Zeolite vs Silica Gel: Deathmatch

zeoliteThe Horticulture Collaborative Research Support Program at UCDavis has a nice factsheet out about Zeolite Desiccant Beads. Why?

Zeolite beads, used with airtight containers, are a simple, inexpensive and widely adaptable method for drying horticultural seeds and maintaining high seed‐quality during storage. The beads can be reused by baking between use.

And of course we know that’s important:

In tropical climates, high humidity causes rapid seed deterioration, resulting in poor stand establishment, lower productivity, less value and disincentive to invest in improved seeds.

Although farmers seem to be the clients here, I thought perhaps this might be a good, relatively low-cost solution for genebanks too, so I ran the factsheet past some seed experts at Kew and IRRI. Thanks to both of them for allowing me to quote them.

It turned out that Fiona Hay, formerly at the Millennium Seed Bank at Kew and now at IRRI, has had quite a lot of experience with zeolite.

Indeed, we have done some work on these zeolite (=molecular sieve) drying beads on rice … in collaboration with the company, Rhino Research, in Thailand that is marketing them (and holds the patent — I’m not sure in which countries). See attachment..

Yes, they are a good desiccator, my concerns are that they could be too good and that they don’t appear to work as described — they don’t take up the same amount of water from the seeds as they do when they are placed over water. This means that is isn’t obvious how to calculate the right quantity of beads to use to dry seeds to a required moisture content. This is based on our work on rice (three different fresh seed lots), but seems to be at odds with what Kent Bradford (UC-Davis) has found for horticultural crops.

In terms of their use by farmers — I don’t think this technology is what they need. If the HORTCRSP project helps them to understand the need to dry seeds, OK; but there may be cheaper, simpler options.

They could be of more use in a genebank situation — once we know how to use them optimally. We are doing more work on this. One of Rhino’s latest products using the beads are bins containing a core of beads which is in contact with some indicating silica gel. Seeds are put in the bin and the silica is used to know when to regenerate the beads. This could be useful for genebanks without proper drying and/or storage facilities. I’d like to get hold of a couple of these to try them out.

Robin Probert at the Millennium Seed Bank then added:

What annoys me most about the USAID fact sheet promotion of Zeolite beads is that it brags the value of Zeolite beads over silica gel for small farmers drying seeds for sowing. We have known for decades that the problem facing local farmers is the rapid loss in viability that can occur if seeds remain at high ambient relative humidity combined with warm temperatures. We also know that if farmers were able to dry seeds from say 80% equilibrium relative humidity (eRH) to below 50% eRH, seed longevity would be improved by several fold. This could mean the difference between seeds surviving for only a few months to a few years.

The fact sheet boldly states (with a nice graph to make the point) that ‘Zeolite beads are more effective than silica gel in absorbing water at low relative humidity’. But this could be written another way: ‘silica gel is more effective than Zeolite beads in absorbing water at high humidities’. Fiona’s work published in Seed Science and Technology last year [Fig 5 in Hay et al (2012) SS&T 40, 374-395] elegantly confirms this.

What this means is that a farmer would need less silica gel than Zeolite beads to dry seeds from ambient humidity to a safe moisture content for short-medium term storage (≤ 50% eRH). But what about cost? The USAID leaflet states that Zeolite beads can be bought for 10-20 US $ per Kg. We buy silica gel beads that we use in our drying drums designed for small-scale seed drying for less than 10 US $ a Kg.

Using Zeolite beads to dry seeds down to very low moisture contents for long-term storage is a different matter and as Fiona’s paper demonstrates, Zeolite beads may have it over silica gel for this purpose. However, as the paper also points out, calculating the weight of Zeolite beads needed is not straightforward and compared to silica gel there is a much greater risk of over drying.

All in all, I know where my money is.

