Substrate sterilization: definition, different approaches, when, and where to apply?

7 min readFeb 28, 2021


As every mycologist knows, the main initial goal when growing mushrooms is to colonize a new substrate — growing mycelium (it takes approximately two weeks).

If you do not ensure your mushrooms get a head start, your mushroom substrate will be vulnerable to harmful contaminants.

The most effective way to jump-start your mushrooms in a new substrate is through sterilization. That way, harmful competitors are being killed.

What Exactly Is A Mushroom Substrate?

A mushroom substrate refers to the material that mycelium grows from and establishes itself in. It provides the mushrooms with the energy, moisture, and nutrients necessary for growing and fruiting. It equivalents to using soil to grow plants.

A good substrate must be dense in fibrous, woody materials like hemicellulose, cellulose, and lignin. Below are common mushroom substrate materials to choose from:


If you want a cheap yet effective substrate material, you can opt for straw. You can grow mushrooms in a substrate composed of 100% straw or try adding supplements for additional nutrients.

Coffee grounds

Coffee grounds are another cheap materials to grow mushrooms, and some coffee shops even give them for free. The recipe is of the easiest! All you need is to combine one kg of coffee grounds with 100 g of mushroom spawn.

Coco coir and vermiculite

Coco coir is available in most garden stores and is a material made of ground-up husks and shells. Vermiculite refers to the yellowish-brown mineral used for retaining moisture. Just mix these two materials.

Hardwood sawdust

Beech, maple, and oak are the best hardwoods for making an excellent substrate for different mushroom types. You can combine them with a bran supplement for the best results.

Soy hulls

You can mix soy hulls with hardwood sawdust at varying ratios to produce high-yielding and effective substrates.


You can use cow, chicken, or horse manure for mushrooms. The recipes commonly call for one part of coco coir with two parts manure. Add water to reach the capacity of the field. Ensure to sterilize the entire mixture before inoculating with mushroom spawn.

What Is Substrate Sterilization?

Sterilization is the process of heating the substrate to an extreme temperature that exceeds 250˚F under pressure. The goal is to eliminate any dormant or living contaminants within the substrate entirely.

The process is done by heating the substrate under pressure for an extended time. If you supplement it with sawdust fruiting blocks, the standard protocol is to pressure sterilize at 15 PSI for two and a half hours. That brings the substrate temperature up to 250˚F, which is effective for killing any potential contaminants.

Besides using pressure to sterilize the mushroom substrate, you can try five different sterilization methods without pressure.

What Are the Different Substrate Supplementation?

  • Macro-Nutrient
  • Micro-Nutrient
  • Lightning
  • Bacteria
  • Fungi

(Meavien will have a more detailed blog post about Mushroom Substrate Supplements in future posts.)

There are several reasons why you should use supplements in the mushroom substrate., including:


Supplementation is necessary to promote growth. It can reduce the harvest time, which will result in more production per season.

Fruiting Bodies

You can use supplements for adjusting the fruiting body’s number. That way, each mushroom will get an even overall size.


Another reason to use supplements is to impact yield per flush the entire harvesting period positively.


When you use the right supplement for the mushrooms, you can increase the yield.

How to Sterilize Substrate without a pressure cooker?

If you want to improve your mushroom cultivation, the sterilization process is an advantage. Like any other plant, mushrooms need a suitable environment. However, if the substrate is contaminated, the entire growth process and cultivation will be a total mess.

That is why you need to perform substrate sterilization. If you have no pressure cooker or do not want to use it, you can try the following methods:


Composting is best for growing button mushrooms (Agaricus). However, you can also use it to grow oyster mushrooms (Pleurotus). The method comes in a two-step process.

Phase 1

This phase is the first step for decomposing the mixed raw materials. It is a biological and chemical process and is commonly done outside. During this phase, you can notice that the substrate heats up to 80˚C.

Phase 2

This time, the second phase is done inside. It is a biological process that finalizes the decomposing. During this phase, the temperature is rigorously controlled for a specific amount of time.

