Laura Smith
Sterilization is a crucial step in the mushroom cultivation process, particularly for nutrient-rich substrates. It involves heating the substrate to temperatures exceeding 120°C under pressure to eliminate all living organisms, spores, and pathogens. This process is essential for growing certain mushroom varieties, such as gourmet or medicinal species, and ensures the mushrooms have the best possible start by reducing competition from mould and bacteria. While sterilization is not always necessary, it significantly reduces the risk of contamination-related issues and sickness, making it an important consideration for cultivators. This introduction sets the context for exploring the specific methods and techniques used to sterilize mushroom substrates effectively.
How to Sterilize Mushroom Substrate
| Characteristics | Values |
|---|---|
| When to sterilize | When growing fussy, delicate, or medicinal mushrooms that need nutrient-rich substrates |
| Purpose | To kill all living organisms, spores, and dormant contaminants |
| Temperature | Above 120°C (248°F) |
| Time | At least 2 hours |
| Pressure | 15 PSI |
| Equipment | Pressure cooker, jars or bags of prepared substrate, a wire rack or jar lids, water |
| Alternative methods | Pasteurization, submerging substrate in hot water, using hydrated lime-treated cold water, hydrogen peroxide |
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What You'll Learn
When to sterilise
Sterilization is not always necessary when preparing mushroom substrates. The nutritional value of the substrate and the type of mushrooms you want to grow will determine whether sterilization or pasteurization is required.
If you are growing mushrooms that require nutrient-rich substrates, such as medicinal mushrooms or gourmet species, sterilization is essential. This is because the more nutrients a substrate contains, the more likely it is to be contaminated by mould, bacteria, or other organisms that can grow faster than mushroom mycelium. Sterilization kills all living and dormant organisms and fungal spores, giving your mushroom mycelium the best possible start. Examples of substrates that typically require sterilization include manure, soy hulls, grains, and master's mix.
On the other hand, if you are growing mushrooms that do not require nutrient-rich substrates, such as oyster mushrooms, pasteurization may be sufficient. Pasteurization reduces the number of competing organisms in the substrate but does not kill all potential contaminants. Examples of substrates that typically only require pasteurization include straw, sugarcane bagasse, coco coir, hardwood sawdust, and cardboard.
It is also important to consider the growth environment when deciding whether to sterilize or pasteurize. If your grow environment is prone to contamination, such as if you have carpets, textiles, curtains, or pets in the room, sterilization may be necessary to prevent issues.
Additionally, if you are repurposing spent mushroom blocks to grow new mushrooms, sterilization may be required to kill any remaining spores or contaminants. However, keep in mind that your yields will likely drop with each new batch.
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When to pasteurise
The need for pasteurisation depends on the type of mushroom and substrate you are using. Pasteurisation is the process of heating a substrate to temperatures between 65 and 85°C for 1.5 to 2 hours. It will not get rid of all impurities, but it will reduce the population of other bacteria to a point where the mushroom species will have an advantage.
If you are using low-nutrient substrates, such as straw, sugarcane bagasse, coco coir, hardwood sawdust, or cardboard, pasteurisation is sufficient. These substrates are less appealing to other organisms, so there is less competition for your mushrooms.
However, if you are growing more delicate, gourmet, or medicinal mushrooms that require nutrient-rich substrates, you will need to sterilise your substrate. This is because the more nutrients a substrate contains, the greater the chances of contamination. Moist, nutrient-rich substrates are appealing not only to mushroom mycelium but also to mould, bacteria, and other organisms, which often grow faster than mushroom mycelium.
If you are using substrates with high levels of supplementation, such as manure, soy hulls, grains, or master's mix, you will need to sterilise them. In this case, pasteurisation will not be enough to prevent contamination and give your mushrooms the best chance of growing.
It is worth noting that some mushrooms cannot be cultivated on sterile media and must be grown outdoors or in nutrient-rich environments. Additionally, sterilisation can kill beneficial microbiology, which can help prevent contamination by not leaving space for outside contamination to occupy. Therefore, it is important to consider the specific needs of the mushrooms you are growing and the characteristics of your substrate when deciding whether to pasteurise or sterilise.
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Using a pressure cooker
Small-scale mushroom growers typically use pressure cookers for sterilisation. This method is ideal for those with some mushroom-growing experience who want to try growing more challenging gourmet or medicinal species. Pressure cookers can be used to sterilise high-nutrient substrates, which are more prone to contamination by bacteria and mould.
To sterilise a substrate, you need temperatures higher than 250°F (121°C) for a minimum of 2 hours. This temperature cannot be achieved by boiling water or steam alone, so pressure needs to be added. Most mushroom growers will need 15 PSI of pressure to increase the temperature to the required level. Before using your pressure cooker, check that all parts are in good working order and there are no dents, bulges or cracks.
