Michael Davis
Mushroom sterilization is an important step in mushroom cultivation. While boiling water or steam is not sufficient to sterilize mushroom substrates, sterilization equipment can be expensive. Atmospheric sterilization is one way to bypass this bottleneck, with growers heating substrates to 91°C for 8 to 12 hours. For those looking to build their own sterilization equipment, a drum sterilizer is a straightforward piece of equipment to construct. This can be done by following a step-by-step guide, with the key consideration being the need to control the steam.
| Characteristics | Values |
|---|---|
| Purpose | To sterilise mushroom substrate |
| Sterilisation Temperature | 121° C |
| Pasteurisation Temperature | 70° C |
| Atmospheric Sterilisation Temperature | 91° C |
| Time | 8-12 hours |
| Equipment | 55-gallon drum steam boiler sterilizer, low-pressure boiler, PID controller, Inkbird Brewing Temp controller, neoprene sheeting, ball valve, hose, exhaust port, rolling racks, water spray, air filter, thermocouples, propane tank, autoclave, Hepa laminar flow hood |
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What You'll Learn
Building a sterilizer barrel
Planning and Materials
Start by gathering the necessary materials and planning the construction process. Sterilizer barrels typically use heat and time to kill contaminants hiding inside the mushroom substrate. You will need a barrel, a heat source, insulation, a temperature controller, and various other components. Decide on the size and type of barrel that best suits your needs, considering factors such as the volume of mushrooms you intend to sterilize and the available space for the barrel.
Barrel Preparation
Prepare the barrel by cleaning and treating it to prevent rust and other issues. If using a mild steel or galvanized barrel, it is important to treat the components with a suitable primer, such as red oxide primer, to prevent rust. Ensure that the barrel is properly sealed to avoid leaks.
Insulation and Temperature Control
Choose an insulation material such as neoprene sheeting, radiator reflection foil, Kingspan insulation, or rockwool. Cut the insulation to the appropriate size and secure it to the barrel using adhesive or other suitable methods. Decide on an appropriate temperature controller, such as an Inkbird Brewing Temp controller, ensuring that it can handle the wattage/amps of your heat source. Install the temperature controller according to the manufacturer's instructions.
Steam Escape and Pressure Control
Sterilizer barrels are not designed to be pressurized, so you must provide a controlled steam escape mechanism. One common method is to drill an exhaust port in the lid of the barrel. If using the sterilizer indoors, consider attaching a ball valve threaded into the bung hole of the barrel lid, along with a length of high-temperature hosing, to direct and control the flow of steam. This also allows you to minimize heat loss by closing the barrel during the sterilization process.
Safety Features
Install a small spacer at the top of the barrel to prevent bags from blocking the steam escape hole and potentially pressurizing the barrel. This safety feature ensures that even if a bag inflates, it would need to cover the entire perimeter of the barrel to cause a blockage, reducing the risk of unsafe pressure buildup.
Testing and Operation
Once the barrel sterilizer is assembled, test it thoroughly to ensure it functions correctly and safely. Follow the operating instructions provided by the manufacturer of your specific components. Remember to drain the barrel after each use to prevent limescale buildup and to keep it free from debris. Proper maintenance will extend the lifespan of your sterilizer barrel.
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Pasteurisation vs sterilisation
When it comes to growing mushrooms, pasteurisation and sterilisation are both essential processes to ensure the successful cultivation of fungi. However, the two methods differ in terms of their effectiveness in eliminating contaminants and the equipment required.
Pasteurisation
Pasteurisation is a process that reduces the number of competing pathogens in the substrate, creating an environment conducive to mushroom growth. It involves heating the substrate to temperatures between 150 and 180 degrees Fahrenheit for around one and a half to two hours. While pasteurisation does not eliminate all contaminants, it weakens microorganisms and kills some pathogenic bacteria, making it sufficient for growing certain mushroom varieties, particularly those in non-supplemented straw substrates. Popular edible mushrooms, such as Oyster mushrooms, can thrive with pasteurisation. Additionally, pasteurisation is favoured by newcomers to mushroom cultivation as it requires minimal equipment and can be performed in most environments. All that is needed is a large pot, spawn, bags, and straw. However, one drawback of pasteurisation is that the substrate must be inoculated immediately after the process to prevent the growth of unwanted spores and bacteria.
Sterilisation
Sterilisation, on the other hand, refers to methods of substrate preparation that aim to completely eliminate all organisms and spores. This process involves heating the substrate to temperatures of 250 degrees Fahrenheit under pressure or holding it at close to 100 degrees Celsius for extended periods, sometimes up to 18 hours. Sterilisation provides a "clean canvas" for mushroom growth, allowing uninterrupted colonisation of the substrate by the desired fungi. While sterilisation requires more specialised equipment, such as autoclaves and pressure vessels, it offers better yields, yield per flush, and mushroom quality when compared to pasteurisation. Additionally, sterilised substrates can be left for future use without the immediate concern for contamination that pasteurised substrates present.
Building a Mushroom Steriliser
When building a mushroom steriliser, it is important to consider the insulation material and temperature controller. Various materials can be used for insulation, such as neoprene sheeting, radiator reflection foil, or rockwool, which can be wrapped around the steriliser barrel and secured with aluminium tape. An Inkbird Brewing Temp controller can be used to regulate temperature, but it is important to ensure that the wattage/amps of the element do not overpower the controller. Additionally, a spacer at the top of the barrel can prevent potential blockages caused by bags inflating during the sterilisation cycle. To allow for the escape of steam, an exhaust port can be drilled into the lid of the barrel, or a ball valve and hosepipe can be attached to drain water and control steam flow.
