Leaching acorns to make them edible
A step by step guide to convert shelled acorns into meal, flour, starch, coffee, acorn water, and acorn milk
By the Manzanita Cooperative
Even after the nuts are dried and shelled, Acorns are not safe to eat raw. They produce large volumes of Tannins, natural preservatives that have a wide range of uses but can make humans very sick when eaten at high concentrations. Small amounts of Oak tannins, however, have been shown by peer-reviewed studies to have a long list of positive health benefits for the human microbiome and digestion. Their flavor will be familiar to anyone who has enjoyed wine or spirits aged in Oak.
To safely get tannin levels down to an acceptable level, there are a few different leaching methods. Of these, Cold leaching produces the best quality food product with the most intact nutritional content. It is also the slowest.
Wash nutmeats before leaching
Make sure the shelled nuts are clean before beginning. Discard anything that floats. As a bonus, small pieces of shell will often float to the surface during this stage. Remove skins if possible - they are very high in tannins and will slow your leaching process.
Before Leaching
Before beginning the leaching process, most people will grind the shelled nuts into a flour or meal. There are several ways to do this. Smaller scale processors often use a food processor with a spinning blade to break up the nuts into smaller particles and this works well, but is very time intensive because of the very small batches. For larger scale processing, a bur grinder designed to grind nuts, soy, and other higher-oil foods into flour can be a worthwhile investment. Unlike other nuts, Acorn is only 10-20% fat, depending on the species. Coffee beans are roughly 23% fat by volume and we have heard anecdotally that coffee grinders may work well.
How fine you grind depends on how you intend to use the finished product. For an acorn polenta or as a replacement for oats, go with a courser grind. For use in breads and pasta, you’ll want something much finer. For acorn coffee, mimic the grind you would use for your favorite coffee grind.
It is possible to leach whole nuts, but the process takes longer since grinds have more surface area. Leaching whole nuts may be preferable though if you intend to toast, salt, and eat the leached nuts as a snack food - one of the main ways they’re traditionally eaten in China. Leaching intact nuts may also mitigate the loss of oils and starches during hot leaching. Further experimentation is needed.
Grinding to a course meal
Blade grinding with a food processor works well for small-scale production and requires no special equipment.
Hot leaching
Hot leaching of acorns involves thermal cycling between hot and cold water. One ingenious method developed by some Native American tribes was place the acorn meal or flour submerged in cold clean water. Stones, heated in a fire, would then be added - instantly bringing the water to a boil. Once cool, the water would be poured off and replaced. The process can be repeated until the water remains clear after heating. This last round of Acorn Water could be drunk as a beverage and the residual trace tannins and starches reputedly have health benefits.
You can replicate this process in your kitchen by boiling water on your stove top and then adding it to your acorn, stirring well, pouring it off, and then adding cold water,, stirring, and draining. Repeat this process as needed. Note that the rapid thermal cycling is a key part of the process. Simply boiling acorn on your stove top will result in an inferior product.
When the hot water runs clear, try a small portion of acorn. Insufficiently leached nuts will have a bitter astringent flavor reminiscent of uncooked artichoke or asparagus.
At the end of the process, the last water used will be clear and, if you taste it, will not have the bitterness of the tannins. Placing the mash onto a cheese cloth or nut milk bag, squeeze it to remove as much of the excess water as you can. You now have acorn flour or meal! Set aside the final water and squeeezings if you want to produce an acorn water or milk beverage, as described below.
Using this process you can produce edible acorn flour in a few hours, but you will lose many of the beneficial fats and starch. Since the natural oils and starch are each around 20% of the nut mass in most species, you will be left with a smaller volume of flour at the end.
Cold Leaching
The cold leaching process is the slowest but is generally regarded as the best process to reliably produce acorn flour with the starches uncooked and oils intact at home.
As with hot leaching, nuts are typically first ground to flour and then placed in a large covered pot or mason jar.
Leaching in a Mason jar
Cold leaching in a large mason jar works well. The white in the water is starch, which will settle over time.
Add clean cold water to the container and leave the flour to soak. When the water turns dark from the tannins, skim off any oil on the surface and gently pour the water off through cheesecloth to capture starches. Repeat this step until the water remains clear. Depending on the species, this process can take anywhere from three days to two weeks.
Many Native Californians would accelerate this cold leaching process by placing the grinds in a tightly woven basket that helped to retain the starch, and then submerging the basket in a river or spring. Having the water continuously flow through speeds up the leaching process. Unfortunately, modern rivers are so polluted this is not practical in most places, not to mention the impact on downstream water quality if this was done at commercial scale.
Letting the discarded water settle in a separate large container may allow you to reclaim starch and fat that escapes your cheesecloth. Oils can be skimmed off the top and starches will settle to the bottom. The Korean acorn starch industry traditionally used a scaled-up version of this process to separate acorn starch.
