Equilibrium Moisture Content: the level a seed settles at

Spread seed on a tray to dry and after a few days the weight stops changing. The seed has arrived: it has reached the moisture level that matches the air around it. Dried to a target value it is not yet. That level is called the equilibrium moisture content. From there on waiting longer changes nothing; the seed goes lower only in drier air.

How the equilibrium comes about

A seed is permeable to water vapour. Between its interior and the surrounding air, vapour travels one way or the other until the pressure is equal on both sides. Humid air drives water in, dry air pulls it out, and both processes end at the same point.

Something counter-intuitive follows: drying time sets how fast that point is reached, not where it lies. A batch sitting at 50 % relative humidity gets about as far in a week as in a month.

What the level depends on

The main influence is relative humidity, the share of water vapour the air carries against the maximum it could hold. Temperature acts more weakly in the same direction: at equal humidity the level sits slightly lower in warmer air.

The third influence is the make-up of the seed. Oil binds no water, so an oil-rich seed has less mass in which water can be held at all and reaches a lower level in the same air than a starch-rich one. Hemp seeds hold 28.5 % to 36.0 % oil, close to a third of their weight; rice and wheat sit at a few per cent. Flax was measured at 3.3 % moisture content at 10 % humidity, groundnut at 3.0 %, soya bean at 4.1 %.

Estimating it for a species

The seed bank manual gives an equation that derives an estimate from oil fraction, humidity and temperature. For hemp at an oil fraction of 32 % it returns, at 20 Β°C, around 3.4 % moisture content at 10 % humidity, 4.2 % at 15 %, 5.5 % at 25 % and 8.2 % at 50 %.

For hemp the equation can be checked at two points, and the measurement record offers no more than that. At 34 % humidity and 35 Β°C the varieties gave 5.8 % and 6.1 %; the calculation returns 5.8 %, so it matches one variety and sits 0.3 percentage points under the other. At 94 % humidity it returns 16.5 % against 13.7 % measured. Below 34 % humidity no hemp measurement exists to check it against: the figures in the paragraph above are unverified there.

Why seed banks regulate humidity rather than moisture content

The international seed bank standard prescribes drying at 10 % to 25 % relative humidity and an environment of 15 % humidity for long-term storage. It names the air and not the seed, and there is a reason: the same drying room produces different moisture contents in different species because their oil content differs. A rule about humidity applies to every species alike, whereas a rule about moisture content would need its own figure for each.

In practice the equilibrium moisture content is also the easier quantity to determine. A sample goes into a sealed jar with a small humidity meter, and the reading gives the humidity that establishes itself. The documented counterpart is oven drying, which determines the moisture content directly while consuming the sample; which of the two fits depends on whether a single anchor value or a running check is wanted.

What to do with it

Anyone after a target value chooses the air; the duration will not set it. Two routes are in common use. A desiccant such as silica gel draws water as long as it can absorb, so it can also overdry a batch. A saturated salt solution in a sealed container sets a fixed humidity instead and limits the end point by itself: lithium chloride holds 13 % at 20 Β°C, calcium bromide 18 %, calcium chloride 30 % at 25 Β°C.

The equilibrium works both ways, and that is where the trap sits. Open a dried jar in humid air and the same process runs backwards. A cold jar out of the freezer is the worst case, because water condenses on the seed as well; it is therefore opened only once it has reached room temperature.

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