Carbon dioxide lowers the intra and extracellular pH of tissues and possibly that of microorganisms. Further it
may affect the membrane potential of microorganisms and influence on the equilibrium of decarboxylating enzymes
of microorganisms. The gases normally employed are carbon dioxide, mixtures of carbon dioxide and nitrogen,
carbon dioxide and oxygen and carbon dioxide, oxygen & nitrogen with the sole objective to extend the shelf life of
the product beyond that obtained in conventional refrigerated storages. Inhibition by Carbon dioxide manifests in an
increased lag phase and a slower rate of growth of microorganisms during logorathmic phase. Inhibition by Carbon
dioxide was found to be more effective when the product was stored at the lowest range of refrigerated temperatures
(Daniels et al., 1985). Packaging materials generally employed for this purpose are flexible films of nylon/surylyn
laminates, PVC moulded trays laminated with polythene, polyester/low density polythene film etc. The use of high
barrier film along with MAP that contains CO2 effectively inhibits bacterial growth during refrigerated storage of
packaged fresh fishery products (Farber, 1991).
The composition of the gas mixtures used for MAP of fresh fish varies, depending upon whether the fish in the
package is lean or oily fish. For lean fish, a ratio of 30 % Oxygen, 40 % Carbon dioxide, 30 % Nitrogen is
recommended. Higher values of Carbon dioxide are used for fatty and oily fish with a comparable reduction in level
of Oxygen in the mixture leading to 40 % to 60 % Nitrogen. By excluding oxygen, the development of oxidative
rancidity in fatty fish is slowed. On the other hand, oxygen can inhibit the growth of strictly anaerobic bacteria like
C. botulinum although there is a very wide variation in the sensitivity of anaerobes to Oxygen. It is also seen that
inclusion of only some Oxygen with Nitrogen or Carbon dioxide will not prevent botulism with absolute certainty.