The Chemistry of Fermentation, Explained Simply

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

Fermentation in one sentence

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

Meet the microbes

Different ferments rely on different microbes.

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

• Yeasts work alongside bacteria in kombucha and tibicos, and they are what makes bread rise. They turn sugars into carbon dioxide and a whole range of aromas.

• Acetic acid bacteria give kombucha its pleasant bite, producing acetic acid and other organic acids.

• Moulds make tempeh, knitting soybeans, chickpeas or lentils together into a firm cake 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 vegetables, keeps unwanted microbes out and creates the tang. In kombucha, yeasts and bacteria work as a team: the yeasts produce the carbon dioxide that gives the fizz, and the bacteria produce the acids you taste as sharpness.

Why ferments taste 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 a block of tempeh has a texture and flavour all of its own.

What ends up in the jar

Unpasteurised ferments contain live cultures, along with the acids, aromas and other compounds the microbes produce along the way. Preservation was the original goal. The flavour turned out to be a generous bonus, and scientists are still studying what else these foods have to offer.

Time and temperature are ingredients

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 palate, 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.