Learning how to pasteurize straw is necessary if you want to start growing certain types of mushrooms on your own. Although it may seem daunting at first, this article will take you through the ins and outs of straw pasteurization.
Cereal straws (not hay) such as oat or wheat are used as a base, or substrate, upon which mycelium grows. Mycelium is a thread-like collection of cells that represent the vegetative growth of a fungus. Much like you need an apple tree to produce apples, you need healthy mycelia to produce mushrooms.
Pasteurization is simply the process by which amounts of microscopic competitors in a substrate are reduced. This gives the mycelium an advantage over harmful organisms, allowing it to take over the substrate and eventually produce mushrooms.
So how does one prepare straw? There are a variety of different methods, with hot water being the most common. There are other ways as well, such as using cold, steam, or certain chemicals.
Let's go over straw pasteurization for growing mushrooms in greater detail. We'll start with the theory behind it and explain why we use pasteurization instead of sterilization. Next we'll look at a few different methods. If in the end this all seems like too much work, we'll review some other alternatives.
When learning how to grow mushrooms, you may come across the term 'sterilization'. Yet for substrates such as straw and dung, you want to pasteurize them, not sterilize.
To pasteurize means to reduce the amount of harmful competing organisms. When the process is over, there is still some micro-activity going on in the substrate, usually in the form of beneficial bacteria.
To sterilize means to remove all living organisms in a substrate. When you're done, there should be nothing left alive. This process is usually done with chemicals or very high heat and pressure, such as with the autoclave on the right.
Sterilizing simply leaves the straw too susceptible to contaminants. Without the beneficial bacteria to guard against foreign competitors, the substrate becomes a free for all for anything to grow.
Having beneficial microorganisms left over allows you to inoculate the straw without using special sterile procedures. Thus you can do it outside in the open air without a flow hood or fancy equipment.
Think of it like having a house sitter when you're away. Sterilizing is like leaving the doors unlocked with no one home. Pasteurizing is like leaving a friend at your house. Someone may still break in, but it's less likely.
Now let's take a look at some different ways to pasteurize straw in order to grow mushrooms.
Before using any of these techniques, you should first cut your straw into 1 to 3 inch segments. Use a lawn mower, blender, or any other machine that will do the job. Note that if you do it by hand, your fingers will be really sore if you have a lot of straw!
Don't skip this step. Mycelium will colonize smaller pieces of straw much faster and easier. It will also make the whole thing much easier to work with.
Pasteurization occurs between 160 and 180 degrees Fahrenheit. Anything more than that and you risk killing good bacteria and allowing the bad to bloom.
With a water bath, you pasteurize by soaking the straw in 160-degree water for an hour. You can do whatever you want to accomplish this, but here are common methods for larger and smaller amounts of straw:
Note that the large-scale water bath merits some caution. You'll be working with a lot of hot water and a heat source. Make sure everything is safe, secure, and sturdy.
Also keep in mind that wet straw is very heavy. Depending on the amount you use, lifting it out of the barrel may be a two-person job. You may want to tie your bag to a long branch so it's easier to lift out of the water.
Mycologist Paul Stamets developed the idea of cold incubation. While not pasteurization per se, it is a way to manipulate straw to give mycelium an advantage. This works on the idea that the mycelium of some cold-loving mushrooms will colonize at a low temperature. This lower temperature slows down the microscopic competitors.
You must use mushrooms that fruit in lower temperatures for this to work, no heat lovers here!
I have used this method before with success. It isn't as successful as straw prepared with a heat bath, but it is less work!
Hydrogen peroxide will kill competitor spores and foreign microbes, but won't hurt mycelium. Treating your straw with hydrogen peroxide will give it the mycelium a head start.
This method is far less effective as a hot water pasteurization bath. However, it is far less work and much cheaper.
Here are a few other ways to pasteurize straw for growing mushrooms. I have not tried these, so I have no comment as to their effectiveness:
Okay, so I've done the oven method and yes, it does work. However it also makes your kitchen smell like baking straw and the inside of your oven nasty. Best for those who never use their oven and consider it as an oversized kitchen ornament!
Are the above methods too much work? Does the idea of babysitting a water bath of straw make you yawn?
If you don't want to pasteurize straw you can still grow mushrooms. Here are some ways around it:
Whatever method you use, pasteurized straw makes for a great mushroom substrate. With the right attitude, it's an easy and fun part of growing mushrooms.
Go ahead and try to pasteurize some straw. If it turns out to be too much work, use one of the alternative methods. Keep growing!