Reasons for Mold on a Sourdough Starter
Is there mold on your sourdough starter? I’ll show you how to prevent it, and effective ways to revive that bad sourdough starter.
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Nothing is quite as frustrating as a bad sourdough starter, especially when it becomes moldy. When this happens to you, don’t lose hope, because sometimes it can be fixed and useable again!
In this article, I will share common causes of a moldy sourdough starter. Some of these causes may be obvious, but others may completely surprise you. The good news is that understanding the various reasons will help you avoid them in the future, or possibly even revive your active starter!
If you don’t have an active starter yet, you can read this article on how to make a sourdough starter.
Common causes for mold on a sourdough starter
There are several reasons why a starter can go bad, including:
- The ingredients
- The supplies or tools
- Water
- Improper storage
- The environment
I go into detail about each of these further down in the post, so keep reading!
What does bad sourdough starter look like?
Most likely, you will know by sight and/or smell if you have mold on a your sourdough starter. It is often a gray or black fuzzy growth on the inside of the jar, or sitting on the surface of the starter. Bad bacteria can also appear as an orange tinge or pink streaks.
Note
If you see brown-ish liquid sitting on the surface of your sourdough starter, that is called hooch. It is not dangerous, but is often a sign that your starter is hungry and should be fed. Some people recommend pouring it off. I usually stir it back in and then feed the starter some flour.
Kahm yeast is something else you might find on your starter, but it is not really a type of mold. It often looks white and powdery, and is a harmless sign of yeast overgrowth.
Contaminated Ingredients
There are 3 ways ingredients can cause mold on your sourdough starter:
- Your flour might contain mold spores. It could be that the flour is old or was stored in a damp environment. Solution: try a new or fresh flour
- The other reason can be bleached flour or conventional flour. Bleaching flour can kill the wild yeasts and good bacteria that are on the grains that we are trying to cultivate. If you are using conventional flour, the grains have likely been sprayed with synthetic pesticides and herbicides that can also kill the natural yeast. Solution: try using at least unbleached flour but better yet organic flour.
Tools and Supplies
It is very important to clean your sourdough tools and equipment. By clean, I don’t mean bleached or sterilized necessarily. However, even soap residue in the jar can cause mold growth.
Solution: Thoroughly rinse the storage jar, or use a new jar for each batch of starter.
Many people have concerns about using metal utensils. Using metal utensils does not create mold! Especially if you’re only using a metal fork or spoon to stir your sourdough starter.
How water can cause mold on a sourdough starter
The type of water you are using can also cause a bad sourdough starter. City water often contains chlorine, which can inhibit the growth of a good sourdough culture. This is especially true if you have a very new sourdough starter.
This makes sense, because municipalities add chlorine to kill microorganisms – and that can kill the good bacteria in your starter as well!
Although rare in the US, your water supply might also be contaminated with something.
Solution: Use filtered or bottled water.
If you only have tap water available, you can leave it standing in an open container at room temperature for at least 24 hours, and the chlorine will evaporate. Alternatively, you can boil tap water for 15 minutes to remove any toxins or contaminants.
Improper Storage
If you are keeping your sourdough starter on the counter for regular feedings, it is growing in a warm environment.
If you are following my no-discard, no-feeding sourdough maintenance method, you will keep it in the refrigerator for the most part.
Even a healthy sourdough starter can grow mold if you store it in high heat and/or humidity. Also, any heat above 140˚F will most likely kill the wild yeast in sourdough starter, which happens in a hot oven. If you keep your sourdough starter in the oven with the light on, be sure to take it out before your turn on the oven.
Solution: Keep your sourdough starter in a dry area, with a temperature of 70-85˚F. If you need help keeping the conditions perfect, consider using a sourdough Goldie.
The Environment
This is something that people don’t talk about often. However, there are a few ways in which the environment can cause mold on sourdough starters.
- Mold spores from moldy fruit can jump over to your sourdough starter and cause mold growth. If you keep fruit or vegetables next to your sourdough starter or in the bread dough, and they have mold on them, the mold spores can easily spread to the starter and start growing there.
- An over-sanitized home can also potentially harm the beneficial bacteria and yeasts in a sourdough starter. While cleanliness and good hygiene practices are important for preventing the growth of harmful bacteria, excessive use of sanitizers and disinfectants can create an environment that is too sterile and can kill off beneficial microorganisms.
- Your hands. As Vanessa Kimbell says in her book Sourdough School: “I believe, however, that there is another influence in determining which microbes end up in starters. The cultures are nurtured by human hands, and I think that this interdependent relationship with the baker is the missing part of our understanding of where the microbes come from. In other words, the lactic acid bacteria in each baker’s pot might well be influenced by the lactic acid bacteria from the baker’s own skin microflora.”
