Laura Smith
Sterilising mushroom substrate is an important step in the mushroom cultivation process, particularly for those looking to grow gourmet, medicinal or more delicate mushrooms. The process involves using a combination of steam, pressure, time and temperature to kill any living organisms and spores that may hinder the growth of mushrooms. Sterilisation is necessary when working with moist, nutrient-rich substrates that are prone to contamination by moulds, bacteria and other unwanted organisms. The most common method for sterilising mushroom substrate is the use of a pressure cooker, which can maintain a temperature of at least 121°C (250°F) for a minimum of 2 hours. Other methods such as pasteurisation, which does not kill all organisms, are also available but may not be sufficient for certain types of mushrooms.
How to Sterilise Mushroom Substrate
| Characteristics | Values |
|---|---|
| When to sterilise | When growing fussy, gourmet, or medicinal mushrooms that require nutrient-rich substrates |
| Why sterilise | To kill all potential contaminants, including living organisms, dormant organisms, and fungal spores |
| Pasteurisation vs sterilisation | Pasteurisation only eliminates most competing organisms, while sterilisation kills all bacteria and pathogens |
| Required temperature | 121°C (250°F) |
| Minimum time | 2 hours |
| Pressure | 15 PSI |
| Equipment | Pressure cooker, autoclave, retort, or pressure canner |
| Alternative methods | Submerging straw or wood chips in a hydrated lime solution, cold water fermentation, boiling water incubation |
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What You'll Learn
When to sterilise mushroom substrate
Sterilising mushroom substrates is a crucial step in the mushroom cultivation process, especially for novice growers. It involves preparing the substrate using a combination of steam, pressure, time, and temperature to kill living organisms, spores, and potential contaminants. However, the decision to sterilise depends on various factors, including the type of mushrooms and the nutritional content of the substrate.
Firstly, it is essential to understand that not all mushroom substrates require sterilisation. If you are a beginner, consider starting with easy-to-cultivate mushroom varieties such as oyster mushrooms, which can be grown with minimal equipment and without the need for sterilisation. Additionally, some substrates, like coffee grounds, do not require sterilisation if handled with proper hygiene and care. The brewing process for coffee naturally pasteurises the grounds, providing a unique and low-tech substrate option.
Secondly, the need for sterilisation arises when cultivating more delicate, gourmet, or medicinal mushroom species that require nutrient-rich substrates. These substrates are highly susceptible to contamination by other organisms, such as bacteria and mould, which can outcompete the mushroom mycelium and lead to growth issues or even sickness. Therefore, if you intend to grow these fussier types of mushrooms, sterilisation becomes a necessary step to ensure a successful yield.
Thirdly, the decision to sterilise depends on the specific substrate you are using. Substrates with high levels of supplementation, such as manure, soy hulls, grains, and master's mix, require sterilisation due to their high nutritional content. On the other hand, low-nutrient substrates like straw, sugarcane bagasse, coco coir, hardwood sawdust, and cardboard can be adequately pasteurised instead of sterilised. Pasteurisation reduces the population of competing organisms, giving fast-growing mushroom species an advantage.
Lastly, the timing of sterilisation is crucial. It is recommended to sterilise the substrate before introducing mushroom spores or mycelium into the mix. This ensures that the substrate is free from any potential contaminants that could hinder the growth of the desired mushroom species. After sterilisation, maintain aseptic conditions and handle the substrate carefully to prevent any post-sterilisation contamination.
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Sterilisation vs pasteurisation
Sterilisation and pasteurisation are two important processes in mushroom cultivation. They are pivotal in determining the quality of the produce and play a significant role in the yield. However, they differ significantly in terms of process, objective, and application.
Sterilisation is the process of preparing a substrate using a combination of steam, time, temperature, and pressure to kill living organisms and spores. It aims to eliminate all potential contaminants and destroy all forms of microbial life. The temperature required for sterilisation is above 120°C, and it is achieved under pressure using methods like autoclaving or pressure cooking. Sterilisation is necessary for cultivating mushrooms that are more sensitive to contaminants, such as shiitake mushrooms, and it ensures a completely aseptic environment. It is also beneficial for mushrooms that require specific conditions or are sensitive to competing organisms. The main drawback of sterilisation is the energy and time required. Additionally, if contamination occurs post-sterilisation, the substrate can quickly become overrun.
On the other hand, pasteurisation is a less intense process named after Louis Pasteur. It involves heating the substrate to moderate temperatures, generally between 60°C and 85°C, for a specified period. The goal of pasteurisation is not to kill all organisms but to reduce populations of undesirable microorganisms while preserving certain beneficial bacteria. Pasteurisation is often used in mushroom cultivation, such as oyster mushrooms and straw substrates, as it is less expensive and does not require complex equipment. It creates optimal conditions for mushroom growth, increasing the chances of success. However, there is always a slight risk that harmful microbes will survive the pasteurisation process, which could result in substrate contamination.
The choice between sterilisation and pasteurisation depends on the specific requirements of the mushroom species being cultivated, as well as factors like budget, scale of cultivation, and personal preferences. While sterilisation provides a completely uncontested environment for mushrooms to grow, pasteurisation offers a natural defence against harmful pathogens due to the presence of beneficial microbes.
