The first signs

As early as mid-June, you can see whether our most common oak, the pedunculate oak (Quercus robur), will produce acorns that year. On trees with low-hanging branches, this is easy to observe, but otherwise a pair of binoculars can be helpful for looking up into the canopy. It is a good strategy for planning early which trees to focus on for the harvest.

Oaks have so-called mast years, meaning they do not produce the same yield every year. Instead, they produce very large quantities of acorns in certain years, followed by resting years with little or no production. Many oaks within the same area follow similar cycles, though not all, and the harvest can also be influenced by weather and climate.

Planning the harvest

The most common method is to gather by hand, but there are also various tools available. Different types of nut harvesters work well if the ground is even. Another method is to stretch fine-meshed nets beneath the trees.

When gathering, it is helpful to bring something to carry the acorns in. A fresh acorn of average size weighs about 5–8 grams, so the weight can quickly add up to several kilos, especially if you are using a nut harvester. A good backpack or a shopping trolley works well.

Acorns fall from August to October, but those released early are often infested or damaged. During strong winds, large quantities can fall at once, so it is useful to keep an eye on the weather.

Planning the processing

Depending on how you intend to use acorns, it is important to think through how they will be processed and what you will need. Will they be used right away, or stored for a longer period? Before gathering larger quantities, it is wise to have a plan for how and where they will be stored. Otherwise, they can easily end up in a pile and begin to mold.

Will you have time to shell them while fresh, or will you need a place to spread them out for drying?

What tools will you need? How will you leach them?

Planning storage

Acorns can be stored for more than ten years if handled correctly. At first, they can take up a lot of space, as they need to be dried. This can take several months if you are not using an oven or a drying cabinet.

At the same time, they need to be protected from mice and other animals for whom acorns are high on the menu.

Collecting and storing acorns is an investment in the future.

Are you allowed to gather them?

Yes! Acorns that have fallen from the trees are included in the right of public access (Allemansrätten).

There is a persistent misconception that acorns and nuts on the ground may not be collected. This idea partly comes from older legislation in the Swedish Penal Code, which states that it is not permitted to pick acorns and nuts directly from living plants. This means that you may not take them from trees and shrubs – but the law says nothing about acorns and nuts that have already fallen to the ground.

Despite this, some interpret the rules as if acorns and nuts are not included in the right of public access at all. However, according to the Swedish Environmental Protection Agency (Naturvårdsverket), they may be collected from the ground. More information can be found in this brochure:
https://www.naturvardsverket.se/.../978-91-620-1304-2.pdf...

There may of course be exceptions, for example in nature reserves or similar protected areas.

When can you gather them?

I usually start as soon as the first green acorns begin to fall, from late August. These are often infested or damaged, but many are still usable. Most acorns fall between mid-September and mid-October, once they have matured. Strong winds can bring down large quantities at once, so it is a good idea to head out after a brisk autumn storm. At the same time, strong winds can also cause many acorns to fall earlier than expected.

The best time to gather acorns is probably from mid-September to mid-October. By then, most are in good condition, and not too many leaves have fallen to cover the ground.

It is also possible to gather acorns into early spring, as long as they have not begun to germinate too much.

At first, acorns are green and have a softer shell, but over time they turn brown and harder. The shells are still relatively thin, and can usually be cracked by hand.

What should you gather?

Always choose whole, undamaged acorns. If you see small holes in the shell, it means a larva has developed inside and eaten its way out. Good acorns should feel heavy and solid when you squeeze them – there should be no gap between the shell and the kernel.

Keep in mind, however, that acorns that have been lying on the ground for a long time during dry periods can sometimes feel light or hollow without necessarily being bad. Over time, you will develop a sense for which acorns are worth using.

If you gather later in the season, you may find acorns with cracked shells that have begun to sprout (see image). These are also perfectly usable and a sign that the acorn is healthy.

How should you gather them?

Traditionally, acorns are gathered by hand, but this can be quite time-consuming and physically demanding.

There are various tools you can use, most of which work best on even ground such as lawns. These include rolling collectors or wire nut gatherers (see image). In the United States, there are also other tools available, such as the “Bag-A-Nut.”

It is also possible to spread out or hang collection nets beneath the tree. This is the same technique used when harvesting olives.

Whatever method you use, it is important to have a way to carry the acorns you collect. A backpack, a shopping trolley, or a small cart are all good options.