In the first part of phase 2, the pasteurization, the temperature is set to 56–60˚C for eight hours. Next is the conditioning period, where the temperature drops to 45˚C for up to seven days. Once the volatile ammonium cleared from the process air, the conditioning period ends.’

Chemical Sterilization

Many mushroom growers typically use chemical sterilization because of its affordability. However, different chemicals are involved in the process, including Na hypochlorite, Formaldehyde, Carbendazim 0.5%, and Derosal 0.01%. These chemicals are poisonous and hazardous to handle.

Alternatively, you can go for cold sterilization. It is a cheap method, where you have to soak the substrate (typically straw) for 12–24 hours in non-toxic chemicals like powdered chlorine or hydrated lime.

The lime is known to increase the pH level while killing off any contaminants. The process is ideal for oyster mushrooms. You need to ensure that the lime contains less than 2% magnesium. As for the chlorine, it is used in its neutralized form. It provides sterility and an appropriate environment for all mushroom types.


You can perform the scalding process with the following steps:

Step 1

Chop the substrate like straws into 1–6 cm long pieces.

Step 2

Heat the water up to 80˚C and add substrate then. Mix it thoroughly and let the process run for an hour.

Step 3

Remove the substrate from the water and let it drain for a maximum of 120 minutes. Ensure not to drain it too long to prevent it from getting contaminated again.

Step 4

Inoculate the substrate with 5% on the w/w wet weight basis. Finally, place the inoculated substrates in the bags and into the grow chamber.


The pasteurization process is used widely to sterilize the substrate. All you have to do is to follow the steps below:

Step 1

Chop the substrate like straws into 1–6 cm long pieces. Place them into your bags.

Step 2

Heat the water to 100˚C. Add the bags with the pressure relief valve.

Step 3

Introduce the hot stream from the water tank into the barrel. Let the process run between 12 and 15 hours, and ensure that your tank has enough water.

Step 4

Remove your bags and let them cool down for a minimum of 24 hours. Mind the draining process depends on the substrate’s density. The temperature within the bags must be below 28˚C. If not, the heat will end up killing the mycelium.

Step 5

Inoculate the substrate with 5% on the w/w wet weight basis. After that, put the inoculated substrate into the grow chamber.


Developed by John Tyndall, tyndallization is a sterilization process that involves treating the products in cycles. This sterilization process lasts up to six days. You can perform the process with the following steps:

Step 1

Sterilize the substrate for 30 minutes at 100˚C.

Step 2

Sterilize the substrate for about 12 hours at 37˚C.

Step 3

Sterilize the substrate for 30 minutes at 100˚C.

Step 4:

Repeat steps 1 and 2 for three days.

How to Sterilize Substrate Using A Pressure Cooker?

Fighting contaminants is your constant battle when growing mushrooms. So, the best thing to do is to sterilize your substrate. You can use a pressure cooker to perform the sterilization process. Ensure to use a pressure cooker that is capable of 15 PSI.

Below are the steps to follow when using pressure to sterilize the mushroom substrate.

Step 1

Fill your pressure cooker with 2.5 cm of cold water. Ensure to have enough water in the pressure cooker to prevent it from running dry during the sterilization process.

Step 2

Place a rack in the pressure cooker. Next is to place the jars or spawn bags on top of the rack. In case you have no rack, the lids of the jars are good alternatives. This will prevent contact between the pressure cooker and jar that can lead to glass breakage.

Step 3

Close the pressure cooker and ensure the lid is closed appropriately.

Step 4

Let the pressure cooker build up pressure. Ensure to heat it to do that slowly. Once you notice that it is on 15 PSI, let it do its job for 45 minutes.

Step 5

Turn off the heat. Let the jars cool down for about five hours.


The sterilization process is an essential step in mushroom cultivation. It is the best way to kill off any harmful competitors in the substrate. You can do this by using a pressure cooker. If you do not have it, you can try the five methods above or buy the sterilized substrate in a bulk directly from Meavien. By following proper instructions, you can get the highest results possible.

Let us know if you have questions about how to cultivate mushrooms or feel free to purchase our sterilized spawn bags in bulk. We will love to hear from you soon.

By Paul S.

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