So it turns out that, on balance, according to these experts at any rate, the Zeolite beads may actually be more promising as a solution for resource-strapped genebanks around the world than for seed-saving farmers in the humid tropics. Which was, however, presumably not the aim of the USAID-supported project that came up with that factsheet. But let me tweet this to HortCRSP and see what they say. Stay tuned…

5 Replies to “Zeolite vs Silica Gel: Deathmatch”

  1. Hi Luigi,
    Thanks for looking into Horticulture CRSP’s work with drying beads. I’m glad you came across our fact sheet and had more questions to ask–that’s what we hope these simple flyers do. There is a lot more information about our team’s work with drying beads here and here. The drying beads team also has its own info-filled site. These projects are led by Kent Bradford and Peetambar Dahal of UC Davis with Johan Van Asbrouck of Rhino Research as one of the partners, along with collaborators in Thailand, Nepal, Bangladesh, India and Kenya.

    I asked Bradford and Dahal to respond to some of the more technical aspects of this post and here is what they had to say:

    We would somewhat take issue with the title of your blog post: “Zeolite vs Silica Gel: Deathmatch.” Clearly, the objective is to preserve seeds and commodities in humid climates for the benefit of farmers, and there are a number of ways that this can be achieved. Both silica gel and zeolites have their relative advantages and disadvantages, which I will discuss below, so it should not be considered as a “deathmatch” between them, or between us and our seed science colleagues. It should also be noted that our project is targeted primarily at horticultural seeds, which are produced and stored in much lower quantities than agronomic seeds, and their value per unit weight or volume is much greater. Thus, economic arguments about the costs and benefits of using zeolite beads or other drying methods need to be crop-specific.

    Addressing first the comments of Fiona Hay, it is not quite correct to say that the beads do not work as described. In fact, when the dry beads are enclosed in a hermetic container with moist seeds, they will reduce the ambient relative humidity to very low levels, and will absorb water that evaporates from the seeds under these conditions, lowering their moisture content. This is what they are claimed to do, and they are very effective at doing it. Her comment that they absorb somewhat different amounts of water when enclosed with seeds versus when they are incubated over water is correct, as Figure 6 in her publication shows, and as we have also observed in our laboratory. However, these discrepancies occur in the RH range where the highest-affinity binding sites in the beads have already been saturated, as shown by the isotherms in her Figure 6. That is, they occur when limiting amounts of beads have been added to an excess of seeds and the primary bead binding sites are saturated. The secondary binding sites in the beads (above about 5-10% RH) are then in competition with those in the seeds for remaining water in the closed system, whereas when beads are placed over salt solutions, the water in the air is able to be replenished from the solutions as it is absorbed by the beads. We are still learning about the sorption properties of the beads once their tight binding capacity is saturated, but this does not mean that their ability to absorb up to 12% of their weight in water even below 5% RH (actually about 17.5% in more recent formulations of the beads) is not performing as described. It is actually a simple matter to calibrate this binding capacity based on the isotherms. If Fiona had used those bead capacity values from her Figure 6 (e.g., 12%) rather than one determined over water (17.5%), the discrepancies between her predicted and actual seed moisture content values (Figs. 4, 5 in her paper) would largely disappear.

    We have developed a spreadsheet that makes it easy to calculate the bead:seed ratios needed to dry seeds from a specific starting moisture content (or equilibrium RH) to a desired final moisture content (available here), and have confirmed its accuracy for seeds of several horticultural species when accurate bead capacity values are used.

    We also note that drying beads can dry seeds more rapidly to low humidities than can silica gel. Independent studies by John McShane of Stover Seed Co., Los Angeles, CA, found that “Silica gel took about 28 days to reduce MC of Zoysia seeds from 10% to 5.2% whereas beads achieved the same in 7 days.” The rapid moisture intake by beads at lower humidity enables a user to dry more batches of seeds and get more rapid turnover of the beads than with silica gel.

    Regarding Fiona’s comments that there may be cheaper, simpler solutions for farmers, we would note that silica gel has been around for a long time, but has not been taken up for this purpose except by specialists. I will address its drying properties below in response to Robin Probert’s comments, but one characteristic that is not mentioned is that with repeated use (absorption and redrying), silica gel loses some of its water-absorbing capacity with each cycle. Thus, over time it becomes less effective and eventually must be replaced. According to our present information, drying beads do not lose drying capacity with repeated use. Thus, while drying beads initially cost more, they may be more economical over time when the replacement costs for silica gel are included. In addition, as with any new product, the cost of zeolite beads can be expected to drop if use and production expand. A current advantage of silica gel over drying beads is that a lower temperature is required to reactivate silica gel compared to the beads. This is related to the tighter binding of water by the beads, but we are also working with the bead manufacturers to try to lower this reactivation temperature. At present, drying beads are probably economical only for high-value vegetable seeds or germplasm repositories, but there are simple ways to utilize drying beads in forced-air drying systems that could be adaptable to larger seed or commodity volumes, such as in combination with large plastic grain bags that are coming into use.

