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In order to elucidate the sterilization mechanism underlying the explosive decompression system, baker's yeast was pressurized with COz, NzO, N z , or Ar gas at 40atm and 40°C for 4h, and then explosively discharged. The survival ratio was markedly decreased only by the treatments with COz and NzO, which are relatively soluble gases in water, suggesting that the microorganisms' death may be highly correlated with gas absorption by the cells. Lower decompression rates to atmospheric pressure, however, led to neither any lower reduction of remaining cells nor any smaller release of total cellular proteins. Furthermore, operating with a longer treatment time and smaller number of repetitions was usually more lethal than with a shorter time and more frequent repetition. From these results, most of the yeast cells appear to have been sterilized during the pressurization process. The spore cells of B. megaterium are considered to have been killed in a somewhat different manner, because of their distinct sensitivity to the applied gases.Key words: sterilization; high-pressure carbon dioxide; food microorganism; survival ratio; decompression rateAlthough heat sterilization is the traditional and most popular method for protecting foods from microbial spoilage, it may often cause hostile changes in the nutritional and sensory properties of the products because of the high temperature involved. To develop and establish a novel and effective alternative to heat treatment, the lethal action of high-pressure COz on microorganisms, with no or only a minimal heating process, has recently received a great deal of attention.Among the methods, the explosive decompression system consists of a pressurization stage to induce penetration of the applied gas into the microbial cells and a subsequent explosive discharge that results in rapid gas expansion within the cells. With this system, the greater part of the microbial cells are believed to be mechanically ruptured like a popped ballon at the moment of flash depressurization.1 6) This technique was first reported by Fraser in 1951, and he found that E. coli cells were significantly disrupted by the quick release of COz gas from 5001bf/in 2 (about 34.0 atm) to atmospheric pressure. 1) This method has subsequently been developed mainly for unit operation to recover intracellular enzymes, recombinant-DNA proteins and nucleic acids from microbial cell cultures.1 5) We have recently demonstrated with a similar system that 10 8 cells/ml of baker's yeast could be thoroughly killed after COz saturation at 40 atm and 40°C for 3 h, and proposed this as a new method for sterilizing food microorganisms. 7.8) Other approaches involving no flash pressure drop, using high-pressure COz treatment alone, have also been widely studied to provide the required lethal action on various kinds of microbes. 9 -17) 15 ) From these investigations, most microbial cells appear to be sterilized mainly due to the inactivating effect of COz under pressure; this may be in conflict with the foregoing explanatio...

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