Kombucha

What it is

Kombucha is sweetened tea fermented by a SCOBY, a symbiotic culture of bacteria and yeasts that floats on top as a rubbery cellulose mat ¹. The yeasts turn sugar into alcohol and carbon dioxide; the bacteria (Acetobacter and Gluconobacter among them) turn the alcohol into acids. The result is a tart, slightly fizzy drink.

Unlike sauerkraut, kombucha has no tested consumer recipe from a food-safety lab; the safety rests on sanitation, a good starter, and getting the acidity right ^{1 2}.

The brew

Kansas State’s guidance ¹:

• Tea: black or green tea steeped in boiling water; Virginia Tech says water above 165 °F ².
• Sugar: 5 to 10% of the tea by weight, dissolved while hot. A quart of water weighs about 2 lb, so 5-10% is roughly 1/4 to 1/2 cup of sugar per quart.
• Cool the sweet tea to room temperature before adding the culture.
• Starter: a SCOBY, or about 10% of the volume as finished kombucha from a previous batch. Start your first batch with a commercial SCOBY: it’s produced under controlled conditions, free of contaminants, with a reliable mix of microbes ¹.
• Ferment 7 to 10 days at 60-70 °F, covered with a clean, porous cloth, in a non-metallic food-grade container such as glass ^{1 2}.

Why pH matters

The acid is the safety factor. Virginia Tech’s rule: the brew must reach pH 4.2 or lower during fermentation, and should never drop below 2.5, which is dangerously acidic ². Over-fermented kombucha can end up vinegar-sour; a weak, slow ferment can leave the pH high enough for harmful organisms. Kansas State calls regular pH monitoring and a controlled fermentation time essential ¹.

At home that means pH strips or an inexpensive meter, checked at day 7 and again before bottling.

Bottling and second fermentation

After the first fermentation ¹:

1. Lift out the SCOBY and set it aside with some kombucha for the next batch.
2. Put flavorings (chopped fruit, ginger, herbs) into clean bottles.
3. Fill with kombucha, leaving about 1 inch of headspace, and cap tightly.
4. Leave at room temperature 1 to 3 days to build carbonation.
5. Refrigerate once it tastes right, which stops the fermentation.

Fruit adds sugar, so pressure builds fast in warm bottles. Use bottles made for carbonated drinks, and open one test bottle a day over the sink.

A healthy culture, and mold

A healthy SCOBY is pale tan to cream, sometimes with brown stringy yeast strands hanging from it; new layers form on top with each batch. Mold looks different: dry, fuzzy spots, white with blue or green, sitting on the surface. Don’t reuse a SCOBY that smells unusual or shows mold; throw out the batch and start over with a fresh culture ².

Alcohol

Fermentation always makes some alcohol. Under federal law, kombucha with 0.5% alcohol by volume or more at any point is an alcoholic beverage, which is why sellers have to test and track it ¹. Home brew, especially with a long, warm second fermentation, can pass that line.

Try it

Brew a gallon with a commercial SCOBY: 1 gallon of water, 8 tea bags of black tea, 1 cup of sugar (about 5%), and 2 cups of starter liquid. Check the pH at days 5, 7, and 10 and taste each time; bottle when it’s below 4.2 and tart enough, with a few slices of ginger per bottle.

What to take from this

Kombucha is sweet tea (5-10% sugar) fermented by a SCOBY for 7-10 days at 60-70 °F in glass. Start with a commercial culture, then keep a SCOBY and about 10% of the brew for the next batch. Check that the pH falls to 4.2 or below but not under 2.5. Bottle with about an inch of headspace, second-ferment 1-3 days, and refrigerate. Discard any moldy or off-smelling culture, and remember that 0.5% alcohol is the legal line.