In the absence of oxygen, fermenting yeast obtains energy by converting sugars into carbon dioxide and ethanol.

C6H12O6=2C2H5OH+2CO2+energy

During the process of baking bread or steaming steamed buns, carbon dioxide rises the dough and the alcohol evaporates.

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You should be able to understand it if you have studied chemistry. Glucose (sugar) is decomposed and alcohol is produced (evaporated). and CO2, so it fermented~