Kombucha Fermentation

Fermentation is a highly intricate process involving both metabolic and microbial activity. Its specifics depend on the substrates, yeast strains, bacterial strains, and environmental factors. Kombucha, an ancient fermented beverage believed to originate in China (like Jun Kombucha), is made by fermenting sweetened tea. Today, it is brewed worldwide, both commercially and at home.

The term "SCOBY" is commonly used—though inaccurately—to refer to the cellulose biofilm that forms on the surface of the finished kombucha. SCOBY is actually an acronym that stands for "Symbiotic Culture Of Bacteria and Yeast."

Traditional kombucha is crafted using black tea of the Camellia Sinensis plant, sugar (sucrose), and a starter liquid containing a healthy, established SCOBY colony. But what fascinating transformations are taking place as you wait for your kombucha to mature into a tangy, fizzy, functional beverage?

Image is credited to "Kombucha: a novel model system for cooperation and conflict in a complex multi-species microbial ecosystem." Authored by: Alexander May, Shrinath Narayanan, Joe Alcock, Arvind Varsani1, Carlo Maley, and Athena Aktipis

Though more intricate than depicted in the image, the process can be simplified as follows:

The yeast in the starter culture interacts with sugar, tea, and oxygen, releasing an enzyme called invertase. This enzyme breaks down sugar into two fundamental molecules: glucose and fructose. The yeast then metabolizes these sugars, producing ethanol (alcohol) and carbon dioxide (CO₂) as its primary byproducts. This step mirrors fermentation processes in beverages like beer, cider, and sparkling wine.

Meanwhile, the bacteria in the starter culture utilize ethanol and oxygen to generate organic acids, primarily gluconic and acetic, along with carbon dioxide and cellulose (the biofilm). Together, these transformations result in the tangy, fizzy beverage we know as kombucha.

Why does my homebrew Kombucha give me a woosy feeling? 

The fermentation curve depicted above illustrates the progression of kombucha fermentation over time. Since most homebrewers lack access to specialized testing equipment, they often rely on taste to judge when their kombucha is ready. This typically occurs midway through the fermentation curve, when sugar levels (pink line) and alcohol content (yellow curve) are higher than they might realize. Additionally, flavoring kombucha with fruit can introduce extra sugars, which further elevate sugar and alcohol levels as part of a secondary fermentation curve.

Kombucha that is low in sugar and alcohol but high in organic acids can be quite tart or acidic, which may not appeal to the average consumer's palate.