"Acetone" smell on bread

Question

I sometimes buy a brand of multi-grain salt-free bread *. After 2-3 days, it gets a very distinct smell of acetone, like paint stripper or nail polish remover. If I toast the bread, the smell goes away.

The bread is well within its sell-by date, so I don't typically put it into the fridge. This is not a one-off thing - I've noticed it many times for this type of bread.

I am confused by where acetone would come from in the typical grain/yeast mixture used for bread. What could be causing this? Is the bread still safe to eat?

* example: Trader Joe's Sprouted Multi-grain Salt-free Bread

Answer 1

I wouldn't see it as safe to eat. I cannot imagine that the acetone was present when you bought the bread (QA should have caught this if it happens every now and then), so it must be the byproduct of some microorganism chomping down on the bread. Especially if you say that it develops over time.

On the chance that it is mold (this is more common in bread than bacteria), try keeping it with less humidity. If it is in a plastic bag or in a bread box, stop doing that. Use a paper bag or leave it naked, and not in the direct way of fumes (e.g. a shelf above the stove).

Seeing that this bread has sprouted grains in it, it could even be moist enough to have bacteria colonizing it. Still, the remedy is the same. Store it in a dry location.

Answer 2

This sounds like a yeast issue to me. Wild yeast contamination can sometimes occur in bread factories after baking, and some of them can produce acetone (or similar smells). Salt also acts as a preservative in inhibiting growth like this, so a salt-free bread may be more prone to such growth.

Acetone notes are sometimes found in sourdough cultures (usually ones that aren't growing well). It would also not surprise me to find such odors in sprouted grains. Some of these odors can stay in the bread even after baking. And it's possible that the outer layers of the bread may have offgassed those odors during baking, but the stuff from the inner layers works its way out over several days after the bread is bagged. If that were so, though, I'd expect the bread to taste odd too, with acetone or organic chemical notes.

As for safety, it's tough to say. In most cases this would indicate contamination or at least something unbalanced in the bread production. I personally would discard it, and if I encountered another loaf from the same manufacturer with the same characteristics, I'd consider contacting them to see if they can explain it (or, if not, that they're at least aware of the problem).