← Tea guide

Kombucha - fermented tea

Lightly sparkling, tangy, refreshing and surrounded by an aura of a health drink - kombucha has stormed onto shop shelves around the world. Few people, however, know that it is not a newfangled invention, but a fermented tea with roots reaching back thousands of years to ancient China. Its secret is a gelatinous disc called a SCOBY, that is, a symbiotic culture of bacteria and yeast, which turns sweetened tea into a slightly sour, fizzy drink. In this post we look at what kombucha actually is, where it comes from, how its fermentation works, which organic acids give it its characteristic flavour and what we really know about its alleged health properties. Time to break this trendy drink down into its parts.

What kombucha is

Kombucha is a lightly sparkling drink produced from the fermentation of sweetened tea. The base is most often black or green tea, sometimes oolong, sweetened with sugar, and the fermentation is carried out by a symbiotic culture of bacteria and yeast, a SCOBY for short. The result is a drink with a tangy, slightly vinegary, refreshing flavour, with a delicate fizz. Despite the tea base, finished kombucha tastes completely different from an infusion - it is closer to a dry, fruity cider than to a cup of tea. It is still tea, but processed by microorganisms that eat the sugar and produce acids and carbon dioxide. It thus joins two worlds: tea craft and fermentation. For many people it is an interesting alternative to sweet fizzy drinks, with a more complex, mature profile.

SCOBY - the symbiotic culture

SCOBY is an abbreviation of the English term meaning a symbiotic culture of bacteria and yeast. In practice it is a gelatinous, pale disc floating on the surface of the drink, often called the mother or tea mushroom, although it has nothing to do with mushrooms. Physically it is a mat of cellulose produced by acetic acid bacteria. Inside this structure and in the liquid itself two kinds of microorganisms coexist: yeast and bacteria, which complement each other. With every subsequent fermentation a new layer of cellulose grows on the surface, so the SCOBY thickens over time and can be divided to start new batches. It is a living, reusable culture, which is why enthusiasts pass pieces of SCOBY to each other like a precious starter.

Ancient roots

Kombucha is not an invention of recent years. It is generally accepted that the drink comes from northeast China, from the region historically known as Manchuria. According to tradition its consumption was recorded already in antiquity, hundreds of years before our era. From there the custom of fermenting sweetened tea gradually spread westward. The drink reached Russia, and then Europe, most likely no later than the early 20th century. In many countries of Eastern Europe kombucha was for decades a home drink, known under local names, long before it became a global trend. Its modern popularity in the West is thus more of a return and rediscovery of an old tradition than a true novelty. It is a drink with a really long and cross-cultural history.

How fermentation proceeds

The process begins by brewing strong tea and dissolving sugar in it, usually at a concentration of a few percent. After cooling, the SCOBY and a portion of finished, sour kombucha from a previous batch are added to the drink as a starter that lowers the pH and protects against mould. Then the work of the microorganisms begins. The yeast breaks down sugar into alcohol and carbon dioxide. Almost simultaneously, acetic acid bacteria oxidise the resulting alcohol into acetic acid, which lowers the pH and gives the drink its characteristic tang. It is precisely this two-step cooperation that turns sweet tea into a dry, sour drink. The whole process usually takes place at room temperature over about one to two weeks, and the longer it lasts, the more sour and less sweet the drink becomes.

Organic acids and flavour

The tangy character of kombucha is the result of a mixture of organic acids that form during fermentation. Analyses show above all acetic acid, usually dominant and responsible for the vinegary, sharp note, as well as gluconic and glucuronic acids. Acetic acid forms from the oxidation of alcohol by bacteria, and gluconic from glucose. It is these compounds, alongside the remaining sugar, that build the balance between sour and sweet that decides the flavour of the drink. The longer the fermentation, the more acids and less sugar, so the drink becomes ever drier and more vinegary. Stopping the process at the right moment is the key to a pleasant, balanced profile. Too long a fermentation gives a drink resembling diluted vinegar, not very pleasant to drink.

A table: kombucha’s main acids

Let us gather the most important acids in one place:

The table shows that the flavour of kombucha is not one acid, but a composition of them. That is why the drink has a complex, multi-dimensional tang, not a flat sourness like plain vinegar.