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Using steam to sterilise
To build a steam steriliser, you will need a plastic 55-gallon barrel. At the bottom of the barrel, install a float valve so that the water refills automatically. This will regulate the water level as it is heated and boiled off into steam. Some of the steam will condense and return to the reservoir, but a large part will be lost, so the float valve is necessary to keep the reservoir topped up. The float valve must be able to withstand high temperatures and mains water pressure. The UK mains water pressure averages 15 psi, and cheaper float valves may leak at this pressure, so a pressure regulator may be required to prevent flooding.
Next, add a heating element. An electric element is more efficient as it sits directly in the water, minimising energy loss. Make sure the wattage/amps of your element won't overpower your temperature controller. Wrap the barrel with insulation, such as R-Tech foam, to retain heat.
Finally, add a meat thermometer probe and hook up the steriliser to a temperature controller and a timer. Ensure that the temperature controller can support the amps/watts of your heating element.
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Required temperatures and pressure
The required temperature and pressure for sterilizing mushrooms depend on the type of mushroom and the sterilization method being used. Atmospheric sterilization, for example, involves heating the substrate to 91°C for 8 to 12 hours, which is effective for Shiitake mushrooms and other similar varieties. This method uses heat and time to kill most contaminants in the substrate.
Pasteurization is another method that utilizes lower temperatures, typically around 70°C (158°F), but requires a longer duration. It aims to preserve beneficial organisms while eliminating others.
For effective sterilization, temperatures above 100°C are necessary, as boiling water at sea level will not exceed this temperature. Certain bacteria and endospores can survive boiling water, requiring temperatures of 121°C or higher for complete eradication.
When using a pressure cooker or canner, the sterilization process creates positive pressure by forcing air out of the container. As the container cools, negative pressure develops, which can lead to the contamination of bag contents if air is sucked back in. This issue can be mitigated by using sterilization indicator tape, which changes appearance after the required sterilization temperature has been attained.
Additionally, it is important to consider the pressure inside the sterilization barrel. Bags placed inside the barrel may inflate and block the steam escape hole, potentially leading to unsafe pressure buildup. This can be prevented by using a small spacer at the top of the barrel, ensuring that any inflated bag would need to cover the entire perimeter to cause a blockage.
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DIY PID controller
Building a mushroom sterilizer is a complex process that requires careful planning and execution. While there are pre-built options available, such as the Mushroom Sterilizer Heat Control Panel (240V 30A 7200W), you can also choose to build your own drum sterilizer. Here are some detailed instructions for creating a DIY PID controller for your mushroom sterilizer:
Parts and Materials
Firstly, you will need to gather the necessary parts and materials. For the PID controller, you will require a temperature controller such as an Inkbird Brewing Temp controller, ensuring that the wattage/amps of your element do not overpower the controller. Additionally, you will need insulation materials, such as neoprene sheeting, and adhesives to secure them. Other components include a float switch and a stainless steel float valve to protect your element from dry firing.
Assembly
The assembly process involves several steps:
- Cut the insulation material: Cut the neoprene sheeting to the appropriate circumference and height to fit your barrel.
- Create holes: Knock out three holes for the inlet, outlet, and element. Ensure that the holes are properly aligned and sized for the corresponding components.
- Secure the insulation: Attach the insulation to the barrel using contact adhesive along one strip. Ensure that the insulation is securely fastened and sealed to maximize its effectiveness.
- Install the float valve: As float valves may not handle high water pressure, you have two options. The first is to use a gravity-fed system, where the water level remains above the barrel's waterline. The second option is to install a PRV (pressure-reducing valve) inline with the hose.
- Install the float switch: Place the stainless steel float switch just above the element to break the power to the control side of the SSR if the water level drops too low.
- Add a ball valve and hosepipe: To drain your barrel after each use, attach a ball valve to the lowest point and connect a hosepipe to channel the draining water away.
- Control steam release: If using the sterilizer indoors, drill an exhaust port in the lid to control the flow of steam. Alternatively, you can create a small spacer at the top of the barrel to prevent bag inflation from blocking the steam escape hole.
Operation
Once your mushroom sterilizer with the DIY PID controller is assembled, you can begin the sterilization process:
- Temperature and pressure: Sterilization depends on both time and pressure. Aim for a temperature of 98-100°C and adjust the pressure accordingly.
- Maintain temperature: Once the desired temperature is reached, the PID controller will maintain it by running the element at 5 to 10% power.
- Monitor the process: Depending on your setup, it may take varying times to reach the target temperature. Monitor the process and make adjustments as needed.
- Homemade steam generator: You can also explore building a homemade steam generator for sterilizing mushroom substrates, which involves a different set of instructions.
Building a DIY PID controller for a mushroom sterilizer requires careful consideration of parts, assembly, and operation. Ensure that you follow safety guidelines and make adjustments as needed for your specific setup.
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Frequently asked questions
A mushroom sterilizer is a piece of equipment that uses heat and time to kill contaminants that can hide inside the mushroom substrate.
There are various types of mushroom sterilizers, including atmospheric sterilizers, pressure sterilizers, and autoclaves. Atmospheric sterilizers use steam to heat and moisten the air, pressure sterilizers use pressure and heat to sterilize, and autoclaves are large sterilizers that use steam and pressure.
The materials used to build a mushroom sterilizer can vary. Some common materials include stainless steel, drums (ceramic-coated or galvanized steel), propane tanks, and insulative materials such as neoprene sheeting, radiator reflection foil, or rock wool.
When building a mushroom sterilizer, it is important to consider the size and type of sterilizer, the heating method (electric, gas, or flame), and the temperature control mechanism. Additionally, ensuring proper ventilation and vacuum systems are in place is crucial to prevent contamination and control steam flow.
