210 Novel food packaging techniques Another possible factor contributing to the growth-inhibitory effect of Co could be an alteration of the membrane properties(Daniels et al, 1985, Dixon and Kell, 1989). It was suggested that CO2 interacts with lipids in the cell membrane, decreasing the ability of the cell wall to uptake various ions Moreover, perturbations in membrane fluidity, caused by the disordering of the lipid bilayer, are postulated to alter the function of membrane proteins( Chin et a.,1976;Roth,1980) Studies examining the effect of a CO2 enriched atmosphere on the growth of microorganisms are often difficult to compare because of the lack of information regarding the packaging configurations applied. The gas/product volume ratio and the permeability of the applied film for O and CO, will influence the mount of dissolved CO2 and thus the microbial inhibition of the atmosphere For this reason, the concentration of dissolved CO2 in the aqueous phase of the food should always be measured and mentioned in publications concerning MAP (Devlieghere et al, 1998) Only a few publications deal with the effect of MAP on specific spoilage microorganisms. Gill and Tan(1980) compared the effect of CO2 on the growth of some fresh meat spoilage bacteria at 30oC. Molin(1983)determined the resistance to COz of several food spoilage bacteria. Boskou and Debevere(1997 1998) investigated the effect of CO2 on the growth and trimethy lamine ocLc ion of Shewanella putrefaciens in marine fish, and Devlieghere and Debevere(2000) compared the sensitivity for dissolved CO2 of different ilage bacteria at 7C. In general, Gram-negative microorganisms such as Pseudomonas, Shewanella and Aeromonas are very sensitive to CO2. Gram positive bacteria show less sensitivity and lactic acid bacteria are the most resistant. Most yeasts and moulds are also sensitive to CO2. The effect of CO2 on psychrotrophic food pathogens is discussed in Section 11.3 11.3 The microbial safety of MAP: Clostridium botulinum and Listeria monocytogenes Modified atmospheres containing CO] are effective in extending the shelf-life of many food products. However, one major concern is the inhibition of normal aerobic spoilage bacteria and the possible growth of psychrotrophic food pathogens, which may result in the food becoming unsafe for consumption before it appears to be organoleptically unacceptable. Most of the pathogenic bacteria can be inhibited by low temperatures(<7C). At these conditions, only psychrotrophic pathogens can proliferate. The effect of CO2 on the different psychrotrophic foodborne pathogens is described below a particular concern is the possibility that psychrotrophic, non strains of C. botulinum types B, E and F are able to grow and prod under MAP conditions. Little is known about the effects of modified atmosphere storage conditions on toxin production by C. botulinum. The possibility of nhibiting C. botulinum by incorporating low levels of Oz in the package doesAnother possible factor contributing to the growth-inhibitory effect of CO2 could be an alteration of the membrane properties (Daniels et al., 1985; Dixon and Kell, 1989). It was suggested that CO2 interacts with lipids in the cell membrane, decreasing the ability of the cell wall to uptake various ions. Moreover, perturbations in membrane fluidity, caused by the disordering of the lipid bilayer, are postulated to alter the function of membrane proteins (Chin et al., 1976; Roth, 1980). Studies examining the effect of a CO2 enriched atmosphere on the growth of microorganisms are often difficult to compare because of the lack of information regarding the packaging configurations applied. The gas/product volume ratio and the permeability of the applied film for O2 and CO2 will influence the amount of dissolved CO2 and thus the microbial inhibition of the atmosphere. For this reason, the concentration of dissolved CO2 in the aqueous phase of the food should always be measured and mentioned in publications concerning MAP (Devlieghere et al., 1998). Only a few publications deal with the effect of MAP on specific spoilage microorganisms. Gill and Tan (1980) compared the effect of CO2 on the growth of some fresh meat spoilage bacteria at 30ºC. Molin (1983) determined the resistance to CO2 of several food spoilage bacteria. Boskou and Debevere (1997; 1998) investigated the effect of CO2 on the growth and trimethylamine production of Shewanella putrefaciens in marine fish, and Devlieghere and Debevere (2000) compared the sensitivity for dissolved CO2 of different spoilage bacteria at 7ºC. In general, Gram-negative microorganisms such as Pseudomonas, Shewanella and Aeromonas are very sensitive to CO2. Gram￾positive bacteria show less sensitivity and lactic acid bacteria are the most resistant. Most yeasts and moulds are also sensitive to CO2. The effect of CO2 on psychrotrophic food pathogens is discussed in Section 11.3. 11.3 The microbial safety of MAP: Clostridium botulinum and Listeria monocytogenes Modified atmospheres containing CO2 are effective in extending the shelf-life of many food products. However, one major concern is the inhibition of normal aerobic spoilage bacteria and the possible growth of psychrotrophic food pathogens, which may result in the food becoming unsafe for consumption before it appears to be organoleptically unacceptable. Most of the pathogenic bacteria can be inhibited by low temperatures (<7ºC). At these conditions, only psychrotrophic pathogens can proliferate. The effect of CO2 on the different psychrotrophic foodborne pathogens is described below. A particular concern is the possibility that psychrotrophic, non-proteolytic strains of C. botulinum types B, E and F are able to grow and produce toxins under MAP conditions. Little is known about the effects of modified atmosphere storage conditions on toxin production by C. botulinum. The possibility of inhibiting C. botulinum by incorporating low levels of O2 in the package does 210 Novel food packaging techniques