Introduction to Pasteurization and Sterilization

Microorganisms are present all around us and in every part of our environment. Microscopic bacteria, yeasts, and the spores of other fungi cover the surfaces and interiors of just about everything. From your hands, to your electronic device, and even the air we breathe. 

For mushroom growers, this microscopic life must be carefully controlled so it does not interfere with the cultivation process. After all, many naturally occurring bacteria and fungi have antagonist relationships with mycelium, either attacking it directly or competing with it for habitat.

Pasteurization and Sterilization are processes that work to eliminate or control these harmful microorganisms. While the processes share similarities, they are distinct and play unique roles in cultivating mushrooms. One of the common beginner mistakes is mixing and misusing these terms.

Pasteurization and Sterilization: The Difference

• Pasteurization is less energy intensive and less disruptive towards microbial life than sterilization. It intends to eliminate the majority (but not all) of microorganisms present on a substrate. It preserves some beneficial microorganisms and eliminates the harmful organisms to a point where they will no longer interfere with the cultivation process. Since a small percentage of harmful organisms persist in this process, its use is limited to low-nutrient substrates.
  
  
• Sterilization is a more intensive process that eliminates all microorganisms present on a substrate or material. It is used for high-nutrient substrates and typically requires greater care as it is more prone to harmful organisms post-sterilization (during inoculation, incubation, and fruiting). Sterilization is also used for the materials/equipment used during laboratory processes and spawn production.

Pasteurization

Pasteurization is a low-tech method that can be done with minimal equipment or infrastructure. It is great for beginners and folks who want to grow mushrooms with the least investment possible. It is also perfect for contexts with minimal cleanliness or aseptic conditions like outdoor gardens.

While pasteurization is completely sufficient for anyone just hoping to grow some mushrooms, it does have some limitations.

• Pasteurization is mostly suitable for Oyster Mushrooms as they are vigorous, fast-growing, and can easily outcompete competition. This being said, there are exceptions, especially if using processed substrates like Hardwood Fuel Pellets or Compressed Sawdust Blocks.
  
  
• Pasteurization is only suitable for low-nutrient unsupplemented substrates. This includes plain straw or sawdust. While these tend to have lower yields, it is much easier to pasteurize large quantities of substrate than sterilize.
  
  
• Pasteurization is not suitable for laboratory work. This means cloning, producing spawn, and maintaining/isolating cultures.

How To Pasteurize Substrates

There are many ways to pasteurize a substrate. You can use steam, boiling water, heat, alkaline baths, and some folks even use microwaves, soap, or bleach. If you search online, you’re bound to find dozens of different guides and techniques that creative growers and hobbyists have developed. Generally speaking, most of these techniques fall into 1 of 2 categories.

• Heat Pasteurization involves treating your substrate with temperatures exceeding 60°C for about 1-2 hours.
• Alkaline Pasteurization involves increasing the pH of the substrate to around 12, which will inhibit the growth of harmful microorganisms.

Alkaline Immersion, AKA “Cold Water Pasteurization”

Alkaline Immersion is a pasteurization technique utilized primarily for Oyster Mushrooms, and it is one of the most employed in rural parts of the world.

By modifying the pH of your substrate, you can successfully ward off harmful microorganisms while promoting the growth of your fungal mycelium. While some microorganisms are killed from this drastic change in pH, many of them are also simply inhibited and incapable of growing under these conditions. Meanwhile, species like Oyster can thrive and successfully myceliate the substrate.

This change in pH is almost always conducted using an immersion in a Hydrated Lime (calcium hydroxide) solution. It is important to clarify not to use Calcium Carbonate which will not achieve the same results.

• Utilize Low Magnesium Hydrated Lime: It also goes by other names like caustic lime, builders lime, slaked lime, cal, and pickling lime. Make sure to get the low-magnesium stuff.
  
  
• Do Not Utilize Calcium Carbonate Lime: Calcium Carbonate will not guarantee the same results as hydrated lime. This is basically chalk but also goes by names like dolomitic lime, garden lime, quicklime, and limestone. It is often used as a soil amendment.

This technique is primarily suitable for straw substrates, although it may be applicable to others. It is not recommended for sawdust or fine particulate substrates as it may be difficult to achieve ideal hydration after submerging your substrate.

Heat Pasteurization

Heat pasteurization is a go-to method which can be used for many different types of substrate. It requires temperatures between 140-170°F (60-80°C) that are achieved by steaming the substrate or by immersion in hot water. Hobby growers could also use an oven.

Below are some basic principles to consider before we move onto the methods.

• Your pasteurization time starts when internal substrate temperatures reach 140°F (60°C). A thermometer can be great for measuring temperatures inside a substrate.
  
  
• Higher temperatures and longer cooking times are not always better. Exceeding temperatures of 170°F (80°C) or pasteurization for excessive time can actually increase risk of contamination, presumably because it destroys beneficial bacteria.
  
  
• Use your judgment for temps/time within the indicated parameters. Dirtier and less hygienic substrates should be at the higher end of the temp/time spectrum.

