Dry processing encompasses all the cleaning and sorting steps necessary for seeds from dry seeded crops to be ready for storage. That includes threshing (coarse separation of the seeds from the rest of the plant material), winnowing (further separation from other plant material based on weight), and screening (further separation based on size) techniques. In some cases, seeds are sometimes left to dry further between those steps. If so, they should be dried in thin layers (half a centimeter) on a hard, non-sticky surface (plate, tray, plywood, window screen), in a moderately warm and well-aerated environment, out of direct sunlight (as it can detrimentally alter the viability of the seeds and, in some cases, induce dormancy), and stirred regularly to ensure even drying. This process can be speeded up by using fans. A label can be placed over each drying tray in order to remember which seeds are which. Alternatively, seeds can be dried outside, provided they are placed in a shaded location with good air circulation.
Threshing
Threshing aims to separate the seeds from bigger chaff (non-seed material such as leaves, stems, and pods from the seed crop), dirt and stones, as well as facilitate the subsequent separations involved in the next steps when cleaning seeds. There are several different threshing techniques, some presumably more suitable to your situation than others. Common methods used for small-scale seed production include rubbing by hand, stomping or beating, as well as using a vehicle.
While somewhat time-consuming, the first technique is particularly appropriate for breaking seed pods open (wearing gloves might be necessary) as they will easily shatter when fully dried. In cases where bigger batches are to be processed, stomping on top of the seeds and chaff might be more efficient. The plant material is first laid down on a tarp (and covered by another one) or inside a big and shallow bucket, then trampled over with the help of your feet. A vehicle can also be used and driven several times over the tarp. To limit potential damages, the material should be laid down in a thick layer while threshing it by wearing smooth-soled shoes. The tarp can also be put on a softer surface to reduce the chances of damaging the seeds. Note that the pressure exerted should be carefully controlled, as certain seeds are known to be more fragile than others and may be damaged if trampled on or driven over. When using this technique, seeds should be appropriately dry. Seeds that are too dry might crack under pressure, while others that still have a high humidity content will likely be crushed. Putting the plant material in a bag and beating it with a stick is also another method commonly used.
Winnowing
Winnowing is the process used to separate smaller debris and chaff from the seeds. Removing chaff is an important part of the cleaning process as it can easily absorb humidity and carry diseases, which may greatly impact the integrity and longevity of the seeds when stored. Several different winnowing techniques exist, either using wind, fans of all kinds, or by blowing. The seeds and the chaff are usually dropped before a wind source, where the heavier material falls closer to it (in a bucket or bowl) while lighter debris are carried further. While natural wind is the most economical and easiest wind source to use, it is somewhat unpredictable as gusts or changes of direction can easily result in loosing a significant amount of seeds. Regardless of the method used, the area should always be covered with a tarp in order to facilitate the recovery of seeds that are accidentally blown away with the chaff. When the seeds and the chaff are nearly the same weight, winnowing can sometimes be challenging and other processing methods such as screening might be required.
Fans are usually more flexible and efficient, as they can be operated to deliver a constant and stable wind source that can be easily controlled and adjusted if needed. A typical winnowing method consists in using two buckets or bins placed in front of one another and a fan placed right in front. The seeds and the chaff are then slowly dropped in front of the fan, where the heaviest material will fall into the first bin, and the lighter one in the second bin. Most (but not all) of the non-seed material will be lighter than the lightest seeds and will land past the second bin.
When small amounts of seeds are being cleaned, another separation method involving bowls can be used, requiring a small fan or sturdy lungs. The seeds and the chaff are placed in the bowl and swirled around. The chaff and debris will begin to collect on the surface while the seeds remain underneath. The bowl is then tipped, and the chaff blown out. This process is repeated until only clean seeds remain. Another alternative consists in using two bowls, with the seeds poured from one bowl to another. Light chaff will float away from the bowl receiving the seeds. Some practice might be required to determine the correct amount of seeds to put in the bowl and the optimal distance of the wind source from the bowl. The cleaning can be finished by using an air column separator, which will effectively separate the very last pieces of chaff and debris. Such devices are easy to operate and can be built inexpensively using low-tech material. They are especially appropriate for small-scale seed savers and producers.
Note that considering the nature of the winnowing process and the amount – sometimes significant – of chaff involved, wearing a mask might sometimes be necessary in order to keep chaff and dust out of your lungs, especially if done frequently.
Screening
In some cases, seeds might require additional and more suitable cleaning techniques. Screening methods can be used to further separate seeds from non-seed material based on size. They involve the use of several screens and sieves of different sizes and hole types based on the plant material and seeds that need to be processed. Some seeds with specific physical characteristics might require the use of screens with particular holes, such as lettuce seeds which are easier to clean when going through a screen with oblong holes. Screens and sieves specifically designed for seed cleaning can be either bought from specialized vendors or built easily by using wood (for the frame) and wire mesh or hardware cloth with a wide range of different hole types and sizes adapted to your needs and seed types. Most are nestable, which allow them to be put away with ease.
The most common screening technique used is a method called “reverse screening”. A first screen is used, by shaking or rubbing a small batch of plant material against the screen to help with the separation process, large enough to exclude large pieces of debris and chaff while allowing the seeds to pass through. All the remaining material (seeds and smaller chaff) goes under the same process but through a second screen that is smaller than the seeds, which will remove any chaff left.
Sorting
After all the necessary cleaning steps have been completed, the seeds must be properly sorted before going through a final drying process (if required) and stored. That entails discarding any seed that is damaged, stunted, or that exhibits odd colors, as most will likely not germinate well in the field. In some cases, a different color might actually indicate that cross-pollination occurred, therefore compromising the purity of the harvested variety. In doubt and if several seeds are affected, the whole batch may need to be disposed and other batches double-checked. Sorting will obviously be more challenging when smaller seeds (and sometimes bigger batches) are to be processed. No matter what method is used, a small amount of seeds will always be lost. Knowing to what extent you want to sort the seeds is therefore important. It is also a great occasion to further involve your community, neighbors, or friends and work together.
Specific harvesting and processing techniques for each crop are discussed in Section VII.
Header photo credit: Michael Foley (CC BY-NC-ND 2.0)