If the kombucha is too sweet after the expected number of days, fermentation is too slow or hasn’t started. The most common causes: temperature too low, weakened SCOBY, insufficient starter liquid. It’s not a lost batch - in most cases you just need to wait longer or correct the conditions. pH is the only reliable indicator: above 3.5 after a week, there’s a problem to investigate.
You followed the recipe, waited the indicated number of days, and tasted it.
Sweet. Still sweet. It tastes like sugary tea with a vague acidic hint, not the sharp, tangy kombucha you were expecting.
The opposite problem to over-fermentation is less discussed, but just as common - especially in winter or for those just starting to brew kombucha at home.
01 — What “too sweet” means
The pH hasn’t dropped far enough
In a batch progressing normally, pH drops progressively during fermentation. It starts around 4.5–5.0 (thanks to the starter liquid) and drops to 2.5–3.5 within 7–14 days, depending on conditions.
If the kombucha is still sweet after the expected time, pH is still high - probably above 3.5–4.0. The bacteria haven’t produced enough organic acids.
This isn’t about subjective taste: pH is measurable, and it tells you exactly where you are.
02 — The most common causes
Why fermentation is slow or stalled
Temperature too low
This is the most frequent cause, especially in winter. Below 18°C, microorganisms slow dramatically. At 15°C, a batch that normally takes 8 days can take 20 or more. At 12°C, fermentation is almost stopped.
If you brew in the kitchen in winter, check the actual temperature. It’s not the thermostat temperature - it’s the temperature of the worktop, the cupboard where you keep the jar, often 2–4°C lower.
Insufficient or old starter liquid
Starter liquid doesn’t just bring low pH - it brings the active bacteria and yeasts that start fermentation. If you use too little (less than 80–100ml per litre), the culture starts from a weak base.
If the starter liquid had been stored in the fridge for a long time, or was from a very old batch, the microorganisms may be less active than needed.
Weakened SCOBY
A SCOBY that’s been stored for a long time without nutrition, or that has experienced extreme temperatures, may be less effective. A weak SCOBY produces fewer acids per fermentation cycle.
Unsuitable tea or water
Very hard water (high calcium and magnesium content) can interfere with fermentation acidity. If you suspect this, try with filtered or natural mineral water.
Tea with essential oils (Earl Grey, flavoured teas) can inhibit bacteria with variable effects.
Too much initial sugar
It seems counterintuitive, but too much sugar can slow fermentation. Above a certain concentration, the osmotic environment becomes unfavourable for microorganisms. The standard amount is 70–80g per litre - more doesn’t speed things up, it often slows them down.
03 — How to diagnose it
Before doing anything, measure the pH
Don’t rely only on taste. “Sweet” taste is subjective and depends on other factors too (type of tea, individually perceived acidity).
Measure the pH:
- pH above 4.0 after 7 days: fermentation is definitely slow. Investigate the causes.
- pH between 3.5 and 4.0 after 7 days: fermentation is underway but slow. Probably just low temperature. Wait another 3–5 days and check.
- pH between 3.0 and 3.5 after 7 days: nearly ready. Wait another 1–2 days.
- pH unchanged for more than 48 hours: fermentation may have stalled. See section 04.
04 — What to do
Actions to try, in order
1. Move the jar somewhere warmer.
First action, always. Even 3–4°C more makes an enormous difference. Near the oven, on top of the fridge (which emits heat from its upper surface), in a closed cupboard - find the warmest spot in your home.
If you have a thermometer, aim for 22–26°C.
2. Wait longer.
If the temperature was only slightly low, the batch is delayed - not stalled. Give it another 5–7 days and check every 2 days with pH.
3. Add fresh starter liquid.
If you have starter liquid from a recent batch (pH 2.5–3.0), adding 50–100ml to the slow batch can restart fermentation. Remove the same amount of kombucha first to make room.
4. Check the SCOBY.
If it’s very thin, translucent, almost absent, or has a strange smell, it may be weakened. A new SCOBY from a reliable source solves the problem.
5. Restart with different water.
If you’ve ruled out temperature and SCOBY as causes, try the next batch with filtered water. If the problem resolves, it was the water.
05 — Can I still drink or use it?
The slow batch isn’t lost
Sweet, under-fermented kombucha is not dangerous. It has a high pH, but has been protected by the starter liquid and initial acidity.
If pH is above 4.0 after many days and fermentation doesn’t restart, you have two options:
Use it as sweet fermented tea. It’s not the kombucha you wanted, but it’s a safe and pleasant drink.
Use it as a base to restart. Add a new SCOBY, 150ml of fresh starter liquid, correct the temperature. Often the batch picks up again.
There’s no need to throw it away unless it has developed mould or unpleasant smells (other than fermentation).
06 — How to prevent it in future batches
Three habits that make the difference
Measure pH from day 4–5. Don’t wait a week to “taste” - a pH reading on day 5 tells you immediately whether fermentation is on track or running behind. If pH is still above 4.5 on day 5, intervene immediately on temperature.
Check the jar’s actual temperature. Not the perceived room temperature - the actual temperature of the liquid in the jar. A kitchen thermometer in the liquid on day 1 gives you the definitive answer.
Always use enough starter liquid. At least 100ml per litre, preferably from a recent batch. It’s the most effective preventive measure against slow fermentations.
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07 — FAQ
FAQ
My kombucha never tastes of anything - is that normal?
If the taste is flat and sweet even after 10+ days, fermentation is slow. Measure the pH. If instead the taste is simply delicate (acidic but not sharp), you might want to wait a few more days or try with less starter liquid in the next batch.
I added too little starter liquid - can I add more now?
Yes, if the batch is less than 48 hours old. Beyond that, the pH may have already dropped enough. Measure first before adding: if pH is already below 4.0, no intervention is needed.
Is sweet kombucha harmful?
No. Under-fermented kombucha is not dangerous. It has a microbiological profile closer to sweet tea than mature kombucha, but it is not toxic.
In summer I never have problems, but in winter I always do. What can I do?
In winter, the problem is almost always temperature. Find the warmest spot available (even 22–24°C is enough) or use a fermentation heating mat, available from homebrewing shops.
I’ve waited 3 weeks and pH isn’t dropping. Is it all lost?
If pH hasn’t moved for days, fermentation is stalled. Before throwing everything away, try adding 100ml of fresh starter liquid and moving the jar somewhere warmer. If nothing changes after another 3–4 days, start over with new SCOBY and starter liquid.
Read also: Your kombucha is too sour → · Complete guide to kombucha fermentation →