Acorns: Harvesting & A Brown Butter Tart Recipe
Our #EatNZKaitaki Adam Harrison is a chef and forager based in Ōtautahi, where he currently sells foraged produce to top NZ restaurants. He shares his thoughts about Acorns & a delicious acorn recipe!
History of Acorns
Acorn have been an integral part of the “human food story” for millennia, developing a process by which to turn the bitter nut of the oak tree into a viable and nutritious food staple was one of the greater revelations of mankind, the original processed food.
Harvesting acorn has been a tradition worldwide wherever oak trees grow, the name acorn comes from an old Norse name ‘Akran’ meaning fruit, originally the mast of any forest tree, gradually restricted to the most important of the forest offerings.
In Europe as civilization coincided with agriculture, and acorns were merely considered a food for swine, was born jamón ibérico de bellota what is now considered to be the finest ham in the world. The ancient oak pastures of Spain and the humble acorn once again highly regarded in culture as part of a romantic food relationship.
The North Americas referred to it as “The grain of the tree”, prior to the 1600’s indigenous tribes would have relied upon the acorn as a staple food source. The harvest of acorns would have also been shared by many animals of the wild lands that depended upon this food source as well, attracting pigeons, ducks, small mammals, rodents, and larger mammals such as deer to the same ground for their feed. The native Americans living as part of the landscape would have taken the opportunity to secure meat as well as gathering acorns in these areas establishing a deep connection to the acorn and oak tree.
In other places around the world, countries have flirted with the use of acorns in times of need, during famine and drought as the crops failed, during war as supplies were cut off. As more desired crops grew abundant for the rich, the acorn became known as peasant food, but in 1549 they made it for the first time into a cookbook, being fed to kings and nobles. Cooked with game meats and spices to differentiate from the poor with ingredients they could not afford.
Nowadays they are also consumed in various forms, although on a minimal scale. In Algeria and Morocco, acorn oil is produced. In Europe, Asia, North Africa, the Mid-East, and North America they are also consumed directly in such products as bread, cake, ice cream, desserts and liqueurs and coffee. Still popular in Korean food today in Dotoriguksu, is a noodle soup made from Acorn flour.
Abundance & food security mast years
The average oak tree starts acorn production at around 25 years, it will continue to increase acorn production for the next 75 years of its life before plateauing off around 100 years old at an average of about 2,500 acorns per year or between 225kg and 450kg. Incredibly, the average natural lifespan of a white oak is 550 years old.
An average person consumes 60kg of flour per year That means a single oak tree that produces 225kg of acorns could technically provide 3 people with all the flour they’d need for an entire year.
Not all these acorns reach full maturity and immature acorn are cast off with the cup still attached if they are attacked by the acorn weevil or disease.
Oak trees produce acorns every year in mast cycles. The mast cycles of the oak tree are not fully understood but it is though that by producing large crops around every 5 years is a strategy to control the acorn weevil population and overwhelm any animals that may eat the acorns with untold abundance to ensure some I left behind to grow.
Nutrition, starch & tannins
Starchy foods are our main source of carbohydrate and play an important role in a healthy diet, and are the most challenging thing to find in a wild food diet. They are also a good source of energy and the main source of a range of nutrients in our diet.
Acorns are a powerhouse of nutrition, mostly constituting of complex carbohydrates stored starch, complete proteins, and omega -3 fatty acids, vitamins B6, Potassium Manganese, Iron, Copper, fibre and biologically active antioxidant.
Starch has a great importance when it comes to baking and creating food. Starch assists with texture, viscosity, gel formation, adhesion, binding, moisture retention and can be used as a fat substitute. It also works as an emulsifier, stabilizer, and a clouding or glazing agent.
Starch in acorn start to gelatinize at 55°C, this giving two options for processing, a cold water leeching that keeps the starch intact and useable or a hot water leeching method the binds starches.
Starch gelatinization is the physical process in which starches and the presence of excess water are subjected to heat, causing the starch granules to swell and bind, as a result, the water is gradually absorbed in an irreversible manner.
