食品微生物に対するガンマ線の作用-I ガンマ線照射を受けた細菌の増殖(その1)

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In the dose range for commercial sterilization or radiopasteurization, gamma radiation generally produces certain limited changes in living cells of microorganisms. By these treat-ments, most of the microorganisms are inactivated and usually fail to grow any more. Some of the cells of microorganisms treated with ionizing radiations at such low or medium dose levels, however, are known to recover to a certain extent if exposed to some suitable condi-tions after irradiation, e. g. STAPLETON et all-3) demonstrated that incubation of Escherichia coli B/r at suboptimal temperatures after irradiation produced a greater survival than incuba-tion at the optimum temperature. Since the first commercial applications of irradiation of foodstuff seem to be in the low or medium dosage field, the practical importance of these postirradiation effects will need no emphasis. Most of the investigations in the literature dealing with the recovery of radiation damage of microorganisms by postirradiation treatments, however, have been made with Es-cherichia coli, which seems to be atypical from the viewpoint of food microbiology, and little is still known about these problems with common food microorganisms. The present work is concerned mainly with the effects of some postirradiation treatments on the growth of several food microorganisms and Escherichia coli irradiated with 60Co gam-ma rays. The results obtained were as shown in figures 1-9, and may be summarized as follows: 1) With respect to the effect of postirradiation incubation temperature on the observed survival, there was relatively wide difference between Escherichia coli and Proteus morganii or spores of Bacillus mesentericus. The strain of Escherichia coli employed in this experi-ment showed marked increase in the observed survival at suboptimal postirradiation incubation temperatures. Proteus morganii also showed a slight increase in the survival at suboptimal postirradiation temperatures, but its increasing tendency was not so marked as was the case with Escherichia coli. With the strain of Bacillus mesentericus, on the other hand, such an increase in the survival was never found at any suboptimal temperature; this organism rather showed the highest survival at its optimum temperature. 2) In all cases the surviving fractions differed to some extent with pH values of the post-irradiation culture media. In the cases of Escherichia coli and Proteus morganii the survi-ving fraction more or less decreased with increasing pH value of the media within certain limited range. The pH optima for the survival of irradiated cells of these bacteria, therefore, were somewhat lower than the optimum pH for the growth of unirradiated cells of these or-ganisms. On the contrary, Bacillus mesentericus showed the highest survival at pH 7 which is the optimum pH for its growth. It may be concluded that the effects of postirradiation treatments on the survival of bac-teria entirely differ with species. It appears that these effects generally decrease with the increase of the variety of the nutritional substance required for bacteria.

食品微生物に対するガンマ線の作用-I ガンマ線照射を受けた細菌の増殖(その1)

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