Unlike drying or freezing, fermenting actively transforms what
you've foraged through the activity of microorganisms —
changing flavour, texture, and nutritional content along the way.
It's one of the more rewarding preservation methods,
though it asks for a bit more attention than drying or freezing.

A traditional fermented drink made from nettle, sugar, and water,
sometimes with a little lemon or rye bread added to help start
fermentation. Left at room temperature for a few days, it
develops a mild, slightly tangy, effervescent quality.
A good way to use a larger nettle harvest from spring,
as covered in section 5.1.

Berries — bilberry, lingonberry, or others from section 5.2 —
can be fermented into a fruit vinegar by combining with sugar
and water and letting natural yeasts do their work over several
weeks, eventually straining out the solids. The result is a
flavourful vinegar useful in dressings and cooking long after
the berry season has ended.

Fermenting relies on the right microorganisms doing the work,
so clean equipment matters more here than in drying or freezing.
Use clean jars and utensils, and keep ferments covered
(often with a cloth rather than a tight seal, to let gas escape)
while they're active.

A healthy ferment usually smells pleasantly sour or tangy,
not foul or rotten. Some bubbling or fizzing is a good sign
of active fermentation. If something smells distinctly off,
looks unusually slimy beyond what's expected, or grows
fuzzy mould rather than the expected surface activity,
it's better to discard it and start again.