How does leaving food to ferment keep it from rotting?
The sour bite of yogurt and kimchi is the waste product of tiny living microbes - and it is the same thing that keeps the food from going bad.
Fermentation puts friendly microbes to work. Lactic acid bacteria and yeasts eat the sugars in milk, cabbage or dough and give off acid, alcohol and gas. That makes the food sour and starves it of oxygen, so the germs that cause rot cannot move in. The food is preserved, and those same by-products are exactly the tangy, fizzy, savory flavors we love. So fermenting is not slow rotting - it is the good microbes winning and locking the bad ones out.
A jar of cabbage with a bit of salt left on the counter turns into sour, crunchy dua cai in a few days. The cabbage did not rot - friendly bacteria turned its sugars into acid that both preserves it and gives that sharp, mouth-watering tang.
Fermentation preserves food because good microbes make acid and alcohol that block spoilage germs, and that acid is also the new flavor.
It explains why yogurt, kimchi, fish sauce and sourdough last longer than the fresh food they came from, and why they taste so different.
Good germs eat first, leave acid behind: too sour to rot, and that sour is the taste.
Learn the idea and practice English at the same time.