Kombucha brewing involves fermentation brought about by a symbiotic combination of bacteria and yeast (SCOBY). A major obstacle in the commercial brewing of non-alcoholic kombucha is maintaining an alcohol by volume (ABV) less than the legal limit, which is 0.5%. Many factors influence the production of alcohol, including the identity and abundance of bacteria and yeast, the food source for the microbes, the temperature of the brew, and the brewing vessel. Traditional brewing vessels that restrict exposure of the SCOBY to oxygen tend to produce ABV levels that are too large as the bacteria require the oxygen to convert ethanol to acetic acid. This project focused on small-scale brewing of kombucha in a vessel that maximizes exposure of the SCOBY to oxygen. Two batches of kombucha were brewed in different vessels using the same ingredients while the pH, sugar levels, ABV, and gluconic and acetic acid concentrations were monitored over time. One vessel was a silicone air permeable bag, which allowed for oxygen to enter from all sides. The second vessel was a traditional gallon-sized wide-mouth jar. We found that air permeability associated with the silicone bag enhanced the bacteria’s activity relative to that of the yeast when measured against the control vessel. This observation was supported by the fact that the kombucha brewed in the air-permeable bag exhibited a decreased rate of alcohol production while also exhibiting a lower pH, higher titratable acidity, and higher concentrations of acetic acid and gluconic acid over the time of study. These preliminary results could act as a model for scale-up efforts by kombucha brewers as they strive to maintain a legal alcohol level in their brews.

Evolution of kombucha in an air-permeable bag

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