How to Avoid Contamination: Proven Techniques From Our Lab

Contamination is one of the biggest challenges in mushroom cultivation, whether you’re a brand-new grower or a seasoned hobbyist. At Colorado Cultures, we work in a sterile lab every day producing liquid cultures, grain bags, substrates, agar plates, and more — which means we’ve seen every type of contam and learned exactly how to prevent it. This guide breaks down real, practical contamination-prevention methods used in our lab that you can apply to your home setup for cleaner, faster, more reliable grows.

What Causes Contamination?

Contamination happens when unwanted microorganisms — mold spores, bacteria, yeast, or environmental pathogens — make their way into your grain, substrate, or liquid culture. These invaders compete with your mycelium and will nearly always win.

The goal is simple:Create an environment where your mushroom culture has every advantage and contaminants have none.

1. Start With Clean, Quality Inputs

The easiest way to reduce contamination is to use high-quality, properly sterilized products.

What to look for:

• Grain bags sterilized at 15 PSI or dual sterilized

• Fresh liquid cultures with no separation, cloudiness, or off smell

• Substrates produced in a clean, controlled environment

• Agar plates poured in a sterile space and sealed immediately

At Colorado Cultures, every LC, grain bag, plate, and monotub kit is produced in a controlled environment using commercial sterilizers and flow hoods — and that’s the standard we encourage growers to look for.

2. Practice Proper Sterile Technique

Even the best supplies can contaminate if you introduce dirty air or unclean tools. Here are the most important principles:

• Work in a still-air or low-airflow environment

Turn off fans, AC, and vents. Air movement is your enemy.

• Sanitize hands and gloves constantly

Use 70%-99% isopropyl alcohol — keep everything extremely clean and sterile

• Don't reuse or flame sterilize needles, use single use needles that come sterilized

Needles are inexpensive and using pre-sterilized single packaged needles is the preferred method is possible.

• Spray/wipe injection ports with alcohol before AND after use

It's a good habit to get into spraying the injection ports before and after.

• Minimize talking, coughing, or exhaling near your workspace

Masking up helps more than most people think.

Sterile technique matters more than nearly anything else in the process.

3. Control Your Environment

Your grow area doesn't need to be a professional lab, but it should be:

• Cleaned before each use

• Free of pets

• Low-traffic

• Wiped down with alcohol or Lysol

• Protected from drafts

A simple bathroom or small closet often works better than a large room with moving air.

4. Know the Early Signs of Contamination

Catching contamination early saves time, money, and frustration. Look for:

On Grain:

• Bright green patches → Trichoderma (mold) doubles in size every 24 hours

• Wet, slimy kernels → bacterial contamination

• Black, grey, or fuzzy growth → mold colonies

• Sour or foul smell

On Agar:

• Fast-growing white fuzz with no rhizomorphic structure

• Discoloration (pink, yellow, green, grey)

• Satellite colonies not touching your main culture

In Substrate:

• Strong smell

• Green blotches

• Wet, slimy sections

• Black pinpoints or “dusty” mold

If you suspect contamination, it’s better to toss it. Trying to save a contaminated grow almost never ends well.

5. Use Proper Inoculation Ratios

Under-inoculating slows colonization and gives contaminants time to take hold.Over-inoculating can cause pooling or bacteria buildup.

General guideline: 5-10 mL per 3 lb grain bag.

Fast colonization = less time for contamination to develop.

6. Store Your Supplies Correctly

Temperature swings and poor storage can introduce moisture issues or slow down mycelium.

Best practice:

• Liquid cultures: store at room temperature for the first 30 days and in the fridge for up to 7 months (do not freeze)

• Grain bags: use within 30–60 days for best results

• Substrate: keep sealed until ready to use, Denver Dirt can be stored for up to 10 months

• Agar plates: store upside down in a cool, clean space

Good storage extends shelf life and maintains sterility.

7. Keep It Simple

Most contamination comes from over-handling. The fewer steps you take, the fewer chances contaminants have to sneak in.

This is why many beginners love all-in-one grow bags and liquid culture: fewer variables, fewer points of failure.

Final Thoughts

Contamination will happen — even to experienced mycologist — but with the right techniques, you can reduce it dramatically. Every clean grow teaches you more about your process, your environment, and your genetics.

At Colorado Cultures, we’re committed to helping the Colorado mycology community grow confidently and successfully. If you ever run into contamination issues, bring in a photo or message us — we’re always here to help troubleshoot.