A ferment in two acts
Kombucha begins simply: tea is brewed, sweetened, and left to ferment. What follows is a collaboration between two distinct microbial partners. Yeasts go first, converting dissolved sugar into alcohol. Acetic acid bacteria step in next, feeding on that alcohol and producing acids — primarily acetic and gluconic — that give kombucha its characteristic sharpness. Neither partner could produce the finished drink alone.
The sugar is fuel rather than flavour. By the time fermentation is complete, most of it has been consumed, which is why a good kombucha tastes tart and complex rather than sweet. A trace of alcohol always remains as a byproduct — very little, but it never reaches zero. That is simply the chemistry.
What the SCOBY actually is
The rubbery, opaque disc floating at the surface of a fermenting batch is widely misunderstood. People speak of it as though it were a single creature, a kind of starter in its own right. It is not. The SCOBY — Symbiotic Culture Of Bacteria and Yeast — is more accurately a home than an organism. The bacteria build it: a dense mat of bacterial cellulose that holds the microbial community in place at the liquid's surface, where oxygen is available for the acetic acid bacteria that need it.
The mat grows with each batch, layering upwards, which is why long-running SCOBYs become thick and unwieldy. New brewers sometimes treat the disc with reverential caution, but it is the living culture within and around it — not the cellulose structure — that does the work. Slice off a layer, pass one to a friend with some starter liquid, and the community re-establishes itself perfectly well.
Live versus pasteurised
Walk into a health food shop and you will find kombucha kept cold. Walk into a supermarket and you may find a different product on an ambient shelf — often sweeter, sometimes carrying the same name. These are not the same thing.
Genuine fermented kombucha is unpasteurised. It contains live cultures and continues fermenting slowly after bottling, which is why it must be refrigerated to keep secondary fermentation in check. Bottles can over-carbonate if left at room temperature — the yeasts and bacteria are still active, still producing gas. Opening a warm bottle over a sink is simply prudent.
Pasteurised kombucha has been heat-treated to stop fermentation entirely. The live cultures are gone. What remains is a flavoured, acidic drink that may taste similar but has undergone a fundamentally different process. Sweetness tends to be higher because the ferment was halted before sugar was fully consumed, or because sugar was added back afterwards. Neither version is a fraud — they are different products, and labelling does not always make that obvious. Check whether the bottle is refrigerated in the shop and whether the label says unpasteurised or mentions live cultures.
Where kombucha sits among live drinks
Kombucha belongs to a broader family of fermented drinks that includes water kefir, milk kefir, and jun — each produced by a different microbial community, each arriving at acidity by a slightly different route. As covered in the kefir and fermentation articles, milk kefir uses a community bound in a protein-and-fat matrix; water kefir grains are a gelatinous polysaccharide structure. The principle throughout is the same: microorganisms transform a simple substrate — milk, sugar water, sweet tea — into something acidic, complex, and alive.
What distinguishes kombucha within that group is the tea base. The tannins, bitterness, and aromatic compounds in the leaf all shape the finished flavour in ways that vary considerably by tea variety. A batch brewed on a lightly oxidised green tea tastes quite different from one built on a dark Assam. Brewers who care about this treat tea selection as seriously as the fermentation itself.
Buying and storing it well
For live kombucha, cold storage from shop to home matters. A bottle left in a warm car for an afternoon is not ruined, but it will have continued fermenting and the flavour balance may have shifted. Once open, keep it refrigerated and finish it within a few days — carbonation drops and acidity climbs the longer it sits.
Visible sediment or a thin film at the bottom is not a problem; it is a sign of a product that has not been filtered or pasteurised. Small variations in flavour between batches from independent producers are equally normal. Fermentation is a living process, and variables — season, brewery temperature, the mineral content of the water — all leave their mark.
