Moldy Sourdough Starter

It’s always alarming whenever you suspect you’re dealing with mold. In this post, I want to address some common experiences, questions, and concerns, hopefully providing some helpful solutions concerning moldy sourdough starter – or what you suspect to be moldy sourdough starter.

All in all, sourdough is pleasant and easy, especially once you’ve been going for a couple of years.

Occasionally you’ll run into some questions.

When those questions are concerning potential mold, I agree it can feel stressful. I hope this post is informative and answers many, if not all, of your questions.

I will add that most of the questionable conditions we’ll discover in our sourdough starters are simply due to some form of neglect.

Neglect will probably make you feel like a bad parent, even if it is only concerning starter. Oftentimes we aren’t intentionally neglecting it – there’s a range. It can simply be learning about what variables need adjusted to satisfy our jar of friendly bacteria.

I don’t think it’s the starter’s fault, and I don’t think it’s your fault. It’s somewhere in the middle. So don’t take it too hard. 

What is mold?

Mold is a fungus, a living organism, and it thrives in warm, humid environments. Don’t think it won’t make a living in the cold, though.

Mold is a hardy, living thing. It requires oxygen, food, and water, all of which can be found on the surface of your sourdough starter. 

It presents itself in a variety of fuzzy ways: fuzzy spots, velvety patches, furry little spreads. Growing in an array of sickening, but fascinating colors, and ultimately, it can be quite harmful.

Some molds, such as those introduced to cheeses like blue cheese and brie, are edible and surprisingly appetizing. We’re not talking about those.

Others are potentially harmful, but can be and have been accidentally eaten with no physical side effects. Like when you notice significant mold on remaining slices of bread after you’ve finished your sandwich, leaving you to surely feel sick – even if it’s only in your head. 

Consuming moldy food can cause nausea, diarrhea, and vomiting. Some molds have sparked severe allergic reactions and even lasting illness.

It can be pesky, and it can be life-altering, and it should be taken seriously.

However, the good news is that not every suspicious discovery is mold. It’s important to calmly diagnose to the best of our ability, and so I’ve listed below some potential problems you might experience.

Potential Sourdough Starter Issues:

Black patches or a layer of black liquid, not fuzzy: 

Hooch: A byproduct of fermentation, hooch occurs in starters that are underfed. Sometimes they aren’t fed often enough, and sometimes they simply aren’t fed enough at all. 

Beginning stages of hooch are dark-colored patches on the surface of.your starter. This develops into a thin, but increasing layer of dark liquid on top. This often occurs in starters stored for longer periods in the refrigerator. 

What to do: You can pour off the hooch or mix it back in, then resume feedings. Considering the development of hooch, you may need to consider more frequent feedings or increasing the amount of flour/water.

Strong smell of acetone or nail polish remover:

Your starter is underfed and overripe: If you’ve missed enough feedings or simply haven’t fed your starter enough, the yeast and good bacteria will let you know by a strong and unpleasant smell. It has been over fermented and will require a bit of attention to cheer it up.

What to do: Feed your starter. You might consider a large discard, a new, clean container, and additional feedings to encourage your starter to bounce back.

White to light tan wavy, bumpy, or rippling surface growth:

Kahm yeast: No need for alarm. This wavy layer of suspicion is a generally harmless growth of yeast, feeding off the oxygen at the surface.

Unlike mold, kahm yeast is not fuzzy, but kind of resembles the wavy sand on the ocean bottom. It can also look like a shallow white film at the top of your starter in its beginning stages. While kahm yeast isn’t necessarily harmful, it can effect the taste of your starter.

What to do: Scrape out the bulk of it, then transfer to a clean jar and resume feedings. If a small amount is stirred back in, your starter should be fine, although it may likely struggle with further kahm yeast outbreaks in the future. 

Pink streaks, slight pink tinge, or orange streaks in the top of your starter:

Serratia marcescens: This is a common, potentially infectious bacteria that takes advantage of underfed starters. A starter that is underfed or experiencing severe neglect runs the risk of the bad bacteria overcoming the good bacteria.

It’s in this weakened state that serratia marcescens bacteria, which were already present in your starter, may increase and overtake the good bacteria. While baking temperatures are most likely to kill this bacteria, you should avoid it.

What to do: Remove the visible signs of pink on your starter. Transfer the remaining starter to a clean jar, then begin feedings daily, or even twice daily, to strengthen your starter and lift the levels of the good bacteria.

Of course, if you are still concerned, you can always dispose of it and start over. I wouldn’t be too concerned, though.

Fuzzy spots, velvety patches, differing colors: 

I’m sorry, but this sounds like mold. While there are many different reasons that you might be wary of some kind of strange growth on your starter, mold really is worth worrying over. It’s also pretty easy to recognize.

The characteristic furry, fuzzy, hairy colors sprouting on the surface are a sure sign you should throw it out. I personally wouldn’t risk saving it, considering the extensiveness of the thread-like roots that reach out beneath the visible spores.

What to do: Toss it. While I don’t recommend it, I have heard of starters being saved by taking a few tablespoons of starter without visible mold (nearest the bottom of the jar), rehoming this into a new jar, and then encouraging a sour starter with delayed feedings of equal parts water and flour.

The higher acidity, in theory, kills the spores. However, it’s very likely there will remain mold spores in your starter that will resurface.

Can you save your mold-contaminated starter?

This is entirely up to you. The safest bet is to toss the entire starter and start fresh. Or ask another friend with a healthy sourdough starter for some of theirs. Bakers are usually always willing to part with a little bit of starter.