Fresh acorns

Acorns can remain fresh for several months, depending on weather and climate. Some begin to germinate shortly after falling to the ground, while others may wait for months for the right conditions. When fresh, the kernel is pale yellowish-white. The shells are initially green and soft, but gradually become brown and hard. As long as conditions are cool or moist, they stay fresh, but exposure to wind and sun causes them to dry more quickly. Once germination begins, the kernel often takes on a reddish-pink tone and gradually becomes more woody.

Fresh acorns differ from dried ones, both in texture and in how they can be used. Depending on how you leach them, you can achieve different properties. If you slice them thinly and cold-leach them, you can, through steaming, produce a texture similar to “glutinous rice.” If you leach them by boiling, they become more buttery – a texture that is harder to achieve with dried acorns.

To keep acorns fresh for longer, they should be stored in a cool place.

Dried acorns

Drying acorns before use has many advantages. The most important is that you don’t have to rush to shell and process them right away. Dried acorns can also be stored for several years without molding or rotting.

Drying makes them much easier to shell, as the kernel shrinks and separates from the thin shell. Often, it is enough to press them between your fingers to crack them open. They are also easier to transport, as they weigh less and take up less space than fresh acorns.

The main challenge is that they become very hard. Before they can be ground into flour, they need to be softened through soaking, which can take up to a day. Using warm water makes the process significantly faster.

Coarsely ground

I mainly use coarsely ground acorn flour to improve the texture of food, as it provides a pleasant bite. To create thin shreds or small pieces, I use a rotary grater, an almond mill, or similar tools.

This coarse grind works well in everything from patties, porridge, and mince to kebab and baked goods. It can also be roasted to create crisp, crunchy bits. The possibilities are many!

Finely ground

You can obtain finely ground flour by milling coarsely ground acorns in a proper grain mill. It is of course possible to use something like a coffee grinder, but it is difficult to achieve the same level of fineness, which can be important for drinks.

I use finely ground flour in baking, pancakes, sauces, beverages, and more.

Starch

The starch is extracted from the flour through cold leaching.

With the starch, you can make noodles, tofu, papadums, and more. It can also be used effectively in batter for frying, where it works similarly to corn starch. I also use it as a binding agent in dishes such as patties, or as a thickener in sauces.

1. Surface moisture and dirt

The first step is to clean and wash the acorns to remove debris, dirt, and mold spores. Since I rarely gather acorns by hand, I often end up with quite a bit of material such as twigs, leaves, acorn caps, and other debris.

The simplest way to sort this out is to submerge everything in water, which separates the acorns from the debris. Fresh acorns will sink to the bottom, while most of the debris floats to the surface and can easily be removed with a colander. Some acorns may also float, which is usually because they are damaged, infested, or have already begun to dry out. It is best to carry out this step as soon as possible after gathering. If the acorns have dried too much, they will float, making the cleaning process significantly more difficult.

There is a common rule of thumb that all acorns that float are bad, while those that sink are good. This is not entirely accurate. It depends greatly on the circumstances – for example, when in the season they were gathered, how long they have been lying on the ground, and whether conditions have been dry or moist.

2. Drying step 1 

After the water bath, I spread the acorns out on a blanket, a sheet, or another piece of fabric. I wrap them up and roll them around to remove most of the surface moisture. At the same time, any remaining dirt is rubbed off. I also take the opportunity to remove acorns that are too small or appear damaged.

3. Pests

There are pests that can infest acorns. The most common is the acorn weevil. It is a small insect that, in early summer, drills a tiny hole into newly formed acorns and lays an egg inside. This egg then develops into a small larva.

The acorns that fall early – from mid-August to mid-September – are often infested, which is why the oak tends to shed them first. Once the acorn hits the ground, this triggers the larva to begin chewing its way out. This process can take anywhere from a few hours up to about a week.

After emerging, the larva looks for a place to pupate. In nature, it buries itself in the soil, but if it is indoors, it may crawl around in search of a suitable spot. So if you are drying acorns and want to avoid larvae ending up everywhere, it is a good idea to limit their movement by creating edges they cannot cross – for example, by placing the acorns in a baking tray.

Acorn weevil larvae consist almost entirely of acorn material, so they are not as unpleasant as they might seem.

4. Drying step 2

There are different ways to prepare acorns for long-term storage. Traditionally, they were dried in the sun or roasted before being stored in granaries, clay vessels, or similar containers.

I live in a small apartment and therefore don’t have much space. For that reason, I pre-dry the acorns in the oven at 60–75°C (140–165°F) with the door slightly ajar for a couple of hours. Some of the acorns may crack open, but that doesn’t matter. The important thing at this stage is to remove most of the moisture that has been absorbed into the shell.

