GROWING GOURMET - Anto2ni.it

172 GROWING GOURMET MUSHROOMS ON ENRICHED SAWDUST
Figure 139. Bags of Shiitake mycelia incubating on
sterilized sawdust. Note that bags are narrowly
spaced, but are not touching, which aids in the loss
of heat.
212° F. unless the pressure within 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 endospore-formiflg
bacteria. Every microcosm,
every microscopic niche, must be subjected to
250° F. (12 1° C.) for at least 15 minutes to effect
true sterilization. When processing tons of
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 help the
cultivator determine sterilization profiles. 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 precautions
so that contaminants are not
introduced. The best alternative is to design the
inoculation room with a positive-pressurized
HEPA filtration system. Many cultivators use
bags or bottles fitted with a filter—either
plugged cotton or a specially designed filter
disc 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
can begin.
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 sterilization is employed.
The greater yield from sterilized sawdust, according
to Royse et al. (1985), is not due to the
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 hardwoods
as a substrate base, such as alder, have not found yields
on super-pasteurized sawdust to be depressed compared
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).
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