Pink oyster mushroom fruiting from colonized bulk substrate

Substrate Pasteurization

Survival for fungi, or any mode of life, is a two-pronged approach: 1) acquiring and converting nutrients to energy and 2) defending against intruders and competitors. 

In nature, much of a fungus’s energy pours into defending itself from unwelcome guests, from bugs, bacteria, parasites, to competitive fungi vying for the same food source. The remaining energy goes towards building mycelia and the fruiting body. The intense struggle for resources often means that most fungi die without fruiting. 

In a culture, the energy sources and environment change, but the rules remain the same. In a culture, by design, the food supply is ample and most competition and predators are eliminated. Fungi in culture, therefore, only need to carry out the first prong of survival: converting nutrients to energy to grow and reproduce. As a result, fungi in culture have a better chance of fruiting and producing a higher yield compared to those grown in nature. 

But because fungi in culture dedicate most of their energy towards growing and reproducing, when an intruder appears (i.e. contamination), fungi in culture have less immunity to defend themselves. 

Contamination, therefore, is the culprit of most unsuccessful mushroom cultivation. 

To prevent contamination, mycologists adapt sterile techniques when carrying out their mushroom work. Some basic techniques include: 

  1. Use a Still Air Box, or a Laminar Flow Hood, to filter and limit air flow. Consider the Bella Bora Still Air Box for this purpose.
  2. Employ heat and chemical treatment to sterilize equipment and pasteurize substrate.
  3. Work with quick and precise movements.
  4. Discard contaminated mycelium and substrate as soon as contamination is detected, followed by deep and thorough sterilization of the workspace.

What is the difference between sterilization and pasteurization? 

Sterilization = a complete removal and/or destruction of all microbial life forms, often through extreme heat (steam), pressure, or irradiation. Sterilization and disinfection techniques are rigorously observed in healthcare facilities and are required for infection control. 

Pasteurization = an incomplete removal of microbes, often through high heat or high pH. The microbes that survive pasteurization are typically harmless, serving as non-competitive placeholders to prevent growth from intruders and competitors.

Between sterilization and pasteurization, we believe pasteurization reduces contamination better in the long term. 

Our reason is as follows: sterilization strips bare all microbial life forms on a given substrate, which means post-sterilization, the substrate is now a fertile ground for all microbes, welcomed or otherwise. One mistake in handling the workspace and/or equipment can result in sure contamination. 

On the other hand, by not leveling the playing field, pasteurization shifts the environment more favorably towards our microbes of interest. The fungi in culture receive a major head start, and with plenty of food supply from 100% colonized grain, have a higher chance of outcompeting contamination. 

Many mushroom substrates can be pasteurized, from straw, sawdust, agricultural wastes, to compost. We therefore will describe in detail two pasteurization strategies that have been proven effective in mushroom farming: (1) chemical treatment and (2) heat.

Chemical Treatment:

There are two effective chemical treatments that utilize readily available ingredients: hydrogen peroxide (H2O2) and high pH (basic) solution. 

Hydrogen Peroxide: 

How does it work?

Hydrogen peroxide (H2O2) induces oxidative stress in most microbes, killing off bacteria, yeasts, and other spores. Fungal mycelia are naturally resistant to the oxidative effects because they produce similar molecular compounds for their own self-defense and metabolism. 


  1. 3% H2O2, over the counter
  2. Water.


  1. Make 0.3% H2O2 solution: add 1 cup of 3% H2O2 to 9 cups of water. 
  2. Submerge substrate in the 0.3% H2O2 solution for 12 hours at room temperature (pasteurization step).
  3. Drain substrate thoroughly (until it stops dripping). 
  4. Substrate is now ready for inoculation. This method can be used for straw and other dry substrates.

High pH Treatment: 

How does it work?

This approach uses hydrated lime to raise the pH to 9.5 in the final substrate. At pH 9.5, the substrate pushes the high end of the mushroom mycelium's tolerance and beyond the limits of competitors. This method is commonly used for growing Oyster mushrooms (Pleurotus ostreatus).


  1. Hydrated lime (calcium hydroxide), from home improvement stores. Choose lime that is low in magnesium. 
  2. Water


  1. Make 0.5-1% hydrated lime solution: Dissolve 5 to 10 gram of hydrated lime into 1 liter of water. Wear gloves when handling the solution.
  2. Submerge the substrate in 0.5-1% hydrated lime solution for 12-24 hours. 
  3. Drain the substrate thoroughly and let dry to field capacity. You can check for field capacity by squeezing the substrate with your hands, and only a few droplets should come out.
    1. Dilute the waste hydrated lime solution with excess water to neutralize the pH before disposing. The neutralized pH lime solution is beneficial to plants, and can be applied to lawns and gardens.
  4. The substrate is now ready to inoculate. 

Heat Pasteurization:

How does it work?

Pasteurization requires heating the substrate to 140-170°F (60-77°C) for at least 1 hour. The high heat kills off filamentous fungi, their spores, and bacteria. This method is good for straw, agricultural wastes, and other dry substrates.


  1. Permeable heat-resistant bags (burlap sacks, onion bags, spawn bags, etc…) 
  2. Twines for tying bags. 
  3. A large pot.
  4. A heavy weight to keep the substrate submerged in the water.
  5. A thermometer.
  6. Water.


  1. Load substrate into permeable bags and close the ends with twine. 
  2. Heat water in a large pot to 180°F (82°C). Note: This is not boiling water - water boils at 211.9°F.  
  3. Put the bags of substrate in the hot water. Weigh down the substrate to keep them fully submerged. The temperature should drop in this process. 
  4. Insert thermometer probe into the center of the bag. Cover the lid to reduce heat loss. 
  5. Monitor the bag internal temperature for at least 1 hour, keeping it between 140-170°F (60-77°C). If the temperature drops below the desired range, add boiling water. Take extra precaution in dealing with boiling water. 
  6. Drain the substrate thoroughly to field capacity.
    1. The excess water can be used for one more batch, but no more than two total batches. The nutrient-rich water can be diluted as a fertilizer for plants, or used full-strength as a biodegradable weed killer. 
  7. Allow the substrate to cool before inoculation.  

Cultivators have successfully pasteurized substrate in jars at high heat for 1 hour. For inspiration, you can check out the following set-up:

  1. Stro's Take On Pasteurization:
  2. Frank's Proper Pasteurization Tek 3.0:

Once you've pasteurized your bulk substrate, you can then inoculate it with your 100% colonized grain spawn. Some mushroom growers perform this step in the open air, but you can use a laminar flow hood or a still air box to reduce the chances of contamination from spore floating in the air. 

The Bella Bora Still Air Box will help you with this next step in the process.


  1. Arevalo, W. DIY Mushroom Cultivation: Growing Mushrooms at Home for Food, Medicine, and Soil (Homegrown City Life, 6). New Society Publishers. 16 July 2019. 
  2. Stropharis. “Stro’s Take On Pasteurization - Mushroom Cultivation - Shroomery Message Board.” Shroomery, 13 May 2013,
  3. Horrigan, Frank. “Frank's Proper Pasteurization Tek 3.0 - Mushroom Cultivation - Shroomery Message Board.” Shroomery, 21 Nov. 2012, 


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