Kombucha

How sweet tea becomes sour

It happens in two stages that overlap. Yeasts get to the sugar first and do what yeasts do: break it into ethanol and carbon dioxide. Then acetic acid bacteria, which need oxygen and which is why the vessel is covered with cloth rather than sealed, oxidise most of that ethanol into acetic acid — the same acid that makes vinegar sharp.

Alongside it, other organisms make gluconic and other organic acids, which is why kombucha does not simply taste of vinegar. The finished drink is a balance: some sugar left over, a mixture of acids, a little carbon dioxide, a trace of alcohol.

Where the balance lands is a matter of time and temperature. Stopped early it is sweet and barely sour. Left too long it goes past drinking and becomes, genuinely, a usable vinegar.

What the SCOBY is

The rubbery beige disc that forms on the surface is a mat of bacterial cellulose with the culture living in and around it. SCOBY stands for symbiotic culture of bacteria and yeast, and it is worth knowing that the mat is not the culture. It is something the culture builds — a raft, in effect — and most of the fermenting is done by organisms suspended in the liquid.

This is why a starter liquid from a previous batch matters more than the disc does, and why a beautiful thick mat in a jar of insipid tea has told you very little.

A new layer forms on every batch, so the mat thickens and is periodically peeled and divided. Brown stringy trails hanging from it are yeast, and are normal.

What it tastes of, and how it is flavoured

• Sour, with a cider-vinegar edge, over a base that still tastes of tea. Black tea kombucha is fuller and more tannic; green tea kombucha is lighter and grassier.
• Lightly fizzy rather than sharply carbonated, unless it has had a second fermentation.
• The second fermentation is where most flavour comes from: the strained drink is bottled with fruit, juice, ginger or herbs and sealed, so the remaining yeasts make carbon dioxide with nowhere to go. That is what produces a properly fizzy bottle.
• Sealed bottles build real pressure. This is the part of home kombucha that actually goes wrong — a forgotten bottle in a warm room can burst. Refrigerate to slow it, and open over a sink.
• Ginger, lemon, berry, hibiscus and stone fruit are the standard flavourings, and they work because they are all acidic or aromatic enough to survive a sour base.
• Overshot kombucha becomes vinegar, and it is a good one. Use it in dressings rather than pouring it away.

The health language, and what it actually supports

Kombucha is sold with more health claim attached to it than almost any other drink, so it is worth being precise about what is and is not established.

What is straightforwardly true: it is a fermented drink, it contains live microorganisms unless it has been pasteurised, it contains organic acids, and it contains rather less sugar than the sweet tea it was made from. Those are facts about composition.

What does not follow from any of them is that drinking it produces a particular effect in a person. A food containing a substance, that substance having a known function in the body, and eating the food producing a health outcome are three different statements, and the marketing routinely presents the first as if it were the third. FoodHQ does not publish health outcomes for foods, and where you see them claimed on a bottle it is worth asking which of the three you are being sold.

Drink it because it is a good sour drink. That is a sufficient reason for a food to exist.

FoodHQ does not publish salt percentages, pH figures or keeping times for home preserving. Those are safety numbers, they are set by a food safety authority rather than by a cookery writer, and the right ones depend on what you are fermenting and where you are. In the UK the Food Standards Agency is the authority — use a recipe from them or from another body with the same standing, and treat this page as the explanation of why that recipe is shaped the way it is.

Common questions