How long to ferment kombucha: timing, temperature and taste

What happens during the first fermentation

The first fermentation is a slow hand-over from sweet to sour. Yeast splits the sugar and turns it into a little alcohol and carbon dioxide; acetic acid bacteria turn that alcohol into acetic acid and make gluconic acid from the glucose. The pH starts around 4.5 with a good dose of starter and settles between 3.0 and 3.5 by the time most people bottle. Sugar drops from 60 or 70 g per liter to somewhere between 20 and 40 g, depending on how long you go.

None of that happens in a straight line. A 2026 study that fermented kombucha at different temperatures measured a pH of 3.47 after 7 days at 23 °C and 2.96 after 15 days at 28 °C, with acidity climbing steadily the whole time. There is no moment when kombucha is “done”. There is only the point on that curve you like, and your job is to catch it.

Temperature is the biggest lever

The comfortable range for kombucha is 20 to 26 °C (68 to 79 °F), and extension guides give similar figures. Below about 18 °C the culture slows, the yeast gets ahead of the bacteria, and the batch sits longer above the acidity that keeps it protected, which is when mold gets its chance. Above 30 °C it races: sour in days, with a harsher, more vinegary character.

If your kitchen runs cold, put the jar on top of the fridge, near (not on) a warm appliance, or on a heat mat made for fermenting. If it runs hot, a cupboard on a cool wall or a slightly smaller batch helps. What the culture dislikes most is swings, so pick a spot and leave it there.

Other things that change the timing

• Starter liquid. More starter means a faster, safer start. Ten percent is the minimum; twenty gets going quickly.
• The culture itself. A yeast-heavy culture sours fast and fizzes a lot; an old or stressed one is slow. Strong hotel liquid evens this out.
• Batch size and shape. Bigger and deeper batches take longer, because the bacteria work at the surface.
• Sugar. More sugar does not speed anything up; it just leaves more sweetness at the end.
• Tea. Green tea tends to ferment a little faster and lighter than black.
• Airflow. A tightly sealed jar starves the bacteria; a breathable cloth cover keeps them working.

Summer and winter brewing

The same recipe behaves like two different recipes across the year. In summer a batch that used to take nine days is sour by day five, carbonates violently in the bottle, and grows a thick pellicle in a week. In winter it dawdles, the yeast strands multiply, and the surface stays sweet and unprotected for longer. Adjust the habits, not the recipe.

• Summer: taste from day 2 or 3, bottle a little earlier, use less sugar in the second fermentation, and move the jar to the coolest room. A slightly larger batch also takes a little longer.
• Winter: use the higher end of the starter range, find a warm spot or a heat mat, expect 10 to 14 days, and give the second fermentation an extra day or two.
• Any season: keep the jar out of direct sun and away from radiators, ovens and windows, which swing the temperature through the day.

How to tell when it is ready

Taste

Slip a straw or a clean spoon under the pellicle and taste a little. Early on it is sweet tea with a faint tang. In the middle, sweet and sour balance, with a light prickle on the tongue. Late, it is dry and sharp. Bottle when the balance suits you. If you plan a second fermentation, stop slightly on the sweet side, because the bottle will use some of that sugar.

Smell and look

A ready batch smells tangy and lightly vinegary, never rotten or musty. The new pellicle is a few millimetres thick, brown yeast strands hang below it, and fine bubbles rise when you nudge the jar. None of these tells you the exact day, but together they tell you the batch is healthy.

pH, if you want numbers

pH strips or a cheap meter give you an anchor. Finished kombucha usually sits between 2.5 and 3.5. Food-safety guidance for commercial brewers uses 4.2 as the level the brew should drop below within a few days, and 2.5 as the floor below which it is unpleasantly acidic. pH will not tell you how it tastes, so taste anyway, but it is a useful check that a slow batch is actually fermenting.

What if it goes too long?

It turns into something close to vinegar: too acidic to drink by the glass. It may still contain sugar or alcohol, and a pH below 2.5 is too acidic for use as a drink. Public-health guidance warns that very acidic kombucha drunk in quantity is a bad idea, especially for anyone with a sensitive stomach, so treat an over-fermented batch as an ingredient. It makes excellent starter liquid, tops up a SCOBY hotel, and works diluted with water or juice, or in a salad dressing.

Too short is the opposite problem: the brew is still sweet and may not be acidic enough to be protected. If it remains like sweet tea after a week and its pH is above 4.2, do not bottle or drink it. Check the temperature and starter; if it fails to acidify, discard it and start again with healthy starter.

Getting the same result every batch

• Use the same ratio of tea, sugar and starter every time. How to make kombucha has the numbers.
• Note the room temperature when you start, or keep the jar in the same spot year round.
• Taste on a schedule: day 3, 5, 7, 9. Write down what you noticed.
• Change one thing at a time. A new tea and a new spot in the same week tells you nothing.
• Keep a strong hotel, so every batch starts from starter of similar strength.

After a few batches you will have your own rule, something like “nine days in winter, six in summer”, and that rule is worth more than any recipe.

Frequently asked questions