Mushroom cultivation has become increasingly popular over the last 5 years. With gourmet mushrooms in great demand, psychedelic mushrooms becoming legalized, and medicinal mushrooms on the rise, it is no wonder that there is an increase in mushroom cultivation across the board. Whether you are a hobbyist, or the most seasoned industrial cultivator, one thing remains the same- the need for sterile environments, and sterile material. In this article we will discuss why sterilization is so important in mycology, what the most popular sterilization methods are, as well as what can and should be sterilized in your laboratory.
Mushrooms are a type of fungus– they grow in the exact same conditions as molds, yeasts, and bacteria. In our everyday environment there are millions of spores floating around, just waiting for the right conditions to present themselves, so that they may grow and propagate. When we create the perfect substrate, and growing conditions, for specific mushrooms, we are also inviting countless other spores to take hold and grow. When this happens the batch becomes contaminated, and often the target mycelium will not even colonize, or grow. Imagine seeding your lawn with grass, but 80% of the seeds were weeds, this is essentially how an un-sterilized bag of substrate behaves. The best way to ensure that a mushroom substrate is going to only yield the desired product, is to ensure that that mushroom is the only fungus present & growing. This is done by means of sterilization. By destroying all bacteria and fungi spores, we ensure that our starting media is a blank slate, ready for the target mushroom to take hold and grow, without competition from other fungi and bacteria.
Substrate is the growing material for the mycelium, this media can take the form of organic matter in solid state, or it can be a liquid agar mixture, regardless of the type of substrate, keeping it sterile before inoculation is one of the crucial first steps in mushroom cultivation. After that, it is about ensuring all tools, and new material remain sterile while interacting with the mycelium cultures. This is done by keeping all tools sterile, as well as working in a clean room environment.
Sterilization vs Pasteurization
While we have been discussing sterilization up to this point, it is good to make a quick note on the distinction between sterilization and pasteurization. Sterilization, whether done by dry heat, steam, UV-light, or chemicals, is the process of completely destroying all microbes that are present. Pasteurization is a method used to greatly reduce the amount of microbes present, but not completely destroy them all. Pasteurization is a process used in many food industries to increase the shelf life of products, and is also popular in large scale edible mushroom cultivation. Because the pasteurization process is less demanding than sterilization, it can be more affordable, and easier to scale up than sterilization. Having said that, pasteurization does have its limitations, and there are many substrates (like hard wood saw dust) that cannot be effectively prepared via pasteurization.
Sterilization – kills every single microbe, steam sterilization is most popular method.
Pasteurization – reduces the amount of microbes present, traditional heat-based methods are popular.
Autoclaves for sterilization
When discussing sterilization, the most popular laboratory tool used is the autoclave. Autoclaves can be found in every microbiology laboratory, every medical facility, dentist, tattoo parlor, Q/A lab, food processing facility, and mushroom grow. While there are many methods of sterilization, the autoclave is one of the most efficient and safe options. Autoclaves utilize superheated steam to penetrate and sterilize the samples that are placed within. At standard atmosphere water boils at 100C, but if the pressure increases, that boiling point will also increase. This principle allows autoclaves to produced super-heated steam. The autoclave is a pressure rated chamber, the material to be sterilized is placed within it, the autoclave is filled with water, and then sealed. As the water within is heated it begins to boil, creating pressure in the chamber, this pressure increases the temperature that the remaining water must reach in order to turn to steam, as a result we have a pressurized chamber with super-heated steam. Generally, autoclaves run at 121C and 15 PSI, or 135C and roughly 30 PSI. This is a considerable amount of heat and pressure, as such it is a very effective method for penetrating surfaces, and sterilizing media.
What needs to be autoclaved?
In the commercial mushroom grow, mycology laboratory, or home grow, there are many stages where contamination can come into play. One of the best ways to reduce this is to ensure everything is sterile. Autoclaves are essential for sterilizing bulk substrate, as well as sterilizing agar intended for liquid cultures. Bags of media, water, jars of agar, all these types of substrates must be sterilized before use. In addition to the actual growing media, all lab glass, petri dishes, and tools should be regularly sterilized within an autoclave. Glass and metal items are safe for use in an autoclave, as are many types of plastic, silicone, and cloths.
Items that can be autoclaved
- Bulk substrate
- Agar
- Liquid cultures
- Glassware
- Tools and lab utensils
- Hoses and equipment
- Bags and jars
- Waste material
Sterilization in summary
No matter the scale, sterilization is an essential part of every mushroom operation. In some instances, a pasteurization process may be adequate for substrate preparation, but the most surefire method is to sterilize. Equipment like autoclaves are key for sterilization; by using super-heated steam, all living microbes can be killed. Without sterilization equipment mycology would be very limited. Sterilization is the only way to ensure minimal contamination and optimal results.
