Sarah Brown
Mushroom substrate sterilization is a crucial step in the cultivation process, but it's not always necessary. Mushrooms can grow in the wild without sterile material, but sterilization gives your mushrooms the best chance of success by killing bacteria, moulds, and other potential contaminants. The need for sterilization depends on the type of mushroom and the substrate's nutritional content. High-nutrient substrates like manure, grains, and soy hulls require sterilization, while less nutritious substrates like straw and cardboard may only need pasteurization. Sterilization utilizes steam, pressure, time, and temperature to achieve a minimum of 250°F (121°C) for 2 hours, killing all living organisms and spores. Pasteurization, on the other hand, reduces competing organisms and is sufficient for most mushroom cultivation.
| Characteristics | Values |
|---|---|
| Purpose of sterilization | To kill all bacteria and pathogens |
| Purpose of pasteurization | To reduce the number of competing organisms within the substrate |
| When to use sterilization | When growing fussy or medicinal mushrooms that need nutrient-rich substrates |
| When to use pasteurization | When growing mushrooms that do not require nutrient-rich substrates |
| Sterilization methods | Steam, pressure, time, and temperature |
| Sterilization temperature | 250°F (121°C) for at least 2 hours |
| Pasteurization methods | Submerging the substrate in hot water for at least 1-2 hours, microwaving, or using a barrel steam sterilizer |
| Pasteurization temperature | 194-212°F (90-100°C) for up to 18 hours |
| Substrates that require sterilization | High-nutrient substrates like manure, soy hulls, grains, and master's mix |
| Substrates that require pasteurization | Less nutritious substrates like straw, sugarcane bagasse, coco coir, hardwood sawdust, and cardboard |
| Sterilization equipment | Pressure cooker, autoclave, retort, or pressure canner |
| Pasteurization equipment | Oven, microwave, or pressure cooker |
| Additional considerations | Keep the substrate moist, maintain proper hygiene, and store in a room free of germs to prevent contamination |
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What You'll Learn
Sterilisation is not always necessary
While sterilisation is important for growing mushrooms to kill bacteria and pathogens, it is not always necessary. The need for sterilisation depends on the type of substrate being used and the mushroom species being grown.
Some substrates, such as coffee grounds, do not require sterilisation or pasteurisation. Coffee grounds can be used as a substrate as long as they are utilised quickly and handled with proper hygiene. The brewing process pasteurises the grounds, eliminating the need for additional steps required for other substrates.
Additionally, some mushrooms cannot be cultivated on sterile media. These mushrooms must be grown outdoors or in nutrient-rich environments that do not allow for sterilisation.
The nutrient level of the substrate is a crucial factor in determining the need for sterilisation. Substrates that are considered food, such as rye grain, popcorn, brown rice, and wheatberries, should be sterilised to eliminate germs and microbial life. However, it is important to note that sterilising a substrate can reduce its nutrient content, which may negatively impact mushroom growth.
Furthermore, mushrooms have their own immune system that can fight off contaminants. While sterilisation minimises the risk of contamination-related growth issues, it is not always necessary for successful mushroom cultivation.
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Pasteurisation may be sufficient
Mushroom substrates are the nutrient source or 'food' for mushrooms. Sterilization of mushroom substrates is a method of preparing substrates using a combination of steam, time, temperature, and pressure to kill living organisms and spores. However, pasteurization may be sufficient for some mushroom substrates.
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. The more nutrients a substrate contains, the greater the chances of contamination. This is because other organisms, like mould and bacteria, also thrive in moist, nutrient-rich environments and often grow faster than mushroom mycelium.
Sterilization aims to kill all potential contaminants, while pasteurization eliminates most but not all competing organisms. There are several pasteurization methods that reduce the competing organisms in less nutritious substrates, like straw and sugarcane bagasse, enough to give fast-growing mushroom species an advantage.
Coffee grounds, for example, do not require pasteurization or sterilization as long as they are put to use quickly and handled with proper hygiene and care. This is because the brewing process pasteurizes the grounds.
For other substrates, pasteurization can be accomplished in a number of ways. One approach is to submerge the substrate in hot water for at least one or two hours. Pasteurization is sufficient for most enterprises to grow mushrooms with a minimum risk of contamination.
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Nutrient levels determine sterilisation
The nutritional value of a substrate and the type of mushrooms you want to grow will dictate whether the substrate needs sterilisation or pasteurisation. The more nutrients a substrate contains, the greater the chances of contamination. Moist, nutrient-rich substrates are appealing to mushroom mycelium, but they also attract other organisms, like mould and bacteria, which often grow faster than mushroom mycelium.
High-nutrient substrates or substrates with high levels of supplementation, such as manure, soy hulls, grains, and master's mix, require sterilisation. Sterilisation kills all living and dormant organisms and fungal spores and gives your mushroom mycelium the best possible head start.