It is worth noting that not all mushroom substrates require sterilisation or pasteurisation. Coffee grounds, for example, do not need these treatments as long as they are used quickly and handled with proper hygiene.
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Using a pressure cooker
To sterilise mushroom substrate using a pressure cooker, you will need the following items:
- A pressure cooker – big enough to hold more than one bag at a time.
- Jars or bags of prepared substrate – mushroom grow bags are recommended as they can withstand high sterilisation temperatures, but you can also use jars.
- A metal cooking rack or jar lids – to place at the bottom of the pressure cooker.
- Water – enough to last the duration of the sterilisation without running dry.
Before using your pressure cooker, check that all parts are in good working order and there are no dents, bulges or cracks. Examine the cooker lid and ensure that nothing is blocking the vent pipe. Confirm that all screws are securely tightened and that the pressure gauge reads zero and is free of damage.
Place your metal rack or jar lids at the bottom of the pressure cooker and put your substrate jars or bags on top. Ensure that your jars or bags are not touching the sides of the pot, as this can cause damage. If necessary, line the lower sides with a kitchen towel or cloth.
Pour in the required amount of water. Most growers recommend starting with around 3 quarts (2.8 litres) of water or enough water to go 1 to 2 inches (2.5 to 5 cm) up the side of the bags or jars.
Seal the pressure cooker and heat it on a burner according to the manufacturer’s instructions. To achieve sterilisation, a temperature of at least 250°F (121°C) must be maintained for a minimum of 2 hours. Most mushroom growers will need to add pressure to raise the temperature to the required level. Typically, a pressure of 15 PSI is required to reach 250°F.
After sterilisation, 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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Using an autoclave
Autoclaves are a popular method for sterilising mushroom substrates, especially for commercial growers. They are considered the fastest, safest, and most efficient way to sterilise mushroom substrate or grain spawn. Autoclaves use pressurised steam to sterilise quickly and easily.
To use an autoclave, you will need to reach a temperature of at least 121°C (250°F) for a minimum of 2 hours. This temperature is required to kill bacteria and fungal spores. To reach this temperature, you will need to apply a pressure of 15 PSI (pounds per square inch). The time required will depend on the amount of substrate being sterilised, with a typical load of 18-20kg requiring at least 3 hours. It is important not to overload the autoclave as this can affect the sterilisation process.
Autoclaves are a good option for mushroom growers as they can be left unattended during their cycle. They are also a good choice for growers who need to sterilise large amounts of substrate, as they have a much larger capacity than pressure cookers. Additionally, autoclaves are a good option for sterilising grain spawn, which can be challenging to sterilise completely.
It is worth noting that autoclaves are more expensive than other sterilisation methods and may be out of reach for small-scale growers. However, they can save money in the long run by reducing utility bills and minimising contamination.
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Alternative sterilisation methods
While pressure cookers are commonly used for sterilisation, they are not the only method. Here are some alternative ways to sterilise mushroom substrates:
Oven sterilisation: One method involves dampening your grain, placing it in a Mason jar with ventilation, and then putting it in the oven at 240° Fahrenheit for 2 hours. This technique achieves perfect sterilisation without the need for a pressure cooker. However, it is crucial to maintain the specified temperature and duration for effective results.
Pot and jar method: Another approach is to place your jars in a large pot, on top of a layer of washrags or ball jar lids to prevent scorching. Fill the pot with water, ensuring it doesn't exceed half the height of the jar, and then simmer with a tight-fitting lid for about 90 minutes. This method has been successfully used for cultivating psilocybe cubensis.
Pasteurisation: Pasteurisation is a process that involves heating the substrate to temperatures between 65°C and 85°C for 1.5 to 2 hours. While it doesn't eliminate all impurities, it reduces the population of competing bacteria, giving your desired mushroom species an advantage. This method is suitable for substrates like straw, sugarcane bagasse, coco coir, hardwood sawdust, and cardboard.
Coffee grounds: Using coffee grounds as a substrate is an interesting alternative that doesn't require pasteurisation or sterilisation. The brewing process pasteurises the grounds, simplifying the overall procedure. Local coffee shops often provide wasted coffee grounds for free, making it an eco-friendly and cost-effective option. However, proper hygiene and prompt utilisation of the grounds are essential for successful mushroom cultivation.
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Frequently asked questions
It depends on the type of mushroom and substrate you are using. If you are using a low-nutrient substrate like straw, cardboard, or sawdust, you only need to pasteurise it. If you are using a high-nutrient substrate like manure, grains, or soy hulls, you should sterilise it to prevent contamination.
Sterilisation aims to kill all potential contaminants, whereas pasteurisation only eliminates most competing organisms. Pasteurisation is a simpler process that requires lower temperatures and can be done in open air, but it may increase the risk of contamination.
A temperature of at least 121°C (250°F) is required for at least 2 hours. This temperature cannot be achieved by boiling water or steam alone, so a pressure cooker or autoclave is typically used.
You will need a pressure cooker, jars or bags of prepared substrate, and a wire rack or jar lids for the bottom of the pressure cooker. Autoclaves are another option but are more expensive and better suited for larger-scale growers.
You can use sterilisation indicator tape, which will display black lines after the sterilisation temperature has been reached.
