Autoclaves for Mushroom Cultivation

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Written by: Scott Mechler

BS Mechanical Engineering, Mechanical Engineer

For mushroom growers, contamination from trichoderma, bacteria, mold, and more all pose a risk to your crop. Some of the leading causes of contamination are airborne contaminants and contamination from incorrectly sterilized or improperly pasteurized tools, substrates, and/or cultivators. One of the most effective ways for mycologists to avoid mushroom contamination is to establish a highly controlled and repeatable approach to substrate preparation — ideally, one that makes use of an autoclave designed for mushroom cultivation.

What Is a Mushroom Substrate?

A mushroom substrate simply refers to any material that provides mushrooms with the energy, nutrients, and moisture they need to grow.

There are a wide variety of substrates growers can use, including straw, soil, coco coir and vermiculite, coffee grounds, logs, and manure; however, some species of mushroom are more selective than others and respond best to certain substrates. For example, lion’s mane, reishi, and maitake (also known as hen of the woods) mushrooms — all of which are naturally found in forested areas — work best with wood-based substrates, such as elm, beech, or ash logs.

Although there are a diverse array of substrates from which to choose, they all have a few things in common. According to the expert cultivators at GroCycle, good mushroom substrates must:

  • Be dense in woody, fibrous materials such as lignin, cellulose, and hemicellulose
  • Contain 1%–2% nitrogen
  • Contain a small amount of magnesium, potassium, calcium, sulfur, and phosphorus
  • Be slightly acidic, with a pH level of ~5–6.5 (though some mushrooms, such as oyster mushrooms, can tolerate higher pH levels)
  • Have a structure that enables air exchange
  • Have a moisture content of 50%–70%
  • Be free from microbes, mold, and bacteria

This last item is especially important because any microbes, mold, or bacteria that remain in a substrate will compete for available water, nutrients, and energy — preventing spores from germinating, mycelium from developing, and mushrooms from growing. To completely eradicate contaminants from a substrate, cultivators must first prepare the substrate using one of two preparation methods: pasteurization or sterilization.

Substrate Preparation Methods: Pasteurization vs. Sterilization

Both pasteurization and sterilization are common methods for preparing mushroom substrates for spore inoculation.

Pasteurization involves exposing a substrate to mild dry or steam heat — usually less than 100° C (212° F) — for a brief period of time to kill off pathogens. Pasteurization is popular with smaller growing facilities because it does not require any special equipment — just a reliable heat source — and with larger facilities because it is easy to use and highly scalable.

That said, pasteurization does not completely eradicate all microorganisms, only pathogenic bacteria. That means that any substrate prepared using pasteurization could still contain other microbes (albeit in a weakened state), including fungi, viruses, mold spores, and other forms of bacteria, which could prevent mushroom spores from taking root and growing. It’s also worth noting that certain substrates, particularly supplemented substrates, cannot be prepared through pasteurization.

Sterilization is another popular method of substrate preparation, one that totally eliminates all bacteria, microorganisms, and other contaminants from your preferred substrate. There are quite a few sterilization techniques growers can use, including exposure to ultraviolet light or chemicals and hot water immersion, but steam sterilization is the most effective.

Steam sterilization involves using steam, which is a very efficient medium for heat transference, to raise the temperature of a cell to a degree at which the proteins in the cell wall break down and coagulate. Steam sterilization is especially useful in medium- and large-scale growing operations because it uses a highly repeatable process and offers rapid turnaround times with reliable results.

Using a Pressure Cooker vs. an Autoclave for Mushroom Substrates

Although it is technically possible to use a pressure cooker for steam sterilization — certain models are capable of sustaining a temperature of 121° C (249.8° F) at 15 psi — they are not designed for this particular purpose, which means results could be inconsistent. Furthermore, most pressure cookers max out at roughly 8 L in capacity, making it an unsustainable option for all but the smallest of mushroom cultivators.

By comparison, mushroom autoclaves are designed for the sole purpose of using steam to sterilize substrates and other mycology equipment, allowing for more control over the preparation process. As a piece of industrial equipment, a mushroom autoclave can also offer far greater capacity than any pressure cooker — anywhere from 100 L to over 1,000 L depending on the model — making it ideal for growth operations of all sizes.

Another key reason why mushroom cultivators should opt to use an industrial-grade sterilizer over a pressure cooker for substrate preparation is that mushroom autoclaves often come equipped with additional features, such as a vacuum system, to ensure that goods come out completely dry. When using a pressure cooker for steam sterilization, growers risk substrate coming out wet, which can cause mold to form on the substrate. A mushroom autoclave with a built-in vacuum system and specialized cycles will evacuate air from the chamber before a sterilization cycle is complete, removing any remaining liquid from in — in the case of porous materials — or around the substrate, thereby ensuring total sterility.

