By Eric Garza
December 4, 2019
Acorns were an important food source for people around the world prior to the rise of industrial agriculture, though few people eat them today. One reason for this is that acorns are not edible straight from the tree. Their high tannin content gives many raw acorns an intensely bitter taste and astringent mouthfeel, making them unpleasant to eat and toxic if eaten in large quantities. With thoughtful processing enough of these tannins can be removed to yield a food that is nutritious and calorie dense. In this essay I follow 100 northern red oak (Quercus rubra) acorns through the process of gathering, shelling, grinding, and leaching I use to turn them into a delicious and nourishing flour.
Before I share details about the process, I first want to make the case that, despite how complicated processing acorns for food might seem, it is absolutely a worthwhile endeavor. To challenge common prejudices against foods that require multi-stage processing, I will offer data on the amount of acorn flour I get relative to the processing time, and on the energetic return I achieve with this food. I have not seen this data presented elsewhere, so hope it proves interesting to casual readers and researchers alike.
Taking 100 acorns gathered near my home from whole, unshelled nuts to finished flour required 92 minutes of labor. I spent 5 minutes gathering the acorns, 20 minutes shelling them, and 27 minutes grinding the dry nuts into flour. Leaching the ground acorns and straining the leached flour required 14 and 15 minutes, respectively, with the remaining time allotted among smaller tasks. This labor was invested over about 5 weeks, since some steps require waiting periods between them. The acorns weighed 539 grams (19 ounces) when gathered, and after water and material losses are accounted for yielded 130 grams (4.6 oz) of finished flour, or just over 1.5 cups. The following graphic illustrates weight losses throughout processing.
The 130 grams of flour I ended up with might seem a paltry yield for 92 minutes of effort, but we need to be careful what we compare this figure to. How much time would you invest planting, tending, harvesting, and processing enough wheat or corn to yield that much finished flour or meal, start to finish, if you did it all yourself rather than outsource parts of the process to other people, or to machines? I suspect it would take significantly longer to grow and process either of these cereal grains, and the end product would be nutritionally inferior.
Beyond time efficiency, realize that acorn flour is very calorie dense. It offers over 500 kilocalories of bioavailable energy per 100 grams, which is significantly more than the 332 kilocalories provided by wheat flour or the 364 kilocalories from corn meal. Acorn flour is more calorie dense because most of its calories come from fat, a macronutrient more than twice as energy dense as the carbohydrates and protein that provide most of the calories in wheat and corn.
Some readers are familiar with the idea of return on investment from finance. This framework is useful when studying the bioenergetic value of different foods. If I were to invest more energy while gathering and processing a food than I get back when I eat it, I am operating at a calorie deficit. This is fine over the short term or for particular foods that provide rare nutrients, but if my diet fails to yield a positive energy return over an extended period of time I will lose weight and eventually starve.
Estimating the amount of bioavailable energy I get from processing 100 red oak acorns is fairly easy. Using the calorie density of acorn flour I noted earlier, 130 grams of it translates to about 650 kilocalories. Assuming my body burns 2500 kilocalories per day (reasonable given my age, weight, and activity level), that averages to 1.7 kilocalories per minute. Applying this figure to the time I invested processing the 100 acorns yields 160 kilocalories of energy input. All told, I get 4 times as much bioavailable energy from eating acorn flour as I invest processing it. Had I chosen to gather data while processing a few thousand acorns instead of only 100, the resulting energy return would have been higher owing to efficiencies of scale. Acorns’ calorie density and solid energy return are no doubt why they served as dietary staples for many people around the world.
With my case for processing acorns made, I will explain the steps I use to gather and make them edible. I gather only acorns that have fallen from the tree, as those still attached are not yet mature. I avoid gathering smaller acorns with caps still on them, which were shed prematurely because of defects, and those discolored by fungal infection. I finally avoid gathering acorns with holes in them, which suggest the nut has been nibbled on by various larval insects. Since it takes the same amount of time to shell a large acorn as it does a smaller one, I preferentially gather larger acorns to maximize their return on investment.
After gathering comes drying, which makes acorns easier to shell. I dry acorns by spreading them out over a large tarp on my screened-in front porch where there is good air circulation and where they are protected from marauding squirrels. Be forewarned that even perfect looking acorns can still contain insect larvae that will bore out to continue their life cycle. For this reason, I do not recommend drying acorns inside your house lest you end up with little white larvae crawling over your floor.
I typically dry acorns for 2-3 weeks before shelling if my goal is to make them into flour, though you can dry them longer without doing harm. I shell the dried acorns by placing their pointy end on a solid piece of wood—I use an old cutting board—and tapping the rounded end with a small hammer. A modest tap is generally enough to split the shell so I can remove the nut. I know it is possible to shell acorns more quickly with commercial nut crackers, but those are out of my price range.
Once the acorns have been shelled, I dry the nuts on a towel for 7-10 days. This additional drying makes it easier to grind them into flour without clogging up the corn grinding mill that I use. Sending the dried acorn nuts through this mill yields a fairly uniform meal, and a finer flour by sending it through a second time.
After the acorns have been ground, the next step is to leach out the tannins. I leach acorns after grinding because ground material offers more surface area from which tannins can dissolve. To leach ground acorns, I fill half-gallon jars 3/4 full of clean, alkaline water before adding enough flour to fill the bottom third of the jar once it settles. I stress the importance of using alkaline water because acidic or even neutral water does not dissolve tannins well, so unless you make your water alkaline the leaching process will take much longer. The easiest way I have found to make water alkaline is to add enough baking soda to bring the pH up to about 8. You can easily measure your water’s pH using strips available at most pharmacies or grocery stores.
This method of removing tannins is often referred to as cold-leaching to differentiate it from hot-leaching, which involves boiling the acorns. I cold-leach exclusively because I do not want to lose or damage any of the fat or starches present in the acorns. My cold-leaching regimen takes place at room temperature, and I change the water in the jars twice each day, once in the morning when I wake up and once in the evening before I go to bed. I determine when the flour is ready by tasting it. I find this method yields palatable flour from red oak acorns after about 5 or 6 water changes over 3 or 4 days. I have heard of people leaching acorn flour for weeks to make it palatable, and suspect they were using water that was too acidic to effectively dissolve tannins, prolonging the process unnecessarily.
Once the bitterness and astringency associated with the tannins are gone, I use a few layers of cheese cloth to strain the flour before sprinkling it over wax paper to air dry. Since the fat content of red oaks is quite high, the flour will go rancid if left at room temperature for too long. I generally store acorn flour in an airtight container in the freezer.
While the above description of acorn processing might seem onerous, it is actually quite easy once you get the hang of it. It is also quite efficient, in terms of the energy return it yields, as I noted earlier. I hope the data I presented will compel readers to try their hand at processing acorns, even if the process seems complex. Acorns are a winner, even if some effort is needed to leach their tannins and render them edible.
I am grateful for the oak trees, first and foremost, for providing such an amazing food source and for making me work for it. I am also grateful to Arthur Haines, a botanist in Maine, whose video on acorn processing originally inspired me to investigate acorns as a source of food. Finally, I am grateful for my patrons on Patreon who support my creative pursuits financially, including my writing. In return for their support they can listen to audio versions of my essays, and download nicely formatted PDF versions.