On rare occasions, bread can have a chemical odor that smells like acetone. This is often caused by the bread being past its Best Before Date or by wild yeast contamination.
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How a chemical smell in bread occurs
Yeast contamination can occasionally occur in bread after baking which can produce a chemical smell that is similar to acetone. Yeast does not survive the baking process, but bread can become contaminated with "wild" yeast during the cooling, slicing or packaging processes (post processing contamination).
Product safety
Eating bread that has a chemical smell due to yeast contamination might taste unpleasant and lead to minor digestive symptoms, but it does not pose a health risk.
What to do if your bread has a chemical smell
If you find that your bread has a chemical smell, like acetone, you may throw it out or return it to the store where it was purchased.