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Abstract: Since spawn media are moist porous solids by nature, it may be necessary to apply the principle of sterilization by dry heat to destroying bacteria. However, there is a lack of quantitative data on the sterilization process by dry heat. Therefore the authors concentrate the present study on heat penetration in porous media and take 'bulk density' (B.D.) as a tentative index for the porosity, or the air spaces of spawn media, because the air in the pore spaces of the media retards heat penetration. An electric precision thermometer was used for the determination of the central temperature of the bottled spawn media and the temperature of the steam or the temperature inside the autoclave. The spawn media which principally consisted of rice straw composts were obtained from local spawn makers. In the laboratory the pattern of heating curves of bottled spawn media is S-shaped. A marked heat retardation by spawn media was observed (fig. 3). The higher the B.D., the greater was the retardation in repeated experiments (figs. 4 and 5). More moist media showed less heat retardation (fig. 6). During practical sterilization with the use of non-temperature regulated steam-heated autoclaves, multiple heating inflection points (fig. 7) and heterogeneous distribution of temperatures inside the autoclaves (table 1) were observed. In conclusion, there was practically no effect of air spaces of the bottled spawn media on heating curves, whereas the effect of the solids in the bottled media was most significant. However, the type of the autoclave, the bad design of which sometimes causes misoperation of the sterilization, seems to be one of the factors affecting contamination of spawn media.
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