Antimicrobial Effects Of Coz

Although the preservative action of C02 in foods has been known for many years, its mechanism of antimicrobial action has not been fully determined. Several theories have been postulated, however. One theory was that the displacement of 02 was the main reason for the antimicrobial properties of C02. This theory was refuted by Coyne, who showed that aerobic spoilage organisms of fish grew well in 100% N2 but not in 100% C02, indicating that displacement of 02 was not the only reason for the antimicrobial effect of C02. Valley and Rettger (1) suggested that C02 acted by lowering extracellular pH as a result of the dissolution of C02 in the aqueous phase of the product. However, several studies have shown that when the pH is lowered by inorganic acid to values equivalent to those achieved under C02 atmospheres, bacterial and mold growth was less inhibited. Furthermore, C02 will inhibit microbial growth in buffered media and in naturally buffered foods such as meat. In response to these observations, Wolfe suggested that the inhibitory effect of C02 may be due to intracellular, rather than extracellular, pH changes that could interfere with enzymatic activities associated with cell metabolism. Several studies have shown that C02 inhibits oxaloacetate decarboxylase, succinate dehydrogenase, and cytochrome oxidase activity. However, King and Nagel (2) observed that C02 did not inhibit extracts of these enzymes. They observed that C02 specifically inhibited malic and isocitric dehydrogenase activity in vitro and concluded that the inhibitory effect of C02 may be due to its mass action effect on decarboxylases within the cell. Another theory suggests that C02 acts on the cell membrane, affecting the permeability characteristic of the membrane and its external environment by redistributing lipids at the surface. This has been demonstrated using a model system by Sears and Eisenberg (3) and has been proposed as the mechanism by which C02 may inhibit aerobic spore germination.

In conclusion, though many studies have been performed on the effect of C02 on microorganisms, there is little conclusive evidence of its mechanism of action. In a review on the effects of carbon dioxide on microbial growth and food quality by Daniels et al. (4) the following appear to be the salient points of investigations:

• The exclusion of oxygen by replacement with carbon dioxide may contribute to its overall antimicrobial effect by slowing the growth of aerobic spoilage microorganisms.

• The carbon dioxide/bicarbonate ion has an observed effect on the permeability of cell membranes.

• Carbon dioxide is able to produce a rapid acidification of the internal pH of the microbial cell with possible ramifications relating to metabolic activities.

• Carbon dioxide appears to exert an effect on certain enzyme systems.

Whatever the reason for its antimicrobial effect, C02 is effective in extending the shelf life of perishable foods by retarding microbiological growth. The overall effect of C02, in conjunction with refrigeration, is to increase both the lag phase and the generation time of spoilage microorganisms.

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