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The wonders of silica for seed drying

Sarah Cody
14 May 2014

Following on from the previous post on the importance of seed drying for long-term seed storage, Sarah Cody focuses on the desiccating powers of silica beads.

Curiosities of a seed bank technician’s work bench

John Adams is the man to turn to regarding things of a technical nature. With quite possibly the coolest job in the seed bank, he is responsible for finding and inventing low-tech solutions for seed collecting, processing and storage as well as watching over the thermostat to check the staff don’t freeze and the seeds don’t defrost. His work bench is a medley of both strange and familiar objects. Jars of many different sizes, some empty, some containing seeds or silica, or other indeterminate items, dominate the space, but in among them you will find other curiosities such as a moth catcher and sock darner, as well as things that wouldn’t look amiss in your kitchen, such as cling film and table salt.

Photo of John Adams at his desk

John at his desk

With a desk like that, the man is surely a genius, which is why I frequently seek his advice (and ask him to turn the heating up). Wishing to hear it from the horse’s mouth, and having little patience for reading technical information sheets, this post passes on some of John’s wisdom on low-tech drying solutions.

Why the need for low-tech solutions in the first place?

Leaving aside the Arctic and the Antarctic, plants grow pretty much everywhere on this planet. Due to the changes in land use and climate over the past few hundred years, plants are more threatened than ever and are in need of conservation. Seed banking is one way of protecting plant biodiversity and, owing to the amazing ability of seeds to remain alive until conditions are optimal for growth, seeds can remain viable for hundreds of years when kept dry and frozen. The Millennium Seed Bank has some of the best equipment for cleaning, drying and processing seeds. However, out in the field you may be far away from the nearest seed bank and low-tech solutions are the only option available. In some cases using low-tech solutions are preferred because they are simpler, easier and more cost-effective than faffing about with fancy machines.

Photo of drying seeds with charcoal

Drying seeds with charcoal

There are a few low-tech solutions out there for drying seeds. Some involve charcoal which, after being heated becomes a desiccant (it’s important to let the charcoal cool down first though – roasted seeds are only good for eating!). Others such as dried rice are also effective. Silica beads are a popular choice because some kinds, known as 'indicating silica gel', change colour to show how much moisture they have absorbed from the surrounding atmosphere. The chemical in the indicating silica gel is called methyl violet and is dark green when wet and orange when dry.

Photo of silica sachets

Silica sachets: orange when dry and green when wet

 

Being able to gauge the moisture content of the seeds is really useful as you can determine which seeds are ready for storage and which need more time to dry.

Using silica as a desiccant

The big blue drums used by the Millennium Seed Bank and partners in the Adapting Agriculture to Climate Change project are a low-tech solution which uses silica to dry seeds. These 60-litre capacity containers, along with their contents, are the perfect portable seed drying kit, ideal for when you are out on a collecting trip and do not have the facilities of a seed bank nearby.

Photo of a Low-tech seed drying drum

Low-tech seed drying drum

Starting the drying process early saves seeds from getting mouldy and has also been scientifically shown to prolong the viability of seeds once they go into storage at -20°C. No doubt the long shelf life of many of the seeds at the Millennium Seed Bank can be attributed to proper post-harvest handling and getting the seeds dried to 15% relative humidity as quickly as possible after they have matured.

Now for some technicalities: The blue drums are filled two-tenths of the way with a mix of clear and indicating silica beads. The colour of the indicating beads shows up more clearly when mixed in with clear beads than when used alone. (Incidentally, it also looks prettier.)

Photo of a mix of clear and indicating silica beads

A mix of clear and indicating silica beads: both dry (yellow) and wet (green)

The methyl violet in the indicating silica should be orange to indicate that the silica is dry and that its desiccating powers are at full strength. Next, take your rolled-up mesh and bury the bottom end vertically in the silica. The mesh acts as a support for the seed bags which can be hooked onto it. This arrangement has the advantage that the seeds are kept suspended in the air and can dry more quickly. The seed bags are made from either cloth or paper and are breathable, which is crucial for the silica to do its job effectively. Now all you need to do is put the lid on the drum, seal it tight, have a cup of tea and then continue with your seed collecting. With a target of 20,000 species to be banked by 2020 there’s no time to waste!

After some time (sooner or later depending on how moist the seeds were to begin with) the silica should start to change from orange to green indicating that it has absorbed moisture from the air in the container and therefore from the seeds, leaving them drier than they were at the start. If your seeds are dry enough you can seal them into a foil packet to prevent them from reabsorbing any more moisture from the atmosphere. To start the process again simply dry out the silica in an oven so that it becomes orange again. Typically, the silica will dry the seeds to around 25% relative humidity which is pretty good for a low-tech job. Once they are in a seed bank dry room they will be dried down to 15% relative humidity which is optimal for long-term storage.

