Michael Davis
Sterilising mushroom substrate is an essential step in cultivating certain types of mushrooms. While not all mushroom substrates need to be sterilised, it is a necessary step for growing delicate, medicinal, or gourmet mushrooms that require nutrient-rich substrates. Sterilisation aims to kill all potential contaminants, including living organisms and fungal spores, to give mushroom mycelium the best chance to grow. This process requires temperatures above 120°C, typically reaching 121°C (250°F) at 15 PSI, for at least 2 hours. Various methods can be used for sterilisation, including pressure cookers, autoclaves, and retorts, each requiring specific preparation and equipment.
How to Sterilise Mushroom Substrate
| Characteristics | Values |
|---|---|
| When to sterilise | Sterilisation is required for substrates with high levels of supplementation, such as manure, soy hulls, grains, and master's mix, or nutrient-rich substrates. It is also necessary for cultivating gourmet, medicinal, or delicate mushrooms. |
| When pasteurisation is enough | Less nutritious substrates like straw, sugarcane bagasse, coco coir, hardwood sawdust, and cardboard only need pasteurisation. Pasteurisation is also sufficient for most enterprises to grow mushrooms with a minimum risk of contamination. |
| Sterilisation methods | Sterilisation can be achieved through pressure cookers, autoclaves, retorts, or pressure canners. |
| Sterilisation conditions | A temperature of at least 250°F (121°C) for a minimum of 2 hours is required to sterilise a substrate. A pressure of 15 psi is typically required to reach this temperature. |
| Pasteurisation methods | Pasteurisation can be achieved by submerging the substrate in hot water for at least one to two hours, using a DIY approach to build the necessary equipment, or through solar hot water. |
| Pasteurisation conditions | The ideal pasteurisation temperature is around 70°C (158°F), although pasteurisation can occur as low as 63°C-65°C and up to 85°C. |
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What You'll Learn
When to sterilise mushroom substrate
Sterilization of mushroom substrates is necessary when growing certain types of mushrooms, particularly those that are more susceptible to contamination or require nutrient-rich substrates.
The need for sterilization arises from the fact that moist, nutrient-rich substrates, which are ideal for mushroom growth, also attract unwanted organisms such as mould, bacteria, and other fungi. These contaminants often grow faster than the desired mushrooms, leading to competition for resources and potential sickness in the mushrooms.
Therefore, sterilization is crucial when dealing with high-nutrient substrates or substrates with high levels of supplementation, such as manure, soy hulls, grains, and master's mix. In these cases, sterilization kills all living and dormant organisms, giving the desired mushroom species a competitive advantage and reducing the risk of contamination-related issues.
On the other hand, less nutritious substrates like straw, sugarcane bagasse, coco coir, hardwood sawdust, and cardboard may not require sterilization. Instead, pasteurization, which eliminates most but not all competing organisms, can be sufficient for growing faster-growing mushroom species.
Additionally, some mushrooms cannot be cultivated on sterile media and must be grown outdoors or in specific nutrient-rich environments. Therefore, it is essential to consider the type of mushrooms being cultivated and their unique requirements when deciding on the need for sterilization.
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When to pasteurise mushroom substrate
The timing of mushroom substrate pasteurisation depends on several factors, including the type of mushroom, the substrate, and the equipment available.
Firstly, not all mushroom substrates require sterilisation or pasteurisation. If you are growing mushrooms that require a sterile medium, such as certain types of mushrooms that cannot be cultivated outdoors or in nutrient-rich environments, you must sterilise the substrate. However, pasteurisation may be sufficient for most other mushroom types.
If you decide to pasteurise your mushroom substrate, the timing will depend on the method you choose. There are two main types of pasteurisation: heat pasteurisation and cold pasteurisation. Heat pasteurisation involves heating the substrate to temperatures between 65°C and 85°C for 1.5 to 2 hours. The ideal pasteurisation temperature is around 70°C (158°F), which requires a thermostat to achieve. This method may require a longer 'run' time and more energy to maintain the temperature. Cold pasteurisation, on the other hand, does not require heating the substrate and can be achieved through other means, such as submerging the substrate in cold water treated with hydrated lime to raise the pH and destroy pollutants.
The timing of pasteurisation will also depend on the amount of substrate you are working with. A typical load of a substrate (18-20kg) will require at least 3 hours for pasteurisation. However, for smaller loads, such as Petri dishes, 15-30 minutes may be sufficient.
Additionally, the type of mushroom and substrate will influence the timing of pasteurisation. If you are growing mushrooms that require a more nutritious substrate or have longer colonisation times, you may need to pasteurise for longer to ensure that competing organisms are reduced as much as possible.
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Using a pressure cooker to sterilise mushroom substrate
Small-scale mushroom growers typically use pressure cookers for sterilisation. To sterilise a substrate, temperatures above 250°F (121°C) must be maintained for at least 2 hours. This level of heat cannot be achieved by boiling water or steam alone, so pressure cookers are an effective solution.