How a Mushroom Autoclave Works

An autoclave/sterilizer used for mushroom cultivation works in much the same way as any other type of autoclave:

  1. Bags of substrate are loaded into the autoclave chamber, and the door is locked to form a sealed chamber.
  2. Steam flows in and begins to displace the air within the autoclave chamber. The temperature and pressure within the chamber gradually increase to a continuous flow purge; this step in the process is often referred to as the “purge phase” for this reason.
  3. The autoclave’s control system closes the exhaust valve, thereby causing the interior temperature and pressure to increase to the operator’s desired set point. The control system maintains the desired temperature (dwells) for the desired duration. This step of the process is known as the exposure phase, or the sterilization phase.
  4. During the exhaust phase, pressure is released from the chamber through an exhaust valve and the interior is restored to an ambient pressure (though its contents remain relatively hot). If the operator chooses to run a vacuum cycle, the autoclave’s vacuum system will remove all liquid from the chamber, ensuring that the load is completely dry.

Installing a Mushroom Autoclave

With the help of a qualified installation partner, installing a mushroom autoclave in your growing facility or mycology laboratory should be fairly straightforward. With that said, here are a few best practices cultivators should keep in mind to make things a little easier:

  • If you know that your facility has specific sterilization requirements, work with an autoclave manufacturer that can custom-build a unit according to your exact specifications.
  • If you’re not accustomed to working with large capital equipment, first make sure you have the proper utilities to run the unit. For example, steam generators require significant amperage to operate, so you’ll want to make sure you have access to three-phase power. Not having access to the proper utilities can not only cause operational issues, but also increase your fire hazard. A little research about the needs of such mycology equipment can save you trouble further down the road.
  • Speaking of utilities, autoclaves require a significant amount of water to operate, so it’s also worth looking into water-saving features to reduce operational overhead.
  • Reach out to the autoclave manufacturer prior to installation to see if they can explain what you can expect in terms of throughput, and whether you can add extendable shelving and/or a cart and carriage to process large quantities of substrate.
  • Consider the ergonomics of loading and unloading your mushroom sterilizer prior to installation and figure out a good workflow for getting goods in and out of your designated sterile processing room without introducing new microorganisms or compromising sterility. Again, an autoclave with a cart and carriage could be a smart investment, as sterilized substrate can be placed in the carriage without compromising sterility.
  • Avoid recycling steam from your autoclave back into the fruiting room, as this can create positive pressure in the fruiting room and increase your risk of dispersing spores or contaminants.
  • Set your autoclave up so that it vents steam outside of the building (using a heat-resistant pipe no less than 30 mm in diameter) and plumb a direct connection to your facility’s sanitary sewer drain. This is especially important when processing potentially hazardous waste, as it reduces the risk of contamination.

Best Practices for Substrate Sterilization

Once you’ve successfully installed an autoclave for mushroom substrates in your growing facility or laboratory, you’re ready to prepare substrate for inoculation. Here are a few best practices to help you get started:

  • Before running your first load, embed a biological indicator into a substrate tray to validate your autoclave. If the biological indicator within your sample tray does not activate, repeat the process using an identical substrate.
  • Use your mushroom autoclave’s automatic shut-off functionality to streamline operations and more easily manage multiple sterilization cycles for different equipment and substrates.
  • Rather than try to fit in large quantities of substrate all at once to reduce your total number of sterilization cycles, load trays evenly with substrate, leaving space between each bag so that the autoclave can sterilize everything completely.
  • Save time by running a vacuum cycle instead of a gravity cycle, which typically involves running a liquid load and requires an additional step for drying.
  • Install a load probe in your unit. Since substrate preparation involves sterilizing multiple bulky containers of similar size, inserting a load probe into the center of the load can help you determine whether you’re achieved uniform temperature-based sterility.

Get State-of-the-Art Mycology Equipment from Consolidated Sterilizer Systems

Since 1946, Consolidated Sterilizer Systems has produced industrial autoclaves and other sterilization equipment for laboratories around the world, including mycology laboratories and grow facilities.

Each of our autoclaves for mushroom cultivation features water- and energy-saving features that help cultivators reduce operational overhead, as well as an EcoCalendar feature to support strict growing schedules (turning the autoclave on and off at pre-set times and dates.) Available in a variety of models and sizes with optional features including a vacuum system, our mushroom sterilizers support maximum throughput and deliver total sterility.

To learn more about any of our mycology sterilization equipment, talk to the experts at Consolidated today.

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