A Primer on Desiccation and Cryopreservation (a.k.a. Storing Seeds)

I would like to take a brief break from energy issues for a moment to discuss a different topic: the science of freezing plant seeds to preserve them for the future. The cultivars of produce that we purchase in the supermarket are typically pretty bland, especially if they have to be shipped by reefer truck from California or worse, Chile. On the other hand, I can buy heritage vegetables from my local farmer's market, figure out which varieties I like, and seed them. This isn't possible for all varieties (i.e. root vegetables like carrots) but the results can be impressive. The other goal here is to get a good yield, so that most of your seeds sprout, by following good practices.

Most of this comes from knowledge I gleaned from a graduate-level mechanical engineering course I took on cryogenics. (Note: cryogenics is the science of liquefaction and refrigeration at temperatures far below zero; cryopreservation is the science of freezing biological tissues with minimal damage; cryonics is freezing dead people's heads. Can you spot the quack?) This is not to say that I have years of experience successfully freezing seeds. This is simply some of my scientific knowledge of how one should approach the problem. I had a yield of 7.5 sprouts from 9 seeds planted from a heritage tomato I bought and seeded.

Cryopreservation can be used to freeze (small) whole organisms — I have read in books on cryopreservation that commercial goldfish can be frozen by liquid nitrogen, shipped by air from China, and defrosted. About half live, the rest die if you look at them wrong when you take them home. Cold-water fish are probably uniquely well suited to protection from freezing damage, since they have natural protective agents.

The basic problem with freezing tissue is that: 1.) water increases in volume when it freezes, and 2.) water forms potentially sharp crystals (dendrites) when it freezes. If an ice crystal punctures a cell wall, it tends to cause irreparable damage and the cell will lyse (die) upon defrosting.

It takes a great deal of energy to thaw ice — the equivalent of heating water by 80 °C, actually. Freezing ice requires taking away the same amount of energy. When water freezes, the process always starts at some local density variation (homogenous nucleation) or some feature such as a protein (heterogenous nucleation). When you freeze slowly only a few nucleation sites form and then the bulk of the water amalgamates onto existing crystals. This leads to a relatively small number of large crystals. Faster freezing encourages more nucleation sites to form, so the end product is many, small crystals. Hence the plunge into liquid nitrogen as the basis of most cryopreservation techniques.

There are two basic methods of cryopreservation: replacing the water with another fluid or solution (such as ethylene glycol) which vitrifies (freezes as an amorphous glass) rather than forms crystals, or dehydrate and freeze. Since I'm talking about plants here, we're going to ignore the vitrification process, since it is technically much more challenging and impractical outside of a laboratory.

The approach then, is to dehydrate the seeds before freezing. This acts to increase the concentration of solutes (sugar, etc.) in the cytoplasm which in turn tends to inhibit the formation of large ice crystals. It reduces the likelihood that the cell walls won't be burst when the water expands as it freezes. Aside: a lot of the literature on cryopreservation discusses the concept that intracellular ice observed in a cell tends to imply that the cell is not going survive thawing. Functionally, what this really means is that if ice crystals large enough to observe with an optical microscope form inside the cell, then the cell will probably die.

Most people recommend drying seeds before storage. This page from Colorado State states that most seeds should be reduced to 8 % moisture content before storage. The typical process is to air dry over a week or two. If you live in a wet climate or are impatient I would think, however, that it is safe to use a food dehydrator to speed up the process. The keys would be don't allow the temperature to rise too high such that it denatures protein (< 45 °C to be safe) and don't overdo it. Plants can tolerate greater dehydration than animals due to the cellulose in their cell walls but you can still kill them. Unfortunately it's quite difficult to assess the moisture content of a seed. Also note the discussion on 'hard seed' at the above link.

After dehydration, you want to put the seeds into a dry environment so that they don't try and germinate. For this, you need an air-tight box (a desiccator) and some material that strongly adsorbs water (a desiccant). For home use, a hundred-dollar laboratory desiccation cabinet is overkill. A glass mason jar with a rubber seal and a tight clamping mechanism should work fine. You may want to grease the rubber gasket with a silicone grease so that it remains pliable in the freezer. When greasing a gasket, you want to work the grease into the material, and then wipe it clean with a paper towel until it no longer feels tacky. Avoid greases with petroleum (i.e. Vaseline) in them as they will break down rubber polymers. Ideally you should also wear latex or nitrile gloves to protect the gasket from the oil on your hands but that would be overkill for non-vacuum applications.

All you need then is a desiccant to put into your cheap desiccator before you pop it into the freezer. You've probably seen a desiccant before in a pill bottle marked "Silica gel - Do Not Eat."
Desiccants are hydroscopic (they adsorb water very readily) so they will basically suck all the water out of the atmosphere of your jar. Personally I would probably want to buy a cartridge (such as this one from Fisher Scientific) for convenience but their are likely cheaper suppliers. Most desiccants can be regenerated by putting them into an oven and baking them above the boiling point of water for awhile. This will drive away the water they have adsorbed onto their surface. Keep in mind that if you leave them exposed to atmosphere for very long they will fully adsorb and no longer fulfill their function.

As I mentioned previously, one of the primary damage mechanisms in cryopreservation is the sintering of ice crystals during thawing. Hence repeatedly defrosting and re-freezing is very harmful and likely to reduce your yields significantly. As such, you need to store them in a deep freeze or freezer with no automatic defrost cycle. Incidentally, this is why stuff stored in a 'frost-free' freezer tends to turn into mush after enough time.

Commercial cryopreservation processes often use microwaves to defrost material quickly. I wouldn't recommend this for home use, however. Home microwaves are generally too powerful and will cook the seeds. Turning down the power won't help, as microwaves simply run 1/10th the time when set to 10 % power. The obvious solution would be to soak them in tepid water. This will unfreeze them fast and start the rehydration process as well, encouraging them to germinate.

If you observe the above steps, and remember to sterilize your soil mix in the oven before you plant your seeds, I think you'll be pleasantly surprised at just how many of your seeds germinate.