How to Pasteurize Mushroom Substrate in 5 Easy Steps - Nublume

How to Pasteurize Mushroom Substrate in 5 Easy Steps

What is Pasteurization?

Pasteurization is a crucial process for growing mushrooms, as it helps to eliminate harmful competitor organisms that may thrive in the same moist and nutrient-rich environment that the mushroom mycelium needs to grow. Mycelium is the vegetative part of the fungus that eventually gives rise to the fruiting body or mushroom that we consume.


Through pasteurization, the mushroom substrate is treated to create an environment where the mushroom mycelium can thrive and grow without competition from other organisms. This process helps to ensure that the mushroom spawn has a higher chance of success, resulting in a higher yield of edible mushrooms.


Fortunately, there are several different pasteurization methods that are available, including low-tech approaches that are ideal for beginner mushroom growers. By learning about pasteurization and the various methods that we will be discussing in this blog, individuals interested in growing their own mushrooms can take the first step towards producing a bountiful crop of delicious and nutritious fungi.


Does Mushroom Substrate Need to be Pasteurized?

If you're looking to cultivate mushrooms successfully, it's crucial to take steps to minimize the presence of harmful competitor organisms. The growing conditions that are optimal for mushrooms are also the ideal environment for organisms such as mold and bacteria to thrive. Unfortunately, these competitors grow at a faster rate than the mushroom mycelium, and if left unchecked, they will quickly take over the substrate before the mycelium can establish itself.


To avoid this, you'll need to either pasteurize or sterilize your mushroom substrate before inoculating it with mushroom spawn. By doing so, you can create an environment that is more conducive to the growth of the mushroom mycelium and less hospitable to other organisms that could potentially harm your crop. With the right techniques and knowledge, you can increase your chances of success and produce a bountiful yield of high-quality mushrooms.


What’s the Difference Between Pasteurization and Sterilization?

When it comes to preparing mushroom substrate, it's important to understand the differences between pasteurization and sterilization. Pasteurization is a process that reduces the number of living organisms in the substrate, while sterilization aims to eliminate all living organisms completely.


Pasteurizing mushroom substrate can be compared to removing as many weeds as possible from a garden before planting vegetables. This process is less aggressive than sterilization, which is like scorching the soil and killing every living plant and seed before planting vegetables. One benefit of pasteurization is that it leaves behind beneficial heat-tolerant bacteria that can protect the substrate from competing organisms during colonization.


The presence of beneficial bacteria in the substrate means that it's possible to inoculate it without needing completely sterile conditions. In contrast, sterilizing the substrate kills absolutely everything in it, leaving a blank canvas on which anything can grow. As a result, sterilized substrates need very sterile conditions during inoculation to avoid introducing contaminants.


Understanding the differences between these two methods can help you choose the best approach for your mushroom cultivation needs and achieve optimal results.


How to Pasteurize Substrate: Different Methods

Mushroom cultivators employ diverse techniques to pasteurize their mushroom substrates, the materials in which mushrooms grow. To select the most effective method, various factors should be taken into consideration, such as your proficiency, the specific species of mushrooms you intend to grow, as well as the type and quantity of substrate you plan to use.


Two primary pasteurization methods are heat pasteurization and cold pasteurization. Heat pasteurization involves heating the substrate to high temperatures to eliminate unwanted bacteria and other microbes, while cold pasteurization utilizes chemicals such as hydrogen peroxide or ultraviolet light to disinfect the substrate. In the following sections, we will delve into more details on these methods.


How Long Does it Take to Pasteurize Mushroom Substrate?

The duration for pasteurizing mushroom substrate varies depending on the method used and the amount of substrate to be pasteurized.


For instance, hot water pasteurization takes about two hours, while cold water fermentation can take one to two weeks depending on the amount of substrate. Cold water bath pasteurization falls in between, with the recommended duration of soaking the substrate being 16 to 20 hours.


