We measured the germinating and inactivating effects of reciprocal pressurization (consisting of six cycles of 5 min compression and rapid decompression) (RP) on bacterial spores compared with the effects of continuous pressurization (CP) by using heat sensitivity (70 °C, 30 min) as a criterion for germination. There was no significant difference in the germination of spores by RP and CP, but there was a significant difference in spore inactivation by RP and CP except for 200 MPa‐25 and 35 °C. The number of germinated spores was more than the number of inactivated spores following RP and CP. These results indicated that compression could initiate germination, whereas rapid decompression could not initiate germination, but could inactivate the germinated spores. Thus, RP was more effective than CP in inactivating spores.

Mechanical analysis of injury of spores of B. subtilis by quick decompression was investigated. The stress on the stress concentration point of germinated spores under 200, 300 and 400 MPa treatment was calculated to be 800 MPa, 1200 MPa and 1600 MPa, respectively, where this point was the highest point of stress concentration on the sporecoat. The stress showed values 4 times larger than treatment pressure. Sporecoat is a keratin-like scleroprotein. The tensile strengths of some polymers were 70 MPa (polyester), 50 MPa (vinyl chloride), 70 MPa (nylon) and 20 MPa (polyethylene). From stress analysis, minimum stress of the spore model on the stress concentration point (800 MPa) was higher than the tensile strength of the above polymers. It was indicated that the sporecoat is disrupted by the stresses produced by expansion of inner-spore-water with quick decompression.