Equilibrium Moisture Content: where drying stops by itself

The equilibrium moisture content is the moisture content that a seed finally takes on in a given air and then holds. The term comes from seed storage, where it explains why drying stops by itself. Seed spread out in a room and weighed daily shows this on the balance: in the end the weight stops changing. The batch is not yet dried to a target value, it only matches its air. Seed therefore only gets drier in drier air, and waiting longer changes nothing.

Why does drying stop by itself at some point?

Drying ends as soon as seed and air give off and take up equal amounts of water vapour. A seed lets water vapour through, and vapour moves between its inside and the air towards the side with less. Dry air draws water out until this balance is reached, and then no further. Moist air pushes water in the other way until the same point is reached.

The air is described by its relative humidity, the amount of water vapour compared with the highest possible amount at that temperature. The humidity a batch sets up around itself at equilibrium is called the equilibrium relative humidity. It is the air’s side of the balance, and the equilibrium moisture content is the seed’s side.

Why does hemp get drier than wheat in the same air?

Hemp reaches a lower moisture content in the same air, because oil binds no water. Hemp seeds carry 31.9 % oil on average, measured in 20 genotypes, which is just under a third of their weight.

At 25 °C and 10 % humidity, linseed reaches 3.3 % moisture content, peanut 3.0 % and soybean 4.1 %. Wheat reaches 5.5 % in the same air and so carries almost twice as much water as peanut. For hemp an estimating equation gives a calculated 3.4 % at 10 % humidity, but at 20 °C and so not comparable. The measured hemp values and the fit of the equation are in the article on the sorption isotherm.

What does temperature change?

At the same humidity, colder air leaves more water in the seed. At about 23 % humidity, drying FINOLA seed carried 6.5 % water at 10 °C and 5.9 % at 20 °C. Both values refer to dry matter. In turn, at the same moisture content the equilibrium relative humidity falls with temperature, most strongly in the middle humidity range. A hygrometer, the instrument for humidity, therefore reads a lower value on cold seed than on warm seed.

In a well-filled, sealed container the moisture content still hardly changes on cooling. 100 cm³ of air carries only about 0.0004 g of water, and no balance with 0.01 g resolution shows that. The seed therefore sets the humidity in the container, not the little air enclosed with it. How to bring seed and measuring jar into line before reading is covered in the article on drying seed.

Why do seed banks regulate the air and not the seed?

Humidity is the same for all species, whereas a moisture content would have to be set for each species. Seed banks therefore dry to an international standard at 10–25 % relative humidity and 5–20 °C until the batch is at equilibrium. The same drying room produces different moisture contents in linseed and wheat, and yet one setting serves both.

For hemp, the relationship between humidity and moisture content has been measured down to about 20 % humidity, not below. Which humidity leads to the target range for storage is covered in the article on drying seed.

How do I use the equilibrium moisture content when drying?

It is used through a sealed container in which a desiccant or a salt solution sets the air around the seed. A large amount of desiccant pulls a batch below the target range, while a saturated salt solution holds a fixed humidity. Amounts and salts are covered in the article on drying seed.

Choose an air in which the equilibrium moisture content of the seed lies within the target range.