Every fall acorns lie by the thousand under oak trees, and the oak is on the short list of four wild plants most worth a forager’s time. What keeps most people from eating them is tannin, the compound that makes a raw acorn so bitter and astringent it puckers the mouth. Tannin dissolves in water, so the whole job comes down to breaking the nuts into small pieces and washing it out. Native peoples across North America made acorns a staple this way for thousands of years.
Step 1: Gather
Collect them from the ground once they drop, and start early; squirrels, deer, jays and weevils want the same crop.
Sort as you go. Throw out any acorn with a small round hole (a weevil has been in it), a cracked or moldy shell, or a dark, shriveled nut. A quick test at home: drop them in a bucket of water and discard the floaters, which are usually hollow or damaged. All North American oak acorns are edible once leached. White oak acorns carry less tannin and leach faster; red and black oak acorns are more bitter and take longer.
Step 2: Shell
A nutcracker or a light tap with a hammer opens them. For a big batch it is quicker to halve each acorn with a heavy kitchen knife and pry the halves out with the tip of a paring knife.
Step 3: Grind
Break the nutmeats down as finely as you can. More surface area means faster leaching. The traditional tools are a broad flat stone and a smaller round one, used as a mortar and pestle. A food processor, blender or heavy-duty juicer does the job in minutes; acorns are softer than peanuts and will not hurt the machine.
Step 4: Leach out the tannins
There are two methods, and which to use depends on what the flour is for.
Hot leaching is fast. Stir the ground meal into rapidly boiling water and keep it at a boil for five minutes or more, stirring so nothing scorches. Strain as in step 5, taste, and repeat with fresh boiling water until the bitterness is gone. Boiling whole nutmeats also works but takes far longer and burns far more fuel.
The drawback is that boiling cooks the starch. Hot-leached meal loses the ability to thicken and bind, so it behaves more like ground nuts than flour. It is fine in porridge, soup and cereal, and in bread it needs wheat flour or egg to hold together.
Cold leaching keeps the starch intact and gives a better baking flour. Put the ground meal in a large jar, cover it generously with cold water and refrigerate. The water turns tea-brown. Pour it off and replace it once or twice a day, stirring each time. Depending on the acorns and how finely they are ground, it takes from a few days to over a week.
Step 5: Strain and taste
Set a colander in the sink and line it with cheesecloth or a clean, thin cotton cloth such as an old T-shirt, pressed down to form a bowl. Pour the meal and water in. After a hot leach, run cold water over the slurry until it is cool enough to handle, then gather the cloth and squeeze out the rest of the water.
Taste a pinch. If it is still bitter, go back to step 4. When it tastes mild and faintly sweet, it is done.
Final steps: dry and store
What comes out of the cloth is a damp ball of acorn dough. Use it straight away, or spread it thin and dry it completely in a dehydrator, in the oven at its lowest setting, or outdoors in warm, dry weather. Because of its fat the dried meal cakes together; break it up and grind it again to a powder.
That fat also means acorn flour can go rancid. Keep it in an airtight container somewhere cool, or in the freezer for long storage. Use it for pancakes, breads, muffins, and as a thickener or an addition to hot cereal and soup.
Nutrition
USDA figures for dried acorns work out to roughly the following per ounce: 144 calories, 8.9 grams of fat, 15.2 grams of carbohydrate, 2.3 grams of protein and about 200 milligrams of potassium. Full profiles for dried acorns and acorn flour are listed in USDA FoodData Central.
Anyone with a tree-nut allergy should treat acorns with the same caution as other nuts.