Acorns are a hugely abundant food source, but they’re also packed with bitter tannins which cause a bad taste in your mouth. Native populations have been using acorns as a food source for centuries, but they’ve fallen out of fashion because of the amount of preparation that it takes in order to get rid of the bitter taste.
It isn’t just the flavor, though. Tannins can also cause upset stomachs, headaches and even an allergic reaction in some people. The traditional method of removing tannins is to slowly leech them out of the nuts, and some native populations used to bury them in riverbanks for months at a time in order to let the water render the acorns fit to eat.
Boiling is a newer method of acorn processing, although it’s been used for a long time to leech out harmful chemicals from leafy greens. The real innovation comes from the sheer amount of time that these people boil their nuts, and the fact that it works on whole nuts instead of ones that have been prepared by grinding.