Unfortunately, most acorns contain high levels of tannins that make them inedible in their raw. Tannins are less of a taste and more of a “mouthfeel” and it’ll make your mouth pucker and feel dry. Tannic acid is a bitter-tasting, water soluble polyphenol, which also happens to be an antinutrient. Tannins have been shown to impair the digestion of various nutrients by decreasing the efficiency with which the body can derive benefit from other bioavailable substances. For this reason, we have to leech out the tannins to unluck the rest of the nutritional value the acorn has to offer.
Finding your oak tree
Oaks, which all belong to the genus Quercus, have roughly 435 species found today on five continents.
All oak tree leaves are lobed, but the lobes of the red oak leaves tend to be much sharper and more pronounced. The margins of white oak leaves tend to be lobed in a much more rounded fashion. Acorns of the white oak produce caps that are bristled, while red oak caps have scales. Red oak acorns tend to be more round and squat, while white oak nuts are often oblong and somewhat football shaped.
It is said that acorns from red oaks contain more tannins than white oak, requiring more leeching.
Acorn Weevil
The most common infestation of the acorn crop is the acorn weevil. The females of these insects bore small holes in the immature nuts where they lay their eggs. When the eggs hatch, the larvae feed on the nut inside, which destroys it, then they burrow their way out the acorn and exit through a tiny hole that looks like someone has drilled into the acorn leaving it hollow in the centre. This is where the weevil made its exit from the acorn and left behind only a shell full of its excrement. This causes the tree to reject the acorn and drop it from the tree early and still immature cap attached.
Gathering
Acorns begin to drop around April and will fall in waves across the season. The first wave will be the tree’s castoffs, those affected my weevils or disease. Ignore these, there will be others to collect. It’s the second and third waves that will really be the harvest.
You will naturally want to choose the largest possible acorns to gather so you have less shelling to do.
Red oak acorns do not begin to sprout until the following spring and can be gathered late into winter. White oak acorns, on the other hand, begin their life cycle as soon as they fall, sprouting and sending down roots within a week of falling from the tree, shortening the harvest season substantially.
The best way I’ve found to gather acorns is to simply pick a nice dry, sunny day as soon as the acorns begin to drop and take a basket, stroll around under the oak trees and as you pick the acorn up from the ground.
Processing & leaching methods
It takes effort to process the unpalatable, tannin rich oak nuts into a flavour-state that can be used in the kitchen, but the effort is worth it. In order to make acorns edible and palatable, it is essential that you leach out the tannins. The process of leeching is one where the acorn is submerged in either boiling or cold water and the nut swells and the soluble tannins are removed.
First, sort your acorns by placing them in water. The acorns that float to the top should be discarded, because their lightness indicates that the nut inside is damaged. The next step is to dry the acorn while it is still in the the shell. This shrinks the nut slightly, pulling it away from the shell and making the acorn easier to crack later.
Shelling acorns can be done tediously by hand, with a hammer or pair of nut crackers. The shell is pliable and quite thin. All it takes is to grasp them with a pair of nut crackers and give a good squeeze or gentle tap with the hammer, you don’t want to mash the nut meat to much at this stage. Simply crack the shell, then peel it off and toss the nutmeat into a bowl.
The process of hot leaching removes a lot of the water-soluble vitamins and nutrients from the acorns, but not the fat-soluble ones. Cold leaching removes fewer nutrients.
Cold Water Leaching
There is no single right way to cold leach acorns. Cold water leaching is the lengthier process done over a 10-14 day period and a lot of water changes or a constant running water which yields acorns with un-bound starches that are good for making flour.
The trick to effectively leeching the tannins with the cold water method is break the nut meat up into small enough chunks that will increase the surface area of the acorns allowing the nut meat to swell and the soluble tannins be removed but not fine enough that the nut meat gets washed away with the water changes.
