7 Ways to Prevent Contamination in Mushroom Cultivation
How to increases your chances of success in no time with little to no work.
Contamination can be frustrating, but it’s unfortunately part of the journey. Every mycologist faces this challenge at some point, as no one is immune to contamination issues. Personally, it wasn’t until my third attempt that I finally succeeded, but it was worth the effort.
Most contaminations occur when the grain spawn isn’t properly sterilized, or during the agar-to-grain transfer if it’s not done under clean conditions. To help you improve your success rate, especially if you’re using the Starter Kit, here are seven practical tips to increse your chances of success.
1. Work more thoroughly & follow the instructions
It might sound obvious, but it's 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’ll never know for sure.
Here are some common big mistakes that are easy to avoid:
- Using a still air box incorrectly – No proper airflow or movement control can lead to higher contamination rates.
- Not sterilizing grain long enough – Under-sterilized grain is one of the main causes of contamination.
- Overcooking the grain – This causes the grains to burst, making them more accessible to contaminants.
- Using too much or too little water when preparing coco coir – This can result in a substrate that’s either too wet or too dry, which negatively affects your flush.
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, healthy mycelium—it's basically the base of your entire grow.
Steam sterilisation works well, but like almost any method, it isn’t 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’s a simple way to improve this: using more than one sterilisation cycle. This method is called tyndallisation. Instead of just one steaming session, you do three:
- Day 1: steam for 2 hours
- Day 2: steam for 2 hours
- Day 3: steam for 4 hours
Doing three rounds of steaming significantly increases your success rate. But of course, sterilising for a total of 8 hours over three days doesn’t 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 (some more prone to contamination)
- Different working environments (more or fewer contaminants in the air)
- Different altitudes (lower altitude makes sterilisation more effective)
- And other unique factors
That’s why it’s important to experiment and find out what works best for you. And if it didn’t work this time—don’t worry. The next attempt will be better.
Personal tip:
Shake the jars and mix the grain between each session. This helps the heat reach the grain in the center of the jar more effectively.
3. 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.
Even better, you can often find small, second-hand pressure cookers for as little as €20, for example on Facebook Marketplace.
Which one to get:
- Ideally, choose a model that reaches 15 PSI (pounds per square inch).
- 12 PSI is also sufficient—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 hoursAt 12 PSI: 2.5 hours
If you plan to grow mushrooms more than once, a pressure cooker is a must-have for every hobby mycologist.
Bonus tip: All items in the Starter Kit—like Mason jars, scalpels, and micropore tape—are pressure-cooker safe up to 15 PSI.
4. 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’s why some growers swear by brown rice, while others prefer corn or rye. There’s no one-size-fits-all—so take some time to experiment and find out which grain works best for you.
5. Use the Still Air Box (SAB)
Use the still air box (SAB) as often as possible—it’s 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’s not possible, try to reduce airflow as much as you can.
- Let the SAB sit for at least 15 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 contamination-free.
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’s less material that needs to be sterilized, which reduces the time and energy required to reach sterilisation temperatures throughout the jar—especially in the center.
In large, tightly packed jars, the heat or pressure takes longer to penetrate to the middle, which 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’re having trouble with contamination even after long sterilisation sessions
- You don’t own a pressure cooker
- You want to try a cleaner, more consistent approach before upgrading your setup
Bonus benefits:
- Smaller batches are easier to handle and observe
- If a jar gets contaminated, you lose less grain overall
- Colonisation is often faster in smaller jars with more airflow around each grain
So, if you’re still struggling despite being careful, try reducing the amount of grain per jar—it’s a simple change that can dramatically increase your success rate.
7. Watch the 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’s 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’s important to remember that even after proper sterilisation, some bacteria might survive. However, that doesn’t 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're 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 don’t 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.
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—combining proper techniques with persistence. Don’t let contamination discourage you; 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.