5. Drying step 3

Even if the acorns are dry on the outside, the kernel still contains moisture, and it can take several months for it to become fully dry through air drying. If you use a drying cabinet or similar, the process can of course be faster. There are various machines and techniques that can speed up the drying process.

I usually use drying racks in the form of stacked frames. These are simple and inexpensive to build yourself. It is important to leave space between the frames and to use a breathable base so that moisture can evaporate more easily. Good materials for the base include jute fabric or other materials that can absorb moisture.

When the next season approaches, I transfer the fully dried acorns to other storage containers, where they can be stored more densely.

6. Leave them undisturbed

Even if the acorns appear intact, there are microscopic openings where the cap was attached. This is often where moisture slowly escapes, but also where mold spores can take hold.

Closed acorns that have not yet fully dried can easily become a breeding ground for mold. When such acorns are moved around, any spores present may spread to others. For this reason, it is important to leave them undisturbed until the kernel is completely dry.

If you shake an acorn and feel that the kernel has loosened from the shell, it is well on its way to drying. You can always crack a few acorns open to check how far along they are in the drying process.

7. Long-term storage

Dried acorns can be stored for many years if handled properly. However, it is important that they are thoroughly dried, both on the outside and inside, before being gathered together in baskets, storage boxes, or similar containers. Depending on where you plan to store them, it is also important to keep in mind that rodents love acorns.

After that, they are ready to be used as needed.

8. Other considerations

It is also possible to shell the acorns and store the dried kernels, but to my knowledge there is limited information on how this may affect shelf life and nutritional value. This practice has been used by some Indigenous peoples in North America.

The advantage is that they take up less space.

Grater mill

An almond mill was the first mill I acquired when I began working with acorns. It is a very inexpensive and simple tool. It works perfectly for this purpose, and it is easy to find second-hand. The main drawback is that it is a bit small if you need to process larger quantities. There are also larger, modern grater mills available.

The almond mill produces thin shreds, similar to grated cheese. This size is very suitable for quickly and easily leaching out the tannins. The result is light and airy, which allows water to pass through more easily than if the acorns were finely ground.

When the acorn kernels are soft, they are easy to process with this type of mill. Acorns are soft when they are fresh, or after soaking dried kernels in water for a couple of days.

Grain mill

To obtain a truly fine flour, you need a grain mill. A spice or coffee grinder produces a coarser result, and it is difficult to achieve a fine flour even if you grind it several times.

Crusher mill

If you want to process dried kernels without softening them first, you need a robust mill that can crush the rock-hard kernels into smaller pieces.

Nut sheller

Even though it is fairly easy to shell our native acorns by hand, it is still time-consuming.

For that reason, I use a Davebilt. It is made by a small family-run company in California. The machine was originally designed for shelling walnuts, but since it is adjustable, it works well with most types of nuts.

I like the Davebilt because it has a large hopper, so you don’t have to refill it constantly. It is adjustable, well-designed, and above all, durable.

Cold leaching

Filtration (coarsely ground), approx. 3 hours

This method involves placing coarsely ground acorn meal in a filter and letting cold water run through it – much like brewing coffee, but without heat. For this, the acorns need to be grated, for example with an almond mill, into thin pieces with a large surface area. This method also makes it easy to collect the starch.

Water bath (larger pieces), 1–2 months

If you are not in a hurry and want to leach larger pieces, you can place them in a water bath for an extended period. It can take one to two months for whole acorn halves to fully lose their tannins. Change the water a few times a week.

Streams, lakes, etc.

Traditionally, flowing water and lakes have been used for leaching, where whole acorns were submerged for several months. Grinding the acorns significantly speeds up the process. Acorns were also sometimes buried in clay near rivers and streams, where they gradually leached over time.

Jar method, 1–2 weeks

This method involves leaching acorn meal in cold water for one to two weeks. The meal is stored in a jar of water, which is changed regularly. By turning the jar occasionally, the tannins are drawn out efficiently, while nutrients and starch are retained.

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Hot leaching

Simmering, 1–3 hours

By placing shelled acorns in simmering water, the tannins are removed more quickly than with cold leaching. However, this method requires more energy and breaks down the starch. To effectively leach out the tannins, the water also needs to be replaced at regular intervals, about every 15 minutes. To make this process smoother, it is common to use two pots and alternate between them.

A useful tip is to taste the leaching water after each change – this helps you learn to recognize the difference in astringency between each batch. When the water is no longer bitter, the acorns are fully leached.

Keep in mind that the steam condensation also carries tannins, so ensure good ventilation to avoid staining.

Steaming, 1–3 hours

This is a gentler method that works best with thin slices of acorn. Keep in mind that condensation also carries tannins, so make sure the area is well ventilated to prevent discoloration.