A cool low-tech way of knowing when your seeds are dry enough not to go mouldy

Fill a glass jar half-way with a mix of common salt and the seeds you want to test. Leave for 20 minutes and then shake the jar gently. If the seeds are dry the salt will fall to the bottom and if the seeds are wet the salt will stick to the sides of the jar, in which case they will need further drying.

Photo of vials containing salt

The salt test

Using silica to assess the moisture content of the seeds in the vault

The seeds in the -20°C freezer have all gone through the cleaning and drying process and they are sealed in tried and tested air-tight containers to prevent any moisture from coming in. Some seeds are stored in small jars, several of which are housed in bigger jars, which are then kept in even bigger jars (a bit like Russian dolls).

Photo of seeds within jars within jars

Seeds within jars within jars

Not only does this maximise the space for storage but it also means that there is less chance of any moist air getting to the seeds. Each of the jars in the vault has a small sachet of indicating silica gel inside them. When the silica starts to change from orange to dark green it means that the seal has broken and moisture has gotten inside. Bringing the seeds back to the dry room and replacing the seal will rectify the situation and prepare them for the freezer once again. 

Final message

If you’ve read both my posts about seed drying, I hope that message has filtered through loud and clear. For a long shelf-life, keep your seeds dry!

- Sarah -

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Comments

5 June 2014
Comment: 
I think the point of a lot of John's research is that although using the 'perfect' solution or desiccant is ideal, the MSB deals with many people living in areas of the world where, for example, the purchase of calcium chloride salt is either prohibitively expensive or impossible. Try telling a poor African farmer that he must use calcium chloride instead of table salt if he wants to save his seeds. It all sounds great until you leave the world-class laboratory and have to do this stuff in the field with little resources or funding. I've been using silica with orange/green indicator beads for 3 years now at my seed bank and find them to be reliable and regenerate very well. Perhaps it depends on your situation and usage levels. I believe this blog is meant as an 'interest' piece and not a referenced source of information for serious scientific work. I know for a fact that John continuously conducts numerous well-designed scientific tests before making recommendations but unfortunately he almost never finds the time to publish. If you want the details and reasoning behind John's work then find the technical information sheets on seed moisture and low-cost seed drying on the Kew website or contact John directly.
22 May 2014
Comment: 
With mixed feelings I read the interesting article on drying seeds using simple methods. It is perfect to use low cost methods, but they should be appropriate. Here some serious warnings and suggestions. 1. Table salt should never be used as indicator for adequate seed moisture level. The equilibrium relative humidity of NaCl is 75%, which is far too high for maintaining seeds viable. In fact seed biologists studying seed longevity use storage above a saturated NaCl solution to accelerate the ageing process. Calcium chloride (CaCl2) salt is more appropriate for seed drying, since the equilibrium RH above this salt is around 32% at 20 °C. 2. I wonder very much about the drying capacity of fresh charcoal. My gut feeling is that it is very limited especially after a cooling in the open air. 3. Rice kernels indeed can absorb moisture, but that depends very much on their previous storage. If stored at high humidity before, they may in fact result in transfer of water from the rice kernels to the seeds you want to dry further. Both the rice skernels and your seed sample will equilibrate with the air surrounding them. 4. Silica gel is a good desiccant, although it is not as easy with the regeneration, since the blue cobalt indicator is abandoned. The modern orangs/green non-toxic indicator disintegrates when the temperature during regeneration is too high. 5. In my experience zeolite drying beads are the most convenient for seed drying (see for instance https://www.seedtest.org/upload/cms/user/Symposium-June13-0830-2b-Timple.pdf or http://hortcrsp.ucdavis.edu/main/overview_beads.pdf). These are easy to regenerate at 200 °C without the need for a very accurate temperature control and a simple oven can do the job or just in a pan above the fire. The beads may be a little more expensive than silica gel, but it seems they can be regenerated indefinitely, which does not hold for silica gel. 6. It is always important that once you have regenerated (dried) your desiccant, you should keep it dry, by storing under hermetic conditions, e.g. in a glass jar with rubber seal or a jam jar. 7. Even when the suggested simple methods of drying are only used for sampling seeds in the field and temporary storage for a few days or weeks, not enough drying will result in an unnecessary rapid loss of the total defence capacity of the seeds towards oxidation and result in a shortening of the potential longevity. Remember, when for logistic reasons seeds have to be sampled before full maturity they are even more sensitive to deterioration during storage. In conclusion, don’t underestimate the skill of drying seeds to an appropriate level. Ignorance of the importance of the right seed moisture level can result in loss of seed viability. It may look cheap but can be expensive on the long term if the seeds die too fast.
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