Before using a pressure cooker, check that all parts are in good working order, with no dents, bulges or cracks. Examine the cooker lid to ensure the vent pipe is clear, and confirm that all screws are tightened and the pressure gauge is functioning and reads zero.
To prepare, place a metal rack or jar lids at the bottom of the pressure cooker, then put your substrate jars or bags on top. Ensure these do not touch the sides of the pot, as this can cause damage. Line the sides with a kitchen towel or cloth if necessary. Pour in enough water to last the duration of the sterilisation without running dry—around 2.8 litres is recommended.
Finally, seal the pressure cooker and heat it on a burner according to the manufacturer's instructions. A pressure of 15 PSI is required to reach the necessary temperature to kill bacteria and fungal spores.
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Using an autoclave to sterilise mushroom substrate
An autoclave is a piece of industrial equipment that is highly effective at sterilising mushroom substrates. It is a large, sealed chamber that uses steam, pressure, time, and temperature to sterilise the substrate. Autoclaves are often used in labs and hospitals and can be expensive due to their automation and certification requirements. They are a good option for commercial mushroom growers as they offer greater capacity than pressure cookers, ranging from 100L to over 1000L.
To use an autoclave for sterilising mushroom substrate, follow these steps:
Prepare the Autoclave:
Before using the autoclave, ensure that all parts are in good working condition, with no dents, bulges, or cracks. Check any seals, the cooker lid, and the pressure gauge for any signs of damage. Adjust the settings to your desired temperature and duration.
Load the Substrate:
Place the bags or jars of substrate into the autoclave chamber. It is important to leave some space in the centre and between items to ensure thorough sterilisation. Cover any jar filters with foil to prevent steam from entering through the filter patch.
Sterilisation Process:
Close and lock the autoclave door to form a sealed chamber. The autoclave will automatically control the process from this point. Steam will flow into the chamber, increasing the temperature and pressure. This is known as the "purge phase". The autoclave's control system will then adjust the temperature and pressure to the desired set points, maintaining these conditions for the specified duration. This is the "exposure" or "sterilisation phase". Finally, the pressure is released through an exhaust valve, returning the chamber to ambient pressure.
Post-Sterilisation Considerations:
To maintain sterility, consider using a cart and carriage to unload the sterilised substrate. Avoid recycling steam from the autoclave into the fruiting room, as this can increase the risk of dispersing spores or contaminants.
Using an autoclave is an efficient and controlled method for sterilising mushroom substrate, offering a repeatable approach to substrate preparation. It is important to follow the manufacturer's instructions and maintain the equipment properly to ensure effective sterilisation.
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Other sterilisation methods
While pressure cookers are a common method for sterilising mushroom substrates, there are other methods that can be used, especially for larger-scale operations.
Autoclaves
Autoclaves are a popular choice for mushroom growers looking to scale up their operations. They are fast, safe, and efficient, using pressurised steam to sterilise quickly and easily. Autoclaves are controlled programmatically, so there is no need for supervision during the sterilisation process. They can also accommodate larger quantities than pressure cookers.
Hydrated lime
Submerging straw or wood chips in a solution of hydrated lime and water can effectively sterilise mushroom substrates. The solution's pH is raised to a level that kills most contaminants. It is important to wear protective gear when handling hydrated lime as it can be harmful to the eyes, skin, and airways if inhaled.
Cold water fermentation
This method involves submerging the substrate in cold water for 5-7 days, followed by draining, rinsing, and draining again until the substrate reaches the appropriate level of hydration.
Boiling water incubation
Add the substrate to a heat-safe container and enough boiling water to hydrate it to the appropriate level. Drain the water if overhydrated.
Pasteurisation
While not a sterilisation method per se, pasteurisation is worth mentioning as it is a common alternative to sterilisation for mushroom substrates. Pasteurisation involves heating the substrate to temperatures between 65-85°C for 1.5-2 hours. It does not kill all impurities but reduces the population of competing bacteria to give the mushroom species an advantage. This method is simpler and more convenient for small-scale or hobby growers and does not require any specialised equipment. However, it carries a slightly higher risk of contamination compared to sterilisation.
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Frequently asked questions
No, you don't always need to sterilise mushroom substrates. It depends on the nutritional content and the type of mushrooms you want to grow. Sterilisation is required for substrates with high levels of supplementation, such as manure, soy hulls, grains and master's mix.
Sterilisation aims to kill all potential contaminants, whereas pasteurisation only eliminates most competing organisms. Pasteurisation is best thought of as partial sterilisation.
A temperature of at least 250°F (121°C) is required for sterilisation. This temperature must be maintained for a minimum of 2 hours.
You can use a pressure cooker or an autoclave for sterilisation. A pressure cooker is a good option for small-scale growers, whereas an autoclave is a larger, automated piece of equipment used by commercial growers.
Before sterilisation, you should submerge the substrate in water for at least 1 hour. You can also soak it in a hydrogen peroxide water bath for a day.
