Emma Wilson
Sterilizing mushroom substrates is an essential step in cultivating delicate, gourmet, or medicinal mushroom species. Sterilization involves using a combination of steam, pressure, time, and temperature to kill all living organisms, spores, and pathogens. The ideal temperature for sterilization is 250°F (121°C) for at least 2 hours, which requires a pressure of 15 PSI. This process ensures that the substrate, which serves as the primary source of nutrients, moisture, and energy for mushrooms, is free from contamination. While boiling water or steam alone cannot achieve sterilization, pressure cookers, autoclaves, retorts, and pressure canners are commonly used for this purpose. Additionally, pasteurization, which targets most competing organisms, is an alternative method for low-nutrient substrates. Before sterilization, it is crucial to inspect the equipment for any damage and ensure proper cleaning and disinfection to prevent the carryover of diseases and residues.
| Characteristics | Values |
|---|---|
| When to disinfect | If you want to grow fussy, delicate, or medicinal mushrooms that require nutrient-rich substrates, or if your substrate has high levels of supplementation (e.g., manure, soy hulls, grains) |
| Disinfection method | Sterilization (not pasteurization) |
| Temperature | At least 250°F (121°C) |
| Time | At least 2 hours |
| Pressure | 15 PSI |
| Equipment | Pressure cooker, jars or bags of prepared substrate, wire rack or jar lids, water |
| Substrate properties | Slightly acidic (pH 5–6.5), good structure for air exchange, 50–70% moisture content, 1–2% nitrogen, small amount of magnesium, potassium, calcium, sulfur, and phosphorus |
| Precautions | Check pressure cooker for damage, fold bags to prevent steam from entering through the filter patch, cover jar filters with foil, clean equipment before dipping in disinfectant |
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What You'll Learn
- When to disinfect: before growing, and after harvesting?
- Why disinfect: to kill pathogens and prevent contamination?
- Disinfecting equipment: pressure cookers, autoclaves, retorts, or pressure canners
- Disinfecting substrates: pasteurisation vs sterilisation
- Preparing substrates: temperature, pressure, time, and pH
When to disinfect: before growing, and after harvesting
Before growing mushrooms, it is essential to choose the right mushroom substrate, as this can significantly impact the success of your mushroom-growing project. Different species of mushrooms prefer different substrates, so understanding how mushroom mycelium develops is crucial.
For more delicate, gourmet, or medicinal mushrooms that require nutrient-rich substrates, sterilizing the substrate before use is necessary. Sterilization aims to kill all potential contaminants, including living organisms and spores, giving your mushroom mycelium the best possible start. To sterilize a substrate effectively, you need temperatures higher than 250°F (121°C) for at least 2 hours, which can be achieved using a pressure cooker or steam steriliser.
On the other hand, less nutritious substrates, such as straw, sugarcane bagasse, coco coir, hardwood sawdust, and cardboard, only require pasteurization. Pasteurization eliminates most competing organisms, giving fast-growing mushroom species an advantage. However, it is important to note that pasteurization may not be sufficient for slow-growing species that require more nutritious substrates or longer colonization times.
After harvesting your mushrooms, the leftover material, known as the spent mushroom substrate (SMS), still has many potential uses. SMS is full of nutrients and can be reused in your garden, compost, or even for growing another batch of mushrooms. If you have a tiny amount of spent substrate, composting is a great option. Simply pile your substrate outside and allow it to naturally decompose, and you might even get an additional flush or two of mushrooms in your compost pile!
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Why disinfect: to kill pathogens and prevent contamination
Sterilizing a mushroom substrate is essential to kill pathogens and prevent contamination. Mushrooms obtain all their nutrients from the substrate, which provides the necessary moisture, energy, and nutrients for growth and fruiting. The substrate is dense in woody, fibrous materials like lignin, cellulose, and hemicellulose, which contain carbon, the main food source for mycelium.
However, moist and nutrient-rich substrates not only attract mushroom mycelium but also unwanted organisms like bacteria and mould, which often grow faster. To prevent this, sterilization kills all living and dormant organisms and fungal spores, giving the mushroom mycelium a head start. This is especially important for slow-growing mushroom species that require more time to colonize and are at higher risk of contamination.
The sterilization process involves achieving a temperature of at least 250°F (121°C) for a minimum of 2 hours, which cannot be reached by boiling water or steam alone. This temperature ensures the eradication of bacteria and spores that are only invalidated at temperatures above 121°C. To achieve this, mushroom cultivators typically use pressure cookers or autoclaves, retorts, or pressure canners for larger operations.
Additionally, it is crucial to maintain aseptic conditions after sterilization to prevent contamination. The sterilized substrate becomes a desirable habitat for the fastest-growing organism to claim it, which is often mould or yeast. Therefore, disinfecting equipment and maintaining clean conditions are essential to successful mushroom cultivation.
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Disinfecting equipment: pressure cookers, autoclaves, retorts, or pressure canners
Pressure cookers, autoclaves, retorts, or pressure canners are commonly used to sterilize mushroom substrates. These methods utilize a combination of steam, pressure, time, and temperature to eradicate living organisms and spores. To achieve sterilization, a temperature of at least 250°F (121°C) must be maintained for a minimum of 2 hours. This temperature cannot be reached by boiling water or steam alone, even at higher altitudes where the boiling point is lower. Therefore, specialized equipment is necessary to reach and maintain the required temperature.
