How Do I Prevent Contamination?

In order to sell mushrooms, you must have continuous production of mushrooms going on. To achieve that you need a tight contamination control. In the next paragraphs, I will talk about different aspects of it.

In this article, I will talk about the about six vectors of contamination which Paul Stamets talks about in his book Growing Gourmet and Medicinal Mushrooms and add my thoughts to it.

  • Cultivator
  • Mobil Contamination Unit
  • Air
  • Media
  • Inoculum
  • Tools

Vector 1 & 2: Cultivator & Mobil Contamination Unit

The first and last vector (cultivator and tools) is obvious but important to remember. As I wrote in my article about farm design to reduce the risk of contamination, all people are only allowed to move from clean areas to dirty areas and not the other way around. This means that when you start your day, you should begin to, for example, in the lab and end with harvesting.

People contribute between 30% and 40% to all contaminations because on average, we as humans release between 100,000 to 500,000 particles and germs per minute! Table 1 illustrates the level of particles in relation to different activities.

Table 1: The Austin’s Contamination Index. Number of particles generated by person per minute, at different degrees of activity, wearing two different types of clothing (≥0.3mm).

Table 1: The Austin’s Contamination Index. Number of particles generated by person per minute, at different degrees of activity, wearing two different types of clothing (≥0.3mm).

To get a better idea about the mentioned size of particle (here ≥ 0.3mm) figure 1 will give you some references. According to this figure, a bacterium has a size of 1,000nm or 1mm. Whereas a virus has a size of 100nm or 0.1mm.

If we compare these two sizes to the size of a mushroom spore which is about 10-12mm, we will find a factor of 10-1000 between them. Therefore, if we want to avoid bacteria or viruses spreading, we need at least a filter system which stops everything below 10mm.

Figure 1: Size comparison

Figure 1: Size comparison [1]

Besides, people, animals like ants, flies, mites, or cats, and dogs are helping to spread contamination. Stamets calls them mobile contamination units because they can spread contaminants from one site to another[2].

With that said, you as a farmer must make sure that you take measures against them. You can do this by using, for example, a fly screen for your doors and windows. Add fly strips in your rooms and check them frequently.

A more inexpensive version of a fly strip can be seen in figure 2. Here the farmer created his own fly traps by mixing his own recipe for a paste and placing it on bottles (red circle).

Figure 2: Simple hand-made fly traps (green bottles)

Figure 2: Simple hand-made fly traps (green bottles)

If you want to learn more about how you can prevent insects coming into your rooms, I put together a list of easy and cheap ways to prevent them from entering your fruiting room.

Another good prevention is using double doors and a small overpressure in between them. The overpressure suppresses the number of particles coming into the room.

Third, if you must deal with, for example, contaminated bags do so after you went to the clean areas or change at least your closes and sanitize yourself properly to prevent spreading of the contamination. This procedure refers to what I said in the beginning.

Always move from clean to dirty. To do so, you have to follow some design principles which I wrote about more in detail in my article farm design. In this article, I will go over four steps, including site selection, components, layout, and phases of a mushroom company.

Vector 3: Air

But not only we have to wear special clothes like gloves and mask (especially in the lab) we also must filter the incoming air. To illustrate what I mean, I got a diagram of the average value of fine dust over Germany measured on the 01.01.2018 (figure 3).

As you can see in figure 3, some cities overshot the level of 50 mg/m³ (red dots), which is the maximum allowed level governed by law.

Figure 3: Average values of fine dust in different German cities on the 01.01.2018

Figure 3: Average values of fine dust in different German cities on the 01.01.2018

The incoming air has therefore directed through a filter (figure 4). Especially inside the lab (inoculation area). Depending on the quality of the incoming air, different filters will be used, or if necessary, several filters will be combined.

The more filter you are stacking together, the higher the pressure has to be, and hence, the more energy you need to get the air through the filters.

Figure 4:  HEPA Filter (top), principle (bottom)

Figure 4:  HEPA Filter (top), principle (bottom)[3]

Vector 4: Media

The media (substrate) you are using is another vector of contamination. To reduce the number of contaminants inside your substrate, you have to sterilize it. During this process, we reduce the number of bacteria, viruses, and fungus so that our mushroom spores have a higher chance to colonize the now sterile substrate first.

To achieve proper sterilization of the media temperature and time at that temperature play an important role. The time at a specific temperature depends on the amount of media we want to sterilize and the sterilization process itself.

For example, if you use a drum sterilizer, it makes a difference in how you place your blocks inside the sterilizer. The more compact (dense) you fill the sterilizer, the longer you must run the process. Because the heat needs more time to penetrate all blocks. If you let small spaces in between the blocks, the process will be shorter.

If you want to learn more about sterilization, I wrote an in-depth article Sterilization 201 in which I talk about different sterilization methods, their pros, and cons, as well as their efficiency.

Vector 5: Tools

On the same token, we must sterilize our instruments. For your scalpel you can, for example, using an Infrared (IR) sterilizer or flame sterilizer. Use this tool every time you want to transfer from a petri dish to another petri dish or from a petri dish to a spawn bag. 

This is one step of many to avoid contamination spreading.

The main activity of you will be cleaning the tables, shelves and so on with an alcohol/water mixture and or hydrogen peroxide (max. 5% conc.).

If you are flipping a room over, follow the following procedure:

  • Remove all shelves.
  • Clean the shelves.
  • Clean the room from debris.
  • Clean the floor.
  • Clean the walls.
  • Let the room dry.
  • Refill the room with shelves.

A good strategy for flipping a room over is to have more rooms that you need. This allows you to clean a room properly while at the same time, you keep your production up and running.

Vector 6: Inoculum

Inoculum is the tissue that is being transferred, whether this tissue is part of a living mushroom or spores. Bacteria and molds can infect mushrooms tissue and be carried with it every time a transfer is made[4].

It is, therefore, a good practice to wear gloves (figure 7) and a mask while handling cultures or spawn.

  • In the back of the figure, you can see the filter which blows air towards the cultivator.
  • Second, the cultivator is wearing gloves while handlings the petri dishes.
  • Third, the cultivator will place the petri dishes during the process onto the metal shelf in front of the filter.
  • Fourth, before touching the cultures, the cultivator cleans the gloves with an alcohol/water mixture.
Figure 7: Handling cultures

Figure 7: Handling cultures[5]

Another good practice is to check always before you transfer tissue or spawn if the petri dish, the tube, or the bag is contaminated.

After the transfer is done the petri dishes are sealed with a paraffin tape.

Besides these steps, a good source of cultures and or spawn producer is crucial. The higher the quality you start with, the better you are off.

With this information at hand, it should be possible for you to be ahead of contamination.

If you want to know more about 18 Ways to Keep Pests in Check. Then this article is for you. Starting with the basics, I will address different strategies, tactics, and tools you can use to reduce the risk.


[1] Particle science | Size comparison

[2] Paul Stamets 1993 Growing Gourmet and Medicinal Mushrooms

[3] Wikipedia | HEPA Filter

[4] Paul Stamets 1993 Growing Gourmet and Medicinal Mushrooms

[5] Own figure

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