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Throughout History, humans have been eating acorns. Indeed, they are a nutritionally rich food. But acorns contain tannins that are harmful in high quantities.
How were humans dealing with acorn tannins? How do we know when it is ready to eat?
Let’s go on a journey through the history of culinary practices…!
Humans have been eating acorns for thousands of years everywhere where oak trees grew1. Along the description of examples, I show some pictures of my own experiments.
In North America, native people were making gruel and unleavened acorn bread, from acorn flour. They also ate boiled or roasted whole acorn kernels2.
Acorn flour
In Morocco and Algeria, the oil extracted from acorns has been used for cooking1. Its flavor and its composition are very similar to those of olive oil3.
In Europe, acorns were roasted to make coffee, Eichel kaffee, in Germany. They were transformed into an alcoholic drink, Liquor de Bolota, in Portugal. They were used to make bread in Sardinia, Italy.4
Acorn coffee: unshelled acorns roasted 200°C (390°F) for 10 min. On the left, the seed coats has detached while roasting.
In Turkey, they made a drink from acorn flour, called racahout5, that might be the ancestor of hot chocolate.
Racahout-inspired drink: boiled acorn flour and water + ginger, cinnamon, galanga, a pinch of salt — Hsiao says “Yum!”
The Korean Peninsula may be one of the few remaining places where acorn is still commonly used as food. It is eaten as pasta, pancakes6, but the most iconic Korean acorn dish is a kind of tofu or jelly, called mook7.
Acorn pancake: 100% acorn flour, egg, deodorized coconut oil + a pinch of salt
We only eat the cotyledons of acorns, after the removal of the two protective layers: the tough pericarp and the thin seed coat.
The nutrient composition of acorn cotyledons is very variable, depending on the oak variety, the climate, the soil. But in general, flour made from acorn mainly contains starch (~40%)8. Thus, acorns are unusual nuts whose composition is closer to cereals rather than other tree nuts. For this reason, it has been described as “the grain that grows on tree”1.
Compared to cereal flours, acorn flour is a bit lower in protein, a bit higher in oil, and has more fibers than whole grain flours. It is also a rich source of minerals, mainly calcium, iron, and copper. And, it is gluten-free.8
While I was researching for this article, I realized that an increasing number of of scientific studies about acorns as food have been published over the last decade. A friend of mine recently told me that we can already find coffee-like drinks that contain roasted acorns. I would not be surprised if I find in France, within a couple of years, acorn flour in organic shops!
But, haven’t you heard like me that acorns are toxic?
Some oak varieties produce acorns that are actually edible raw1.
But a lot of varieties contain high amounts of tannins, or tannic acid, a kind of astringent and bitter molecule, that protects the seed from oxidation, microbes, and animal predation.
Tannins are very common molecules found in plants. They have the ability to bind with other kinds of molecules, like proteins or minerals9.
When you drink tea or wine, tannins bind to the proteins of your tongue, producing this strange feeling of astringency. This property is used to tan animal skin, that is, to bind the proteins of animal skin to make leather. In our digestive system, tannins can even bind to our digestive enzymes, preventing us from digesting food properly9.
Have you already heard that you should not drink tea during a meal rich in iron? This is because tannins also bind to the iron in food, preventing us to assimilate it properly10.
As you see, tannins do really well in their job of protecting the seeds! This is why they are considered “anti-nutrients”.
Yet, have you also heard like me, that tea and wine are good for health, precisely because they are rich in tannins?
Indeed, to protect the seeds, tannins are also anti-oxidants, anti-microbial, and have many other beneficial effects for human health9.
So, are they bad or good?
Well, it seems that, as for all nutrients, tannins are not really good or bad. It is just a question of finding the right balance. Eat too many tannins and you will be sick, eat too less, and you will be sick!
Therefore, we can probably conclude that acorns are not toxic, they are healthy, as long as their tannin content is reduced to an optimal amount.
To reduce tannins content of acorns, some native people in North America immersed whole acorns — peeled or not — into a stream for a few weeks or months. Others peeled and ground acorns into powder, and leached this powder with water along a river. Others just boiled the whole peeled acorns several times and discarded the water.2
All these techniques are called “tannin leaching”. It is also the way acorn tannins are removed in the Korean Peninsula to make the gelly mook7.
This works because tannins are water-soluble. They are the ones you see diffusing in water when you put a tea bag in your mug. Later, you can even see those tannins accumulating as a kind of skin on top of your tea, as in this picture, when I had soaked acorns for a few days.
Tannins are not destroyed at the boiling temperature of the water, but they dissolve faster in warm water. That is why boiling several times also works.
In my case, though, I had to cook whole acorns more than fifteen times to get a low astringency. It may mean that my acorns are quite strong in tannins, and eventually, I prefer grinding them first to increase the contact area with water when leaching.
When extracting oil, in Marocco and Algeria, tannins are automatically removed, as they stay in the aqueous part! This can be done by pressing acorns, or by boiling them and skimming floating oil2.
When doing Eichel kaffee in Germany, the high heat of about 200°C (390°F) for just 10 minutes destroys some of the tannin molecules, hence reducing tannin content11.
In my case, I found my acorn coffee quite acidic. Perhaps because of some remaining tannic acid? Next time, I will try to leach acorns first, before roasting, as North American native people used to do2.
In Sardinia, an unusual ingredient was added to the acorn bread dough: clay! This is because clay has the ability to neutralize tannins by binding to them12. Native people from North America also knew this technique1.
But that’s not all.
Still in Sardinia, this same acorn bread was sprinkled with wood ashes before eating!12 In North America and other places like China, acorns were also soaked or boiled in wood ash water1,2. This is because ash is alkaline, and hence neutralizes tannic acid. Adding soda powder to the soaking or cooking water will lead to a similar effect1.
Clay and ash as food ingredients. What kind of creative cooks have walked this planet!
I also came across another potential way to extract tannins from ground acorns: vodka! This is because tannins seem even more soluble in alcohol than in water.
And finally, I found some evidence that fermentation was sometimes used to reduce tannins content in acorns, but we will talk about this next week ;)
Tannins are clearly astringent, bitter, and can also feel acidic.
So, we can use our taste to determine when we reached an optimal amount of tannins. As a reference, we can remember the level of astringency, bitterness, or acidity of black and green tea, red wine, or coffee, which are some of the most common tannic food.
And, after some trials to get used to it, I guess the best is just to stop the leaching or roasting process as soon as we enjoy the taste!
The oldest scientific reference I cite was written in 1952 by Harold E. Driver. It is a free-access and pleasant-to-read anthropology article about the acorn in the North American Indian diet2.
This video shows in a very detailed way how native people of North America were harvesting and storing acorns, and how they were leaching tannins and cooking gruel. It is essentially showing what Harold E. Driver describes in his paper.
In 2020, the two Poland researchers Emilia Szabłowska and Małgorzata Tańska wrote the most comprehensive, clear and well-structured review paper I found about acorn flour production, composition, and use for bread and pastry baking.8
This blog post compiles many acorn recipes in a list.
I am a kind of researcher-explorer. I am French, introverted, and hypersensitive. I value a lot freedom, creativity, and altruism. I am curious about almost anything, but I do have a preference for topics around simple living: permaculture, nature, craftsmanship, autonomy, philosophy, the mysteries of life… More about me and my work >
