Chemistry of Komucha

Since my doctor prescribed a daily kombucha beverage nearly 3 years ago, I have been drinking a lot of kombucha and even began brewing my own kombucha at home about 9 months ago. While I need a general understanding of the chemistry of kombucha to brew it myself, I thought it was about time I learned what’s really going on in that vinegar-smelling jar in my cabinet!

What is kombucha you ask? In essence, kombucha is a sparkling fermented tea with a slightly sweet and slightly sour acidic taste. Various other fruits or essences can be added after the first brew to make a truly refreshing beverage (I usually make a watermelon mint variety)!

There are really a few steps involved in making sparkly kombucha tea:

  1. The first step involves brewing tea. You can use black or green tea, but not herbal teas because many herbal teas include oils that would ruin the tea fermentation. Boiling water (1L) is added to about 8 tea bags and let to steep, infusing the tea into the water. The boiling water also sterilizes the environment.
  2. After removing the tea from the jar, 50g of sugar is added while the water is still hot to dissolve as much sugar as possible. At this point you have hot sugared tea, but to continue to the next step the tea must cool to room temperature.
  3. The kombucha fungus culture (formal name: Medusomyces gisevii; aka “SCOBY”) is then added to the sugar tea. The fungus culture is made of cellulose and consists of various yeasts and bacteria — the exact composition of a culture cannot be given as it varies from one culture to the next, but it usually includes an Acetobacter. A review on kombucha tea studies lists all the various yeasts and bacteria found in many different studies of kombucha.
  4. To prevent the growth of “undesirable microorganisms” a cup of white vinegar or a previously fermented kombucha tea is added to the mixture. Adding the vinegar immediately lowers the pH of the mixture.
  5. The tea is now ready to incubate in a warm area away from direct sunlight (I usually keep my jar in a cabinet in my kitchen). The optimal range is somewhere between 65–78 degrees F. Because heat speeds up reactions, the tea will brew faster in a 78 degree room than a 65 degree room.

What happens during fermentation?

The yeast in the culture convert the sucrose to glucose and fructose (via hydrolysis using the enyzme invertase) and glucose to ethanol and carbon dioxide via glycolysis. Simply:

C6H12O6 → 2 C2H5OH + 2 CO2
sugar → alcohol + carbon dioxide

The bacteria convert the alcohol to acetic acid (which is why the tea starts to smell more like vinegar after a few days, and if left out for too long, will become primarily vinegar!):

C2H5OH + O2 → CH3COOH + H2O
alcohol + oxygen → acetic acid + water

As the new cellulose culture begins to form on top of the mixture, less carbon dioxide can be released, so the carbon dioxide also reacts with the water to form carbonic acid (this is what makes the tea fizzy!):

CO2 + H2O → H2CO3
carbon dioxide + water → carbonic acid

6. At room temperature, a jar of 1L of tea will usually be ready in 7–10 days. The culture is then removed from the liquid and the mixture is filtered (a cheesecloth works well) and bottled. Some people choose to do a second fermentation to flavor the kombucha and increase the fizz, but it is completely ready to drink at this point. The finished kombucha is stored in the refrigerator until ready to drink!

What’s in a finished bottle of kombucha?

At the end of fermentation, what started as a simple sugared tea now consists of acetic acid, glucuronic acid, gluconic acid, glucose, fructose, and sucrose primarily. You may also find other organic acids such as citric acid, B-vitamins (B1, B2, B12) and Vitamin C, trace amounts of alcohol (ethanol), “14 different amino acids, lipids, proteins, some hydrolytic enzymes, antibiotically active matter, carbon dioxide, phenol, as well as some tea polyphenols and minerals” (Jayabalan, Malbaša, Lončar, Vitas, & Sathishkumar 2014).

Additional Resources:

Frank, Gunther W. Kombucha — Healthy beverage and natural remedy from the Far East. 4th ed. Austria: Wilhelm Ennsthaler, 1994.

Jayabalan, R., Malbaša, R. V., Lončar, E. S., Vitas, J. S. and Sathishkumar, M. (2014), A Review on Kombucha Tea — Microbiology, Composition, Fermentation, Beneficial Effects, Toxicity, and Tea Fungus. Comprehensive Reviews in Food Science and Food Safety, 13: 538–550. doi:10.1111/1541–4337.12073