And no, the ESA isn't trying to turn its astronauts on to the hypothetical health benefits of drinking the pungent brew. They're interested in the resilience of biofilms like the one used to brew the tea, which could one day be used as building materials for space habitats.
First, a little about kombucha: To brew it, you take what's called a "SCOBY" or a "mother" (depending on what mycology nerds you're running with) and plop it into some sugary tea for a week or two. The result is a fizzy, vinegary beverage purported to improve everything from libidos to immune systems, beloved by hippies and hipsters alike.
SCOBY stands for "symbiotic colony of bacteria and yeast." The bacteria — Acetobacter — and the yeast (whatever kind was hanging around at the right time, generally) form what's called a biofilm, which is just to say that the itty bitty microorganisms of the yeast and bacteria grow into something big and interesting together. In the case of a kombucha SCOBY, that big interesting something is a thick, gummy-looking layer.
But in addition to being quite robust and resilient, this biofilm has some properties that space agencies are right to find interesting: When you heat up the culture, it dies and forms a leather-like material called microbial cellulose. I've seen some pretty cool applications for microbial cellulose, including vegan clothing that looks like it's made of prosciutto and weather-proof coatings for structures built from the fluffy material that makes up mushrooms.
So it's a material worth throwing into space, basically. And Earth-based tests have shown that the material gets even stronger when exposed to lunar dust, because it can pick up some of the minerals therein and incorporate them into its sturdy structure.
The ESA is also hoping to gage whether biofilms like this one could presumably exist on other planets. It would be easier to spot a large symbiotic structure than it would be to spot a tiny microbe, and since the symbiotic pairings are so much more resilient than the sum of their parts, it follows that they might have prominent play on worlds harsher than our own.
The samples are in the ESA's Expose-R2 facility, which does exactly what it sounds like it does — it exposes things on the space station to space. The kombucha samples will come back down for analysis in a year or so.