132 Produce Degradation: Reaction Pathways and their Prevention
microbial counts of bacteria of the genera Leuconostoc, Pseudomonas, Micrococcus,
and Bacillus on prickly pear cactus. [98] Gunes and Hotchkiss [99] found that at
abusive temperatures (15 and 20°C), elevated CO 2 (greater than or equal to 15%),
and reduced O 2 (less than 1%), MAs inhibited the growth of E. coli O157:H7 on
apple slices. Bennik et al. [82] observed decreases in the specific growth rates of
Aeromonas hydrophila, Yersinia enterocolitica, Listeria monocytogenes, and a cold-
tolerant strain of Bacillus cereus with increasing CO 2 (0, 5, 20, and 50%) concen-
trations regardless of the O 2 concentrations (1.5 or 20%).
An increase in CO 2 concentration inhibits yeast growth and significantly reduces
mesophilic flora compared to air storage. The CO 2 protective effect against molds
occurs for concentrations above 5% on a synthetic culture medium [100]. El Halouat
et al. [101] found that 40 and 80% CO 2 did not support the growth of Aspergillus
niger. The growth of most molds is completely inhibited above 20% CO 2 , but some,
such as Botrytis cinerea, resist up to 30% (Figure 5.9). Yeasts on inoculated fruits
are more resistant to CO 2. Monilia laxa, which is totally inhibited by 20% when
cultivated in vitro, manages to survive and grows in atmospheres with up to 30%
CO 2 when inoculated on peaches (Figure 5.10). Vial [85] also found that the pro-
tective effect of CO 2 against yeasts and bacteria occurs for concentrations of 20%.
If normal atmosphere is restored after exposure to high CO 2 concentrations, the
molds resume their growth, proving that CO 2 has no lasting effect (Figure 5.10).
Carbon dioxide is a soluble gas both in water and fat, and its impact on micro-
organisms depends on its degree of solubility in produce. In MAP, the solubility of
carbon dioxide depends on the ratio of headspace to medium volume, buffering
capacity of the medium, pH, water activity (aw), and the temperature [102]. As
carbon dioxide dissolves in cells, it hydrates to H 2 CO 3 and so reduces the pH and
damages cell activity. Lowenadler [102] and Devlieghere et al. [103] have shown that
decrease in the growth rate of some bacteria, Pseudomonas fragi, Yersinia enterocolitica,
and Lactobacillus sake, is well correlated with increasing CO 2 solubility in the medium.
FIGURE 5.9Mold growth under varying CO 2 concentrations. (From Chambroy, Y. and Souty,
M., Rôle du CO 2 sur le comportement des fruits à noyau. Utilisation des atmosphères modi-
fiées, in Proc. Symp. Qualité Post-Récolte et Produits Dérivés chez les Fruits à Noyau,
Vendrell, M. and Audergon, J.M. Eds., Lerida, Spain, 1994, p. 139.)
05101520253035400 5 10 15 20 25 30
CO 2 (%)Mold (diameter in mm)Monilia laxa Monilia fruc.
Rhizopus st. Botrytis cin.