However, less nutritious substrates like straw, sugarcane bagasse, coco coir, hardwood sawdust, and cardboard only need pasteurisation. Pasteurisation is sufficient for most enterprises to grow mushrooms with minimal risk of contamination. It reduces the number of competing organisms within the substrate, giving mushroom cultures a competitive advantage.
Coffee grounds are an example of a substrate that does not require sterilisation or pasteurisation as long as they are put to use quickly and handled with proper hygiene and care. The brewing process pasteurises the grounds, allowing growers to skip the additional steps needed for other substrates.
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Sterilisation equipment
The equipment used to sterilise mushroom substrates depends on the scale of the operation and the type of mushrooms being grown. Small-scale growers often use pressure cookers, while larger operations may use autoclaves, retorts, or pressure canners.
Pressure Cookers
Pressure cookers are a common tool for sterilising mushroom substrates, especially for small-scale growers. They are suitable for sterilisation because they can achieve the high temperatures and pressures required to kill microorganisms and spores. The required temperature for sterilisation is typically 121°C (250°F) at 15 PSI, which is much higher than the boiling point of water (100°C at sea level).
Autoclaves
Autoclaves are advanced equipment used for sterilisation, especially in laboratory settings and larger mushroom-growing operations. They are designed to handle challenging loads and consistently sterilise various mushroom substrates, including grain. Autoclaves offer improved control over the sterilisation process and can be purpose-built for mushroom cultivation.
Barrel Steam Sterilisers
Also known as retorts, barrel steam sterilisers are used by some mushroom growers. They are large, insulated barrels that utilise steam to sterilise the substrate. While they may not reach the full sterilisation temperature due to low pressure, they can come close to the required temperature for mushroom growers.
Pasteurisation Equipment
While pasteurisation does not achieve complete sterilisation, it is a common method for treating mushroom substrates, especially those with low nutrient content. Pasteurisation equipment includes large barrels, such as 44-gallon drums, which can be heated using various methods like electricity, gas, fire, or solar hot water. Pasteurisation typically targets temperatures between 60-80°C (140-176°F) for a prolonged period.
Other Equipment
Other equipment mentioned for mushroom substrate sterilisation includes ovens, which can achieve the required temperatures but may dry out or burn the substrate. Additionally, some growers use steam generators in conjunction with insulated barrels for steam sterilisation.
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Rehydration after sterilisation
After sterilisation, aseptic conditions are required for all stages going forward. This is because the substrate is now a perfect habitat for the fastest-growing organism to claim it. Fungi, moulds, yeast, and bacteria can colonise the substrate more quickly than the mushroom culture. Therefore, rehydrating the substrate after sterilisation must be done carefully to prevent contamination.
If the substrate dries out too much during sterilisation, it will need to be rehydrated with distilled water to prevent contamination. This is especially important if you are using an oven to sterilise your substrate, as the dry heat can dry out the substrate and may burn it, reducing the nutrients available for the mycelium.
To rehydrate the substrate, you can use either distilled water or lime-saturated water, depending on the type of substrate. If using straw or woody substrates, they may require soaking for up to 12 hours. For wood pellets, soaking is not required. Instead, use a sufficient amount of lime-saturated water to hydrate the batch to field capacity. The rehydration process should take around 15-20 minutes.
It is important to note that rehydrating the substrate after sterilisation does not guarantee that it will be contaminant-free. To maintain sterility, it is crucial to work in a sterile environment, such as a still-air box or in front of a laminar flow hood, when handling the substrate after rehydration.
Additionally, it is worth mentioning that not all mushroom substrates require sterilisation. Some mushrooms cannot be cultivated on sterile media and must be grown outdoors or in nutrient-rich environments. Low-nutrient substrates like straw, sugarcane bagasse, coco coir, hardwood sawdust, cardboard, and coffee grounds only require pasteurisation or no treatment at all.
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Frequently asked questions
It depends on the type of mushroom and substrate. Sterilization kills all bacteria and pathogens, while pasteurization reduces the number of competing organisms. Sterilization is required for high-nutrient substrates like manure, grains, and soy hulls. Less nutritious substrates like straw, cardboard, and coco coir only need pasteurization.
Sterilization requires a combination of steam, pressure, time, and temperature. A temperature of at least 250°F (121°C) must be maintained for a minimum of 2 hours. This can be achieved using a pressure cooker or an autoclave, retort, or pressure canner for larger operations.
No, sterilization requires temperatures above the boiling point of water, which cannot be achieved in a standard oven or microwave. Microwaves can, however, be used for pasteurization.
Yes, coffee grounds can be used as a substrate without sterilization or pasteurization if they are handled with proper hygiene and used quickly. The brewing process pasteurizes the grounds, eliminating the need for further treatment.