Yeast versus bacteria - the division of labour

At the heart of kombucha is the division of labour between two kinds of microorganisms. The yeast goes first: it breaks sugars into alcohol and carbon dioxide, providing the raw material for the further process and part of the fizz. The acetic acid bacteria take over, oxidising the alcohol into organic acids and at the same time building the cellulose mat of the SCOBY. It is a classic symbiosis: the product of one group is food for the other. Thanks to this the drink does not stay alcoholic but becomes sour, because the bacteria continuously process the ethanol made by the yeast. The balance between these populations decides the character of the drink and can be sensitive to temperature and time. Disturbing this balance, for example by too high a temperature, changes the flavour and composition of the final product.

Two stages: primary and secondary fermentation

In practice kombucha is often made in two stages. The primary fermentation takes place in an open vessel covered with a breathable cloth, where the SCOBY works on the surface and acids plus a new layer of cellulose form. After a few to a dozen or so days the drink is already sour, but usually with little fizz. Then the secondary fermentation begins: the drink is poured into airtight bottles, often with the addition of fruit, juice or a little sugar. The yeast in the closed bottle eats this extra sugar, and the carbon dioxide produced cannot escape, so the drink carbonates and becomes sparkling. It is at this stage that kombucha gains its bubbles and fruity flavours. The two-stage approach lets you separate building the acidity from adding fizz and aroma.

Alcohol, pH and safety

Since the yeast produces alcohol, is kombucha an alcoholic drink? Usually not in a practical sense: the bacteria continuously process the ethanol, so in the finished drink only trace amounts usually remain, often below half a percent, although with longer or uncontrolled fermentation there can be more. What is characteristic of kombucha is instead a very low pH, dropping to around 2.5 to 3.5. This acidity, alongside the presence of a live culture, protects the drink from unwanted microbes. In home production hygiene and the use of a pH-lowering starter from the outset are key, to avoid mould. If a dry, fuzzy mould appears on the surface, the batch should be discarded. A healthy SCOBY and an acidic environment are the best safeguard of the process.

How it tastes and what shapes the profile

The flavour of kombucha is a balance between sour, sweet and sparkling, with a background resembling dry cider or light apple vinegar. Several factors influence the profile. First, the type of base tea: black gives a fuller, more tannic character, green a lighter and more vegetal one. Second, the fermentation time: shorter means sweeter and milder, longer means more sour and vinegary. Third, the additions in the secondary fermentation: fruit, herbs, ginger or juices, which give the drink a specific aroma and boost the fizz. Fourth, the temperature, which governs the pace of the microorganisms’ work. Thanks to these variables every batch can be different, and home producers treat kombucha as a field for flavour experiments, choosing tea and additions to taste.

The health myth - what we know and what we do not

A great many health promises have grown up around kombucha: probiotics, detox, immune boosting and many others. Here one must keep a clear head. It is true that kombucha contains live cultures and organic acids, and fermented drinks are sometimes studied for benefits to the gut microbiome. However, many circulating claims go far beyond what solid science confirms, and some research is preliminary or conducted outside the human body. It is sensible to treat kombucha as a tasty, low-sugar drink with an interesting profile, and not as a medicine or miracle elixir. Sensitive people should remember its acidity and trace alcohol. Drink it for flavour and pleasure, and treat health declarations with caution.

The key points in a nutshell

Kombucha is fermented sweetened tea, in which a SCOBY, a symbiotic culture of bacteria and yeast, turns sugar into organic acids and carbon dioxide. The yeast makes alcohol from sugar, and the bacteria oxidise it into acetic acid, giving a sour, lightly sparkling drink with a very low pH. Its roots reach back to ancient Manchuria, from where the drink reached Russia and Europe. The profile depends on the tea, the fermentation time and the additions, while the fizz forms in the secondary fermentation in the bottle. There are usually traces of alcohol, and the health promises are worth treating with caution. Want to record the flavours of successive kombucha batches and infusions? Keep notes in the GustoNote app. See also our posts on Chinese tea and heicha microbial fermentation.