212° F. unless the pressure with | the vessel is raised above 1 psi. I call this method atmospheric sterilization or super-pasteurization.
Most competitor organisms are easily killed with steam heat, with the exception of some thermotolerant black pin molds, and en-dospore-forming bacteria. Every microcosm, every microscopic niche, must be subjected t > 250° F. (121° C.) for at least 15 minutes to effect true sterilization. When processing ton; >f sawdust, true sterilization is rarely achieved. The cultivator must constantly compromise the ideal in favor of the practical. To this end, temperature-sensitive indicator strips hel the cultivator determine sterilization profil ;s. If sawdust is treated in bulk and not separated into individual bags, the danger of cross-contami nation is likely during the unloading and spawning process.
After 12 hours of heat treatment, the steam is shut off. As the mass cools, air will be drawn into the sawdust. The cultivator must take pre cautions so that contaminants are not introduced. The best alternative is to design hhe inoculation room with a positive-pressurized HEPA filtration system. Many cultivators use bags or bottles fitted with a filter—ei ier plugged cotton or a specially designed filter use that prevents the introduction of airborne contaminants. Often times, one to two days must pass until the mass naturally falls below 100° F. (38° C.), at which point inoculations canoegin.
Super-pasteurization of supplemented oak sawdust substrates, although effective, often results in less total yield than from the same substrate sterilized. Comparative studies by Badham (1988) showed that there are no appreciable differences in yields of Shiitake between supplemented sawdust blocks subjected to high pressure autoclaving vs, atmospheric steam sterilization for the first flush. In comparing total yields, however, more mushrooms can be grown per pound of sawdust if pressure sterfction is employed. The greater yield from sterilized sawdust, according to Royse et al. (1985), is not due tc he survival of contaminants, but a function of the rendering of the sawdust into a form more readily digestible to the Shiitake mycelium.*
* Those using the more rapidly decomposing hardwc is as a substrate base, such as alder, have not found yields on super-pasteurized sawdust to be depressed comparec to sterilized sawdust. Moreover, the density of the wood and moisture content are major factors affecting heat penetration. The addition of buffers, calcium carbonate and calcium sulfate are recommended for the more acidic woods. For more information see Badham (1988) and Miller & Jong (1986).
Figure 141. Pouring sawoust »pawn into bag of sterilized, supplemented sawdust
Pressurized steam essentially softens the saw dust
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