Cleaning and preparing acorn

Preparing Acorn
Written By Manzanita Cooperative

This article was adapted with permission from content created by Mighty Wild and updated to include insight from Manzanita Cooperative members and others.

Nut Quality

How do you remove acorn caps from the seed?

For most species of acorn on the west coast, the caps will fall off on their own. Even on species like Gold Cup Oak that have larger caps, the caps will often fall off on their own when the nut is ripe and dry. Tumbling acorns is an efficient way to remove the caps, but for the most part they can be ignored and removed during processing.

How do you ensure acorn quality?

For more information on the major insect and mite pests of oaks, click here.

Knowing the characteristics of each acorn species can help you maintain quality.  One common method that works well across a range of species is called a float test - using a large tub filled with water, pour in the nuts. If it floats, discard it. As a bonus, you can wash your acorns while they’re in the bath. Just be sure to dry them quickly and thoroughly after removing from the water to prevent mold.

Some larger scale processors use a chlorine wash to clean their nuts and remove any mold spores, insect eggs, or other potential contaminants.  You can use pH strips to test the strength of the solution, but the general rule of thumb is one tablespoon of household bleach in one gallon of water is equivalent to 200 ppm of sanitizing solution. Remember to rinse the acorns after this washing step.  There are also various organic food cleaning solutions available. 

More sophisticated methods used in commercial processing include using mechanical cleaning, air pressure, weight differentials, and even optic scanners to remove low quality or rotten nuts and .then heat to rapidly dry.

Whatever process you use, at the end of it you want clean and thoroughly dried nuts. Acorns are around 45% water by weight, so those dried nuts will weigh far less and can keep for up to 10 years if they’re kept cool and dry.

How do you dry acorns?

Drying is the most important step and can happen at several points along the way depending on throughput scale and processing methods. Improper drying can lead to food safety issues or change the organoleptic properties (taste, color, odor, etc.) of the acorn.  The rate and temperature of drying can also affect the nut properties. For example, bur acorns have a thin shell and can crack when drying in-shell if the heat is too high. Mighty Wild had many trials and errors with drying nuts in shell and recommends shelling after float testing rather than drying nuts in shell.  However, large volumes might require this step. Improper drying can lead to post-harvest loss.

In many rural areas, food products are dried in the open air on tarps.  This is how most coffee and spices are dried in Africa, and drying can last only a few sunny days.  However, this method can lead to food quality issues with pests and weather. Pecan and walnut harvesters in the Midwest bag their nuts and leave them in stacks outside.  This method works in cold areas with enough air flow, but acorns have too high of a moisture content to allow for this. You will experience sprouts and moldy acorns if you employ this method.   

Another method is to create a solar dryer.  This is essentially a large solar oven with a fan to heat the nuts, kill any pathogens, and rapidly dry them. Hohenheim University made the following model that is used by Oakmeal in Kea, Greece. 

Mighty Wild used large drying bins with a Sukup heater attachment.  

Freeze drying can also be effective but the equipment is more expensive.

For the home or small scale producer, most of that equipment is out of reach. Simply spreading your acorns on a tarp (so they can be easily poured back into a bag when you’re done), and stirring them around every so often while a wood fire burns nearby will go a long ways. Storing them in burlap and keeping them in a very dry room is a good plan too. You’ll know they’re dry when you shake them and hear the nut meat rattling around inside the shell. As a bonus, having them spread out this way makes it easy to pull out anything moldy or bug eaten by hand.

When are acorns dry?

For most crops, farmers rely on moisture content or relative humidity meters in order to verify when their crops are dry.   In the end, the best test for moisture is to use water activity as the preferred metric.  Mighty Wild used a Meter Group Pawkit for this purpose.

In terms of avoiding mold growth, the water activity level to aim for is lower than .82. For food safety purposes, Dr. Linda Harris recommends less than .7. In order to pinpoint a target water activity for quality purposes, one needs to be determined by correlating taste and texture data with the water activity. One way to do this is to take several nuts at similar high water activities and dry them to different water activities. Then you can correlate the taste and texture characteristics with each given water activity. 

Acorns have a high moisture content, and drying acorns improperly can lead to rancidity and oxidation.  Acorns tend to turn brown very quickly with handling due to oxidation.   Experimentation with drying at different rates and temperatures is key to resolving these sensitivities as there are differences among each species. Get to know your local acorns. 

When are Acorns Ripe?

Most ground fall acorns will be ripe when they drop. For those harvesting from the tree or using shakers, however, Acorns can be safely harvested green. Oakmeal, in Greece, strongly recomends harvesting green as a method to avoid insects, fungus, and other contaminants.

Once harvested, green acorns will ripen on their own. This process typically takes around 6 weeks, but there is no harm in letting them sit longer as acorn will store for years as long as they have not gotten wet. For this reason, we recommend that if you are going to use a float test to sort out bad nuts, do so when they are ready to process and not immediately after harvest.

Further reading:

Sizing, Sorting, and Shelling Acorns

Shelling acorns by hand is one of the most time consuming parts of processing by hand. Acorns have thin flexible shells that resist many commercial shellers and nut crackers. Most people prefer to smash the acorns and take the pieces out of the shell, but this can result in small bits of shell getting mixed into the nut meat which are difficult and time consuming to remove by hand.

Machines are available to crack and split nuts much faster than can be done by hand, but using them will require sorting your acorns by size first since there is significant variability in size even within the same species.

How do you size acorns?

In order for nuts to properly filter through a sheller, it is more efficient to size nuts beforehand.  This allows for uniformity in cracking. There are two types of sizers: automatic and manual. You can make a manual sizer cheaply by nailing different sizes of chicken wire to a wooden frame, pouring the nuts in, and shaking it, allowing the different sizes to fall through. For most home processors this will be the best option 

Automatic sizers can be adopted from the pecan industry.  Different acorn varieties will require different spacers.  Mighty Wild reported that they liked the sizers that had bars rather than holes for filtering the acorns.

What equipment do you use for shelling?

Shelling equipment preference changes by batch size.  Mighty Wild’s favorite hand cracker was the Texan Nut Sheller.  For small batch processing and lab testing they used the Davebilt nutcracker. 

Mighty Wild’s recommended electric cracker was the universal nut cracker.  Oakmeal recommended this Turkish company for de-hulling.  Manzanita Cooperative uses a customized machine manufactured by Jesse Manufacturing in Chico, California.

What equipment do you use for sorting nutmeat?

For small scale home processors, winnowing the shelled nuts is effective and any remaining small bits of shell tend to float to the surface and are easily removed when the nut meat is washed before leaching.

Acorns are very delicate, and at larger scale an aspirator is the best way to separate the shells.  After removing the shells, the separation method used to eliminate out-of-specification nuts (for cracked, moldy, sprouted nuts) in large nut factories is an optical sorter or color sorter machine.  These machines also may have the capability to identify aflatoxins and other mycotoxins using laser technology. Some nut processing facilities like to then size nutmeat by pieces.  We do not recommend using a vibrating nutmeat sizer as this material handling can lead to browning and oxidation of the nutmeat. Any good nut processing facility finishes its manufacturing line with an inspection table on a conveyor belt.  This step is key for discarding pieces of shell and degraded nutmeat. A magnet is also used to remove any metal.

Manzanita Cooperative https://manzanitacooperative.com
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Leaching acorns to make them edible
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Harvesting Acorn