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I have a sourdough starter which I feed roughly once every 2-3 days (throw away 95% of it, refill with water and flour). Sometimes I forget and wait 5 days between feedings, and by that time it often has developed what looks to me like mold on the top. It looks super interesting and I'd really like to know what it is. A picture is below. It's grown like this a few times, and the starter keeps rising and producing good bread, so it's definitely still alive and well.

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  • All I see is a coating of flour on top of partly dried starter in a nearly empty bowl.
    – Rob
    Apr 24 at 9:36
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    @Rob No, the starter is about two inches deep, and that's certainly not flour Apr 24 at 12:44
  • I see the mold, and the safety-conscious thing to do is to recommend you stop using it. It's unlikely anyone outside of a lab can identify what type it is, though, so I doubt you'll get any other answers here. You might consider any moldy cheeses or other mold-produced foods as potential sources for contamination, otherwise this isn't a mold-friendly site lol.
    – kitukwfyer
    Apr 24 at 13:37
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I don't see anything that has grown there, except for the starter itself. The starter used to be more expanded, its skin dried up in the expanded state, then it fell back down and the skin tore in places and wrinkled up - this is what you are probably picturing as a growth. It also trapped some gas bubbles (the round warts). There also seems to be a layer of hooch formed under the skin. Where the skin is torn, a single untrapped bubble is also visible at roughly (45°, 0.2r).

If you had actually had a mold growing, I would have advised against baking with the starter, because it is unsafe, even if the bread rises.

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  • If you zoom in, those lumps of "flour" have a definite fluff of white mold on them, which is slightly gray on the center. That grey and red discoloration is a bad sign as well..... Mine will oxidize to that grey color if I don't clean the container and it dries onto the side, but the fresh stuff never turned red in the middle...
    – kitukwfyer
    Apr 24 at 13:30
  • The starter rises on day 1, falls on day 2-3, and this white layer only appears on day 5. It's definitely not some "skin" which existed during the "rising and falling" stage. Apr 24 at 18:26

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