7 Ways to Prevent Contamination in Magic Mushroom Cultivation

20TH MAY 2025 • 5 MIN READ

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1. Follow the Instructions 

It might sound obvious, but it is surprisingly easy to skip a step, use the wrong measurements, or take a shortcut. Even small mistakes can add up and make a big difference. Luckily, P. cubensis is very forgiving, which makes it great for beginners, but some errors can still be fatal. Minor issues like forgetting to wear a mask might not always ruin a grow, but you will never know for sure.

Here are some common big mistakes that are easy to avoid:

• Using a still air box incorrectly
• Not sterilizing grain long enough
• Overcooking the grain
• Wrong Field Capacity

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2. Longer Sterilisation

As mentioned before, improperly sterilized grain is one of the biggest enemies of mycologists when it comes to contamination. Properly sterilized grain forms the foundation for strong and healthy mycelium. It is basically the base of your entire grow. Steam sterilisation works well, but like almost any method, it is not perfect. The biggest advantage of steam sterilisation is that anyone can do it in a regular kitchen without any special equipment. However, the downside is that the success rate is generally lower compared to using other methods like a pressure cooker. Luckily, there is a simple way to improve this by using more than one sterilisation cycle.

This method is called tyndallisation:

• Day 1: steam for 2 hours
• Day 2: steam for 2 hours
• Day 3: steam for 4 hours

But of course, sterilising for a total of 8 hours over three days does not sound very appealing. Fortunately, three sessions are usually not necessary. You can also try two sessions of 2 hours each, or one session of 2 hours followed by another of 4 hours. Sometimes even a single 2 hour session is enough. Why?

Because we all live under different conditions:

• Different types of grain
• Different working environments
• Different altitudes
• Many more unique factors

That is why it is important to experiment and find out what works best for you. And if it did not work this time, do not worry. The next attempt will be better.

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3. Use a Pressure Cooker

A pressure cooker might be the solution to all your problems, especially if you feel stuck and your jars keep getting contaminated. A pressure cooker works similarly to a regular cooking pot but builds up pressure inside. The combination of pressure and heat is ideal for killing off bacteria, mold spores, and other contaminants.

Compared to steam sterilisation, a pressure cooker offers one major advantage: higher success rates in much less time. Instead of spending up to 8 hours across several days with steam sterilisation, you only need about 1.5 to 2.5 hours with a pressure cooker. That time saving alone is a great reason to consider getting one.

Which one to get: Ideally, choose a model that reaches 15 PSI (pounds per square inch). 12 PSI is also sufficient, but it just requires slightly longer sterilisation. Look for one with a pressure indicator, which helps you monitor the process.

Sterilisation times:

• At 15 PSI: 1.5 hours
• At 12 PSI: 2.5 hours

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4. Use Different Grain

Sometimes, simply changing the type of grain you use can make a big difference. As mentioned earlier, everyone lives in different environments, and that affects what kind of grain is available, and how clean or contaminated it might be. In some regions, corn might be the cleanest and least contaminated option, while in others it could be brown rice or millet.

Every grain comes with its own advantages and disadvantages:

• Some grains are made for human consumption, making them cleaner and less likely to carry contamination.
• Others might be cheaper, larger, smaller, or more nutritious.
• Some are easier and faster for mycelium to colonize, while others might take longer.
• And depending on where you live, certain grains may not even be available or may vary in quality.

That is why some growers swear by brown rice, while others prefer corn or rye. There is no one size fits all solution, so take some time to experiment and find out which grain works best for you.

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5. Use a Still Air Box (SAB)

Use the still air box (SAB) as often as possible, as it is a real game changer. Some growers have shared that introducing a SAB into their process increased their success rates by up to 200%. But just like with anything else, proper usage is key. Always pay close attention and follow the tutorial step by step.

Tips for using a still air box effectively:

• Work carefully but efficiently: The longer things are open, the higher the contamination risk.
• Sterilize everything beforehand: This includes your jars, tools, and surfaces.
• Sterilize your equipment: Flame your scalpel, wipe down tools with alcohol, and make sure your hands or gloves are clean.
• Wear gloves and a mask: Protect your grow from bacteria and spores that may come from you.
• Use a room with no air circulation: Turn off fans, close windows, and avoid drafts. If that is not possible, try to reduce airflow as much as you can.
• Let the SAB sit for at least 20 minutes before use: The longer the better, as it gives airborne particles time to settle.

By using your still air box properly, you create a controlled environment where your mushroom cultures have a much better chance of staying clean and free of contamination.

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6. Use Less Grain

Another method that has worked really well for me is using less grain per jar. This can be done by simply filling smaller jars or using larger jars with less grain inside.Why does this help? Because when you use less grain, there is less material that needs to be sterilized. This reduces the time and energy required to reach sterilisation temperatures throughout the jar, especially in the center.

In large and tightly packed jars, the heat or pressure takes longer to penetrate to the middle. This means the grain in the center may not be fully sterilized, increasing the risk of contamination. Smaller jars or reduced grain volume allow for more even and thorough sterilisation, even when using a regular steam bath instead of a pressure cooker.

This method can be especially helpful if:

• You are having trouble with contamination even after long sterilisation sessions
• You don´t have a pressure cooker
• You want to try a cleaner and more consistent approach before upgrading your setup

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7. Use Correct Temperature

Temperature plays a crucial role in mushroom cultivation. One of the easiest ways to reduce contamination is to keep your jars or tubs below 28°C (82°F). That is because this temperature range is ideal for bacterial growth, allowing bacteria to outcompete the mycelium, especially if there are even small traces of contamination left in the grain.

It is important to remember that even after proper sterilisation, some bacteria might survive. However, that does not automatically mean the grain is contaminated or unusable. In many cases, strong and healthy mycelium can outgrow small amounts of bacteria, but only if conditions are in its favor. If temperatures are too high, the bacteria multiply faster than the mycelium can spread. So by keeping the environment cooler, you are giving your mycelium the upper hand.

Recommended temperature range: 21°C to 26°C (70°F to 79°F).

Optimal temperature: ~24°C (75°F) for most P. cubensis strains.

If your grow space is naturally warm, try placing your jars in a cooler part of your home, avoid direct sunlight, and do not use heating mats unless you can control the temperature precisely. Controlling temperature may seem like a small detail, but it can make a big difference in your contamination rate and overall success.

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8. Conclusion

Contamination can definitely be frustrating, but fortunately there are many effective ways to prevent and overcome it. The key is to work carefully and patiently by combining proper techniques with persistence. Do not let contamination discourage you, as every mycologist faces it at some point. Just keep trying and learning from each attempt, and I guarantee that if you stick with it long enough, you will succeed.