Other factors influencing water activity

You might be wondering why there are so many different ways of measuring the amount of water inside a seed: moisture content, eRH and water activity. In the Genebank Standards, equilibrium relative humidity (eRH) is considered the best measurement for drying, because it is independent of small differences which can affect moisture content. Let’s go through these.

  

Oil within seeds

The moisture content of a seed can vary with the oil content – in general, the lower the oil content, the greater the amount of water a seed can take in. This means crops containing more or less oil behave differently at the same moisture content. By contrast, equilibrium relative humidity is independent of these differences. Figure 9 (below) shows the isotherms of four different types of seed with increasing oil contents - wheat, soy, lettuce and peanut. Use a ruler to check which region each of these crops is in at any given moisture content.

For example, if you rule a horizontal line across the graph corresponding 10% moisture content, it crosses the curve for wheat seeds in the middle of region II (at about 40% eRH). Here we can expect moderate rates of aging reactions. In groundnut on the other hand, still at 10% moisture content, the seeds are already in region III of the isotherm (>90% eRH) and progressing towards germination.

The graph continues to examine the type of S-shaped curve, or seed moisture isotherm, that you explored in Figures three to eight. The x axis is equilibrated relative humidity (eRH). The y axis is the moisture content of the seeds. Again the graph is broken into regions I, II and III. Four S-shaped curves are plotted for four different crops. The curve for wheat (which contains the least oil - just two point two percent) is the steepest curve. The curve for soybeans (which contain twenty percent of oil) is less steep. The curve for lettuce seeds (which contain thirty-seven percent of oil) is less steep again. The curve for groundnut (which contains the most oil of these four crops, fifty-eight percent), is the most flattened of all. A horizontal line ruled across the graph would cross these curves at different eRH.
Figure 9: differences in oil composition in seeds

Different areas of tissue within a seed can also have different oil contents: neem has a greater oil content in the cotyledons than in the embryo, therefore the water content in the embryo tends to be higher. This means that eRH is preferable to moisture content and is given as the best measure of the amount of water in seeds in the Genebank Standards.

  

Temperature and pressure

What temperature is best for drying? This is a balancing act for genebanks (see Figure 10, below). Faster drying can be achieved at higher temperatures, but higher temperatures also bring on physiological aging, which is reduced at lower temperatures.

In theory, pressure could also affect water in seeds, although in reality, temperature tends to be more of a consideration for genebanks than pressure, because in most genebank situations, pressure is unlikely to deviate far from atmospheric pressure.

This graph continues to investigate the S-shaped curves or seed moisture isotherms you have been studying in this module. Again, the x axis is the equilibrated relative humidity (eRH). The y axis is the moisture content of the seeds. In this graph, there are four S-shaped curves corresponding to four different temperatures: ten, twenty, thirty and forty degrees Celsius. The steepest of these S-shaped curves is at ten degrees Celsius, the least steep is at forty degrees. A horizontal line ruled from fifteen percent moisture content crosses the curve for ten degrees Celsius at a lower eRH than the eRH when it crosses the curve for forty degrees. In other words, if moisture content is fixed, the higher the temperature, the higher the seed eRH. By contrast, a vertical line drawn from forty percent eRH crosses the curve for seeds at forty degrees Celsius at a lower moisture content than seeds at ten degrees Celsius. In other words, at a given eRH, moisture content of seeds is higher, the lower the temperature.
Figure 10: effect of temperature

  

Hysteresis

The movement of water in and out of seeds varies, depending on whether the seed is taking up (adsorbing) or letting go (desorbing) water. As you learned in module 3, seeds that come directly from the field are likely to be losing water (desorbing), whereas seeds that have been dried, stored, and are being taken out of storage, are likely to be adsorbing.

A seed that has once been dried may also be less efficient in taking up water. This means that it is helpful to know the moisture history of a seed when trying to predict how it will take up water.

The graph continues to explore the S-shaped curves or seed moisture isotherms associated with water activity. The x axis is the equilibrated relative humidity (eRH). The y axis is the moisture content of seeds. Again, region I is from eRH zero to eighteen percent. Region II is from eRH eighteen to eighty percent. Region III is from eRH eighty to ninety-nine percent. The curve shows there are differences between seed adsorption (taking in water) and desorption (losing water). The curve for adsorption (moving from a lower region to a higher one) is less deeply bowed than the curve for desorption (moving from a higher region to a lower one).
Figure 11: hysteresis

The effects of species, temperature and hysteresis means that it can be challenging to compare the longevities of seeds dried and stored under different conditions. For instance, as Figure 12 (below) shows, both moisture content and eRH differ, depending on whether seeds are drying at 20°C or 45°C.

The graph continues to examine S-shaped curves or seed moisture isotherms associated with water activity. Again, the x axis is the equilibrated relative humidity (eRH). The y axis is the moisture content of seeds. Two curves are plotted - one for seeds drying at twenty degrees Celsius, the other for seeds drying at forty-five degrees Celsius. The curve is steeper at the lower temperature. This means a horizontal line ruled across from fourteen percent moisture content crosses the curve for seeds drying at twenty degrees at an eRH of forty percent. The same line for fourteen percent moisture content crosses the curve for seeds drying at forty-five degrees at sixty percent eRH.
Figure 12: effect of drying temperature on adsorption

Imbibition

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