When using a pressure cooker, it is crucial to inspect the equipment before each use carefully. Ensure that all parts are in good working order, with no dents, bulges, or cracks. Pay close attention to any rubber seals, as they can degrade over time and cause leaks. Check the cooker lid for obstructions, ensure all screws are tight, and verify that the pressure gauge reads zero and shows no signs of damage. Before placing jars or bags of substrate into the cooker, fold the bags to prevent steam from entering through the filter patch, and cover any jar filters with foil.
For small-scale growers, a pressure cooker is an effective method for sterilizing mushroom substrates. However, as operations expand, it may be more practical to switch to an autoclave, retort, or pressure canner, which are commonly used by commercial mushroom growers. These larger pieces of equipment can handle more substantial loads and are designed to process food items efficiently. However, they come with higher energy costs and require more labour to operate and maintain.
It is important to note that disinfecting equipment is crucial not only for the sterilization of mushroom substrates but also for maintaining the overall hygiene and safety of the mushroom cultivation process. Proper disinfecting procedures help prevent the carry-over of diseases, pathogens, and other contaminants from one crop to another. This includes disinfecting commonly used tools, harvesting equipment, packaging containers, and even foot baths to ensure a clean environment for mushroom growth and worker safety.
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Disinfecting substrates: pasteurisation vs sterilisation
The nutritional content and type of mushrooms cultivated dictate the requirement for substrate sterilisation. Sterilisation is necessary for substrates with high levels of supplementation, such as manure, soy hulls, grains, and master's mix, as well as nutrient-rich substrates. These substrates are more prone to contamination by unwanted organisms due to their moisture and nutrient content.
On the other hand, pasteurisation is sufficient for less nutritious substrates like straw, sugarcane bagasse, coco coir, hardwood sawdust, and cardboard. Pasteurisation aims to eliminate most competing organisms, not all, and can be achieved through various methods, including submerging the substrate in boiling water or soaking it in hydrated lime-treated water to increase the pH and kill contaminants.
Sterilisation, however, requires higher temperatures exceeding 250°F (121°C) for a minimum of 2 hours. This temperature cannot be achieved by boiling water or steam alone, and specialised equipment like a pressure cooker or autoclave is often needed. The pressure cooker method involves placing the substrate in jars or grow bags designed for high temperatures and ensuring the cooker is in good working condition before use.
The decision between pasteurisation and sterilisation depends on the specific mushroom species and the substrate's nutritional content. For novice growers, easy-to-cultivate varieties like oyster mushrooms can be grown with minimal equipment, and pasteurisation is often sufficient. However, for those with more experience interested in cultivating gourmet or medicinal mushrooms, sterilisation becomes necessary to eliminate all potential contaminants and provide the desired mushroom species with the best chance to thrive.
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Preparing substrates: temperature, pressure, time, and pH
The preparation of mushroom substrates involves a careful interplay between temperature, pressure, time, and pH. Here are the key considerations for each factor:
Temperature:
To effectively sterilize a mushroom substrate, a temperature of at least 250°F (121°C) must be achieved and maintained for a minimum of 2 hours. This temperature threshold is crucial because boiling water or steam alone is insufficient to sterilize the substrate. At sea level, water boils at 100°C, and increasing the heat will not raise the temperature beyond this point. However, 121°C is the minimum temperature required to kill most fungal and bacterial spores.
Pressure:
Typically, a pressure of 15 pounds per square inch (PSI) is necessary to attain the desired sterilization temperature of 121°C. The relationship between pressure and temperature is important to understand, as higher altitudes will require either increased pressure or a longer sterilization cycle to compensate for the lower boiling point of water.
Time:
The sterilization time depends on the amount of substrate being processed. A typical load of 18-20kg of substrate will require at least 3 hours, whereas a lighter load, such as Petri dishes, may only need 15-30 minutes. It's important to note that these times do not include the additional time needed to bring the substrate up to the required pressure and temperature.
PH:
Maintaining the correct pH level in the substrate is crucial. The ideal pH range for most mushroom substrates is slightly acidic, between 5 and 6.5. Certain mushroom varieties, such as oyster mushrooms, can tolerate a higher pH of up to 8. To increase the pH and create an alkaline environment, you can soak the substrate in a bath of hydrated lime-treated cold water for 24 hours before sterilization. This process helps to eliminate contaminants and foreign spores.
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Frequently asked questions
A mushroom substrate is a material that mushroom mycelium can grow and establish itself in. It is where mushrooms get their nutrients, moisture, and energy to grow and fruit. Different species of mushrooms have their own preferences, so it's important to match your mushrooms with the correct substrate.
The nutritional value of a substrate and the type of mushrooms you want to grow will dictate whether the substrate needs disinfection. The more nutrients a substrate contains, the greater the chances of contamination. Moist, nutrient-rich substrates are appealing to organisms like mould and bacteria, which often grow faster than mushroom mycelium. Disinfecting a substrate kills all living and dormant organisms and fungal spores, giving your mushroom mycelium the best possible start.
To disinfect mushroom substrate, you need temperatures higher than 250°F (121°C) for at least 2 hours. Boiling water or steam alone is not hot enough to disinfect a substrate as the boiling temperature of water at sea level is 212°F (100°C).
Mushroom cultivators typically use a pressure cooker for disinfection on a small scale. You will also need a wire rack or jar lids for the bottom of the pressure cooker, and water. Before using your pressure cooker, check it carefully to ensure that all the parts are in good working order and there are no dents, bulges, or cracks.