Hot Water Immersion
Immersion methods work best for straw and can be done in several ways. Like steaming, it could be done in a large pot or 55 gallon barrel. The simplest way to do hot-water immersion is to bring water to a boil and then submerge prehydrated straw (hydrated in warm water) into the water inside a mesh vegetable sack or wire basket.

As the temperature equilizes between the substrate and the boiling water, you should end up with temperatures of about 150-160°F. This of course depends on the quantity of water/substrate you are using. Maintain temperatures above 140°F for 90 minutes, then remove your substrate and drain.

Bucket-Tek
One of my favorite low-tech methods is to do hot-water immersion in a 5 gallon bucket. Simply stuff a mesh vegetable sack with as much straw that will fit a 5 gallon bucket. Place this in your bucket, then fill the bucket with boiling water. This is usually about 2-3 gallons, which I heat up in the biggest pots and water kettles I have. After filling my bucket, I place the lid on, and cover it with large blankets to keep the heat. After 90 minutes I remove the sack and drain it.

Steaming
Steaming can be done in large 55 gallon barrels or in a large-pot on your kitchen stove. Steaming works great for sawdust, straw, and most other low-nutrient substrates. You can either steam your substrate as is in a mesh bag (use 2 for finer-particulate substrates), or you can steam mushroom grow bags that are already full of substrate. Steaming in a growbag is not necessary, but it may allow you to use small amounts of supplementation (5-10%) as the process is much cleaner.

Ultra Pasteurization

This is a standard process conducted by many small growers. It allows you to achieve results that approach sterilization, without having to use a pressure cooker (PC) or autoclave. It is typically conducted in a 55-gallon barrel fitted with a steaming rack. Substrate bags are then placed into a barrel, and temperatures of about 212°F (100 °C) are maintained for at least 12 hours. Remember that it can take a couple hours just to reach these temperatures, meaning this process could easily take up 16-24 hours in total.

Sterilization

Sterilization intends to eliminate all biological life from a material, substrate, or piece of equipment. This includes all bacteria and fungi, plus their extremely resistant spores. It is a crucial step in the mushroom cultivation process, especially for laboratory work and spawn production.

Sterilization is considered a more advanced process, as it typically requires more equipment and a more developed infrastructure. At the very least it requires a pressure cooker (PC), but it may also entail improving your infrastructure to ensure clean conditions during inoculation and incubation. It also requires experience with sterile technique.

As mentioned in the above table, sterilized materials are generally more susceptible to contamination. Even low nutrient substrates which are sterilized are more likely to become contaminated compared to their pasteurized equivalent. By removing all living organisms, even the beneficial ones, you open the door for any introduced microorganisms to flourish. Not to mention, many of the materials which are intended for sterilization are naturally rich in nutrients.

All of this being said, there are also many benefits to conducting sterilization;

• You can utilize supplemented substrates which can have up to a 50% greater yield than non-supplemented counterparts.
• You can produce your own spawn and liquid cultures.
• It allows you to do laboratory work, such as isolating and maintaining cultures.

Before we scare anyone away from sterilization and the doors it opens, it’s also important to be clear that it is accessible to anyone with enough enthusiasm. Many enthusiasts produce their own spawn and conduct laboratory work using low-tech and relatively inexpensive solutions. Some folks even conduct sterilization in “instapots” or electric PC devices.

How To Sterilize A Substrate

To sterilize a substrate you need to achieve temperatures of about 250 °F (121°C). These temperatures, which are well above boiling, can only be achieved under pressure which is about 15 PSI at sea level. The duration of sterilization typically ranges anywhere from 15 minutes for small objects like petri-dishes, agar, and liquid culture solutions, and up to 120 minutes for large bags of substrate.

Theoretically, sterilizing a substrate is as easy as utilizing a pressure cooker or autoclave. This being said, there are many nuances that exist depending on the process you are conducting. Below are some things to consider regarding sterilization.

• Sterilization temperatures reach 250 °F (121°C) at 15 PSI when you are at SEA LEVEL. If you are in the mountains you either need to compensate for this by raising the PSI (make sure your pressure cooker can tolerate this) or the time. Generally speaking, an extra 30-60 minutes should be plenty.
  
  
• Make sure your pressure cooker and regulator is designed for 15 PSI. Many pressure cookers are only designed to reach about 12 PSI.
  
  
• Safety is important when utilizing pressure cookers or autoclaves. They can be very dangerous if utilized improperly. Make sure they are in good working condition and only use them as intended.
  
  
• Your sterilization time begins when you reach 15 PSI. For pressure cookers, this is typically when your regulator starts shaking. This means you could easily be waiting 30 - 60 minutes to reach pressure, plus your cooking time, and a couple of hours for the substrate to cool down after.

Conclusion

When it comes to sterilizing or pasteurizing your own substrates for mushroom cultivation, the most important thing to remember is that even under perfect conditions and with experience, you may still experience a few bags going south on you, or some unexpected incursions of Trichoderma. And that’s ok - even professionals experience up to 3% failure with top notch equipment, so just keep on growing, and keep dialing in your preferred method that works best for you!