The wonderful acorn

Did you know the acorn was a staple in the diet of early man? Paul Butler, Forestry Liaison Officer, tells us more and highlights why there really is a lot to be said for planting a few oak trees every year.

Every few years in autumn, the oak woods of Ireland produce enormous quantities of acorns. Of course, the trees do this in order to reproduce. And, from some of these tiny acorns, mighty oak trees do grow - if they encounter the right conditions.

Only a very low proportion actually make it to germination stage however. This is because the acorns are a very palatable and nutritious source of food for a wide variety of animals. In Ireland for example, they are eaten by squirrels and deer. In mainland European countries, they are a major autumn food for wild boar and for domestic pigs destined for high end “Jamon Iberico de Bellota” bacon.  

In order to ensure that at least some acorns survive to germinate and eventually grow, the strategy of the oak tree is to produce an enormous crop of acorns, so many that the chances of at least some surviving are reasonably good. This is very costly to the tree in terms of its biological resources, so it can’t produce this big crop every year. Instead, there are often several quiet years with few acorns produced followed by a so-called mast year, when the oak trees all over the country, or certainly by regions, produce this bumper crop.

Photo credit: Alicia Dempsey, Category Winner of the Teagasc Forest Photo Competition 2023

Human nutrition

As well as being important to certain forest animals, these mast years were, in the past, important to humans too. Acorns have been used as a food source for millennia and on a good mast year, humans were able to make and store flour from the acorns and indeed store the acorns themselves. The acorns had the great advantage that they matured and fell during a short time window, making it easy to collect large quantities. They were also relatively easy to store and process into usable food. 

This food source was so important that there is an idea that the spread of acorns northwards in Europe, after the ice ages, was facilitated by humans who wished to keep these valuable trees in their new settlements and so brought seeds, or maybe even plants, with them on their pioneering journeys into new areas.

And acorns were used to make flour when crops such as wheat or barley failed. For example, a cookbook published in Italy in 1549 outlines a recipe for acorn pie. It is estimated that the diet of early Californians was around 50% acorns and the annual harvest probably exceeded the modern harvest of sweet corn.

In the book Stalking the Wild Asparagus, Euell Gibbons explained: “To primitive man … acorns were often the ‘staff of life’. If we consider the whole sweep of his existence on earth, it seems likely that mankind has consumed many millions of tons more of acorns than he has of the cereal grains, which made their appearance only during the comparatively recent development of agriculture. It seems a pity that the food which nourished the childhood of our race is today nearly everywhere neglected and despised.”

Nutritional characteristics

There are about 500 species of oak around the word. Some produce acorns that are immediately edible by humans, in fact some species are even sweet and tasty.

Our European oaks tend to be a bit bitter, perhaps with the exception of Quercus cerris and Quercus ilex. It is important to note that raw acorns contain tannins, which can be toxic to humans and cause an unpleasant bitter taste. They can also be poisonous to horses, cattle and dogs. But by leaching acorns (thoroughly and carefully) to remove the tannin, they can be made safe for human consumption.

In Ireland, all of the acorns produced by our trees are bitter. This bitterness needs to be leached from the fruits using water. The bitterness is caused by tannins and these of course had their own uses in the process of making leather, for example. Adding small amounts of wood ash or earth to the ground-up acorns also reduced the effect of the tannins.

Once safely treated, the acorns can be used to make bread and also in soups and stews. Acorns from certain species are very tasty roasted over a fire.

Acorn flour contains on average 59% starch, 33% fat, of which over 80% is unsaturated, and approximately 8% protein. In addition, acorn flour contains a considerable amount of electrolytes (calcium, magnesium, potassium and phosphorus), but little or no sodium, and is rich in iron, copper and zinc.

Acorns, including those from Quercus robur, have been used to make coffee. The acorns are roughly ground and roasted.

Another interesting product is acorn oil, extracted from the acorns by pressing or crushing, and has been used as a cooking oil in Algeria and Morocco for example. Some varieties are said to produce 30% oil, which is comparable with the yield from olives.

Of course, there are not as many oak trees as in the past, so if you are collecting acorns, be careful to just collect what you need and leave most where they are for the wildlife.

Additional benefits

So much for the acorns! If we look at the timber, we see more good things - oak timber is one of the best in the world for furniture, veneer and roofing. Oak logs are used in the production of shitake mushrooms. Oak is also an excellent wildlife tree, for example, an Irish oak is associated with supporting over 300 different insect species. Really, there is a lot to be said for planting a few oak trees every year.

Also read: The Woodland Improvement Scheme - supporting sustainable woodland enhancement

Also read: Diary Date: Talking Timber 2024