Sorption Isotherm: the curve between humidity and moisture content

Put seed of the same variety into several chambers of differing humidity, wait until nothing changes any more, and each chamber yields one pair of values. Plotted, they give a curve: the sorption isotherm. It answers the question of which moisture content a given seed comes to rest at in a given air. Isotherm it is called because it holds for one fixed temperature; another temperature needs its own curve.

Two curves, not one

Approaching the equilibrium from above or from below makes a difference. Wet seed drying down follows the desorption curve; dry seed taking up moisture from the air follows the sorption curve. The two do not end at the same point.

That gap is called hysteresis, and it has been measured in hemp: it appeared in all three varieties examined and came out larger at lower temperature. In practice a batch dried too far will not return to the same value on rewetting as one arriving from above. Anyone using a curve should know which of the two it is; for drying, the desorption curve is the right one.

The measured course in hemp

Curves for hemp exist at four temperatures between 10 Β°C and 35 Β°C. The driest measured point sits at 34 % humidity and 35 Β°C with 5.8 % moisture content, the wettest at 94 % humidity and 10 Β°C with 13.7 %. Between those ends the curve does not rise evenly: in the upper range the moisture content grows more per percentage point of humidity than in the lower. Between 80 % and 90 % humidity more percentage points of water are therefore added than between 15 % and 25 %.

In humid air two effects push the same way: the moisture content rises more steeply per percentage point of humidity, and a higher moisture content speeds up ageing.

Why every variety has its own curve

The curve depends on how much of the seed can bind water at all. Oil binds none, starch and protein do. Oil-rich species therefore reach a lower moisture content at the same humidity than starch-rich ones, and hemp at close to a third oil a markedly lower one than cereals.

The differences do not stop at the species. In the hemp study two varieties of different sexual type diverged more from one another than two dioecious varieties from different countries. The authors recommend using the curve of one’s own variety for drying and storage. Individual batches of the same variety can differ too, depending on ripeness and the make-up of the seed.

What it is used for

Three questions can be answered from a curve without sacrificing a single seed. First: which moisture content this batch reaches in the room it currently sits in. Second: which humidity is needed to hit a target value. Third: at which humidity storage turns risky.

Where no curve exists for a species, an estimating equation helps, deriving a value from oil fraction, humidity and temperature. For hemp it can be checked at two points: at 34 % humidity it matches one of two measured varieties, at 94 % it runs almost three percentage points high. Below 34 % no comparison value exists, and there it stays unverified.

Where the curve for hemp stops

The published measurements on hemp seed start at 34 % humidity; nothing below that was found. The international seed bank standard, however, calls for drying at 10 % to 25 % humidity, and long-term storage happens at 15 %. The entire range in which seed is stored for decades is therefore calculated for hemp and not measured. Nobody can check that without climate chambers of their own. What remains is the quantity that can be read off: the humidity inside a sealed container, taken with a humidity meter. The seed bank standard regulates that quantity anyway rather than the moisture content, and converting the one into the other is precisely the step that stays unsupported for hemp below 34 %.

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