Kombucha is a fizzy, slightly tart fermented tea made by adding a live culture of bacteria and yeast to sweetened tea and letting it ferment for several days at room temperature. The result is a tangy, lightly carbonated drink that contains organic acids, trace amounts of alcohol, and living microorganisms. It has become one of the most popular fermented beverages worldwide, but the basic process behind it is surprisingly simple.
How Kombucha Is Made
The process starts with brewed tea (usually black or green) sweetened with regular sugar. A rubbery, pancake-shaped culture called a SCOBY is added to the sweetened tea. SCOBY stands for “symbiotic culture of bacteria and yeast,” and it looks like a thick, pale disc floating on the surface of the liquid. This culture is sometimes called a “tea fungus,” though it isn’t actually a single fungus.
Once the SCOBY is added, fermentation begins. Yeast cells in the culture break down the sugar into simpler sugars, then convert those into ethanol (alcohol) and carbon dioxide, which gives kombucha its natural fizz. Acid-producing bacteria then feed on that ethanol and convert it into acetic acid, the same compound that gives vinegar its sharp taste. This is why kombucha has a sour, tangy flavor and why its alcohol content stays low. The bacteria also convert some of the sugars into gluconic acid and glucuronic acid. Meanwhile, as yeast cells naturally break down over time, they release B vitamins and other nutrients that help the bacteria thrive. It’s a genuine partnership: the yeast feeds the bacteria, and the bacteria keep the alcohol level in check.
The ideal fermentation temperature is between 75°F and 80°F (24°C to 26°C), and the first fermentation typically takes three to seven days. Many brewers do a second fermentation in sealed bottles, often adding fruit or juice for flavor, which builds more carbonation and develops a more complex taste.
What’s Actually in Kombucha
Finished kombucha is a cocktail of organic acids, live microbes, a small amount of residual sugar, and trace alcohol. The dominant acid is usually gluconic acid, followed by acetic acid, succinic acid, and smaller amounts of glucuronic acid and ascorbic acid (vitamin C). These organic acids give kombucha its signature sourness and contribute to its mildly acidic pH.
In the United States, kombucha sold as a non-alcoholic beverage must stay below 0.5% alcohol by volume under federal regulations enforced by the Alcohol and Tobacco Tax and Trade Bureau. If a product reaches or exceeds that threshold at any point, even after bottling due to continued fermentation, it is classified as an alcoholic beverage and must comply with alcohol labeling laws. Most commercial brands are carefully formulated to stay under this limit, but homebrewed kombucha can vary more widely.
Sugar content also varies. During fermentation, yeast consumes a significant portion of the added sugar, but commercial brands often add juice or sweeteners after fermentation to improve taste. A typical serving of store-bought kombucha contains anywhere from 2 to 12 grams of sugar, depending on the brand and flavor. Checking the nutrition label is the most reliable way to know what you’re getting.
The Microbes Inside
Kombucha’s microbial community is diverse but tends to be dominated by a few key groups. On the bacterial side, acetic acid bacteria are the most common, particularly species in the Komagataeibacter group. A study of over 100 commercial kombucha starter cultures found Komagataeibacter to be the most prevalent and abundant genus. Lactic acid bacteria, including Lactobacillus species, also show up but are generally less dominant. Some commercial products contain the probiotic Bacillus coagulans, which may be added during production.
The yeast side is typically dominated by one or two species per product. Brettanomyces (a genus also associated with wine fermentation) and related yeasts are commonly found in commercial kombucha. Saccharomyces cerevisiae, the common brewer’s yeast, has been detected as well, though it’s not always the dominant yeast. The exact microbial profile varies from brand to brand and even batch to batch, which is why two kombuchas can taste quite different from each other.
Health Claims vs. Evidence
Kombucha has a long list of claimed health benefits, from improved digestion to reduced inflammation to blood sugar regulation. Animal studies have shown some promising signals, including anti-inflammatory, antioxidant, and blood-sugar-lowering effects. But human evidence is still limited, and the results so far are modest.
A controlled clinical study published in Scientific Reports tested the effects of daily kombucha consumption over four weeks in healthy adults eating a typical Western diet. The researchers found no significant changes in blood chemistry markers or circulating markers of inflammation. They did observe a small shift in gut microbiome composition: certain beneficial bacteria, including a probiotic species and several types that produce short-chain fatty acids (compounds linked to gut health), became more abundant in kombucha drinkers. However, the researchers noted that the effects were modest and varied widely between individuals.
The takeaway is that kombucha is not a proven treatment for any health condition. It does contain live microorganisms and organic acids that could, in theory, support gut health over time, but the current human evidence doesn’t support the stronger claims you’ll see on social media or product labels.
Antioxidant Activity
One area where kombucha shows a clear, measurable effect is antioxidant content. The fermentation process actually increases the antioxidant capacity and total phenolic content of the tea base. In other words, kombucha made from black or green tea has more available antioxidants than the unfermented tea it started as. The fermentation also improves bioaccessibility, meaning those antioxidants are more readily absorbed by your body. This effect comes from the way microbial enzymes break down complex plant compounds in the tea into simpler, more bioavailable forms during fermentation.
Taste and How to Choose a Brand
Kombucha tastes tart, slightly sweet, and fizzy. The flavor ranges from mildly sour (like a tart apple cider) to quite vinegary, depending on how long it was fermented. Longer fermentation produces a more acidic, less sweet drink. Most commercial brands fall somewhere in the middle and add fruit flavors like ginger, berry, or citrus during the second fermentation.
If you’re new to kombucha, a few things are worth paying attention to. Look for brands that list live cultures on the label if you’re drinking it for the microbial content, since some products are pasteurized, which kills the microorganisms. Check the sugar content per serving, as it can vary significantly. And if you’re avoiding alcohol entirely, be aware that even “non-alcoholic” kombucha contains trace amounts, up to 0.5% ABV. That’s roughly one-tenth the alcohol in a light beer, but it’s not zero.
Homebrewing kombucha is straightforward and inexpensive, requiring only tea, sugar, a SCOBY (easily sourced online or from another brewer), and a glass jar. The main risks with homebrewing are contamination from mold and over-fermentation, which can make the drink unpleasantly acidic or push the alcohol content higher than expected. Keeping your equipment clean and fermenting within that 75°F to 80°F sweet spot reduces both risks considerably.