I'm baking bread at home as a hobby, using sourdough. After skipping one baking day, I haven't used my sourdough starter for two weeks. When opening the jar in which I keep it, I saw the surface had taken on this unusual look. When viewed from the side through the glass, the contents looked normal below the surface.

The sourdough starter consists of whole rye flour at 100% hydration (flour:water = 1:1). I had removed it from the ripe sourdough on the last baking day and have since kept it in the refrigerator with a lid (only loosely screwed on) for 13 days.

Normally I'm using it already after keeping it for 7 days, and I'm used to the surface turning light gray and dry (but smooth and soft) after that time, which didn't ever seem to be a problem. Now the color of the surface was identical, but it has become grainy, like tiny worms.

I have removed and discarded the top layer (3 mm). It was soft (unlike dried out, crusty, dough) and smelled a bit like fresh yeast. Below that, the contents smelled normal, like very ripe sourdough.

The jar is about 6 cm in diameter and was filled to about 2 cm (→ volume of the starter ≈ 60 ml).

  • What are we looking at here? Could it be actual yeast cells (or other microorganisms normally found in sourdough)?

  • Is it harmful?

Surface of sourdough starter after 13 days
After 13 days (click for full-resolution version)

Reference pictures

After only 2 days, the starter looks like this:

Surface of sourdough starter after 2 days Sourdough starter after 2 days
(click for full-resolution versions)

  • $\begingroup$ You're not seeing cells, as you would need a microscope to see them. $\endgroup$
    – mgkrebbs
    Dec 16, 2017 at 18:00
  • 1
    $\begingroup$ I'm not talking about whether we are seeing individual cells. It is clear to me that a single cell is too small to see with the naked eye. But if there are many cells forming colonies, we can see them. My question is if a colony of yeast or other organisms found in sourdough would look like in my picture. $\endgroup$
    – mkrieger1
    Dec 18, 2017 at 0:09
  • $\begingroup$ Somebody else has had this problem. I don't think it's a separate species identification problem. There is some abnormal yeast/bacterial growth. $\endgroup$
    – user38329
    Dec 19, 2017 at 15:10
  • $\begingroup$ Thanks for the link! I hadn't seen that before, it looks very similar. $\endgroup$
    – mkrieger1
    Dec 19, 2017 at 15:11
  • $\begingroup$ No probs. I think it's probably a mold fungi growing over the yeast, or sometimes when yeast is deprived of nitrogen (so if there was reduced air flow) excessive cell division occurs. In any case, in terms of cooking advice I think you probably need to make another starter! $\endgroup$
    – user38329
    Dec 19, 2017 at 15:16

1 Answer 1


Naturally, when you create dough you expect it to rise in size the following few days. That's just a chemical reaction that allows air to enter the dough body and traps it in tiny bubbles. My theory is that if the dough is not as high as expected that the air has escaped it because of the extra time you left it.

The pattern is not unusual and is the result of the deflation I'm talking about.

It's probably still safe to eat, but the bread won't be as good(eat at your own risk)

  • $\begingroup$ The sourdough starter doesn't rise much (because it is taken from already ripened sourdough) and collapses relatively soon after 2 or 3 days. But then the surface is still glossy and darker as in the picture I've included. The starter collapsing is not the issue here. $\endgroup$
    – mkrieger1
    Dec 18, 2017 at 0:03

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