Koso is a Japanese drink which — from translation or otherwise — is called everything from fermented, enzyme, postbiotic, and more.
But after finding it online, and messaging with a lovely Japanese person, I started making lime koso. It’s equal parts sugar and thinly sliced limes. After a few weeks, the syrup with some fresh soda water is the best limeade ever.
You can also do other fruits, even thick/chunky ones which you can then use the fruit for a pancake/ice cream/etc topping (but I’ve so far only done lime a few times).
Fresh batch just put up:
After a month or so (I actually would refresh this one with more water and it seemed to work well):
In the glass with some soda water:
You can search more online or here’s a site I was sent: https://www.masayosugimoto.com/