Michael Davis
Mushroom growers use various methods to sterilize their mushroom substrate, including pressure cookers, autoclaves, retorts, pressure canners, and barrel steam sterilizers. Autoclaves are a popular choice for mushroom sterilization due to their ability to maintain the heat and high humidity required for optimal mushroom growth. They use pressurized steam to sterilize the mushroom substrate or grain, and the sterilization time depends on the weight, volume, and grain type. While autoclaves offer a cost-effective solution, they require proper handling to avoid sudden pressure release, which can cause agar to boil over or jars to dry out. Small-scale growers often start with pressure cookers and transition to autoclaves as their operations expand.
| Characteristics | Values |
|---|---|
| Autoclave sterilization | Used by mushroom growers to sterilize mushroom substrate |
| Autoclave vs pressure cooker | Autoclaves use pressurized steam, pressure cookers are a cheaper alternative |
| Autoclave process time | 45 minutes to two hours depending on the load and type of grain |
| Autoclave temperature | 121°C or 135ºC |
| Autoclave installation | Requires a qualified installation partner |
| Autoclave maintenance | Water-and-energy-saving solutions are available |
| Autoclave tape | Used to record the amount of water added to the jar |
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What You'll Learn
Autoclave vs pressure cooker
Autoclaves and pressure cookers are both used to sterilize mushroom substrates, which is a necessary step when cultivating certain species of mushrooms. However, there are some key differences between the two methods.
Pressure cookers are commonly used by small-scale mushroom growers as they are capable of reaching high temperatures above 250°F (121°C) with pressure. They are also more versatile and can be used for cooking and canning in addition to mushroom cultivation. While stove-top pressure cookers can reach the desired temperature faster, they usually require an external heat source. This can lead to a cumbersome setup, especially for commercial growers who may need a "chain" of pressure cookers and portable burners. Additionally, pressure cookers have limited capacity, and their smaller size may not be suitable for those looking to use larger jars or switch to half-gallon jars.
Autoclaves, on the other hand, are more suitable for larger-scale operations or those with more experience in mushroom cultivation. They sterilize several times more substrate in a single, fully automated cycle, making them more efficient and cost-effective for commercial growers. Autoclaves also have a larger capacity, which can be more convenient for those who need to sterilize a larger volume of substrates. Unlike pressure cookers, autoclaves instantly go to the set pressure and begin the heat cycle, achieving the proper internal temperature more quickly and allowing for shorter 'cook' cycles. However, autoclaves are generally more expensive than pressure cookers.
Both methods are effective for sterilizing mushroom substrates, but the choice between an autoclave and a pressure cooker depends on the scale of the operation, the desired level of automation, and the budget. For small-scale growers or those just starting, a pressure cooker is a good entry-level option. As the operation expands, autoclaves become a more viable and efficient choice.
Additionally, it is worth noting that there are alternative methods to sterilize mushroom substrates without using pressure cookers or autoclaves. Barrel steam sterilizers, for example, can be used to super pasteurize the substrate by maintaining temperatures between 194 and 212°F (90 and 100°C) for up to 18 hours. While this process takes longer, it is a more affordable option for small mushroom farmers who may not have the funds for an autoclave or pressure cooker.
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Autoclave settings and timings
Autoclaves are the fastest, safest, and most efficient way to sterilize mushroom substrate or grain spawn. They are controlled programmatically, removing the need for supervision during the sterilization cycle. The time and temperature settings for autoclaving mushrooms are directly proportional to the size of the material being sterilized and the amount of space between objects placed within the load.
The first step in the autoclave process is the "purge phase," where steam flows into the autoclave chamber and displaces the air within. The temperature and pressure within the chamber gradually increase to a continuous flow purge. The exhaust valve is then closed, causing the interior temperature and pressure to increase to the desired set point. The control system maintains the desired temperature for the desired duration, which is known as the "exposure phase" or the "sterilization phase."
