Holm oak acorns (Quercus ilex)
Foraging and leaching acorns
It is fairly widely known that acorns, the nuts of oak trees, are inedible freshly picked. But things do not have to stay that way. The reason that they are too bitter to eat is because they have high levels of tannins, but tannins (witness a cup of tea) will leach into water. It takes a lot of soaking and is pretty labour intensive, but it is still possible to get a hold of some of that most elusive of foraged foods – a staple quantity of starchy carbs. God forbid that you’ve ever need to do it, but the joy of really getting engaged with where your food comes from is enough reason to want to.
Foraging: In Australia, the northern hemisphere’s oak trees are mostly found as plantings in larger parks and gardens. Near me, there are several hundred holm oaks (Quercus ilex) in Centennial Park, just a few kilometres from the Sydney CBD. The acorns can be collected at any time after they have started naturally falling from the oaks. You can gather them off the ground (as fresh looking as possible), but if there is even the tiniest hole in the shell (pericarp), leave it, because it is probably the entry or exit hole of a weevil or other grub that will already have messed things up inside. You can also pick acorns off the tree if they are fully plump and come easily from the cupule (the characteristic little hat that acorns wear). Late April to early May is the best time for me, and it will be a similar season elsewhere in the southern hemisphere. If you get frosts or snow, it should be possible to collect off the ground for a bit longer, even as just-sprouted nuts in spring, but in frost-free Sydney they are mostly ruined by insects and rot pretty quickly.
Shelling and cleaning: You can either get started on the processing straight away or store them somewhere free of damp for up to a month, although you will get more spoiled ones the longer you wait. Freezing is fine and will stop any hatching grubs in their tracks as well as supposedly have the benefit of making the tannin-rich skins come off easier. Once they are older you can crack the shell a little to get the meat out, but when they are fresh, I find that a sharp knife works best. I top and tail them, run a slit down the shell and can then easily peel it off (check out the video). You can then scrape off some of the tannin-rich skin, particularly if you are going to go straight to a quicker hot water leaching, or you can work it off later during the soaking stage. Pop them in water to stop them oxidizing. They will remain in water, changed many times, until you use or dry them.
There are various methods for leaching the tannins out.
Cold leaching: This can take a month of more of storing the acorns, whole or chopped, in a container with a decent volume of water, which you change at least daily. Before you do the water change get your hands in there and give the all the nuts a hearty rub against each other. Particularly if you didn’t scrape the skins off, this will help dislodge them bit by bit so they can be washed away with the water you pour off.
In many places, this leaching was traditionally done by suspending acorns in a flowing stream. For at least some of my leaching, I use a rather clever equivalent of putting them in a mesh bag and hang them in the cistern of the toilet – every time we flush, we change the water. Take them out every week or less to check on them and give them and the cistern a clean. While they are out, give them a few hard rubs together followed by a water change, again, particularly if you have left the papery skins on. A fair warning with the cistern method is that the tannins may stain the toilet bowl. If your cistern does not drain completely, this method may not work because it might allow a population of yeast and bacteria to persist and work away at your harvest. If you are concerned about this, move to a hot leach, even if it is just one sterilising one.
You can test for doneness first of all by the water running clear and then by having a little nibble. If they still have too much tannin, you will know it. A properly leached acorn tastes a little nutty but otherwise pretty bland.
Hot leaching: This is the quick(er) method, or the alternative if you either get sick of waiting for the cold water way to work or feel like they are starting to decay while still being too tannin-rich. The down side is the loss of more oils and some of the flavour. Put them in a pot, fill it up and bring in to the boil. Then drain it off, give them a rinse and do it all over again until either the water stays clear or the acorns taste fine. If you stop between boils, leave them in a fresh change of water – this will also get dark and should be changed too.
Paste leaching: There is one other method that I haven’t tried where you process them into a paste (see here); then put it in a huge jar with a lot of water, shake it all up and put it in the fridge; the flour is supposed to settle completely, allowing you to pour your tannin water off to repeat the process until the water stays clear and the paste tastes ok (said to happen within a week). I can see how it would give a better product (retaining some oils and flavour lost in hot water) but also how it might lead to losing some flour in the water changes.
Grinding: Once the acorns are leached, you could either blend it into a nutty soup or sauce or dry them and grind it into a flour. I use a dehydrator, but a very low oven or the sun will also work. Don’t go so dry that they become little rocks that will be too hard to grind – you want it so it will grind into a flour rather than a paste. I have heard that coffee grinders are good for getting a reasonably fine flour and I have found that some, but not all, food processors do a decent but not perfect job. Because I have a flour mill (unusual though that may be) I use that. Because acorn has no gluten and my favoured flatbread or pasta recipes require a pliable dough, I mix it 50:50 with spelt or wheat grain when I grind it. This also deals with the warning I have had that oily or too-moist acorns might gunk up the grinding stones, and I have not had any trouble yet.
Cooking: While using it as a replacement for chestnut or even almond meal is possible, acorn flour is usually going in a recipe as a replacement for normal wheat flour, with the proportion depending on how much the recipe needs wheat’s gluten. With that in mind, then consider that acorn flour will bring something earthy and nutty to the dish and think of something from there.
For sauce/soup/gravy thickening, you can go 100% with acorn flour; think earthy contexts, like a game meat pie gravy or a mushroom soup.
With flatbreads, shortbread and pancakes you can go to 50%; maybe only 1/3 acorn if you want a flatbread to be pliable enough to fold.
For pasta, any more than 25% and the risk of it falling apart goes up as well.
For an airy, leavened bread, where wheat or other grain’s gluten governs success or failure (in a normal person’s kitchen), acorn flour has very little place at all.
If searching online for acorn flour recipes - Hank Shaw has some great acorn recipes at honest-food.net.