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CULTURING MUSHROOM MYCELIUM ON AGAR MEDIA 91
lease pressure during the sterilization cycle are
ideal. The old-fashioned pressure canners,
those having weights sitting upon a steam valve,
cause the media to boil as steam is vented. A
huge mess ensues.The pressure cooker should
ideally form a vacuum upon cool-down. If,
upon returning to atmospheric pressure, a
vacuum is not formed, the cultivator must place
the pressure cooker in the clean room or open
it in front of a laminar flow hood while it is still
pressurized.
As the media cools within the pressure
cooker, outside air is sucked in. If this air is
ladened with contaminant spores, the media
contaminates before the cultivator has handled
the flask! One precaution is to saturate a paper
towel with alcohol and drape it over the point
where outside air is being drawn in. The cloth
acts as a filter, lessening the chance of contaminants.
Twenty minutes after the heat source has
been turned off, most media vessels can be
handled without a hot-glove. Prior to that time,
media can be poured, but some form of protection
is needed to prevent burns to the hands.
Agar coagulates water when added in excess
of 10 grams per 1000 ml. H20. Only high grade
agar should be used. Various agars differ substantially
in their ability to gelatinize water,
their mineral and salt content, as well as their
endemic populations of micro-organisms, including
bacteria. (Bacteria, if surviving, often
de-gelatinize the media.) Increasingly, pollution
has affected the refinement of tissuegrade
agar, causing the price to spiral. Agar
substitutes such as Geirite TM are widely used
by the plant tissue culture industry. Although
only a few grams are needed per liter, it does not
result in a media firm enough for most mushroom
cultivators. Mushroom cultivators desire
a media with a semi-solid, firm surface upon
which the mycelium will grow. Plant tissue
culturists seek a softer, gelatinous form so that
plant starts will grow three dimensionally, deep
into the medium.
Sugars are essential for the healthy growth
of mycelium. For media formulation, complex
sources of sugars (carbohydrates and polysaccharides)
are recommended. Cornsteep fermentative,
cooked potatoes, wood, and barley
malt extracts provide sugars and an assortment
of basic minerals, vitamins, and salts helpful in
the growth of the mushroom mycelium. From
my experiences, simple sugars, while they may
support growth, are not recommended as strains
can not be maintained for long without promoting
mutation factors, senescence, or loss of
vitality.
A variety of nitrogen and carbohydrate based
supplements can be added to fortify the media.
Strains grown repeatedly on mono-specific media
for prolonged periods risk limiting the
repertoire of digestive enzymes to just that formulation.
In other words, a strain grown on one
medium adapts to it and may lose its innate ability
to digest larger, more complex and variable
substrates. To prevent a strain from becoming
media-specific, the following compounds are
added to 1 liter of MEA or PDA at various intervals,
often in combinations:
Nitrogen & Carbohydrate
Supplements
2 grams yeast or
1-2 grams peptone
2 grams oatmeal, oat bran
2 gram rye or wheat flour
1 gram soybean meal
1 gram spirolina
2 grams high quality dry dog food
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