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This is part two of a three part series. It is a brief interlude on sterility. Sterility is required during most procedures in this hobby.
Part one: An introduction to mushroom cultivation
Part three: An overview of mushroom cultivation techniques
In order to cultivate mushrooms, we need to reproduce the right growing conditions. This includes providing the fungi with easily accessible nutrients. Unfortunately, the fungus we want to grow is not the only organism interested in the meals we provide.
Bacteria and mold spores are present everywhere in the air around us. Once they are present on our grow medium, they will take hold and consume as much of it as they can. When these organisms come into contact with our fungus, they will try to displace or even consume it. The conflict will put stress on our fungus. Moreover, the contaminants might produce harmful substances like mycotoxins. Because of this, you should immediately throw out any medium that has become contaminated. It is inevitable that you will loose a few grows to contamination, but with some preparation and good technique, this will only happen rarely.
Being able to spot contamination early is a skill that comes with experience. I encourage you to research different contaminants and look at pictures of them. Describing them all goes beyond the scope of this article, but here are the basics:
The mycelium of every species has its own visual appearance. Some is dense, bright white and stringy, some is faint and greyish. When you discover mycelium, that looks different than the desired one, chances are it's an invader. If there are different patches of mycelium that don't merge upon contact, but create a distinct line between them, you can be sure that you've got more than one species at hand.
Mold, which is one of the most common contaminants, will start sporulation as it matures. You will see this as a discoloration, as mold spores are often black or green. At this point you must be especially careful when disposing of the media, because the spores very easily become airborne and will contaminate your workspace if released there.
Bacteria are harder to spot with the eye. They are mostly translucent and will form a slimy substance. You might notice them first, because the mycelium of your fungus avoids a certain spot of you growth medium. Undisturbed mycelium should always grow as a circle or sphere. Once you open the container of your medium and it smells sour or rotten, it's very likely that bacteria are present.
There are more rare contaminants, like parasitic fungi that cause cobweb disease, but mold and bacteria are the most common ones.
The most important measure to avoid contamination is to sterilize all media, media containers and tools. On a hobbyists level this is done with the high heat in a pressure cooker. Due to the pressure, a pressure cooker can reach temperatures around 120°C, which is enough to kill the very most of bacteria and spores. Metal tools can also be sterilized in a flame directly before use. Larger surfaces and gloves are usually sterilized with a 70% isopropyl alcohol solution.
After our growth media is sterilized, we still have to open up its containers in order to get our desired spores or mycelium in there. This comes with the risk of re-contamination. Once our desired organism lives on the medium, we cannot sterilize it anymore, as this would kill our fungus, too. Thus, we have to create an environment where the contaminants in the air are so few, that it's unlikely they will come into contact with our growth medium. On a professional level, this is achieved with flow hoods, where finely filtered air is blown across all work surfaces, but such a setup is expensive. Luckily there is a cheap alternative: the "still air box".
A still air box (SAB) is just a clear plastic box with two holes for your hands in one side. Before use, its walls and the surface it will be placed on are sprayed with soap water. After that, the box is put into place, everything you want to work on is placed inside and the box is left undisturbed for a minute or so. Make sure that the room in which your working has no airflow either - there must be no draft and any radiator, fan or air conditioning must be turned off. Now all airborne particles within the SAB will either stick to the soap water on the walls or fall to the bottom and get stuck there. Now you have a box full of sufficiently clean air. It sounds rather primitive, but works surprisingly well.
When working within your SAB, you should also employ a good "sterile technique". This mostly means never to move any unsterilized surfaces above sterile ones and working quickly but without disturbing the air much. Describing this in detail goes beyond the scope of this article, but if you search for it, you'll find tutorials and videos demonstrating good sterile technique.
Now that you know about sterility, we can finally go ahead, buy some fungus and start growing: