Kombucha is made from just three basic ingredients: brewed tea (usually black or green), sugar and a SCOBY, a rubbery mat that is a symbiotic culture of bacteria and yeast. In a typical process described by New York State’s Department of Agriculture and Markets, tea leaves (about 4–5 g per liter) are steeped in freshly boiled water for about 10 minutes, sugar is added at 50–150 g per liter, and the cooled tea is inoculated with about 10% fresh kombucha containing the culture from a previous batch.
The covered mixture then ferments at room temperature for about 7 to 10 days. The yeast turns the sugar into carbon dioxide and a little alcohol, and the bacteria turn much of that into acids such as acetic acid, which gives kombucha its tart, slightly fizzy taste. Brewers often add fruit juice, ginger or herbs for flavor before bottling.
Source: New York State Department of Agriculture and Markets, “Kombucha Processing”
Image: Kombucha SCOBY by Simon A. Eugster, CC BY-SA 3.0, via Wikimedia Commons.