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[Cerere]

Hi,
This is my first post here, and I wanted to start off by sharing some information that I recently researched about seed drying. I will not go into too much detail and I will share the resources at the bottom. It is not meant to be a tutorial, only to give a general idea about the topic.

For most seeds, proper drying can ensure the viability of the seed for a long period of time. To ensure maximum vitality, it is crucial that seeds be dried at a constant temperature and humidity. This is because the seed must not dry too much (controlled humidity prevents this) or too quickly (controlled temperature). Seeds are permeable to moisture in the air. If the air outside is drier than the inside of the seed, the water will diffuse out of the seed onto the surface and evaporate, until the moisture level equilibrates with the outside. While normal air drying does work, a more controlled drying process can ensure a higher germination rate once the seeds are planted after being stored for a period of time. While constant temperature (at the level that is needed, usually around 20 °C) isn't easy to cheaply achieve at home, a somewhat controlled level of humidity can be achieved with very little.

A solution is saturated when there is undissolved solute on the bottom. Saturated solution of salts (such as table salt), acids (sulphuric acid) and bases (lye) have the peculiar property of maintaining a constant humidity over the surface of the liquid. It is therefore possible to create a desiccator (or buy one, they are usually sold to keep humidity out of wardrobes), basically an enclosed space containing a saturated salt solution, that can keep moisture levels constant. On a rack standing just above the water level, the seeds will be placed to dry, and the lid of the desiccator closed. As the seed dries, the salt solution will absorb water and part of the solid on the bottom will dissolve. The advantage of using a saturated salt solution instead of a common desiccating salt without the water (epsom salt, silica gel, calcium chloride) is that the seed will not dry completely. While using saturated salt solutions, there is no risk of leaving the seed in the desiccator too long, as the seed will dry until it equilibrates with the humidity outside. Since the humidity outside of the seed (inside of the desiccator) is bigger than 0% RH, the seed will not dry to the point of becoming non-viable.

Now on to the main choices when it comes to preparing your saturated salt solution (SSS). Keep in mind that as the temperature varies the humidity provided by the SSS also varies, increasing along with the temperature. I will give some reference values for temperatures around 20-25 °C.
The number one choice, the gold standard, is lithium chloride. It provides a RH of around 13% and is ideal even for seeds that will be stored below 0 °C. The downside, except for the dangers that come from handling it improperly, is its price. Lithium is a rare metal, and lithium chloride is the most expensive salt on this list. On the other hand, unless a large quantity of seeds is being dried, a little amount of lithium chloride is necessary, and it can also be reused by evaporating the water on a stove.
The second choice, which comes closest to lithium, is calcium bromide, with a RH of 17,6%. It is also useful for super-cold storage, but for some reason not too easy to find. It is cheaper and safer to handle than lithium chloride, and the humidity provided is acceptable.
My third choice is calcium chloride, which is by far the cheapest and easiest to find. It is already part of most chemical desiccators you can find for wardrobes and similar. It is not too toxic and apart from being mildly caustic it doesn't pose any significant health hazard. Alas, it produces a RH around 28%, at which the seeds at equilibrium still have too much moisture for super-cold drying. If dried like this though, they should still last several years if preserved in cold storage above 0 °C, definitely more than enough unless you are planning to pass the seeds on to your nephews.

Something I would like to mention before I include some links, is that another choice worth exploring is sodium hydroxide (lye). It's a quite dangerous and corrosive chemical, but it is cheap and can be found easily enough. A saturated solution of sodium hydroxide will provide a surface RH of around 7%. This might be a bit too low for some seeds, but by adding water to dissolve all of the solid (therefore making the solution no longer saturated) higher levels of RH can be achieved. In the links I have included a study about the RH levels of lye solutions at different concentrations, though I highly suggest using a probe hygrometer to measure the humidity inside the desiccator. If a saturated solution isn't used, humidity levels will fluctuate and a larger amount of solution relative to the amount of seeds to be dried will be needed to maintain a somewhat constant RH without having to make constant adjustments to the concentration.

It is necessary to periodically weigh the seeds to monitor the drying process. Once the weight of the seeds, usually after a couple weeks, remains constant, it means the seeds have dried (or to be correct, have reached equilibrium) and are ready to be put into storage.

References:
http://brahmsonline.kew.org/Content/Projects/msbp/resources/Training/09-Seed-air-moisture-relations.pdf
https://cropgenebank.sgrp.cgiar.org/images/file/learning_space/genebankmanual8.pdf

3 Likes sapla, jbz711 and RatsboggleBiological like this.

[sapla]

very interesting, thank you
these explanations are clear.
how to be sure, without material, that the seeds are dry enough to go to the freezer without them damage and be sure of their viability in 15 or 20 years?

weigh the seeds seems complicate cause even if their weight does not vary, their humidity may not be suitable for freezing

Bye

[Cerere]

The traditional way to do it would be to completely dehydrate a portion of seeds from the same sample (like in an oven) to determine how much moisture they contain by weighing them before and after the dehydration. For smaller amounts where a destructive method cannot be employed and averages are less accurate, a hygrometer can be used to measure equilibrium relative humidity of the seeds, which can then be used to obtain an approximation of the seed moisture content through a seed moisture sorption isotherm (basically a graph that correlates the two parameters). Here you will find a more complete explanation:
http://brahmsonline.kew.org/msbp/Training/Resources

For practical purposes though, and especially when it comes to dehydrating seeds on a smaller scale, knowing this data serves little purpose. Eventually one will still dry the seeds over saturated salt solutions until weight equilibrium is reached, and that should generally be good enough

[RatsboggleBiological]

This is a fascinating thread. I am very interested in finding out how to better preserve my seed collection, besides regenerating them whenever I can flower them. I have a lot to learn.