Wet processing encompasses all the cleaning and sorting steps necessary for seeds from wet seeded crops to be ready for storage. That includes soaking, fermentation, rinsing, and decanting techniques – the latter being important as it will help to separate the viable seeds from the ones that are not. The seeds are usually extracted by scrapping out the inside flesh of the fruit (endocarp) using a spoon. The quality of seeds–especially in cucurbits–can be increased by harvesting the fruit after it reaches edibility and leaving it on the crop for several days or weeks to mature further. This will also soften the endocarp material in the seed cavity where it will easily separate from the seeds upon cleaning. Depending on the circumstances, the fruits might need to be harvested before the seeds are fully mature and stored away in a dry, well-aerated place to protect them against damage or disease. The seeds will then continue to ripen inside the fruits during their storage.
Depending on the crop’s characteristics, your situation, and the volume of seeds that needs to be cleaned, one technique might be more suitable than another. Regardless of the method used, cleaned seeds should always be dried in thin layers (half a centimeter) on a hard, non-sticky surface (plate, tray, plywood or window screen should be preferred, as paper towel, cardboard or cloth will make seeds difficult to remove), 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.
Soaking is an easy cleaning step aiming to loosen the pulp clinging to the seeds. The seeds and pulp are simply placed inside a container filled with water and left to soak for 8 to 12 hours. The pulp will then seem easier to separate from the seeds, which will facilitate any subsequent cleaning steps.
Besides helping to further loosen the pulpy residue from the seeds, fermentation will be particularly useful when cleaning tomato or cucumber seeds as it will remove the germination-inhibiting gel covering them and destroy some potential pathogens. This process is akin to what can be observed in nature where fruits fall on the ground, start to rot and ferment, allowing the seeds inside to germinate (provided the environmental conditions are right).
To ferment, place the seeds and pulp in a jar or container. Water should only be added if the mixture is too thick to stir or not liquid enough as it might potentially encourage sprouting. The jar must then be placed in a warm environment (at around 25-30°C) and left 48 to 72 hours to ferment depending on the temperature and seed variety. The mixture should be stirred two to three times a day, which will aerate it and encourage further the fermentation process. In the event a layer of white mold appears on the surface (a normal and harmless occurrence), it can be stirred back in the mixture. The jar should be closely monitored for any seed sprouting, which might indicate that the seeds have been soaked for too long and potentially damaged. The fermentation process is considered complete when all the gel around the seeds has dissolved. This can be checked throughout the process by removing a small sample of the mixture. When ready, the seeds are then cleaned by decanting or rinsing them.
Seeds can be rinsed by simply using a colander or strainer while passing them under pressurized water. The separation of the pulp can be facilitated by rubbing the seeds against the screen of the colander with the help of your hand. The holes must be large enough for the pulp to go through while retaining the seeds. However, cleaning seeds by only using this technique does not remove non-viable seeds.
Decanting is particularly useful, as it will help to separate the pulp and lightweight, less viable seeds from the good and heavy ones. This process will greatly facilitate the sorting of seeds by removing the ones that will not germinate well. Before decanting, the seeds can be quickly rinsed to remove much of the pulp. To decant, place the seeds and pulp in a jar and filled with four times the volume of the mixture. Stir, then wait for the mixture to settle at the bottom of the jar. Good seeds will tend to sink and stay at the bottom while less viable ones will float on the surface. Those can then be easily removed by using a colander or scooped with your hand, along with any other debris and pulp residue. Stir again several times to ensure that no lightweight seeds are left. When the water if fairly clear, pour the seeds into a strainer and rinse.
Specific harvesting and processing techniques for each crop are discussed in Section VII.
Header photo credit: Chiot’s Run (CC BY-NC 2.0)