was unaffected. Generally, greater levels of lactic acid bacteria were found on the mixed salad during storage under conventional MAP gas mixtures than with superatmospheric O2 MAP. Salads treated with superatmospheric O2 also exhibited a longer shelf life, retaining acceptable visual characteristics longer than conventional MAP treatments; the authors did not report whether any other significant organoleptic changes occurred. While superatmospheric O2 has the potential to extend shelf life and maintain produce marketable qualities, these effects may vary depending upon the commodity. Wszelaki and Mitcham  found that superatmospheric storage of Camarosa strawberries resulted in acceptable product firmness, titratable acidity, external color, ethylene production, respiration, and soluble solids, but unacceptable odors and flavors developed as a result of increased production of volatile fermentative metabolites (ethanol, acetaldehyde, and ethyl acetate).
The mode of action of high O2 is thought by some  to be due to oxidative stress and reduction of cell viability due to the generation of intracellular reactive oxygen species such as peroxides or superoxides. Some microorganisms may adapt by producing radical scavengers or inducing O2 decomposing enzymes; repair proteins have been identified for S. typhimurium, E.coli, and L. lactis . It is clear that significantly more work is needed to examine and clarify the effects on the growth parameters of individual spoilage microorganisms and food pathogens of superatmospheric O2, alone or in combination with CO2, applied to different produce commodities and MAP systems. Additionally, a more complete understanding of the underlying basic biological mechanisms of superatmospheric O2 is necessary prior to successful commercialization of this technology .
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