    Robin Probert argues that it is only necessary to dry seeds to 50% RH to extend longevity and that since silica gel can absorb more water at high humidities, it would be better than drying beads for this purpose. While drying to equilibrium with 50% RH is certainly better than 80% RH, it is not low enough to prevent all storage losses. Robin is correct that these relationships have been known for a long time, as the diagram below is based on one published by Eric Roberts in 1972. While 50% RH will prevent most fungi and mites from growing, insects can still grow and reproduce at those moisture levels. That is why we believe that drying beads offer new opportunities to easily dry seeds to these lower RH/MC levels where all of the storage pests are suppressed, as well as extending seed longevity. We have published some preliminary data for both seed longevity and insect suppression for seeds stored at low moisture contents with drying beads (Kunusoth et al., 2012, Seed Times 5(2): 33-38).

    Both Fiona and Robin mention the possibility of overdrying seeds when using drying beads. While there have been vigorous debates on this point, there is clear evidence that very low moisture contents (i.e., at RH values of 10% or lower) can extend seed longevity. It is also clear that for some larger seeds such as beans or peas, rapid water uptake by very dry seeds can cause imbibitional damage. This is why we include a caution in all of our educational materials that such seeds should be rehydrated in ambient air for a few days prior to planting, which is sufficient in most humid climates to increase seed moisture content to a safe level for imbibition. For small vegetable seeds (e.g., lettuce, tomato, pepper, onion, melon), we have specifically tested whether drying over excess beads results in imbibitional damage, and we have not found any. We have taken seeds from equilibration over excess beads and placed them directly in water, with no adverse effects on germination. One has to ask whether much of the literature on the dangers of overdrying is associated with the conditions normally used to dry seeds to these low moisture levels (i.e., extended times in heated air) rather than the moisture content per se. The ability of drying beads to achieve very low seed moisture contents (hardly possible with silica gel) without heat is one of their more interesting features.

    We believe that drying beads should be of particular interest for seed banks in developing countries where power outages make continuous operation of refrigeration and dehumidification equipment problematic. We expect that drying beads combined with hermetic containers could largely replace dehumidification of large storage rooms in seed banks. Enclosing a small quantity of beads in foil packet or glass jar with a seed sample would be a great solution for local seed banks that may not have the drying equipment that Robin mentioned. For example, we are working with the ECHO program seed bank operations in Thailand, which also was supported indirectly by Horticulture CRSP. Refrigeration is always advantageous, but reducing seed moisture content is the most critical factor for extending storage life, and could largely replace the need for cold storage for practical medium-term storage needs such as for local community seed systems or for breeders’ or foundation seed.

    We hope that this provides further information for you and your readers. The Horticulture CRSP teams are still doing research related to using drying beads for drying and storage of various commodities by different types of users. But the results we have found so far continue to encourage further trials and research. We think pursuing these questions is fruitful and will help us find ways to further improve horticultural seed systems.

    Kent J. Bradford
    Peetambar Dahal
    (and Brenda Dawson)

    Kunusoth, K., Dahal, P., Van Asbrouck, J. V. and Bradford, K.J. 2012. New technology for postharvest drying and storage of seeds. Seed Times 5(2): 33-38. The National Seed Association of India, New Delhi.

    Roberts, E.H. 1972. Viability of Seeds. Chapman and Hall Ltd., Syracuse, NY, pp. 14-58.

  2. In our lab at Wageningen University and Research centre, we have achieved positive experiences with the zeolite drying beads, which were provided for free to us by Rhino Research. We see a good potential for their application by genebanks, but in our communications always warn to the risk of ultra-dry storage when an over capacity of beads is used, although that risk may also be present with application of a large amount of silica gel.