At the end of the process, the last water used will be clear and, if you taste it, will not have the bitterness of the tannins. Placing the mash onto a cheese cloth or nut milk bag, squeeze it to remove as much of the excess water as you can. You now have acorn flour or meal! Set aside the final water and squeeezings if you want to produce an acorn water or milk beverage, as described below.
Acorn Starch
Depending on the starch content of the species you used, and how finely you ground the flour, you will likely have a layer of pure starch on the top of your leached flour. Scrape this off and dry it to use as a thickener. It can replace corn starch in any recipe and has a superior flavor. It’s also the main ingredient in many Korean acorn dishes.
Once the nuts are leached, use cheesecloth to squeeze off any remaining water and set the squeezings aside.
From there, you can dry the flour or meal using a dehydrator or freeze drier, or by spreading on a pan and putting it in your oven at 100 degrees or less. Grind to your desired coarseness for use as polenta, meal, or flour.
After Leaching
Whichever process you use, once the tannins are removed you need to get your acorn flour or meal dry to prevent mold. Common methods involve
Sun drying, if you’re someplace sunny with low humidity
Oven drying on a tray at 150F or less (hotter than that will cook the starches)
A dehydrator (but watch the temperature!
A freeze drier
Roasting, if you intend to use it to brew acorn beer or coffee.
Whatever process you use, get it dry quickly and then keep it cool and dry in a sealed container. This is precious stuff and you wouldn’t want it to rot or go stale! While commercially produced acorn flour can keep for up to a year, home made acorn flour will usually only keep for a week or so because it’s very hard to get it dry enough without cooking the starches unless you have access to a freeze drier. Once dry, many people perform a final grind to normalize size and texture.
Acorn water and Acorn milk
If you saved the final water, let any final starches settle out, skim off the oil, and you can drink what’s left. The residual tannins have numerous benefits for your gut microbiome - it’s tasty too!
As for the liquid squeezed off in the final step, let it settle and then pour it off leaving starches behind. Once you have removed the bulk of the starch, you can either use it as-is or add a small amount of sweetener of your choice (we recommend using Manzanita sugar or honey) and then cook the mixture down while stirring it. Cooking will develop the remaining starches and thicken it.
Acorn Milk
The squeezing’s from the acorn nut meat after leaching will be milky white and creamy and makes an ideal non-dairy milk.
Acorn Oil
Acorn oil retrieved from the leaching water can be either added back into the meal or flour or kept separate. It’s amazing as a finishing oil on salads or soups and draws comparisons to high end truffle oils. You can also use a nut oil press to extract more of the oil from your acorn meal, leaving a drier flour behind that may be preferred for some recipes. Typically this is done after leaching.
In theory it should be possible to press un-leached nuts for oil and then “wash” the oil by adding water to remove tannins before skimming off the oil, but more experimentation is needed to perfect this process.
Acorn Beer
While the tannins in raw acorn will prevent it from fermenting, malting before leaching will produce an excellent dark beer. See our guide to malting acorn for brewing for more detailed instructions.
Ethanol Leaching
A new process, invented in China for the Acorn industry there, uses a solution of 40% food-grade ethanol instead of water. This can dramatically reduce leaching time to a few hours while leaving starches and oils intact, but degrades the flavor and nutritional content of the acorn flour produced.
A chemical process is then used to remove the tannins in suspension from the ethanol so that it can be used again. This process has rapidly gained adoption and much of the acorn flour which is increasingly imported from China is processed in this way.
Other Additives and processes
Other additives have been used throughout history and, while much of the traditional knowledge for how this crop has been processed around the world for millennia has been lost, occasionally interesting fragments surface. For example, we have heard anecdotal accounts of some Native Californians using lye, produced from the ashes left after cultural burns, to adjust the ph level of water in order to improve the leaching process. The incredible diversity and ingenuity of Native Californians working with this crop since time immemorial doubtless produced countless innovations. While much of that knowledge was lost during colonization, much remains. There is also continual research into acorn and acorn processing in China and Korea where large acorn industries remain; as well as southern Europe and North Africa where unbroken rural traditions of acorn processing are being carefully revived and built upon.
We are at the very beginning of a global revival in acorn agriculture, processing, and cuisine; and expect rapid innovation over the coming years as this exciting food is brought back to the mainstream.
Further reading:
“Leaching Acorns” by Sara Calvosa Olson
“A Guide to Cold Leaching and Preserving Acorn Flour” by Ellen Zachos
“Recipes for the Mighty Acorn: A Forager Experiments” by Hank Shaw
“The Best Way to Make Acorn Flour” by Hank Shaw
“The Mechanics of Eating Acorns” by Hank Shaw
“Acorns” by Sam Thayer