Other tips

Ash

The ash method involves adding wood ash to the water when leaching acorns. The ash raises the pH of the water, which helps break down the tannins more quickly and reduces bitterness effectively. Traditionally, clean wood ash was used when boiling acorns to speed up the leaching process.

Roasting

Roasting is used to improve the flavor of acorns and make them easier to shell. Heat can reduce the astringency of the tannins somewhat, but roasting does not remove them completely. For this reason, roasting is mainly used after leaching or in combination with other methods.

What do you need?

The method is simple and similar to brewing coffee. All you need is a container with holes in the bottom – for example, a pot – and a fine-meshed cloth or filter. I use linen fabric. And of course, you will need water.

If you want to collect the starch, you will need an additional container without holes, placed beneath the first one. Since starch, like tannins, is also dissolved by water, some of it will be carried along with the liquid.

Coarsely ground acorns

To make the leaching process as efficient as possible, the acorn meal should be airy, with thin pieces that allow water to penetrate throughout. The best option is to use an almond mill or another type of grater that produces thin shreds (similar to grated cheese).

If the meal is too finely ground, the water will not pass through as easily, and the leaching process will take much longer. If it is too coarse (thicker pieces), it will also take longer for the tannins to be leached out.

Leaching

The acorn meal is placed in the linen cloth inside the container. As the meal absorbs water, I usually press it down firmly toward the bottom. It is best to keep the water flow as low as possible, but it is important that all the meal becomes thoroughly moistened. I make sure the meal remains constantly submerged beneath a thin layer of water. Only a minimal flow is needed for the leaching process to work. If the hole in the container is too large, the flow may become too fast.

Some of the starch that leaches out will also help slow the flow through the cloth as the process continues.

Make sure to taste the meal during the process – this will help you learn when the tannins have been sufficiently leached out. When the astringency or bitterness is no longer noticeable, most of the tannins are gone. The process usually takes about three hours.

I often leave it under a gently running tap overnight.

Pressing

Once you consider the meal fully leached, gather the cloth into a bag and press out as much water as possible. The cloudy white substance in the water is starch that has been carried along.

Since starch is heavier than water, it will settle at the bottom after a while, making it possible to collect and use it.

Drying

Once you have pressed out as much liquid as possible, it is time to either dry or freeze the meal, unless you plan to use it right away.

I usually dry the coarsely ground meal in the oven at a low temperature, around 40–50°C (104–122°F). You can stir it occasionally to ensure that all surfaces are exposed. Be very careful to make sure it is completely dry before storing it in sealed containers.

Collecting the starch

The starch can be collected during cold leaching. When I use this method, I often place a bucket underneath to catch the leaching water. In the example shown, I use a bucket with a lid that has a hole in it. On top of the lid, I place a pot with a hole, allowing the water to drip straight down into the bucket. Inside the pot, I secure a piece of linen cloth where I place the acorn meal.

The starch that leaches out will settle at the bottom of the bucket, since it is heavier than water. The tannins remain in the water above, as they are lighter and stay dissolved.

Pressing

Once you consider the meal fully leached, gather the cloth into a bag and press out as much water as possible. The cloudy white substance in the water is starch that has been carried along.

If you want to extract as much starch as possible, you can dip the bag back into the leaching water and continue pressing it out into the bucket until no more cloudy liquid appears. Then leave the bucket to stand for several hours, allowing the starch to settle at the bottom.

Pouring off and drying

Once the starch has settled, it forms a thick paste at the bottom, and the water can be carefully poured off. Transfer the paste to a tray or a bowl with a flat base and let it spread into a thin layer. If it is too thick, you can add a little water and whisk it to make it easier to pour.

After that, let the starch air dry or dry it in the oven at a low temperature (no more than 40°C / 104°F). The starch will then harden into brittle flakes.

Grinding and storage

Once the starch has dried, you can grind it or crush it into a powder using a mortar. It can also be stored as flakes. Be sure that all the starch is completely dry.

A useful tip is not to store it in a fully sealed container right away – leave the lid slightly open or check on it from time to time.

Tannins

Also known as tannic acid, traditionally used primarily for tanning leather. Tannins are also used for flavoring and for clarifying beverages.

Dye

It is possible to extract a dye from acorns with a golden-brown tone. If iron is added during boiling, the color turns black.

Animal feed

Acorns have been used, and are still used, as animal feed in various parts of the world. Iberian pigs are perhaps the most well-known example.

Oil

Acorns from certain oak species contain significant amounts of oil that can be extracted.