To begin the sterilisation process, place a metal rack or jar lids at the bottom of the pressure cooker, then put your substrate jars or bags on top. Ensure that the jars or bags are not touching the sides of the pot, as this can cause damage. You can line the lower sides with a kitchen towel or cloth if necessary. Fill your cooker with your substrates and enough water to cover them. It is typically recommended to add around 3 quarts (2.8 litres) of water, or enough to cover the substrates up to 1 to 2 inches (2.5 to 5 cm) from the sides. Seal the pressure cooker and heat it on a burner according to the manufacturer's instructions.
Once the sterilisation process is complete, remove the pressure cooker from the oven and take out the substrate. Store it in a room free of germs that could cause contamination. Before inoculating the block with spawn, let it cool to at least 30°C.
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Sterilisation methods
Sterilisation is necessary for mushroom substrates with high levels of supplementation, such as manure, soy hulls, grains, and master's mix. It is also required for nutrient-rich substrates, which attract not only mushroom mycelium but also bacteria and mould, which can grow faster than the mycelium.
Sterilisation aims to kill all potential contaminants, whereas pasteurisation only eliminates most competing organisms. Sterilisation is achieved by heating the substrate to temperatures above 120°C under pressure, typically requiring a pressure of 15 PSI to reach this temperature. This process kills all living and dormant organisms and fungal spores, giving your mushroom mycelium the best possible start.
There are several methods for sterilising mushroom substrates:
- Using a pressure cooker: This is the most straightforward method for small-scale growers. You will need a pressure cooker, jars or bags of prepared substrate (mushroom grow bags are recommended as they can withstand high sterilisation temperatures), a wire rack or jar lids for the bottom of the pressure cooker, and water. Before using the pressure cooker, carefully check that all parts are in good working order and there are no dents, bulges, or cracks.
- Submerging the substrate in hot water: This method involves submerging the substrate in hot water for at least one to two hours.
- Using hydrated lime-treated cold water: Immerse your substrate for 24 hours in a bath of hydrated lime-treated cold water. This raises the pH of the water, thus destroying pollutants.
- Hydrogen peroxide bath: Foreign germs and competitor spores can be killed by hydrogen peroxide without harming the mycelium. Soak the substrate for about an hour in water, then drain and clean it before draining again. Allow the straw substrate to soak in a hydrogen peroxide water bath for a day. For every 4.5 litres of water, use 1 litre of hydrogen peroxide. After you’ve finished, rinse and drain your substrate.
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Sterilisation temperature
The ideal temperature for sterilising mushroom substrate is a highly debated topic, with various methods and temperature suggestions available. Sterilisation is necessary for substrates with high levels of supplementation, such as manure, soy hulls, grains, and master's mix. It is also essential for nutrient-rich substrates, as these attract not only mushroom mycelium but also bacteria and mould, which can outcompete the mushrooms.
The general consensus is that a temperature of at least 250°F (121°C) is required for effective sterilisation. This temperature should be maintained for a minimum of 2 hours. At this temperature, all living organisms and spores in the substrate are eradicated, giving your mushroom mycelium the best possible start. It is important to note that boiling water or steam alone cannot achieve sterilisation, as the boiling point of water is lower than the required temperature.
Some sources suggest that a temperature of 350°F is more appropriate for sterilisation. However, it is not clear how long this temperature should be maintained for effective results. Additionally, one source mentions that pasteurisation, a less intensive process, can be achieved by heating the substrate to temperatures between 65°C and 85°C for 1.5 to 2 hours.
To achieve sterilisation temperatures, a pressure cooker is commonly used for small-scale growers. This method requires a pressure of 15 pounds per square inch (PSI) to reach the desired temperature. As the scale of the operation increases, more advanced equipment, such as an autoclave, retort, or pressure canner, may be required.
It is worth noting that not all mushroom substrates require sterilisation. Low-nutrient substrates like straw, sugarcane bagasse, coco coir, hardwood sawdust, and cardboard can be adequately treated through pasteurisation. Additionally, some mushrooms cannot be cultivated on sterile media and must be grown outdoors or in nutrient-rich environments. Therefore, it is essential to consider the type of mushrooms you want to grow and their specific substrate requirements before deciding on the sterilisation temperature.
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Frequently asked questions
Not always. The nutritional value of a substrate and the type of mushrooms you want to grow will dictate whether the substrate needs sterilization or if pasteurization will do.
Sterilization aims to kill all potential contaminants, while pasteurization eliminates most, but not all, competing organisms.
A temperature of at least 250°F (121°C) for a minimum of 2 hours.
You will need a pressure cooker, jars or bags of prepared substrate, a wire rack or jar lids for the bottom of the pressure cooker, and water.
Sterilization kills all living and dormant organisms and fungal spores, giving your mushroom mycelium the best possible start.
