This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
As a home-fermenter, I've largely restricted myself to what Sandor Katz calls "Krautchi" - lacto-fermented vegetables such as sauerkraut and kimchi. Making krautchi is dead simple, and accommodates a huge amount of experimentation. But recently, as a result of a little business venture I'm involved in (more details on that to come), I've been branching out a bit. Over the past couple of weeks, I've been delving into kombucha.
Kombucha is fermented tea. Actually, it's unclear how much of the substance of the tea is actually fermented - you've got to add sugar to make it work. The fermentation requires the cooperation of two very different types of microorganisms - fungi and bacteria.
The first stage of fermentation takes place when the fungus - mostly a species of yeast called Zygosaccharomyces - ferments the sugar to acquire energy and carbon backbones to make molecules necessary for growth. This fermentation is anaerobic - meaning that it does not require oxygen - and produces ethanol, the same alcohol found in beer and wine, as a waste product.
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In the next stage, the bacteria in the brew - which can be a number of different species of acetic acid bacteria - ferment the alcohol secreted by the yeast into acetic acid. This process, unlike a lot of ferments, requires oxygen. Since the oxygen in the liquid is rapidly consumed, most of this process takes place on the surface. Incidentally, this is why wine, if not properly sealed, will turn to vinegar.
This is also the same fermentation that occurs with vinegar mothers, though the specific species involved may differ. In fact, you can probably make kombucha with a vinegar mother and vice versa, though the end result may have a different flavor or acid content.
My recipe (a hybrid from here and here)
Per Liter of Kombucha:
850mL spring water (don't use tap water, it often contains chloramines and other sanitizers that will inhibit growth)
2 black tea bags
120g sugar (~1/4 cup)
150mL starter kombucha (if you don't have starter kombucha, use 75mL vinegar)
First, boil about half of the water and add tea bags, allowing the tea to steep for at least 5-10 minutes (you want it concentrated). Add the sugar to and stir while still hot, then add the other half of the water. This dilutes the tea a bit and cools it down.
When the temperature has dropped below ~40oC (~100oF), you can add the mother, plus vinegar or a previous batch of kombucha. This is super important - you want the mixture to start out acidic, otherwise it's quite easy for molds to get a hold on the surface of the tea.
Unfortunately, Kombucha doesn't really work as a completely wild ferment - you need to get the mother from somewhere. The good news is that mothers grow like mad and constantly slough off new babies, meaning that anyone that makes kombucha will have an ample supply. You can also get them dehydrated from online suppliers, though my first experience with that did not go well, and it takes a while to get them active. YMMV.