    We agree with the findings from Fiona Hay (see previous comments) that the drying capacity of the beads is not as yet described on the supplier’s web site, since that protocol includes in the capacity also the weak binding by the beads. This was also confirmed in the answer by Kent Bradford. Kent suggests using an excess of the beads to compensate for that when he recommends the use of the beads for genebank seed storage. It is especially in relation to their potential use by genebanks and the issue of drying too much that I want to give a warning.

    There has been lengthy discussion in the genebank and seed science community on yes-or-no risks of ultra-dry seed storage. Some research showed deleterious effects, other (including our team) showed a neutral effect. This dispute was settled when it was shown that it is the (relatively unlimited) presence of oxygen during storage that is accelerating oxidation under very dry conditions – a phenomenon that is also known in food science. Ultra-dry storage in hermetically-sealed containers, with the inclusion of a low amount of air (and an impermeability of the containers for oxygen) provides no problem. Late Prof. Gomez-Campo stored Brassica seeds ultra-dry in heat-sealed glass vials after replacement of the air by carbon dioxide. He observed that there was no measurable loss in quality after about 40 years of storage. So, ultra-dry storage is no problem when oxygen is very limited or removed. That warning should be provided when the drying beads are recommended for use by gene banks, even if the same is the case for the use of silica-gel. For the relative short-term storage of farm-saved seeds, as in the Hort-CRSP project, the negative effect will likely be limited and the potential risks will be far less compared those of present storage practice, at higher moisture contents. For those farmers, keep the message simple: over-drying is better than not enough drying. But the genebank community can understand more difficult messages and needs a warning regarding the risks of over-drying. Although it might seem that the risks of ultra-dry storage in the presence of oxygen seems more a long-term risk (and therefore sure for genebanks), we have also observed additional deteriorating effects on ultra-dry stored seeds with storing primed celery seeds for three weeks in the presence of an overcapacity of beads. Priming is a kind of pre-germination treatment to fasten germination and uniformity in the field, during the priming germination processes are initiated and part of the protection is removed. We consider this additional damage, above that as a result from storing in air without beads, due to the over-drying and not to the beads, since no deterioration was observed when the overcapacity of beads was combined with an oxygen absorber. Presently we are working on a manuscript including the results of those experiments, so I cannot go into too much detail, although they are known by Rhino Research.

    The primed celery seeds are reputed for their short shelf life and may be unusual in their very high sensitivity, but genebanks store many different species and it cannot be excluded that some other seeds are also rather sensitive for this. Moreover, genebank storage is aimed on long term storage and with several species sensitivity has been shown during storage for more than one or two years.

    My advice to the supplier of the drying beads, is to provide customers, especially genebanks, with this warning. The solution is very simple, add a package of iron powder as is used in the food industry, they cost only a few cents and are available for various volumes.

    We find working with the round and hard drying beads easier than silica gel, with its tendency to crumble. Also we do not have so many problems with the drying (regeneration) of the beads, although it is carried out at a higher temperature than for silica gel. With the orange-green silica gel, we once had a problem that we dried it at too high a temperature, causing the destruction of the colour indicator. The blue silica gel did not have that problem, but we are no longer allowed to use that, because of cobalt toxicity. We don’t know about problems with loss of capacity after frequent regeneration of silica gel. Silica gel is used in the ‘drying wheel’ of our controlled humidity cabinet, where the same package of silica gel absorbed moisture for many years, while frequently being regenerated by heating.

    Our conclusion from the experiments we have performed is that the drying beads can be an alternative to silica gel. The more they are tested the more information will become available about their advantages and disadvantages compared with silica gel. We consider them safe for use by gene banks when the oxygen is removed from the storage atmosphere. We also think they can be of great value in drying during germplasm collection missions.

    Steven Groot

  3. I am a rice seed researcher of Prachinburi Rice Research Center, Rice Department in Thailand. I have used a drying bead in my experiment.I have known it at the first time when Johan presented it in National Seed conference at Kong Kane many years ago. I thought its potential would be a benefit in rice drying and rice seed storage. So I contacted with him and Patcharin to give me more detail and drying bead to do my research. Morover I had an opportunity to join with Fiona s’ drying bead research at Prachinburi Rice Research Center. After that I have set my research to use it dried rice seed to 6% moisture content and storage it in an aluminium foil at ambient temperature.
    I think its benefit should be to use it for gene bank.

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