The chemistry of fermentation

What actually happens, molecule by molecule, when food ferments. A plain-language walk through the chemistry, no lab coat required.

The one-line version

Microbes eat sugars and give back acids, gases and aromas. That, in a sentence, is fermentation. Everything that follows is just detail worth knowing.

Meet the workers

Different ferments rely on different microbes.

• Lactic acid bacteria drive kimchi and most vegetable ferments, turning sugars into lactic acid.

• Yeasts power kombucha, tibicos, bread and beer, turning sugars into alcohol and carbon dioxide.

• Acetic acid bacteria give kombucha its bite, turning alcohol into acetic acid.

• Moulds make tempeh, knitting the soybeans together and pre-digesting them as they grow.

Sugar in, acid out

Take kimchi. The vegetables carry their own natural sugars. Lactic acid bacteria ferment those sugars into lactic acid, and that acid lowers the pH, which preserves the food, keeps pathogens out and creates the tang. In kombucha, yeasts first make alcohol and the carbon dioxide that gives the fizz, then bacteria convert that alcohol into the acids you taste as sharpness.

Why it tastes and feels different

Fermentation breaks big molecules into smaller ones. Proteins become amino acids, which read on the tongue as savoury and full, and complex carbohydrates become simpler sugars and acids. That is why ferments taste more layered than their raw ingredients, and why they are often gentler to digest: some of the work has been done for you.

Why it's good for you

You end up with live cultures, nutrients that are easier to absorb, sometimes brand new vitamins that the microbes synthesise along the way, and compounds that feed your own gut bacteria. Preservation was the original goal. The nutrition turned out to be a generous bonus.

Time is an ingredient

None of this happens in an instant. Temperature and time set the pace: warmer and longer means more sour. The best instrument you have is your own mouth, so taste as it goes and stop when it tastes right.

Fermentation is really just patient chemistry, run by microbes instead of a stove. Once you can picture the reactions, every jar on the shelf starts to make sense.