Sterilize Straw with Peroxide / Lime

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Also See:  Peroxide Manual Vol. I  &  Peroxide Manual Vol. II

By Nan (Nanook) on Tuesday, October 23, 2001 - 06:55 pm:

Remember, there's two methods, here's the one listed in Volume 1

A number of mushrooms will grow on straw, and straw is the traditional substrate for oyster mushroom cultivation. The standard way to prepare straw for mushroom growing is to steep it in water heated to 180 degrees F for an hour, then drain, cool it down and inoculate. However, without some specialized equipment, it can be rather awkward to heat any significant amount of straw in hot water, and it is often difficult to tell whether the straw has cooled sufficiently for inoculation unless you have a compost thermometer. So, I prefer to use a cold water hydrated lime soak. Hydrated lime (calcium hydroxide or Masonís lime, available at builderís supply stores) is a caustic powder which creates a strongly alkaline solution in water. The alkali both pasteurizes the straw and wets it by penetrating the waxy coating on the straw. When the solution is drained off, the liquid that remains behind on the straw gets converted by carbon dioxide in the air to ordinary calcium carbonate lime, which is alkaline but no longer caustic.

I have a large plastic tub that will take a quarter of a bale of straw. With the straw in place, the tub then takes 25 gallons of water to fill to the rim. I use a one half percent solution of hydrated lime, which means one pound of lime in 25 gallons of water. I mix a slurry of hydrated lime in a separate bucket, then pour it into the tub in stages as the tub is filling with water, to assure thorough mixing. (Caution: hydrated lime will burn skin and eyes. Be sure to wear gloves and eye protection when handling the powder or the solution. Also, store the powder in an airtight container to maintain its potency). I let the straw soak for about 16 hours, then drain it for two hours. You can probably get away with using half as much lime, but then it may prove wise to soak the straw somewhat longer, say 24 hours instead of 16.

I inoculate the straw by filling a plastic trash bag, draped inside a large cardboard apple box. I put in a layer of straw, then a sprinkling of spawn, then another layer of straw, then more spawn, and so forth. Finally I close up the bag and compress the straw by pushing the bag and its contents down into the box as far as I can get it to go. A quarter of a bale of straw fills three apple boxes worth. The bags get loosely twisted closed and covered with old bed sheets to incubate for about a month.

On hearing about the peroxide method, many mushroom growers have the thought that peroxide could be used to replace the cumbersome hot water steeping or steaming required to pasteurize straw for oyster mushroom cultivation. Unfortunately, peroxide cannot be used by itself to pasteurize straw. This is so because the many microscopic and macroscopic particles of live mold in straw are highly resistant to peroxide, and also because straw contains a large amount of active peroxide-decomposing enzymes.

Now here's the Method listed in Volume 2

Despite this complicated explanation, the protocol for preparing straw with peroxide is extremely simple. It goes like this:

1) Place your straw in a large soak vessel.
2) Fill the vessel with the appropriate cold solution (see below) to immerse the straw.
3) For chopped straw, soak about 4 hours at room temperature. For unchopped straw, soak for at least 28 hrs, or until the leachate takes on the color of a good tea.
4) Drain the straw thoroughly, until it is no longer drippy.
5) Remove the straw to your culture containers, mixing in spawn as you go.

Notes on straw preparation (keyed to the step numbers):

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1) If the soak vessel has a heavy or tight fitting lid, this can help keep the straw submerged in the solution.

2) The most effective solution I have found so far uses hydrogen peroxide at a concentration of at least 0.15%, combined with 10 mls. of vinegar per liter of soaking solution, or about 2.5 tablespoons vinegar per gallon (higher concentrations of vinegar did not work in mini-trials).

Curiously enough, an alternative which has proved almost equally effective in mini-trials is hydrated lime (calcium hydroxide, Masons' lime) in peroxide. But here you will use far less hydrated lime than called for by the hydrated lime soak detailed in Volume I. Instead of adding so much hydrated lime that you create something close to a saturated solution, which then creates disposal problems when you drain the straw, you will now add just enough hydrated lime to raise the pH a bit while the peroxide reacts with the straw. You'll use just 1/2-2/3 tsp. hydrated lime per gallon of 0.15% peroxide solution, or 0.4 - 0.5 gms. per liter of solution (higher concentrations of hydrated lime did not work in mini-trials).

The solutions can be prepared with cold water from the tap, but the best bet is to use water that is not far below room temperature.

3) I recommend chopping the straw for best results. Chopping the straw promotes more efficient absorption of water, and the smaller particle size encourages faster mycelial growth upon inoculation.

The wetting of the straw will proceed more quickly in warmer climates, more slowly in colder ones, so adjust your soak times accordingly.

4) How long you have to drain the straw depends on how much straw you are working with, but a couple of hours will probably be the minimum.

5) Straw can be mixed with spawn and loaded into plastic columns, for instance. Pasteurized gypsum, if added, can be mixed in at this stage as well.

I donít recommend adding nitrogen supplements, as most of the standard ones like bran will invite contaminants. But if you must have a supplement, I would suggest using drainable materials resembling straw, such as alfalfa, which can then be soaked in peroxide solution with the straw.