The time it takes to sterilize equipment and supplies in an autoclave depends on the type of autoclave and the equipment being sterilized. Typically, it takes between 30 minutes to one hour, but this can vary depending on the volume and type of substrate being sterilized. For example, small volumes of hardwood sawdust can be sterilized in under 20 minutes, while large bags of the same sawdust could take upwards of 2.5 hours. Jars of grain spawn can take anywhere from 45 minutes to two hours.
It is important to note that overloaded autoclaves cannot effectively sterilize items placed in the center. Therefore, it is recommended to leave some space in the center and between all items in the autoclave, especially for items like grain bags that tend to sag or compress when stacked. Household items such as glass jars and ceramic mugs can be used as spacers.
When using an autoclave, it is crucial to follow safety precautions such as wearing protective clothing and ensuring the autoclave is properly grounded. It is also important to allow the autoclave to cool down before unloading the equipment to prevent burns and damage to the equipment.
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Autoclave tape and labelling
Autoclaving is a common method for mushroom growers to avoid contamination from trichoderma, bacteria, mould, and other sources. It involves using steam to sterilise substrates and other equipment, and it is important to ensure that the correct sterilisation temperature and pressure have been reached. Autoclave tape is a useful tool to indicate whether the correct sterilisation conditions have been met. The tape is placed on mushroom jars or spawn bags before autoclaving, and dark lines appear on the tape if the correct heat and pressure have been reached. This provides reassurance that the pressure cooker or steam steriliser has worked properly.
When using autoclave tape, it is important to place a piece of tape of sufficient length (3-4 inches) on each jar or bag. The tape should be placed in a way that does not interfere with colonisation. Some users have suggested placing the tape in the centre of the jar or checking the tape later when the jar is opened for use. It is also recommended to add extra water to the jars to prevent them from drying out during the autoclaving process.
To label autoclave tape, an indelible marker can be used to write the amount of water used during the process. This labelled tape can then be attached to the outside of the jar, providing a record of the sterilisation conditions. Additionally, the strain for identification can also be marked on the tape. It is important to use a type of tape that can withstand the autoclaving process without interfering with growth. Some recommended types of tape include micropore tape, white cloth medical tape, and autoclave-specific sterilised indicator tape.
Proper labelling and use of autoclave tape can help mushroom growers ensure effective sterilisation and reduce the risk of contamination. By following these steps and choosing the appropriate type of tape, growers can verify that their equipment has been properly sterilised and maintain the health of their mushroom crops.
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Autoclave installation
Autoclaves are used to sterilize mushroom substrates, and small-scale mushroom growers often use pressure cookers for this purpose, upgrading to an autoclave as their operations expand. Autoclaves are powerful tools that can be intimidating for those without experience in operating them. Before using an autoclave, laboratory personnel must complete training under a supervisor. This training covers safety procedures, proper usage, and maintenance of the autoclave.
When installing an autoclave, it is important to follow these steps:
- Verify the power supply: Ensure that the power supply is a 1-phase, 230Vac (220 to 235 volts) 50/60Hz, 15A supply. If the voltage deviates from the specified range, a licensed electrician must install a buck/boost transformer (0.5KVA).
- Separate power source: Connect the autoclave to a dedicated power source to prevent issues like flickering lights or sensitive device interference.
- Verify grounding: Ensure that the autoclave is properly grounded and protected with a current leakage safety relay (GFI).
- Counter surface and dimensions: Place the autoclave on a rigid, level surface that can support a load of at least 225 lbs (102 kg). Verify that the counter dimensions are at least 22" wide x 24" deep (55 cm x 63.5 cm).
- Ventilation: Maintain a 2" (5 cm) gap between the back and sides of the autoclave and the wall to ensure adequate ventilation. If the autoclave is placed in a cabinet, ensure the rear is open for ventilation. Insufficient ventilation will affect the vacuum portion of the cycle. Leave enough space around the autoclave for technician access during servicing. Ensure the room ventilation is at least 10 cycles per hour.
