When CO2 is used in high concentrations (20—I00%}. alone or in combination with N3 or O3, or both, the shelf life of MAP foods is also extended. Several mechanisms by which CO3 increases the lag and exponential phases of microorganisms have been proposed, such as rapid cellular penetration of CO2 and alteration in cell permeability, solubilization of CO2 to carbonic acid (H2CO3) in the cell with the reduction of the pH inside the cells, and interference of CO2 with several enzymatic and biochemical pathways, which in turn slow the microbial growth rate. The inhibitory effect of C02 on microbial growth occurs at the 10% level and increases with the increase in concentration. Too high a concentration can inhibit growth of facultative spoilage bacteria and stimulate growth of Clostridium botulinum