I've pulled a batch out expecting the usual light tang and gotten something closer to salad dressing more than once. It's an unpleasant surprise the first time, and it's tempting to assume something went wrong with the culture. In almost every case, nothing did. A too-sour kombucha is just a batch that fermented longer than you wanted it to, and it's a completely fixable, completely safe situation.
Why it happens
Kombucha sours because the bacteria in the SCOBY convert sugar into acids over time — mainly acetic acid, which is the same thing that makes vinegar taste like vinegar. The longer the first ferment runs, the more of that acid builds up and the less sweetness is left to balance it. Leave a batch sitting an extra week past when it tasted good to you, and it'll taste noticeably sharper. Leave it a month, and you'll basically have kombucha vinegar.
Ambient temperature speeds this up a lot. A jar fermenting on a counter in a warm kitchen will sour faster than the exact same recipe in a cooler room, because the culture is more active at higher temperatures. This is why a 10-day ferment that tasted right in March can taste like nail polish remover by August if you don't adjust for the season.
This is not mold, and it's not dangerous
It's worth saying plainly: a sour, vinegary batch is not contaminated. It's not going to make you sick, and it doesn't mean your SCOBY has gone bad. It just tastes strongly acidic, which is unpleasant but harmless — kombucha's acidity is actually part of what keeps it safe to ferment on a counter in the first place.
Actual mold looks completely different and isn't something you'd confuse with over-souring once you've seen it. Mold is fuzzy and dry, sitting on top of the liquid in raised patches — usually white, blue-green, or black, with a texture more like the mold on old bread than anything you'd expect on a SCOBY. An over-fermented batch, by contrast, still looks like normal kombucha: clear liquid, an intact culture, maybe some stringy yeast strands. The only thing wrong is the taste. If what you're looking at is fuzzy and raised rather than smooth and submerged, that's the one case where you toss the batch. Everything else described here is just about taste.
Fixing it for future batches
The real fix is upstream: shorten your first ferment and start tasting earlier. I check batches from around day 7 now instead of waiting for some fixed number of days, dipping a straw past the SCOBY without disturbing it and tasting a few drops. Once it hits the balance I want — still a little sweet, with a light tang rather than a sharp bite — I bottle it or move it to a second ferment. Catching it a day or two early is much easier to fix than catching it a week too late.
Since temperature changes how fast this happens, the same day count won't give you the same result year-round. A batch that took 10 days to taste right in a cool room in winter might be ready in 6 or 7 days on a warm counter in summer. Rather than brewing on a fixed schedule, treat the tasting step as the real clock and adjust your expectations by season.
Rescuing an already-too-sour batch
If you've already got a jar that's gone past the point of enjoyable, don't dump it. The simplest fix is dilution at second ferment: instead of bottling the sour kombucha straight, cut it with fresh sweet tea — plain black or green tea, sweetened the same way you would for a new batch, but not fermented. A ratio of roughly two parts sour kombucha to one part fresh sweet tea is a reasonable starting point, and you can adjust from there depending on how far gone it is. The fresh sugar rebalances the sweetness, softens the acidity you're tasting, and still gives the culture something to work with, so you get carbonation during the second ferment instead of a flat, diluted drink.
A splash of fruit juice does something similar and adds flavor at the same time — apple, grape, or berry juice all work, added at bottling the same way you'd add any second-ferment flavoring. The natural sugar and fruit character mask the sharpness without you needing to think about ratios as precisely as with plain sweet tea.
Cooking with it instead
If a batch is too sour to enjoy as a drink even after diluting, it hasn't gone to waste — it's just moved categories. Kombucha that's fermented well past drinkable is functionally a mild, fruity vinegar, and it works as one. I've used an overly sour batch in salad dressings in place of apple cider vinegar, in marinades for chicken or pork, and for deglazing a pan after searing something, where the acidity does the same job vinegar would and the flavor is closer to fruit than to plain vinegar. Keep a jar of "cooking kombucha" going if you brew often enough that this happens regularly — it's a better outcome than tipping it down the drain.
The short version
Too sour means too long in first ferment, sped up by warmth, and it isn't a sign anything went wrong with the culture. Taste earlier next time, starting around day 7. For a batch that's already sour, dilute it with fresh sweet tea or juice at second ferment to bring the sweetness back, or set it aside as a vinegar substitute for cooking. Save the "start over" response for actual mold — fuzzy, dry, raised patches — not for a batch that's simply more tart than you wanted.