344 The nutrition handbook for food processors inhibition of microorganisms in modified atmospheres is determined by the con- centration of dissolved CO2 in the water phase. The effect of the gaseous environment on microorganisms in foods is not as well understood by microbiologists and food technologists as are other external factors, such as pH and aw. Despite numerous reports of the effects of CO2 on microbial growth and metabolism, the 'mechanismof COz inhibition still remains unclear (Dixon and Kell, 1989: Day, 2000). The question of whether any specific metabolic pathway or cellular activity is critically sensitive to CO2 inhi- bition has been examined by several workers. The different proposed mechanisms of action are. 1. Lowering the ph of the food. 2. Cellular penetration followed by a decrease in the cytoplasmic pH of the cell. 3. Specific actions on cytoplasmic enzymes 4. Specific actions on biological membranes. When gaseous CO2 is applied to a biological tissue, it first dissolves in the liquid phase, where hydration and dissociation lead to a rapid pH decrease in the tissue. This drop in pH, which depends on the buffering capacity of the medium (Dixon and Kell, 1989), is not large in food products. In fact, the ph drop in cooked meat products only amounted to 0.3 pH units when 80% of CO, was applied in the gas phase with a gas/product volume ratio of 4: 1 (Devlieghere et al, 2000b). Several studies have proved that the observed inhibitory effects of CO2 could not solely be explained by the acidification of the substrate(Becker, 1933; Coyne,1933 Many researchers have documented the rapidity with which CO2 in solution enetrates into the cell. Krogh(1919)discovered that this rate is 30 times faster than for oxygen(O2), under most circumstances. Wolfe(1980) suggested the inhibitory effects of CO2 are the result of internal acidification of the cytoplasm. Eklund(1984)supported this idea by pointing out that the growth inhibition of four bacteria obtained with CO, had the same general form as that obtained with eak organic acids(chemical preservatives), such as sorbic and benzoic acid. Tan and Gill(1982) also found that the intracellular pH of Pseudomonas fluorescens fell by approximately 0.03 units for each I mM rise in extracellular CO concentration CO2 may also exert its influence upon a cell by affecting the rate at which particular enzymatic reactions proceed. One way this may be brought about is to cause an alteration in the production of a specific enzyme, or enzymes, via induc tion or repression of enzyme synthesis (Dixon, 1988; Dixon and Kell, 1989 Jones, 1989). It was also suggested (ones and greenfield, 1982; Dixon and Kell, 1989)that the primary sites where COz exerts its effects are the enzymatic car- boxylation and decarboxylation reactions, although inhibition of other enzymes has also been reported (ones and Greenfield, 1982 nother 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 mem-inhibition of microorganisms in modified atmospheres is determined by the con￾centration of dissolved CO2 in the water phase. The effect of the gaseous environment on microorganisms in foods is not as well understood by microbiologists and food technologists as are other external factors, such as pH and aw. Despite numerous reports of the effects of CO2 on microbial growth and metabolism, the ‘mechanism’ of CO2 inhibition still remains unclear (Dixon and Kell, 1989; Day, 2000). The question of whether any specific metabolic pathway or cellular activity is critically sensitive to CO2 inhi￾bition has been examined by several workers. The different proposed mechanisms of action are: 1. Lowering the pH of the food. 2. Cellular penetration followed by a decrease in the cytoplasmic pH of the cell. 3. Specific actions on cytoplasmic enzymes. 4. Specific actions on biological membranes. When gaseous CO2 is applied to a biological tissue, it first dissolves in the liquid phase, where hydration and dissociation lead to a rapid pH decrease in the tissue. This drop in pH, which depends on the buffering capacity of the medium (Dixon and Kell, 1989), is not large in food products. In fact, the pH drop in cooked meat products only amounted to 0.3 pH units when 80% of CO2 was applied in the gas phase with a gas/product volume ratio of 4 :1 (Devlieghere et al, 2000b). Several studies have proved that the observed inhibitory effects of CO2 could not solely be explained by the acidification of the substrate (Becker, 1933; Coyne, 1933). Many researchers have documented the rapidity with which CO2 in solution penetrates into the cell. Krogh (1919) discovered that this rate is 30 times faster than for oxygen (O2), under most circumstances. Wolfe (1980) suggested the inhibitory effects of CO2 are the result of internal acidification of the cytoplasm. Eklund (1984) supported this idea by pointing out that the growth inhibition of four bacteria obtained with CO2 had the same general form as that obtained with weak organic acids (chemical preservatives), such as sorbic and benzoic acid. Tan and Gill (1982) also found that the intracellular pH of Pseudomonas fluorescens fell by approximately 0.03 units for each 1 mM rise in extracellular CO2 concentration. CO2 may also exert its influence upon a cell by affecting the rate at which particular enzymatic reactions proceed. One way this may be brought about is to cause an alteration in the production of a specific enzyme, or enzymes, via induc￾tion or repression of enzyme synthesis (Dixon, 1988; Dixon and Kell, 1989; Jones, 1989). It was also suggested (Jones and Greenfield, 1982; Dixon and Kell, 1989) that the primary sites where CO2 exerts its effects are the enzymatic car￾boxylation and decarboxylation reactions, although inhibition of other enzymes has also been reported (Jones and Greenfield, 1982). Another 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 mem- 344 The nutrition handbook for food processors