- Unpack and inspect: Unpack the autoclave and inspect for any mechanical damage. Verify that the packing method is correct and retain the packing materials until the unit has been fully inspected. Look for signs of physical damage, such as scratched panel surfaces or broken knobs. Always use a mechanical aid or multiple persons when lifting and carrying the autoclave to prevent injuries.
- Power and water filling: Plug the power cord into the power socket. Adjust the automatic fill by following the procedure in the autoclave's manual, which often involves pressing and holding the Manual Water Fill Button.
- Testing: Before daily operation, perform a leakage current test, test the operation of the current leakage safety relay (GFI), and verify the continuity of the grounding connection.
- Loading: Loosen the lids on containers being loaded into the autoclave, and ensure bags are not sealed shut. Use steam-penetrable bungs if necessary. Place empty glassware upside down to prevent air pockets. For liquid goods, use a secondary containment pan made of autoclave-safe material. Leave space between items and ensure nothing is touching the sidewall, bottom floor, or blocking the drain.
- Closing and cycling: Fully close the door, engaging the locking pins and turning the locking mechanism. Choose the appropriate cycle (Gravity, Liquid, or Vacuum) considering the goods inside. Remember that Gravity and Vacuum cycles can cause liquids to boil over.
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Autoclave alternatives
Autoclaves are considered the fastest, safest, and most efficient way to sterilize mushroom substrates or grain spawn. However, they can be costly and may not be accessible to everyone. Here are some alternatives to consider:
Pressure Cookers
Pressure cookers are a good budget option for those looking to sterilize their mushroom substrates. They are commonly used by successful mushroom growers, especially those starting out. While they may not have the capacity of autoclaves, they can still effectively sterilize smaller batches. Pressure cookers typically max out at around 8 liters in capacity, which is sufficient for home use.
Instant Pot Pressure Cookers
Instant Pot pressure cookers can be a cost-effective alternative to autoclaves for personal use. They can be used for sterilization and save money compared to purchasing an autoclave.
Water-Vapor Units
Old water-vapor units are another alternative to autoclaves. These units are highly efficient, easy to maintain, and have shorter sterilization cycles. They are a good option for those who don't require the high-grade sterilization capabilities of autoclaves.
Mushroom Growing Bags
Mushroom growing bags are designed to make the cultivation process easier. They reduce the chances of contamination and provide optimal conditions for mushroom growth. There are two main types of bags: autoclavable and pasteurized.
- Autoclavable bags are made from polypropylene and can withstand high temperatures required for sterilization in an autoclave, sterilizer, or pressure cooker. These bags are ideal for use with nutrient-rich substrates.
- Pasteurized mushroom bags are made from different materials and are not autoclavable. However, they are still effective for growing certain species of mushrooms.
Other Containers
Besides growing bags, mushrooms can be grown in various containers such as buckets, bottles, jars, totes, and trays. The best container depends on the type of mushrooms, the growing conditions, available space, and budget. Reusable bottles, for example, are more environmentally friendly but require thorough cleaning between crops.
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Frequently asked questions
An autoclave is a machine that uses pressurized steam to sterilize mushroom substrate or grain.
Autoclaves are ideal for mushroom sterilization as they help achieve consistent results and higher yields while creating less waste. They are also cost-effective and energy-saving.
The time depends on the type of jar and the substrate. For grain jars, you want at least 60 minutes at 15 psi. For BRF jars, you need at least 45 minutes at 15 psi.
Yes, small-scale mushroom growers often use pressure cookers for sterilization. Other alternatives include barrel steam sterilizers and pressure canners.
Ensure that the pressure is released slowly after the cycle to avoid drying out the jars. Also, make sure to vent the steam outside using heat-resistant pipes to maintain proper humidity levels.