Mold growth operates much like the roots and branches of a tree. What you see on the surface of your starter, if it is mold, is likely only the start of more activity hidden beneath the surface.

Sure, if you’re like me and find a tiny bit of mold on the end of a loaf of bread, you may toss that end (plus a slice or two) and use the rest.

So far I’ve not suffered any consequences, and I’ve not lost much delicious sourdough bread either.

The risk you run has much to do with the type of mold and your individual reaction to it. 

Some molds, such as the well known and much feared black mold, are extremely toxic and wreak havoc on the respiratory system, nervous system, and beyond.

Black mold is not typically associated with food, but rather products such as wood and paper in a humid environment.

There are many different kinds of molds that can be found in food, any of which might be the culprit if you find it on your starter. Some have a more negative effect than others, but all deserve our concern.

Why does mold develop on a sourdough starter?

Mold spores, when dry, travel airborne until they find conditions suitable for growth. They are found in all kinds of conditions, in all seasons, and most of the time our bodies handle them just fine. 

Here are the main reasons for mold to feel at home in your starter. 

New starters

Young sourdough starters have not had enough time to develop good bacteria necessary to prevent mold. A strong presence of beneficial bacteria wards off pathogenic bacterias, or those which cause disease. 

Frequent feedings and regular feedings, as well as a warm environment, are the right conditions for a starter to thrive. These conditions, continuing on for several weeks or months, produce a healthy starter, growing into maturity and becoming one of your most helpful friends.

A mature starter has established a strong presence of good bacteria, leaving less room for these opportunistic spores to take root. Literally. 

Contaminated tools

Clean tools are very important. They should be washed, rinsed of soap residue, and dried. Sterilized tools are not necessary, but an old-fashioned soap and water wash will suffice.

Contaminated or bleached flour

It’s not uncommon for mold spores to find their way to you through a contaminated flour. You can replace the flour and start again. 

Bleached flour can also be problematic because the bleaching process kills the wild yeasts that we are attempting to nourish. Without sufficient wild yeast, your starter will struggle.

Unfiltered water

The type of water you use actually matters a lot. Unfiltered tap water contains chlorine and other chemicals and heavy metals that kill bacteria, good and bad. It’s important to have a quality water source, which is why we’ve loved our Boroux filter. 

You can use bottled water, as well, though plastic bottles are not my first choice. 

Another option is setting aside an amount of tap water at room temperature for 24 hours or so, giving the chlorine opportunity to evaporate. Alternatively, you can boil tap water for 15 minutes.

Environment

If the temperature in your home is too cool, less than 70 degrees or so, your starter will struggle to remain active and strong. This puts the good bacteria at a disadvantage to warding off pathogenic bacteria. 

You can try feeding your starter with flour and warm water to encourage activity in an otherwise cooler environment. 

Also, a home that has high humidity is favorable to mold, and as such, it can raise the condensation in your starter container, increasing the probability of mold. Furthermore, many homes have increased disinfecting efforts since 2020, using products that kill the harmful bacteria and the good bacteria. This can actually play a part in your starter’s health. 

Overall, you want conditions that encourage a strong and vibrant starter that is less susceptible to contamination.

Proximity to contamination or mold

If your starter is kept in close contact with fruit, vegetables or other foods that you routinely keep on the countertop, it’s possible that mold can carry over.

Mold spores can travel through the air, particularly when dry, and so it is actually quite feasible that moldy food can contaminate your starter if kept nearby. 

Tips

• Keep a backup starter. Did you know you can freeze or even dry sourdough starter just in case? 
• Keep good hygiene practices, ensuring your tools, hands, and home are clean, but not sterile.
• Use an unbleached flour, preferably organic.
• Keep your feedings regular and observe your starter’s activity to determine a proper feeding schedule. This may vary, depending on one’s home.
• If you’ve had to toss your starter for any of these reasons, here’s my post on how to make sourdough starter from scratch.

Sourdough Starter Mold Prevention

• Feeding your starter regularly cultivates a healthy colony of wild yeast and good bacteria, which maintain an average pH of 3.5-5, a level that inhibits the growth of mold spores and other pathogenic bacteria, such as botulism and E. coli. 
• Feeding your starter the correct ratios is important to maintain the health of your starter. If your starter is underfed, the good bacteria are weakened and allow the bad bacteria to abound, producing a variety of negative impacts as outlined above. Feeding your starter too much is less of a concern, but may be unnecessary and an extra cost financially. Common ratios for a new starter are 1:1:1 (starter:flour:water), increasing to 1:2:2 and 1:3:3 for a mature starter.
• Using clean tools, as established earlier, is important in mold prevention. While sterilization is not necessary, washing with soap and thoroughly rinsing with water will help to keep unwanted bacteria out. Tools include your mixing spoon, jar, and your hands.
• Switching out jars occasionally is a good idea. Your starter will be happy in the same jar for a long time, but it helps to keep the top of the jar and rim clean. This is because it is nearest outside exposure and has a higher risk for contamination.
• Changing up your flour may be beneficial, depending on how quickly or how slowly you go through it. If you suspect your flour has been exposed to moisture, change it up. If your starter is showing signs of slowed activity, try a new flour. For instance, if you’ve been using unbleached all-purpose flour, try switching to whole grain or a blend of all-purpose and whole grain.

FAQ

Find More Sourdough Basics

• How To Store Sourdough Starter
• Freezing Sourdough Bread Dough
• How To Cut Sourdough Bread
• Sourdough Baking Glossary Of Terms

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