More experimenting with kvass, a fermented soda popular in Russia that can be made with any number of things.
Our second attempt uses wild strawberries – Fragaria vesca, ‘fraisier des bois’ – collected around the beach at Nida. Evidence from archaeological excavations suggests that Fragaria vesca has been consumed by humans since the Stone Age http://intarch.ac.uk/journal/issue1/tomlinson/part2.html#S711
Recipe from Sandor Katz here
Seems you basically just cover the berries in sugar water, 1:1.5 ratio, and wait.
We would have like to have tried this with the arctic raspberry or arctic bramble, the ‘superior berry’ (rubus arcticus) of the sub-arctic region, but that’s now extinct in Lithuania. In other places it’s also on the decline – in Finland the decrease in forest fires have adversely affected it (it likes the nutrients from forest ash and post-fire gets a better run than competitors); while in Estonia and Scandinavia, agricultural and forestry developments which drain soil have had an impact, since it likes wet soil and in dry conditions is overgrown and replaced by other species.
[image from https://upload.wikimedia.org/wikipedia/commons/thumb/7/79/Rubus_arcticus.jpg/440px-Rubus_arcticus.jpg ]