Fermentation

Fermentation is the process in which microbes, mainly yeasts and bacteria, feed on sugars and starches in food and turn them into other substances, such as gas, alcohol or acid. Cooks use it to raise bread, preserve vegetables, and make cheese, yogurt, wine, beer, vinegar, miso, soy sauce, kimchi and many other foods. The microbes also create new flavors, from tangy to savory.

Why it works

Microbes break down food for their own energy. The by-products they release change the food: gas makes bread rise, acid makes food sour and keeps harmful bacteria out, and enzymes break proteins into savory amino acids. Temperature controls speed. Most bread doughs ferment well around 24 to 27°C (75 to 80°F). Lower temperatures slow it down and allow more flavor to develop.

How to use it

The main kinds you will meet in a kitchen:

Kind	Microbe	Main product	Examples
Yeast	Yeasts	Carbon dioxide and alcohol	Bread, beer, wine
Lactic acid	Lactic acid bacteria	Lactic acid	Yogurt, sauerkraut, kimchi, sourdough
Acetic acid	Acetic acid bacteria	Acetic acid	Vinegar, kombucha
Mold	Molds such as koji	Enzymes that break down starch and protein	Miso, soy sauce, sake

In bread, yeast makes gas that raises the dough during bulk fermentation and proofing. A sourdough starter holds both wild yeast and lactic acid bacteria, which add sourness. For vegetables, see lacto-fermentation.

Common mistakes

• Fermenting too warm, which speeds things up but can give off flavors or overproofed dough.
• Using chlorinated tap water, which can slow some ferments. Let it stand for a few hours or use filtered water.
• Ignoring signs of spoilage. Fuzzy mold in many colors, a slimy texture or a rotten smell mean throw it out. Follow tested recipes for salt levels.
• Sealing a fermenting jar so tightly that gas builds up. Use an airlock or open the lid daily.

In recipes

"Let it ferment" in a bread recipe means let the dough rise. In vegetable recipes it means leave the jar at room temperature, often for days, until sour.