Finding mould in your sourdough starter can feel discouraging especially when you’ve been nurturing it for days. But don’t panic. Mould is one of the most common issues for new bakers, and it’s almost always preventable with a few easy tweaks. In most cases, it comes down to cleanliness, airflow, and feeding frequency. Here's what to look for and how to fix it.
What Causes Mould in a Starter?
Mould thrives in warm, damp environments, exactly the conditions you’re creating in your jar. While a healthy starter can usually fend off bad bacteria, certain habits or oversights can create the perfect conditions for mould growth.
Here are the most common causes:
- Too much moisture: A watery or runny starter creates a damp surface where mould can grow.
- Poor airflow: Sealing your starter with a tight lid or plastic wrap can trap too much moisture and heat.
- Dirty jars or utensils: Even if they look clean, they may carry bacteria that can compete with your starter’s natural microbes.
- Warm, humid environments: High temperatures can accelerate spoilage.
- Infrequent feeding: Letting your starter sit too long between feeds creates an environment where harmful microbes can take over.
Beginner Tip: Avoid screwing your jar lid on tightly. A loose lid or breathable cloth lets air in while keeping contaminants out.
How to Prevent Mould
Good starter hygiene is your first line of defence. Here’s how to keep your culture healthy and mould-free:
- Use a clean jar for each feeding: Rinse with hot water, let it air dry, and don’t reuse unwashed jars.
- Avoid touching the inside of the jar: This reduces the risk of introducing unwanted bacteria.
- Wipe down the jar walls after each feed: Prevent dried starter from crusting and flaking back in.
- Clean the rim: This is a hotspot for dried starter and mould spores.
- Use a breathable cover: A clean cloth or loosely fitted lid keeps air flowing.
- Feed regularly: Don’t let your starter sit for days unfed, especially in a warm room.
Beginner Tip: If you’re not baking every day, store your starter in the fridge and feed it once a week to keep it active and safe.
What to Do If You See Mould
It’s important to know the difference between harmless surface changes and actual spoilage.
- Black, green, or fuzzy white mould: This is actual mould. Carefully scrape it off and transfer your starter to a clean jar. Keep a close eye on it over the next few days.
- Pink, red, or orange streaks or patches: This is a sign of bad bacteria. Unfortunately, it means the starter has spoiled. Best to throw it out and start again.
Beginner Tip: A healthy starter may form a dark liquid called “hooch” on top. That’s not mould it’s a sign it’s hungry! Just pour it off and feed your starter.
How to Restart Your Starter
If mould has taken over, you have two great options:
- Use discard from the fridge: If you’ve saved any, this unfed starter can be reactivated with fresh flour and water.
- Use dried starter flakes: Our kits come with dried sourdough starter that’s shelf-stable and super easy to revive.
Once you’ve restarted, follow the prevention tips above to keep mould from returning.
Final Thoughts
Mould can be annoying, but it’s not the end of the world. A little extra attention to cleanliness, airflow, and feeding routines can keep your sourdough starter strong and healthy. And if something goes wrong, restarting is easy especially with our dried starter flakes or fridge-stored discard.