The Ultimate Guide to Sterilized Grain Jars: Mastering the Art of Mushroom Cultivation at Home

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Why sterilized grains?

Grains need to be sterilized for mushroom growing because it helps to eliminate any potential sources of contamination that could compete with the mushroom mycelium for nutrients and resources.

When mushroom growers inoculate grains with mushroom spores or liquid culture, they introduce a relatively small amount of living organisms to the grain. However, the grains themselves may contain a variety of bacteria, fungi, or other microorganisms that could outcompete the mushroom mycelium or cause infections, which would reduce or even completely stop mushroom growth.

By sterilizing the grains before inoculation, growers can ensure that any potential sources of contamination are eliminated. This creates a sterile environment for the mushroom mycelium to grow and colonize the substrate. This also helps to promote healthy and vigorous mycelial growth, which is necessary for the mushrooms to develop and mature.

Overall, sterilizing the grains helps to create a clean and controlled growing environment, and is a critical step in successful mushroom cultivation.

Shopping list for sterilized grain jars

Soak the grains

Soaking grains is an important step in preparing them for future grain spawn. It helps to create a more suitable and hospitable environment for the mycelium to grow. This is because the outer layers of the grains can be tough and resistant to colonization by the mycelium. Soaking them can help break down these barriers allowing for an easier growth surface. Also, soaking the grains can help release nutrients that are inside the grain making them more readily available for the mycelium to use.

Determine how much grain to use and pour those grains inside of the five pound bucket. One thing to be careful of is that the grains expand after soaking. Expect to have a greater yield of grain than measured. Fill the bucket with water until the grains are completely covered plus some more. The optimal time to allow the grains to soak is anywhere from 12 to 24 hours. Anything less than that may not hydrate the grains enough and anything more than that might cause the grains to sprout or germinate.

soaking rye berries for grain jars
Soaking the grains.

Create the specialized lids

In order for the mycelium to colonize the grains there needs to be FAE (fresh air exchange). Mushrooms are closer to animals than they are to plants in that they breath in oxygen and breath out CO2. To allow the mycelium to breath in the mason jars, there needs to be adequate air exchange which can only be created by modifying the lid. This lid will have a hole for fresh air exchange about 1/4 an inch and an optional hole to be able to inject microspore syringes or liquid culture at 1/8th an inch. The reason that the second hole is optional is that there are many other techniques to inoculate grains other than using a syringe, such as grain to grain transfers and agar transfers.

After drilling the holes in the mason jar lids it is time to finish the modification. With the larger hole in mind, grab a pinch of the polyester fiber fill and insert it into the hole with the fill being on the outside and inside of the hole. If the second hole was drilled, use the red silicone to cover the hole. This is where the syringe will go into. The red silicone acts as a self healing injection port for syringes. Once these steps have been completed the lids are ready to go.

Modified mason jar lids for sterilizing grain jars.
An example of modified mason jar lids.

Simmer the grains

Simmering the grains after they have soaked help the grains retain the moisture and solidify the hydrating process. Take the grains from the five gallon bucket and add them to a pot. Fill the pot with clean water and bring the pot to a boil. Once it reaches a boil, reduce the heat and let the grains simmer in the water for about 15 minutes.

Simmering grains for grain jar sterilization
Simmer the grains for 15 minutes.

Dry the grains

Drying grains after simmering is important when sterilizing grains because excess moisture can create an environment that is conducive to the growth of unwanted bacteria. Bacteria can compete with the desired mycelium and can even completely take over the grain, resulting in contamination.

Additionally, when sterilizing grains, it is important to ensure that the heat can penetrate the grains evenly, which is easier to achieve with dry grains.

Furthermore, excess moisture can also cause the grains to clump together, which can lead to incomplete sterilization and uneven colonization by the mycelium. By drying the grains properly, you can ensure that they are loose and evenly spread out, which helps to promote uniform colonization and successful mushroom cultivation.

Let the grains dry in the sink or in a strainer for a few hours until the grains are uniformly dry.

Fill the mason jars

As a general rule, you should fill the jars about 2/3 to 3/4 full with grain, leaving some headspace at the top for the grain to expand during sterilization.

It’s important not to overfill the jars, as this can cause the grain to pack tightly and prevent proper sterilization. It’s also important to leave enough headspace to allow the grain to expand during sterilization without pushing against the lid and potentially causing it to loosen.

Additionally, it’s a good idea to shake the jars after filling them to distribute the grain evenly and reduce any air pockets that may be present. This can also help to prevent clumping of the grains during sterilization.

Once the jars have been filled, put the specialized lids back onto the jars and cover the tops of the lids with aluminum foil. Water tends to drip down from the top of the pressure cooker and that water can seep back into the grains through the polyester fiber fill and could lead to excess moisture inside of the jars.

Grain jars covered with aluminum foil before sterilzation
Adding aluminum foil protects the jars from water entering from FAE hole.

Pressure cook the grain jars

Pressure cooking is used in grain sterilization because it can effectively kill off any bacteria and other microorganisms that may be present in the grains. When grains are cooked under high pressure, the temperature inside the cooking chamber can reach up to 121°C (250°F) or higher, which is sufficient to kill even the most heat-resistant microorganisms.

By pressure cooking the grains, producers can ensure that they are starting with a sterile product, which can then be inoculated with desired strains of fungi. Pressure cooking is a reliable and efficient method of sterilization, which is why it is widely used in the food industry.

Steps to pressure cooking the grain jars:

  • Place the jars on the pressure cooker rack, making sure they are not touching each other.
  • Add water to the pressure cooker until it reaches the recommended level specified by the manufacturer.
  • Put the lid on the pressure cooker, making sure it is securely in place. Turn the heat on high to begin building pressure.
  • Allow the pressure to build until the desired pressure is reached. This will vary depending on the type of grain and the size of the jars, but a common pressure setting is 15 PSI (pounds per square inch).
  • Once the desired pressure is reached, reduce the heat to maintain the pressure for the recommended sterilization time which is 90 minutes.
  • After the sterilization time is complete, turn off the heat and allow the pressure to decrease naturally. Do not attempt to release the pressure manually as this can be dangerous.
  • Once the pressure has decreased, carefully remove the lid of the pressure cooker and carefully remove the jars from the pressure cooker.
  • Allow the jars to cool to room temperature before inoculating them with mushroom spores or liquid culture.
Sterilizing Grain Jars for Mushroom Cultivation. Pressure cooking grain jars for sterilzation.
An example of a pressure cooker reaching 15psi.

By following these steps, you can effectively sterilize your grain jars using a pressure cooker, ensuring a contamination-free environment for your mushroom cultivation.


In conclusion, grain sterilization plays a critical role in mushroom cultivation as it ensures that the grain substrate is free from any harmful microorganisms that can compete with the mushroom mycelium or contaminate the final product.

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