Kombucha is a fermented tea drink made by adding a living culture of bacteria and yeast to sweetened black or green tea. The culture consumes most of the sugar during fermentation, producing a tart, slightly fizzy beverage that tastes something like a tangy apple cider. It has been brewed for centuries and has surged in popularity over the past decade, largely because of its probiotic content and distinctive flavor.
How Kombucha Is Made
The process starts with brewing regular tea, dissolving sugar into it, and letting it cool. A rubbery, pancake-shaped culture called a SCOBY (short for symbiotic culture of bacteria and yeast) is then placed in the sweetened tea. This living mat of microorganisms kicks off fermentation, which typically takes one to three weeks depending on temperature and taste preference.
During fermentation, the yeast in the SCOBY breaks down sucrose into glucose and fructose, then converts those sugars into small amounts of ethanol. Bacteria in the culture then oxidize that ethanol into acetic acid, which is responsible for kombucha’s signature sour taste and its drop in pH. The bacteria also convert glucose into gluconic and glucuronic acids. By the time fermentation is complete, most of the original sugar has been transformed into these organic acids, carbon dioxide, and trace amounts of alcohol.
What Lives Inside the SCOBY
The SCOBY is a dense, layered community of microbes. The two most dominant groups in commercial starter cultures are a yeast genus called Brettanomyces and a bacterial genus called Komagataeibacter. Komagataeibacter is the main acetic acid bacterium. It produces the cellulose mat that forms the SCOBY’s physical structure and drives much of the acid production that gives kombucha its tang.
Beyond those two, the microbial lineup varies. Other common yeast genera include Zygosaccharomyces, Lachancea, and Starmerella. Lactic acid bacteria such as Lactobacillus and Lactococcus also show up in some cultures. This variability is why two brands of kombucha can taste noticeably different even when brewed from similar tea and sugar.
Nutrition and Probiotic Content
A typical 8-ounce serving of commercial kombucha contains roughly 30 to 50 calories and 2 to 8 grams of sugar, though this varies widely by brand. Some brands add fruit juice after fermentation, which bumps up the sugar content. The drink provides small amounts of B vitamins and organic acids but is not a significant source of any single nutrient on its own.
Where kombucha stands out is its live microbe count. Studies measuring colony-forming units in finished kombucha have found concentrations around 100 million per milliliter, which is comparable to many commercial probiotic supplements. However, the specific strains present in kombucha differ from those in most studied probiotic capsules, and the health effects of kombucha’s particular microbial mix have not been tested in large human clinical trials.
Some of the organic acids produced during fermentation have their own potential benefits. Acetic and gluconic acids can inhibit the growth of certain pathogenic bacteria associated with food poisoning. Gluconic acid also functions as a mineral carrier, potentially improving absorption of calcium, magnesium, and iron. One genus of yeast commonly found in kombucha, Starmerella, ferments fructose rapidly and requires lipid derivatives to grow, which researchers have noted could play a role in lowering cholesterol levels, though this has mainly been observed in laboratory settings rather than in people.
Raw vs. Pasteurized Kombucha
Most kombucha sold in the refrigerated section of a grocery store is raw, meaning it has not been heat-treated after bottling. Raw kombucha retains its live probiotics and active enzymes, and generally has a more complex, tangy flavor with natural effervescence. The tradeoff is a shorter shelf life of roughly 60 days and the fact that fermentation continues slowly inside the bottle, which can increase acidity and carbonation over time.
Pasteurized kombucha, usually found on room-temperature shelves, has been heated to kill both harmful and beneficial microorganisms. This extends shelf life to six months or more and keeps the alcohol content stable, but it significantly reduces or eliminates the live probiotic cultures that many people drink kombucha for in the first place. Pasteurization can also degrade heat-sensitive nutrients like vitamin C and mute the drink’s sharper flavor notes. If probiotics are your reason for drinking kombucha, check the label for “raw” or “unpasteurized” and look for refrigerated bottles.
Alcohol Content
All kombucha contains some alcohol as a natural byproduct of fermentation. In the United States, the Alcohol and Tobacco Tax and Trade Bureau classifies any kombucha that reaches 0.5% alcohol by volume (ABV) or higher as an alcoholic beverage subject to regulation. Most commercial brands are carefully brewed and monitored to stay below that threshold, so what you find in the grocery store is legally non-alcoholic.
Home-brewed kombucha is harder to control. Longer fermentation times, warmer temperatures, and extra sugar can all push alcohol levels above 0.5%, sometimes reaching 2 to 3% ABV. If you’re avoiding alcohol for health, religious, or pregnancy-related reasons, commercial brands with labeled ABV are a safer bet than homemade batches.
Safety and pH
Kombucha’s acidity is what keeps it safe. As fermentation progresses, the pH drops well below the 4.6 threshold where most harmful bacteria can survive. Food safety guidelines from the Pennsylvania Department of Agriculture recommend that unpasteurized kombucha reach a pH of 4.2 or below before it is bottled or served. Most finished kombucha lands around a pH of 2.5 to 3.5, which is acidic enough to inhibit pathogens.
There is a floor, though. A pH below 2.5 is considered too acidic for safe consumption and can irritate the digestive tract. This is mainly a concern with home brewing, where batches left fermenting too long can become vinegar-like and overly harsh. Commercial producers are required to monitor and log pH levels for every batch.
How Much to Drink
The CDC’s guidance is conservative: about 4 ounces per day is unlikely to cause adverse effects in healthy adults. Many people drink more than that without issues, but starting small makes sense if you are new to fermented foods. The organic acids and live microbes can cause bloating or digestive discomfort in some people, especially at first. Gradually increasing your intake over a week or two gives your gut time to adjust.
People who are immunocompromised, pregnant, or have a history of liver or kidney conditions should be more cautious, particularly with unpasteurized versions. The live bacteria and residual alcohol, even in small amounts, may pose risks that outweigh the potential benefits for these groups.