Understanding the sterilization process is a core skill for anyone who wants to grow mushrooms at home. Sterilizing substrate and tool is something you’re going to have to do a lot if you want to grow mushrooms at home. If you’re only doing a few batches it’s not too expensive to buy pre-sterilized substrate, but buying all that growing medium can add up over time. If you’re going to be growing more than a few mushroom colonies at once, sterilizing your own substrate is the way to go.
While there are many ways to sterilize anything the best way to do so is with a moist heat from an autoclave or other pressure vessel. Most home growers won’t have an autoclave, so instead you can use a pressure cooker which works the same way. Today we’re going to look at the process and technique used to sterilize most equipment and growing medium.
How Sterilization Works
Sterilization is the process of killing off all microbial life on any given object or surface. There are different ways to do this with chemicals, UV light, radiation, or heat. For most things that are bulky, cannot be exposed to chemicals, or cannot be penetrated by UV, you only have heat and radiation left. Naturally, heat is the only option for a home setup.
To effectively kill off living organisms, spores, and viruses, you need to achieve high temperatures for long periods of time. This process kills organisms by disrupting proteins within them or by destroying their structural integrity.
How Hot Does Sterilization Have to Be?
For most purposes 250F (121C) will do the trick when applied for between 15-90 minutes. This is a problem for many reasons. First, exposing things to a direct heat source at that temperature will not heat them evenly. This can lead to burning or poor sterilization. Secondly, air does not retain heat very well, so keeping something at this temperature in any vessel is difficult. To overcome these challenges we need to use water and steam. Water holds heat well, keeps and even distribution of temperature, and prevents burning.
Since water boils at 212F (100C) the only way to ensure a moist heat is applied to objects is through the use of pressure. At 15 psi a pressure vessel will raise the boiling point of water to 250F (121C). This is thanks to some basic physics and the Ideal Gas Law (PV=nRT). This essentially means that as the pressure goes up, so too does the boiling point. This will allow you to steam sterilize all of your equipment easily.
Using a pressure vessel to sterilize equipment
No matter what type of pressure vessel you are using to sterilize your equipment, always read and follow manufacturer directions carefully. Never leave a pressure vessel unattended and do not let a pressure vessel boil dry. If the water fully evaporates the vessel can burn and melt its interior contents.
When you are preparing your equipment you may want to do a few things. First adding a raised stand to the bottom of your pressure cooker will help the sterilization process and keep items off the bottom of the pot. Direct contact with the bottom of the pot it too hot for most plastics, even if they are autoclave safe.
Next make sure that you are either sealing items in autoclavable containers or wrapping equipment in tin foil. This helps keep things clean when they come out of the pressure cooker. For any container that forms a perfect seal, make sure the lit is loose or has a vent of some kind. Sealed glass jars can implode under the pressure and similarly plastic containers can rupture. When making your own growing bottles, you will put a hole with a filter in the lid.
Another thing that is a good idea is to use autoclave tap. This material changes color to indicate the correct heat and pressure was reached while sterilizing. If the tap does not change color, the sterilization was either incomplete or the heat didn’t reach the correct temperature.
Heat, Time, and Pressure
To sterilize your equipment you will need to get the vessel up to pressure. 15psi for 15-90 minutes depending on the contents is ideal. Follow your pressure cooker or autoclave instructions on specific operation. If you are unsure on how long to sterilize your equipment, you can always assume 90 minutes. This may be overkill in some cases, but it will ensure quality sterilization.
As a rule of thumb, small things like scalpels and syringes can be autoclaved at shorter time periods as they have low volume and surface area. Empty jars or areas with more void and mass should be heated longer, roughly 30 minutes. Any full jars, grain bags, or larger amounts of substrate should be heated for the full 90 minutes. For Agar, follow the manufacturer directions on sterilization as they may have minimum and maximum times indicated for the particular brand.
What Gets Sterilized for growing mushrooms?
Almost everything that can. You will commonly sterilize scalpels, growing bottles, bags of substrate, agar, glass slides, and various other pieces of lab equipment. The exception to this rule is anything that cannot withstand the high temperatures. Things that would melt or break under these conditions instead need to be cleaned with 10:1 isopropyl alcohol or other solvents. This does not guarantee sterilization, but it’s better than not doing anything.
Also, make sure to check that you can put something in pressure cooker conditions before doing so. Check that your bags, bottles, and other equipment are all autoclave safe. Always use lab grade materials, especially for bags of grain.
Maintaining Sterility When Opening the Pressure Cooker
The point in this whole process is to sterilize things, so you should work to ensure they stay that way. If you don’t need the pressure vessel again once it’s done, leave the items in it until you need them. When you do open the vessel after it has cooled completely, be sure to do so in a clean space and wear gloves when handling the contents. Also be aware that if you set your equipment on an unclean surface you are potentially contaminating the outside of the equipment.
For things like slides, plates, syringes, and scalpels, you can leave them wrapped in their foil and transfer the whole contents to a clean bag. It’s good practice to do this and label the bag with the equipment name and the date it was cleaned. Reducing contact with unclean air can help the equipment stay clean for longer.
Sterilization is a Key Skill and Practice Makes Perfect
If you are going to be growing mushrooms at home with any degree of seriousness you’ll be doing a sterilization process of some sort often. Practice makes perfect, so if you have issues with contamination look at your process and think about how you can better maintain sterile conditions. Over time you’ll end up with good habits and procedures that will work for your particular setup.