Homemade kombucha usually ferments at room temperature, about 21–27°C / 70–80°F, and becomes ready to drink in 7–14 days. The timing depends on the room temperature, the activity of the SCOBY, and the desired level of tartness.
What Happens During Fermentation? The SCOBY consumes the sugar in sweetened tea and produces carbon dioxide, a small amount of alcohol, and acetic acid. During the first few days, the tea is still sweet, with a mildly sweet taste; as the days pass, the sugar decreases and acidity increases, creating the drink's characteristic sweet-and-sour flavor.
How Is the Timing Determined? Starting on the seventh day, taste a small sip each day to track the fermentation. If sweetness still dominates, continue fermenting for a few more days; when it reaches the desired level of tartness, strain and bottle the kombucha. In warmer surroundings, above 25°C / 75°F, fermentation may take as little as 7 days; in a cool kitchen at 18–20°C / 65–70°F, it may take as long as 14–21 days. A new, thin SCOBY ferments more slowly, while a thicker SCOBY that has been used several times may shorten the process by a few days. For this reason, taste more frequently during the first few batches.
How Long Does the Second Fermentation (Bottling) Take? Kombucha strained and flavored with fruit or spices is carbonated by leaving it in a sealed bottle at room temperature for another 2–4 days. At this stage, briefly open the bottle cap a little once a day to release excess gas and prevent the bottle from bursting.
What Happens if It Ferments Too Long? Fermentation lasting more than 3 weeks makes kombucha sharply sour, close to vinegar; at that point, it can be used as a salad dressing instead of a drink. Over-fermented kombucha is also less fizzy and tastes more sour.
Can Fermentation Stop if the Temperature Is Too Low? Below 18°C / 65°F, SCOBY activity slows considerably and fermentation may nearly stop. For this reason, keep the kombucha jar somewhere warm and away from light, rather than in a cold corner of the kitchen.
What Should First-Time Brewers Watch For? A thin, whitish new SCOBY layer forming on top of the jar and a mild vinegar-like smell are signs that fermentation is progressing normally. If black or green mold spots, a fuzzy texture, or a rotten smell appear, consider the entire batch unsafe and discard it.
Should You Use Black or Green Tea? Kombucha made with black tea usually ferments a little faster than kombucha made with green tea because black tea's tannins provide a richer food source for the SCOBY. Kombucha made with green tea is a little milder, with less noticeable tartness, after the same amount of time.
How Should You Cover the Kombucha Jar? Do not seal the jar tightly. Cover its opening with a breathable cloth secured with a rubber band so air can enter while keeping flies and dust out. Tightly closing the jar with a metal lid can let fermentation gas build up and cause the jar to burst. Keep the jar completely away from direct sunlight.