Sorption Isotherm: humidity and moisture content as a curve

A sorption isotherm is the curve that plots a seed batch’s moisture content against humidity. It is measured in chambers with a fixed humidity, where the seeds lie until their weight stops changing. It is called isothermal because it applies to one temperature only. For the grower it answers which moisture content a given air brings the seed to, without sacrificing a sample. For hemp the curve has been measured and an estimating equation checked against it. Humidity and moisture content can thus be translated into each other.

What does the curve show me?

The curve shows the moisture content at which a seed settles in a given air. This end value is the equilibrium moisture content, and the article on equilibrium moisture content explains its link to oil and temperature.

There are two curves: moist seeds that dry follow the drying or desorption curve. Dry seeds that take up water from the air follow the wetting or sorption curve (adsorption). The two curves do not lie on top of each other. For drying freshly harvested seed, the desorption curve applies.

Which measurements exist for hemp?

Two measurement series exist for hemp, and together they cover almost the whole storage range. Three varieties have been measured between 34 % and 94 % humidity at 10–35 °C. When drying, the variety Altair stood at 5.8 % moisture content at the driest point, at 35 °C and 34 % humidity. At the moistest point, at 10 °C and 94 %, it stood at 13.7 %. The variety CanMa lay 0.3–0.5 percentage points higher throughout.

The variety FINOLA has been measured over salt solutions down to about 20 % humidity, at 10–60 °C. At 20 °C and about 23 % humidity it stood at 5.4–5.6 % moisture content, converted to fresh weight. Admixture, broken seed and plant remains in the batch raised the value by about 0.5 percentage points. Below about 20 % humidity nothing has been measured for hemp.

The gap between varieties is not random. A monoecious and a dioecious variety lay further apart than two dioecious varieties from different countries. For drying and storage, the curve of your own variety is therefore the best guide. Where there is none, an uncertainty of a few tenths of a percentage point remains.

How well does the estimating equation fit the measurements?

In the dry range it fits the measurements well, and in the moist range it clearly overestimates. The estimating equation from a genebank manual calculates moisture content from oil content, humidity and temperature, here with 32 % oil. At Altair’s driest point it gives the same value as the measurement. At the moistest point it calculates 16.5 % and so lies 2.8 percentage points above the measured value.

At the temperature of the FINOLA measurement it gives a calculated 5.2 % for about 23 % humidity. The calculation is thus slightly too dry, within the reading accuracy of the measurement. A point that fits only confirms that point. The table showing which humidity sets which moisture content at 20 °C is in the article on drying seed.

What does hysteresis mean for my batch?

When an overdried batch takes up water again, it holds less at the same humidity than one dried down from above. For FINOLA this gap was about 0.2 percentage points at 20 °C and about 0.8 at 10 °C, based on dry matter. The gap between desorption and sorption curve is called hysteresis, and it appeared in both hemp measurement series.

In practice this affects any batch that was dried too far and then brought back up in the air. It shows the same humidity in the measuring jar as a normally dried batch, but weighs slightly less. This is not a measuring error and no reason to moisten it further up to the target weight.

  • Judge an aired batch by the humidity in the measuring jar and not by the target weight.

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