Brewing kombucha in silicone bags makes for less alcohol, faster process

Brewing kombucha tea. Note the trademark gel-like layer of SCOBY (symbiotic culture of bacteria and yeast).
Enlarge / Brewing kombucha tea. Note the trademark gel-like layer of SCOBY (symbiotic culture of bacteria and yeast).

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Kombucha tea continues to grow in popularity as a healthy alternative to alcoholic beverages—and chemistry can help commercial and amateur brewers alike get faster and better results with their brews, according to a presentation yesterday at a meeting of the American Chemical Society in New Orleans.

“Brewers typically see making kombucha as an art more than a science,” Jeb Kegerreis, a physical chemist at Shippensburg University, said of the research. “So when we are doing a consultation, we also walk the brewer through the biochemistry of what’s happening during fermentation.”

As we’ve previously reported, you need just three basic ingredients to make kombucha. Just combine tea and sugar with a kombucha culture known as a SCOBY (symbiotic culture of bacteria and yeast), aka the “mother,” also known as a tea mushroom, tea fungus, or a Manchurian mushroom. It’s basically akin to a sourdough starter. A SCOBY is a firm, gel-like collection of cellulose fiber (biofilm), courtesy of the active bacteria in the culture creating the perfect breeding ground for the yeast and bacteria to flourish. Dissolve the sugar in non-chlorinated boiling water, then steep some tea leaves of your choice in the hot sugar water before discarding them.

Once the tea cools, add the SCOBY and pour the whole thing into a sterilized beaker or jar. Then cover the beaker or jar with a paper towel or cheesecloth to keep out insects, let it sit for two to three weeks, and voila! You’ve got your own home-brewed kombucha. A new “daughter” SCOBY will be floating right at the top of the liquid (technically known in this form as a pellicle).

There are two kinds of fermentation taking place: alcoholic fermentation and acetic acid fermentation. A really good kombucha will strike the perfect balance between the two. The yeast in the SCOBY produces an enzyme (invertase) that breaks the sugar into fructose and glucose. The glucose then breaks down into pyruvate, acetaldehyde, and finally ethanol, releasing carbon dioxide as a byproduct to give kombucha that pleasing touch of carbonation.

It’s not a lot of ethanol, since the bacteria in the SCOBY converts much of it into acetic acid. Too much alcohol would actually stop the fermentation process. So most kombucha teas have less alcohol than even a very light beer. You can get higher alcohol concentrations if you add too much sugar and/or let the stuff ferment too long, but then your kombucha will probably just taste like straight vinegar.

“I think as [kombucha] becomes a more popular beverage it can take the place of soda in somebody’s diet,” Kegerreis said during a media briefing. “It has the carbonation, the fizz, and it doesn’t have nearly the same sugar content in grams per liter. Compared to soda, kombucha is a much healthier alternative. A lot of our research focuses on making sure that it maintains its status as a nonalcoholic beverage. So we focus a lot on keeping ethanol content below the legal limit for a non-alcoholic beverage.”

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