Solutions:
- Keep your sourdough starter away from anything moldy, or even other ferments (such as German sauerkraut or kefir).
- Only use sanitizers, disinfectants, bleach, and so on if absolutely necessary. For more info, read the CDC’s guidelines for cleaning your home.
- Avoid over-sanitizing your hands. For more info, read the CDC’s guidelines on washing your hands.
What to do if you have mold on your sourdough starter
If there is any visible mold on your sourdough starter, you can never go wrong by throwing it out and creating a new sourdough starter.
However, if there is only a tiny bit of mold on the side of the starter jar, you can use a clean knife to scrape it off and discard it.
Then, transfer a small amount of the clean starter to a new jar. Give it a little extra attention in the form of feedings with flour and water.
Ultimately, you should go with what you are comfortable with. If in doubt, just throw out the bad sourdough starter entirely.
Unpleasant odors
You might be worried if your sourdough starter smells like nail polish remover, but most likely, it has not gone bad.
I have actually experienced that after I had kept it in the refrigerator for a long time (more than six months; you can learn more about my sourdough maintenance method).
Usually, that odor is a sign of over-fermentation, which produces high levels of acetone.
Solution: Take a small amount of that old starter and feed it with equal parts of flour and water. You may have to do some additional feedings for a few days until you have a nice, active sourdough starter again.
Myths – these will not cause a bad sourdough starter
Despite what you may have heard, these things will not kill your sourdough starter:
- using metal utensils
- mild neglect: Just resume regular and consistent feedings
- incorrect feeding: If you’re not feeding the starter often enough, hooch may accumulate on the surface. Equally, if you are feeding it too much flour, it may dry out, reducing its rising capability.
- freezing a sourdough starter (if you’d like to keep your starter long-term, I recommend dehydrating it. If you only freeze it for a short time, it won’t kill it, but the longer you freeze it, the more inactive it can become.
Recap- how to prevent mold on sourdough starters
Here are the best practices for preventing mold growth from occurring in the first place.
- Start with a clean jar.
- Use organic, unbleached flour. All-purpose flour is fine, but a better option is whole wheat flour, and the best option is rye flour.
- Keep the starter stored at 70-75˚F. Either follow a discard-and-feed method according to the instructions or use my super easy method to make a sourdough starter. If you are making a starter for the first time, it might be your most challenging one to make.
- Keep the sourdough starter in a clean but not overly sterilized environment. Be sure to keep it away from your other ferments and anything moldy.
Related articles
The Easiest Way To Make A Sourdough Starter
No-Discards, No-Feeding Sourdough Maintenance
Easy No-Knead Sourdough Bread
What to Do With Stale Sourdough Bread
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Hi Anja,
I have started a sour dough starter and on day 1 it got white fluffy mold on top, day 2 the mold had spread to the sides of the jar.
Is this a throw away starter and start again?
Thanks
Mel
The safest for you (and for me to recommend) is to throw it out, especially early on. Hope this helps and don’t give up ~ Anja
I’m a few days into making my new sourdough starter after giving up on the discard/feed method I was trying to do. It felt like such a waste of flour! Today I noticed fuzzy greenish mold around the top edge of my bowl where I didn’t wipe well enough. I wiped it off, transferred my starter into a clean bowl and gave it a good stir. I can visibly see the bubbles coming to the surface and popping, and it smells of yeast with a light beer scent. I’m praying it’s still good because it looks and smell lovely!!
I think it will be fine but use your own best judgement ~ anja
Great article, except for one point, which I encourage you to update. Once there is visible mold on any part of the starter or jar, the starter is already infected. It was infected before you could see the mold. You can’t save any part of it.
Thank you for chiming in to this popular debate. Ultimately, I tell people to use their best judgement and in doubt, to throw it out ~ Anja
The recommendation to keep sourdough starter culture away from other ferments like kefir is somewhat confusing to me because I’ve read (and it makes sense to me that) you can actually use kefir to bootstrap (start up) a sourdough culture; there’s (apparently) quite a lot of overlap between the yeast and lactic acid bacteria species in both cultures. So I don’t see how one would “contaminate” the other. Just FYI.
The way I understand is that while there may be an overlap, in your SD starter you want to cultivate slightly different ratios. However, if it works for you, then no need to separate them ~ Anja
I’ve tried to make 2 starters now. The first was going great and then I spilled 3/4 of it on Day 3 or 4. I tried to continue with the rest but it ended up with a white, powdery looking “crust”. I was afraid that what I was seeing as powdery, might, in fact to be “fuzz”and so I threw it out. The second starter did exactly the same thing on day five. How do I know if I have Kahm yeast or mold?
