Fermentation is a natural process where tiny living organisms called microbes break down sugars and starches, creating new substances in the process. When yeast ferments grape juice, it transforms the sweet liquid into wine. When bacteria ferment milk, they turn it into yogurt or cheese. The microbes eat the sugars and produce different compounds, changing both the flavor and the chemistry of the food.

People discovered fermentation thousands of years ago, probably by accident when someone left food sitting out and noticed it had transformed into something different but still good to eat. Ancient civilizations used fermentation to preserve food before refrigerators existed. A fermented cucumber becomes a pickle that lasts for months. Fermented cabbage becomes sauerkraut or kimchi that lasts longer without spoiling.

Fermentation also makes bread rise. When yeast ferments the sugars in bread dough, it produces tiny bubbles of carbon dioxide gas that make the dough puff up and become light and airy instead of flat and dense.

Scientists study fermentation to understand how microbes work, brewers use it to make beer, and bakers rely on it every time they make a loaf of bread.