Kombucha Won't Fizz? Fix the Second Fermentation

Flat kombucha is almost never a mystery - it is one of four dials set wrong. In order of how often they are the culprit: not enough sugar for the microbes to eat, too little live culture in the bottle, a room too cold for yeast work, or simply not enough days. The arithmetic of the first dial is exactly what the second fermentation calculator does; the full first-fermentation picture lives in the kombucha sugar guide.

Dial 1: sugar for the yeast

Carbonation is yeast eating sugar and exhaling CO2 in a sealed bottle. No sugar, no gas - the first-fermentation sugars are nearly gone by bottling time, which is the point of that stage. Work from 12 g per liter: a 500 ml bottle wants about one and a half level teaspoons (6 g). Fruit juice counts toward the total; a thin slice of ginger or two raisins adds surface and flavor but not much sugar.

Dial 2: live culture in the bottle

Sugar needs yeast to eat it. Bottling from the very bottom of the jar, or from a batch that has been sitting in the fridge, can leave too few active yeast in the liquid. Fill bottles from a jar that still shows bubbles at the surface, include some of the cloudy yeast sediment, or top each bottle with a splash of unfiltered starter. An active continuous brew rarely has this problem; a tired fridge-stored batch often does.

Dial 3: temperature

Second fermentation belongs in the 20-25 °C range, out of the refrigerator and out of direct sun. A 16 °C kitchen slows yeast to a crawl - the same temperature-time relationship the winter fermentation guide covers for every ferment. Find the warm spot first, adjust expectations second.

Dial 4: time - and when to stop

Give bottles 2-3 warm days, then chill one and taste. If the fix list above is right, the test bottle tells you before the whole batch does. Do not let bottles ride for a week "just in case": kombucha that ferments too long swings from flat to gushing, and the bottle, not the brewer, decides when that happens. Chill before opening, always.