Hot Water Pasteurization

Hot water pasteurization is a method that works well for small-scale home cultivators, but can be costly when pasteurizing bulk substrate. This method involves submerging the substrate in hot water between 149 and 167°F (65 and 75°C) for one to two hours.


The hot water at this temperature is effective in killing heat-sensitive competitors while leaving some beneficial heat-tolerant bacteria alive in the substrate. It is important to be mindful of the water temperature, as overheating can kill the beneficial bacteria.


To pasteurize the substrate in hot water, the water should be brought to a boil and then turned down to maintain the desired temperature range. Using a thermometer can help ensure the correct temperature is maintained. The substrate should be added to the hot water, either loose or in an old pillowcase or net bag, with a weight on top to keep it submerged.


After maintaining the required water temperature for around two hours, the heat should be turned off, and the substrate should be cooled. If the substrate is in a pillowcase or bag, it should be hung up to drain and cool. If a quicker cooling process is desired, the substrate can be taken out of the bag and spread out on a clean surface.


Steam Pasteurization

Steam pasteurization, a process similar to hot water pasteurization, involves using steam to heat the substrate instead of water. However, this method requires specialized equipment and a significant amount of energy, making it more costly and less practical for home cultivators. To use this method, steam is released into containers or insulated rooms containing layers or bags of pre-moistened substrate. It is important to take care not to overheat the substrate, as overheating can sterilize it and kill any beneficial bacteria present.


Oven Pasteurization

It is possible to pasteurize substrate in an oven, although this process may not be as reliable as using hot water or steam pasteurization. The amount of substrate that can be pasteurized in the oven is limited by the size of the oven, unlike hot water and steam pasteurization, which can be scaled up for larger quantities. To pasteurize substrate in the oven, the first step is to preheat the oven to 390 to 400°F (200 to 210°C). Then, the substrate is hydrated until it's slightly above field capacity to account for water loss through evaporation. The hydrated substrate is then placed in an aluminum foil baking tray and covered with tightly sealed tin foil to minimize evaporation. The tray is placed in the oven and left to pasteurize. After 60 minutes, the temperature in the middle of the substrate is tested using a thermometer. The aim is to reach a temperature of 160 to 180°F (71 to 82°C). If the temperature is not achieved, the heat should be left on for another 30 minutes. If the correct temperature is reached, the oven should be turned off and the substrate left to cool down slowly.



Large-scale oyster and button mushroom farms use fermentation as a method for pasteurizing substrate.


To begin, they chop and moisten the substrate and turn it regularly for the first few days to promote fermentation. This is known as phase one composting, and it causes the substrate to heat up similarly to a compost pile.


In phase two, they load the substrate into an insulated chamber or tunnel and allow it to heat up using the self-generated heat from the fermentation process.


Once phase two is complete, the substrate is clean and pasteurized, and it's now ready for spawning.


While this method is effective for large-scale production, it's challenging to achieve on a smaller scale as it requires a significant volume of fermented substrate to generate high enough temperatures to pasteurize.


Cold Water Fermentation

Natural fermentation is a pasteurization method that is energy efficient and well-suited for handling large amounts of straw, but it does require a significantly longer processing time than other techniques and can produce an unpleasant odor.


This method involves submerging the straw in non-chlorinated water for approximately a week, during which anaerobic microorganisms (bacteria that do not require oxygen to survive) grow and break down the substrate.


As a result, organisms that require oxygen, such as fungal spores and aerobic bacteria, are eliminated.


Finally, when the straw is removed from the water, drained, and exposed to oxygen, the anaerobic organisms perish, leaving behind pasteurized straw.


Cold Water Bath Pasteurization

Cold water bath pasteurization is a method of pasteurizing substrate by soaking it in a solution that both hydrates the straw and kills harmful organisms.


Several substances can be used in the cold water bath to achieve pasteurization, such as lime, wood ash, soap, bleach, chlorine, vinegar, and hydrogen peroxide. Our preferred method is lime bath pasteurization.