Interesting! Sandor Katz, author of “The Art of Fermentation,” describes Kahm yeast as a harmless creamy-white-to-beige wavy growth that forms on the surface of fermented vegetables or beverages where oxygen meets the liquid. This film stays at the top, is not fuzzy, and doesn’t grow below the surface. Use your best judgement but I would just scrape it off and see how the starter develops ~ Anja
I have what I think is mold, but you mentioned Kham yeast?
Is this a problem or something to stir into the starter and keep going?
Please provide more info on Kham yeast.
TIA
New SD Starterer
Jacqueline
Kahm yeast is harmful overgrowth of yeast. You can scrape it off ~ Anja
I woke up today to mold in my starter. I was devastated! My starter was probably not healthy the past 4-6 weeks as it had a lot of water on top and didn’t seem to bubble up like it used to. I tossed it out and am sanitizing the vessel and my friend is bringing me some of her starter she froze.
I suspect two possible scenarios
#1 I wasn’t feeding it properly
#2 my flour could have been contaminated
Those could be the reasons. If you make a new one, feed it properly and use a different batch of flour, you will quickly know ~ Anja
I have a question regarding mold – what happened to me was that i had my starter in two different jars in the fridge (same starter and everything, I was planning on using one in a discard recipe and the other as my regular starter) and just one of the jars got moldy. Do you know why this could be? It contained the same starter, flour and everything and both were kept in glas jars in the fridge.
Thanks!
-Liz 🙂
That is very interesting – and hard to diagnose. Good thing you had 2 jars, though ~ Anja
I just started this sourdough journey and I have to say I feel kind of like Job but I am NOT GIVING UP!! I developed mold on the inside of the jar around the rim…wiped it off and kept going. I’ve been repeating your words in my head…paraphrasing “don’t be afraid, don’t give up”! My starter has been behaving wonderfully…. made my first loaves of bread yesterday….my kitchen aid mixer stripped the gears and burned up halfway through kneading my dough. No problem…I finished by hand. It rose beautifully and I divided it and put into loaf pans and it rose the 2nd time beautifully! Finished baking at 2 am! Beautiful! Next morning got halfway through slicing it and my electric knife burned out! The bread is a little denser than I hoped it would be but the flavor is great. Overall, for my first time making sourdough I’m pleased…..cant wait to make more….AFTER I replace my mixer!! Thanks for all your guidance and tips!!!
Yay! That makes me so happy to hear and good for you for not giving up. EVER! You got this and happy sourdoughing ~ Anja
I thoroughly enjoy your articles. I’m on day 9 of my starter using your no discard method and it’s looking good! On day 7 when I looked in on it I had mold on the upper edge & hooch. Right or wrong I wiped out the mold and stirred in flour. I haven’t notice anymore mold and good bubbles even on sides but, I think I’ll take your advice and take some to a new clean jar.(but I think I’ll continue my old, keeping it away from new). Thank you it’s been a very gratifying experience – onto bread!
Nice! I think your starter should be good and I am so happy to hear you’re loving your sourdough journey ~ Anja
Would mold look white??? The entire top of my states looks white and the bottom regular brown rye flour.
That’s hard to diagnose remotely and if in doubt, discard it. However, often white stuff can be Kahm yeast.
I have the same thing with rye flour! Never had that with unbleched flour! Hoping it’s the good kind of yeast mentioned not Mold!
I really enjoy your videos on sourdough bread making. I recently put together a starter using your buttermilk method. After the 4th day it began to produce mold. I was so disappointed! But I read your article on mold, and I’m ready to give it another try.
Mold can be frustrating but be sure to go through the list of potential causes – and do not give up! Once you have a mature starter, it will be much less susceptible to mold ~ Anja
Interesting facts here! I have been doing sourdough for 3 years and learned something new! Thanks for sharing!
That makes me so happy to hear ~ Anja
Thank you for this! I haven’t done sourdough in a while but was thinking of starting up again and this post will help me get it going!
Nice! Happy sourdoughing ~ Anja
Anna, this article is so helpful to me right now because I’ve been thinking of getting on the sourdough starter “train” but am still clueless. This info is really good to have. Thank you!
I am so glad to hear that ~ Anja
This is so helpful and thorough, thank you!! I now have a name for that white stuff 😅 and I think I know why a couple of my starters molded.
Yay! I am so glad you found this helpful ~ Anja