Contamination From Substrates

The best substrates for growing mushrooms are also the best substrates for many types of molds and bacteria.  One of the most common sources of contamination in mushroom cultivation is due to incomplete sterilization or inadequate pasteurization of your spawn or bulk substrate.  These competing contaminants and the dormant spores that naturally exist in bulk substrates and grains must be reduced or destroyed to prevent them from out-competing your mushrooms.

Should I Sterilize or Pasteurize? 

Sterilization is required for grain spawn, sawdust spawn, and some mushroom substrate types to avoid contamination.  You can get away with just pasteurizing other kinds of substrates.  The nutrient content of the substrate is an essential consideration when deciding whether to sterilize or pasteurize. 
All substrate materials considered food for mycelium must be sterilized.  Rye grain, wheat berries, wild bird seed, millet, brown rice, and popcorn are commonly used for grain spawn.  These substrates are abundant in nutrients and attract a wide range of fungus and mold.  Mold can start to grow on food in just a week or two, as you’ve undoubtedly seen in your kitchen.
Pasteurization can be used instead of sterilization for less nutrient-dense substrates.  Coco Coir and Straw are two examples of this.  Straw is made up of dried stalks from a variety of grain plants.  While it still contains some nutrients, it is not as nutrient-dense as whole grains. 
Sterilization is not required for coco coir, vermiculite, or straw.  Pasteurization is sufficient to eliminate most contaminants and give it an advantage.  Pasteurization takes place at significantly lower temperatures, allowing beneficial microbes to thrive.  The microbes will help protect the substrate and your mycelium from contamination.  When the substrate is pasteurized rather than sterilized, many species of mushrooms, notably Agaricus species of mushrooms, colonize faster and provide higher yields. 
When mixing substrates, also known as supplementing, there are a few things to consider.  To make a substrate more nutritious, you can add a supplement of a more nutritious substrate and mix them, for example, by adding some manure to hardwood sawdust.  Sterilization isn’t usually required for hardwood pellets or sawdust by themselves.  But, sterilization is necessary when supplemented with another material that makes it easier for mold or bacteria to develop. 

Here’s a quick guide by material type. 

  • Rye Grain
  • Wheat Grain
  • Wild Bird Seed
  • Millet
  • Brown Rice
  • Popcorn
  • Sawdust
  • Wood Plugs
  • Oak & Soy Hull – Masters Mix
  • Oak & Wheat Bran
  • Oak
  • Coco-Coir
  • Vermiculite
  • Manure
  • Straw
  • Peat Moss
Hope this helps  – Reach out with questions or leave a comment on what works for you.
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