Question: how does fermentation work?

Rebecca Dewey answered on 23 Jun 2015:

Fermentation is a chemical process that causes sugar to be converted into other stuff, typically either an acid or a gas and an alcohol. The main two types of fermentation we use are ethanol (alcohol) fermentation and lactic acid fermentation. In both of these, the chemical process is catalysed (this means that something else allows it to happen, or causes it to happen faster) by a living thing such as a yeast (in ethanol fermentation) or by a biological cell such as a bacterium or a muscle cell (in lactic acid fermentation).

In ethanol fermentation, a yeast eats the sugar and makes ethanol (alcohol) and carbon dioxide. If we’re making beer or wine, the alcohol is what we want out of it, and if there is any carbon dioxide gas left then it might be fizzy at the end, which might or might not be good, depending on the wine!! If we’re making bread, then the carbon dioxide is what we want because the bread can stretch using gluten to keep all the carbon dioxide gas in and this makes nice little pockets in between the strands of bread dough and makes the bread springy and light. The alcohol is burned off/evaporated when the bread is baked in the oven.

In lactic acid fermentation, the sugars are converted to lactic acid and energy. Here, we use harmless bacteria to make a food have a sour taste (lactic acid tastes sour), such as certain beers or wines, but also yoghurt, and certain foods like fermented cabbage (Kimchi from East Asia or Sauerkraut from Eastern Europe). Lactic acid fermentation also occurs in muscles as a way of making energy out of sugar when the muscle has used up all its oxygen supply. This build-up of lactic acid during extreme exertion causes a burning burning sensation you sometimes feel in your muscles. This is actually beneficial because it gets us to stop overworking the body, and forces us to pause to recover from the exertion while the lactic acid is diluted and removed via the blood supply.