If you are fortune enough to live next to a running stream with clean, uncontaminated water, you’ve got the perfect setup. Simply break down the acorns in small chunks then put the mash into a pillowcase. Place them in a section of the stream with gentle flow and tie them into a secure position. After a week, take a small nibble of an acorn to see if it’s still bitter. Leave the acorns in the stream until they no longer have a bitter flavour.
If leaving all your hard work and time commitment in the middle of a stream isn’t your thing, I totally get that and there is a second option. Place your acorn mash plus fresh cold water into a large food-grade container. Using 3 time the volume of water to nut meat will speed up the leeching process. A minimum of twice each day, strain them and dump the water refilling with fresh water each time. Continue this process until the water no longer turns yellow and them mash no longer has a bitter taste.
When you are happy that the tannins have been removed, it is time to dry the nut meat out. Sun is hard to find at this time of year so I find it best to spread it out on trays. Then either in a dehydrator or in an oven on the lowest setting until the nut meat is totally dry and no longer sticks together. You must keep the temperature below 55° to avoid cooking the starch.
Once cooled it can be placed in a food processor and ground down to a fine flour.
Hot Water Leaching
The hot water process will leach the tannins faster and can be done on a stovetop. First, break up the nut meat in the same fashion as for the cold water leeching to increase the surface area.
Then, place the mash in water at a 1 to 5 ratio and bring to a simmer for 30 minutes. Replace the water after each boil. At the 3-hour mark, you can start testing the acorns by nibbling them until the nutmeat is palatable and nutty, not bitter. You’ll need to simmer for several hours with many water changes, exactly how long depends on high in tannins the acorns you gathered are.
Once you’ve fully leached the acorns, strain and rinse them. The nutmeat can then be fully dried in a warm oven or dehydrator for long-term preservation.
The actual roasting time depends on your tastes, and just like when you’re roasting coffee. At a minimum, they need to be roasted until they’re fully dry and becomes brittle dry. Further roasting is all about flavour, for my acorns, that means about 35 minutes at 180°. Giving a flavour profile that has a strong nutty flavour with hits of caramel and bitter coffee
Once cooled it can be placed in a food processor and ground down to a course flour. I like to keep this flour course to use in coffee machines or filters as a coffee substitute.
Storing your harvest
Tannins are nature preservative, so fully drying your acorns a single layer deep will allow them to be stored for long periods of time, months, or even years at room temperature if you don’t have time to process them straight away.
Once you have prepared your acorns in flour, no matter which method you used. Given the varying fat content in different types of acorns, there’s no way to say for certain how long your acorn flour will last before going bad. The higher the fat content, the shorter the shelf life. Store your acorn flour in an airtight container in a dry cupboard if you feel you will use it in a short time frame. Otherwise store large quantity in the freezer to ensure all your labour isn’t in vain and the acorn flour doesn’t go rancid.
RECIPE
Acorn and brown butter tart
1 block Sweet Pastry
250g Acorn flour
350g Butter
360g Caster sugar
45g of Flour
1 pinch Sea salt
12 Egg whites
Roll the pastry out to 5mm thick to line a 20cm tart ring, then place back into the fridge to chill once again.
Heat the oven to 160°. Line the chilled pastry case with baking paper, then fill with baking beans. Bake for 15-20 minutes, until a light golden colour is achieved, then remove the beans and paper.
Place the butter in a medium saucepan over a low heat and allow to melt completely. Continue to cook the butter over a higher heat until it turns a nutty golden brown, then remove from the heat and strain through a fine sieve into a bowl. Leave to cool.
While the brown butter cools, combine the sugar, acorn flour, salt and plain flour in a separate bowl. Add the egg whites and stir until incorporated.
Weigh out 250g of the cooled brown butter, then add this to the mixture, stirring well to ensure no lumps remain. Cover and chill in the fridge.
Pour the filling into the tart case, smoothing the top if needed, then bake for 25 30 minutes, until set and deep golden on top. Remove from the oven and allow to cool.
See more from Adam over on @mypatchofdirt.
This is a post in collaboration with #FOODDIVERSITYDAY taking place on Jan 13th in the UK. Learn more here.