Alternatives to the gravimetric method

The image shows a scientist pouring maize seeds into a moisture meter at the International Institute for Tropical Agriculture.

  

A moisture meter provides an alternative method of measuring the amount of water in seeds. Instead of attempting to address the question directly, a moisture meter measures the moisture in seeds indirectly, using a measure such as capacitance or conductance of the seeds. Moisture meters are quick and easy to use. They can be non-destructive, and they do not require large samples. They do, however, require regular calibrating against the oven method.

A water activity meter is based on the idea that a sample of seeds will always reach equilibrium with the air around it. It is quick to use, and although more expensive than the moisture meter, it is more reliable. Water activity meters measure the equilibrium of water vapor in the air around the seeds rather than removing water from the seeds themselves, so are completely non-destructive. Like the moisture meter, water activity meters need to be calibrated regularly.

Watch this video about measuring water in seeds. It explores the different contexts in which genebanks take measurements of water in seeds, and the different techniques they use to do so. As you watch, think about the different types of information about the behavior of water in seeds that scientists at IITA and IRRI need to know.

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The image is a still from the video. It shows water activity meter in action. Check out the video transcript for more detailed video content.
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Transcript: Video 1: measuring water in seeds

Narration:
Out in the fields, crops grow, flower and seed. The survival of these seeds is influenced by movements of water vapor between the seeds and the surrounding air. In this film, we look at how measurements of water activity supply important information at different stages of the conservation process.
Olaniyi Oyotomi:
Measurement of the water activity in seed is important, uh, in order to ensure longevity, because the higher the moisture content of any seed, of course the faster they do degenerate and lose their viability.
Narration:
During the maturation-drying phase, seeds can only lose water as fast as the humidity of the air around them allows. Too much humidity in the air can cause the seeds to age and spoil.
Andres Godwin Sajise:
In the Philippines, we have a very varied weather pattern. So in our case, timing our harvest at nine to twelve noon, just to capture the lowest moisture content that we could get, and we verify that using an MC meter.
Narration:
Small differences in moisture levels can improve these seeds’ survival in storage. This is a moisture content, or ‘MC’ meter. It helps Andy’s team work out when the seeds have dried to equilibrium with ambient conditions.
Andres Godwin Sajise:
Just to give you an idea, the manageable MC that we could get is about 19 to 21. You know we can go, in some very dry days we could go lower, but it’s really a challenging activity for us.
Narration:
In IITA, they’re inspecting Bambara groundnuts. They’ll reach maturity in Nigeria’s dry season. Even so, measuring water content yields important information about when the seeds are ready for harvest.
Sola Owoborode:
The best time to harvest a Bambara groundnut is when the moisture content is between 10 to 13% moisture level. You can use either the auto moisture content analyzer or the gravimetric moisture method for that.
Narration:
The drying process that started in the field, continues in the genebank’s drying room.
Olaniyi Oyotomi:
there must be at an optimum moisture content before they are conserved, either at low temperature or during the, uh, uh, processing period. So it's important to know how much moisture is within the seed. Is it sufficient enough to keep the seed alive? Is it too high to kill the seeds? So that is why it's important to know the level of moisture in the seed before conservation.
Narration:
Are these maize seeds dry enough to go into storage? A moisture meter reassures the scientist that the seeds are now ready for packing. But measuring the moisture content of seeds is not the only way to find out what’s happening inside them.
Narration:
At IRRI they’re using a water activity meter. It measures the equilibrium between rice seeds and the surrounding air.
Olaniyi Oyotomi:
When you have higher relative humidity around the seed, moisture will move into the seeds. And at the same time, when you have a higher moisture content inside the seed, water will move out of the seed if the relative humidity around the seed is lower.
Narration:
Depending on whether the relative humidity is high or low, there are predictable responses inside the seeds.
Fiona Hay:
Water activity is another, uh, measure of the, um, amount of water in the seeds. So it's different from moisture content. Um, basically it's telling us, it's giving us an idea of the type of reactions that can be occurring in the seeds.
Fiona Hay:
So, for example, it's at very high water activity levels that we can start to see the onset of enzyme activity and ultimately at very, very high moisture levels, we'll see seed germination.
Narration:
At very low water activity, the seeds show the best longevity in storage. Unlike moisture content, these effects are stable, irrespective of crop species, or the amount of oil the seed contains.
Fiona Hay:
Between about 0.8 and 0.2, um, on the water activity scale, over that range, we see, um, extension of longevity. So the longevity of the seeds increases as we dry over that phase. And we think that the, maximum longevity would be when we dried the seeds down to about point 0.15, 0.2 on this water activity scale.
Narration:
Water activity is more than just a number. It reflects real reactions going on inside the seed.
Fiona Hay:
So this is why, um, in the CGIAR gene banks, we've started using water activity meters and we are using it to make sure that the seeds have dried enough. So if, the seeds have been drying in a drying room, let's say that's at 15% relative humidity and 15 degrees C, then we'd expect if we measured the water activity, we'd expect the water activity to be 0.15 at the same temperature. And that would mean that we knew that the seeds had equilibrated and were ready for packing.
End transcript: Video 1: measuring water in seeds
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Video 1: measuring water in seeds
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Discussion

In this documentary, you saw measurements of water being used to obtain different types of information. In the field, they are used to help determine the best time to harvest, especially in climates where there is only a narrow window in which it is possible to harvest. They are also used in the drying room, to determine when the seeds are dry enough to go into storage. You may have noticed how portable and convenient the moisture meter is for these two purposes. The water activity meter, by contrast, gives a different type of information: a more accurate idea of the kind of biochemical reactions going on inside seeds. It can tell you, for instance, when seeds are gearing up for germination, or when they are quiescent, and in a good state for storage.

Measuring water in seeds

Why does water activity matter?