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Although C02 is effective against aerobic spoilage microorganisms, it has little or no antimicrobial effect against other spoilage microorganisms. Several studies have shown that C02 has little or no effect on the growth of facultative microorganisms in the Enterobacteriaceae or microaerophilic lactic acid bacteria. These organisms have been reported capable of growth in high concentrations of C02 (75-100%). Anaerobic bacteria, such as the food-poisoning organisms Clostridium botulinum and C. per-fringens, are not affected by the presence of carbon dioxide, and the anaerobic conditions inside MAP foods may be conducive to their growth. There is concern that these organisms represent a potential public health hazard if present in gas-packaged foods, particularly if packaged under completely anaerobic conditions and stored under conditions of temperature abuse. Thus, although low concentrations of C02 (20-30%) may suppress the growth of aerobic microorganisms, they do little to inhibit the growth of facultative anaerobes, microaerophiles, and strict anaerobes. Under MAP conditions, the microbiological population shifts from a predominantly aerobic one to one comprising almost entirely C02-resistant anaerobic bacteria. In effect, the MAP suppresses competitive spoilage microorganisms and offers conditions conducive to pathogenic anaerobic microbiological growth. For this reason, prevention of contamination of MAP foods with pathogenic microorganisms is very important.

The age of the microbiological population also influences the inhibitory effect of C02. It has been shown that, as bacteria move from the lag phase to the log phase, the inhibitory effects of C02 are reduced. Thus, the earlier the product is gas packaged, the more effective C02 will be.

Concentration of C02

Early experiments clearly established that success in controlling aerobic spoilage deterioration of food was not simply dependent on the elimination of oxygen; rather, there was a definite requirement for C02 in the gas atmosphere. Coyne reported that the growth of Achromobacter, Flavo-bacterium, Micrococcus, Bacillus, and Pseudomonas was markedly inhibited by 25% C02 and completely inhibited by 50% C02 (5). In a later study, Coyne (6) reported that the optimal concentration for inhibition of aerobic spoilage microorganisms was 40 to 60% C02. No additional extension of shelf life was obtained by using higher concentrations of C02, and bacterial growth was less inhibited below these concentrations. Gill and Tan (7) examined the effect of various concentrations of C02, equivalent to pressures of 100 to 300 mm Hg, that is, 13 to 39% C02 in air, on the respiration rates of Pseudomonas species, Acinetobacter, Alteromonas putrefaciens, Yersinia enterocolitica, and En-terobacter. They reported that the respiration rates of most of the common spoilage organisms under investigation, with the exception of Enterobacter and Brocothrix ther-mosphacta, were affected by elevated levels of C02 in air. The level of C02 that resulted in maximum inhibition of the common spoilage organisms was approximately 200 mm Hg, or 26% C02 in air. However, the level of C02 investigated in this study had no antimicrobial effect on B. thermosphacta, which requires concentrations of 75% or more C02 for complete inhibition.

Several studies have shown that mold growth is also inhibited by low concentrations of C02. For example, many Aspergillus, Rhizopus, and Cladosporium species are completely inhibited by 5 to 10% C02 at 1°C. Other studies have shown that 20 to 30% C02 was sufficient to prevent the growth of meat-associated molds whereas 30 to 50% C02 was found to completely inhibit all mold species associated with the spoilage of bread and cakes. Again, this was not simply due to lower partial pressures of 02 in the gas atmosphere, because it has been shown that many molds continue to grow normally when the 02 concentration is maintained as low as 1%.

It is evident from these studies that the concentration of C02 in the gas mixture is very important in obtaining the desired extension of microbiological shelf life of the product. For most food products, with the exception of fruits and vegetables, a minimum of 20 to 30% C02 by volume is required to inhibit the growth of aerobic spoilage microorganisms and extend the shelf life of food, whereas for maximum shelf-life extension a concentration of 50 to 60% should be used. Though there is little or no increased antimicrobial effect or extension in shelf life at concentrations of C02 >50 to 60%, slightly higher concentrations are sometimes used to compensate for losses of headspace C02 through packaging films. However, too high a concentration of C02 may result in discoloration problems and drip loss in muscle foods. For fruits and vegetables, the maximum concentration of C02 is approximately 5 to 20% by volume because higher concentrations result in carbon dioxide damage.

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