MAP, product safety and nutritional quality 209 the water phase and the fat phase of the food. This results, after equilibrium, in a certain concentration of dissolved CO2([CO2ldiss )in the water phase of the product. Devlieghere et al. (1998)have demonstrated that the growth inhibition of microorganisms in modified atmospheres is determined by the concentration of dissolved cO, 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 COz 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 CO inhibition has been examined in several studies. 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 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 iquid 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 ration of 4: 1 Devlieghere et al., 2000b). Several studies have proved that the observed inhibitory effects of COz 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 I 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 induction 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 CO2 exerts its effects are the enzymatic carboxylation and decarboxylation reactions, although inhibition of other enzymes has also been reported (Jones and Greenfield, 1982the water phase and the fat phase of the food. This results, after equilibrium, in a certain concentration of dissolved CO2 ([CO2]diss) in the water phase of the product. Devlieghere et al. (1998) have demonstrated that the growth inhibition of microorganisms in modified atmospheres is determined by the concentration 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 inhibition has been examined in several studies. 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 ration 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 induction 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 carboxylation and decarboxylation reactions, although inhibition of other enzymes has also been reported (Jones and Greenfield, 1982). MAP, product safety and nutritional quality 209