Kombucha: The Fungus in Your Tea

For the uninitiated, kombucha, a slightly sweet, slightly acidic, carbonated beverage made from fermented tea, may not sound like an appetizing beverage. But some enthusiastic supporters claim that it is a miracle elixir, reporting that kombucha aids digestion, gives relief from arthritis, acts as a laxative, prevents microbial infections, helps in combating stress and cancer, and vitalizes the physical body.

A simple Google search for “kombucha health benefits” reveals more extreme conceptions about kombucha: that it is spiritually cleansing, comes from outer space, is a natural psychic defense against negative energies and protects from evil thoughts. In this article we will go into a little detail on the background of kombucha, how kombucha is made, and whether its suggested health benefits stand up to science.

Kombucha is made by fermenting sugared tea with a symbiotic culture of bacteria and yeast (scoby). This scoby is also referred to as a kombucha mushroom or tea fungus and is similar to the “mother” used to make vinegar.

Pictured: Kombucha beverage with scoby

Kombucha is sold worldwide in retail stores and online, usually in refrigerated, single-serve bottles. It can also made at home using a starter culture, sugar, and tea. Black tea and white sugar are the preferred substrates for preparation, but green tea can also be used. Fermentation gives the kombucha tea a lightly sparkling fruity sour flavor after a few days and a stronger vinegar flavor after prolonged incubation. While some enjoy the pleasant carbonated acidic beverage, others find it to be too strong; a large variety of flavored kombuchas including ginger, cherry, and guava have been formulated to appeal to varying taste preferences.

Food historians believe kombucha originated in in northeast China, in Manchuria, in 220 B.C. This “Divine Che” was prized during the Tsin Dynasty for its detoxifying and energizing properties. Kombucha is thought to have been given its name when a physician named Kombu brought the tea fungus from China to Japan. It was later traded to Russia and Eastern Europe and became popular in Germany and France in the 1950s. In the 1960s Swiss scientists reported that drinking kombucha was as beneficial as eating yogurt, which helps explain the health hype of kombucha today.  

Home-brewed kombucha is traditionally fermented for a week in gallon-sized glass containers.  During fermentation, the scoby floats as a cellulosic pellicle layer on top of the tea. The scoby consists of acidophilic yeast and acetic acid bacteria embedded in a microbial cellulose layer. The exact microbial composition of kombucha varies depending on the source of the inoculum but is guaranteed to contain various species of Acetobacter including Acetobacter xylinium. During fermentation, A. xylinum produces a thin cellulose film where the cell mass of bacteria and yeasts is attached, enhancing the association between the bacteria and fungi. 

During the brewing process, a new “daughter” tea fungus is formed at the tea surface while the “mother” is submerged below. The Internet abounds with a variety recommended uses for excess mother scobys including facials, smoothies, candy, pet food, compost, and crafts.  The cellulose matrix produced by A. xylinium is also the basis for the chewy Filipino delicacy “nata de coco.” A. xylinum cellulose mats have also shown potential as a novel wound healing system.

As the tea ferments, scoby microbes break down the black tea ingredients and sucrose to produce acetic, lactic, gluconic, and glucuronic acids, ethanol, and glycerol. Kombucha fermentation also produces B-vitamins—scientists found that kombucha contains 161% more vitamin B1 and 231% more vitamin B12 than unfermented sweetened black tea. The final composition and concentration of metabolites depends on the fermentation length, sugar concentration, and the tea fungus itself. Essentially, the yeast cells break down sucrose into fructose and glucose and then metabolize these sugars, mainly fructose, to make ethanol and carbon dioxide. The acetic acid convert the metabolized glucose into gluconic acid and the ethanol into acetic acid. The caffeine and xanthines in tea help A. xylinium stimulate cellulose synthesis. Ethanol and acetic acid are both antimicrobial agents, protecting the tea fungus from contamination.

Yeasts and bacteria in kombucha are involved in metabolic activities that utilize substrates by different and complementary ways. Yeasts hydrolyze sucrose into glucose and fructose by invertase and produce ethanol via glycolysis, with a preference for fructose as a substrate. Acetic acid bacteria make use of glucose to produce gluconic acid and ethanol to produce acetic acid. During fermentation the pH value of kombucha beverage decreases due to the production of organic acids.

Scientific studies suggest kombucha has probiotic, antioxidant, antimicrobial, and detoxifying properties. However, all available research on kombucha was performed in cell or animal models. The lack of human clinical trials means it is impossible to truly substantiate whether these properties translate to real health benefits from regular kombucha consumption. (Read more about how important human studies are versus animal studies here)

Like sauerkraut, kefir, kimchi, yogurt, and a number of other fermented foods, unpasteurized kombucha may contain good-for-you bacteria that can aid digestion and help maintain intestinal health. Kombucha tea fractions have been shown to reduce lung and prostate cancer cell invasion, motility, and survival. Microbes in scoby produce antioxidants from tea polyphenols that protect liver cells against oxidative damage. Due to its acetic acid and catechin content, kombucha has been shown to be effective in inhibiting both Gram positive and Gram negative pathogenic microorganisms. Kombucha also contains glucuronic acid, a compound known to react with toxins or carcinogens forming a glucuronide complex which can then be excreted, hence speeding the elimination of harmful compounds from the body. Glucuronic acid can also be turned into glucosamine, a beneficial substance associated with cartilage, collagen, and fluids related to the treatment of osteoarthirits.

     However, it bears repeating: these studies were all performed in vitro or in animal models—not in human clinical trials! There are therefore no proven benefits to consuming kombucha. Additionally, there are risks associated with kombucha. Consuming kombucha can result in an upset stomach, acidosis, and possible allergic reactions. The unpasteurized tea, while rich in probiotics, may also pose a food safety threat, particularly for those who are pregnant or have compromised immune systems. Even though the scoby protects itself against foreign microorganisms, contamination is always possible. Home fermentation carries an inherent risk and failure to take proper precautions with regards to sterility and acidification can lead to unwanted, harmful bacteria such as Clostridium botulinum. Adherence to strict preparation protocol, particularly maintaining a low pH, is necessary to avoid the risk of serious illness. Therefore any home-production of kombucha should be done with great caution.

        So, in the end, is kombucha truly a health drink? We may never know beyond anecdotal claims. Because kombucha is a living food and it changes from batch to batch, the scientific community is less likely to spend money researching its clinical effects. If you enjoy the taste, and have a healthy immune system, then drink commercial kombucha with pleasure, and homemade brews with caution. The probiotics and antioxidants may provide some small benefit as part of a healthy diet, but don’t expect that kombucha, by itself, will prevent or cure any illness.

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Erica graduated from University of Georgia with a master’s in Sensory Science. Her thesis project was on the emotions of coffee drinking with a focus on coffee connoisseurs. (Follow her on Twitter! @Ericalovesfood)