Why is dung and straw better pasteurized than sterilized?
There are several reasons for this.
There are several reasons for this.
Dung and straw are used as so called bulk substrates. This means you are usually working with a greater quantity of those substrates and inoculate it with grain spawn.
Now, while it's easy to sterilize a bunch of PF-Tek or grain jars in the pressure cooker, it's a much more difficult task to do so with 20 or more pounds of straw or dung. Who has such a big pressure cooker anyway? ;)
The second, probably more important reason is the contamination risk of sterilized substrates.
When you pasteurize substrates at 160-170°F (71-77°C) some beneficial micro organisms, mainly bacteria, stay alive, inhabit the substrate and “guard” it against other more aggressive micro organisms. Mushroom mycelium is still able to grow on this substrate though.
This is the reason why you can inoculate bulk substrates with spawn in open spaces without taking special sterile precautions.
If you sterilize the bulk substrate it becomes as much susceptible to contaminants as for instance rye grain or whole rice. This means that you would have to inoculate the big amount of bulk substrate in sterile conditions, in front of a HEPA hood. And this is not practicable if you have big amounts of the substrate.
The above is only true for pure dung and straw, the only allowed additives are gypsum and calcium carbonate( for pH correction and stabilization).
As soon you add a additive high in nutrients to the substrate, for instance brown rice flour or wheat bran, the substrate must be sterilized and inoculated in sterile conditions.
