Acorns contain bitter, unhealthful tannin in varying concentrations. Here's the way to remove it and make the acorns edible. Ignore this recipe if you’re a squirrel, because you’re already adapted to handling concentrated tannin.
Use acorns in the white oak group, those species with blunt-tipped leaves. Acorns in the red oak group, with pointy-tipped leaves (the tips resemble needles) contain too much bitter, unhealthful tannin to leach out easily with the following method
1. Boil acorns in water 1-2 minutes to loosen their shells.
2. When cool, cut them into quarters with a paring knife to shell more readily.
3. Shell the acorns, discarding any insect-damaged, internally blackened ones.
4. Put 1-1/2 cups of acorns in a blender, fill almost to the top with water, and blend until finely chopped, the size of rice grains (to create more surface area for the boiling water to act on).
5. Boil the acorn bits 5-10 minutes in water, and taste one of the largest pieces. If it has any trace of bitterness, change the water again, repeating this procedure until all the bitterness is gone.
6. Drain and use within a couple of days (acorns are very perishable, raw or cooked) or freeze.
7. You can dry acorns in a food dehydrator, or on a cookie sheet in an oven on the lowest setting, with the door ajar to release the water vapor
8. When the acorns are dry, you can grind them into flour in a grain mill, spice grinder, or blender.