We boil the jars in order to sterilize them in order to destroy all forms of life in the jars. (molds and bacteria, spores and bacterial endosporse). This is done, so we can inoculate the now sterile substrate with the desired fungus.
These organisms which live in the soil, in water, etc. can invide and digest a food source like the one we are providing with the PF-tek recipe. This we call contamination.
Boiling water reaches a temperature of 212° F, which is adequate to kill molds and spores which may reside in the water, vermicuite and the brown rice flour.