I work night shift in a bakery and the people before me always leave unused dough in the mixer. Who knows for how many hours, but during that same day. It's a dough made from dry yeast. One night I went to scrape it out and got a strong spoiled yeasty smell that came from it, almost knocking me to the floor. The other bakers said it was fine and that the dough is still usable for baking bread, but I'm not sure. Is leaving dough in a mixer for hours a health hazard? Does the smell indicate a health hazard?

  • 1
    Checking with a handbook is a good move. Just remember that many bread recipes proof over a really long time (as in days), usually in a refrigerator. Likewise, adding new flour and water to leftover dough "feeds" the existing yeast and makes good bread again. This is how bread was made centuries ago. Over a longer time, you get sourdough, because lactobacillus bacteria ("good bacteria") grow in there, too. Double-check with local health & safety regulations, though. Besides, @Catija has a good point in her answer.
    – Stephie
    Oct 21, 2015 at 19:06
  • 1
    I'd just like to throw in that since the dough has yeast, it'd be very difficult for it to get anything harmful growing in it within a day (or even a couple). The yeast would crowd out any other organisms, and yeast itself can produce some stinky fumes, but they're not harmful.
    – SourDoh
    Oct 23, 2015 at 4:13

1 Answer 1


.... There's no way to know for sure (since we can't test it) but it was probably CO2 (Carbon Dioxide)... Yeast creates CO2 when it converts sugars... that's how you can bottle carbonate beers.

The breaking down of sugars, or fermentation, produces alcohol and carbon dioxide as by-products. Fermentation turns fruit juices into wine and helps turn wort (diluted grain mash) into beer or whiskey. The carbon dioxide produced by fermentation makes the bubbles in beer and some kinds of wine, and causes bread to rise. As bread bakes, the alcohol produced by fermentation evaporates.

We brew beers and we've had our freezer, which we use for a fermentation fridge, fill with the gas when we're brewing and the fermenting overflows. When we have to get into the freezer to clean it out, we have to be really careful as the carbon dioxide is heavier than air, so it stays in the freezer for a while... the same thing would be true of your large, covered mixer bowls. It can make you light-headed because your brain is suffering from oxygen deficiency.

Here's the fact sheet on CO2 inhalation:

  • Inhalation: Low concentrations are not harmful. Higher concentrations can affect respiratory function and cause excitation followed by depression of the central nervous system. A high concentration can displace oxygen in the air. If less oxygen is available to breathe, symptoms such as rapid breathing, rapid heart rate, clumsiness, emotional upsets and fatigue can result. As less oxygen becomes available, nausea and vomiting, collapse, convulsions, coma and death can occur. Symptoms occur more quickly with physical effort. Lack of oxygen can cause permanent damage to organs including the brain and heart.

And, because it's a byproduct of the yeast activation, it can smell really strongly of yeast, and have a really off smell, despite the gas itself being odorless.

  • 1
    I know the smell of which she speaks, and it's almost certainly this, combined with a bit of an alcohol tang and perhaps a bit of whatever acids the yeast produced. All harmless, but when dough proofs that long, it can get a skin with a lot of gas trapped under it and you can get quite a lungful.
    – SourDoh
    Oct 23, 2015 at 4:11
  • 3
    One small problem: CO2 is odorless. I'm sure it's still fermentation byproducts that the OP was smelling, but it's not the CO2 itself.
    – Cascabel
    Oct 28, 2015 at 3:17
  • @Jefromi my last sentence mentions that.
    – Catija
    Oct 28, 2015 at 3:26
  • Sort of, but the answer overall is really implying that you're smelling the CO2, and it's only at the end that you mention actually it's everything except the CO2. And I think it's kind of relevant to know that all the smelly things aren't harmful either, not just the CO2.
    – Cascabel
    Oct 28, 2015 at 3:35
  • I'd be mostly concerned with the displacing of oxygen -- trying to lean over into a large mixer bowl might mean you're getting no oxygen. If you have to exert yourself to clean out the bowl, it's going to be difficult to hold your breath while doing it.
    – Joe
    Oct 28, 2015 at 14:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.