Lime Bath Pasteurization

This pasteurization method is highly regarded by many cultivators due to its effectiveness compared to other cold water bath techniques. It involves using hydrated lime to increase the pH of the water bath to a significant degree. Once the substrate is submerged in the lime bath, the sudden shift in pH damages the cell walls of the microorganisms, resulting in their death.


Wood Ash Bath Pasteurization

Wood ash pasteurization is another technique that works similarly to lime bath pasteurization by increasing the pH of the water bath. Wood ash is used instead of lime and can raise the pH of the water to a level between 11 and 14. This method is considered more environmentally friendly and can result in beautiful mushroom flushes. However, finding a continuous source of untreated hardwood ash can be challenging. Additionally, more wood ash is required compared to lime to achieve a suitable pH level for pasteurization. The recommended amount of ash is 3% of the water weight or 30 g (1 oz) per 1 liter (0.26 gallons) of water. While this method is typically used to pasteurize straw, it can also be used for hardwood pellets.


Soap Bath Pasteurization

Soap pasteurization is a simple and effective method for sterilizing substrate. Growers can use inexpensive dishwashing or laundry detergent to make a soap solution that has successfully pasteurized substrate. However, it is important to use washing powder instead of liquid soaps, as some liquid soaps contain preservatives with antifungal properties. This pasteurization method works by using osmotic pressure to break down the microorganisms' cell walls, causing them to rupture. When you submerge the substrate in the water with added soap, the difference in osmotic pressure between the solution and the organism's cells causes the cell walls to rupture and the organisms to die. Using a small amount of soap, about 0.6 g (0.16 oz) per 1 liter (0.26 gallons), creates a solution that is effective for pasteurizing substrate.


Chlorine Bath Pasteurization

You can use household bleach or swimming pool chlorine for chlorine bath pasteurization. Chemically they’re very similar but you’ll generally need to add more household bleach than swimming pool chlorine to create your solution. 


This method involves adding household bleach or chlorine containing 5.25% sodium hypochlorite to water and submerging the substrate for 16 to 20 hours. When using household bleach you’ll need to add it at a rate of 0.35% of water weight or 355ml (12 oz) per 1 liter (0.26 gallons) of water. The chlorine in the water bath oxidizes, killing off most living organisms and gasses off before you inoculate the substrate, so it should not affect the mushroom mycelium. But to be safe, we recommend neutralizing the chlorine content of the solution before you drain the substrate using a chlorine reducer. 


You’ll find chlorine reducers at swimming pool shops and they will bring the chlorine level in the water back down to a range that’s similar to tap water. When using this method without the chlorine reducer, it’s best to discard the leftover solution somewhere without vegetation to prevent scorching.


Vinegar Bath Pasteurization

Vinegar pasteurization is a process that lowers the pH of the water bath, which is different from lime and wood ash methods. To make a vinegar bath, add 5% white vinegar to water at a rate of 2% of water weight, which is about 20g (0.07 oz) per 1 liter (0.26 gallons). You want the pH of the solution to be between 3.5 and 4.


Before soaking the substrate, check the pH to make sure it falls within the required range. Submerge the substrate in the vinegar solution and let it soak for about 16 to 20 hours. After removing the substrate, let it drain for about an hour, and it's ready for inoculation.



  • cost-effective and simple alternative to sterilization

  • does not require a completely sterile environment for spawning, making it more forgiving and easier for beginners to use

  • the pasteurization process is typically less time-consuming and less expensive than sterilization.




  • you will find that you are limited to certain species cremini, portabella, oyster, wine cap, pioppino, and straw strains of shiitake

  • higher spawn rate when inoculating the substrate,

  • pasteurized substrate is more vulnerable to contamination when using substrates with high nutrient content


The Bottom Line
Cultivating gourmet mushrooms can be a gratifying experience as they not only taste good but also provide a range of health benefits. It can also be enjoyable to witness the growth process. Thankfully, the low-tech pasteurization techniques explained above have made it easy for everyone to grow mushrooms at home. These methods are cost-effective and consume less energy.


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