Drying Seed: Moisture Content and Drying Calculator

Drying removes enough water from cleaned seed for it to stay viable for years. This is measured as moisture content, the share of water in the weight of the seed. Moist seed ages quickly, and this is worst without cooling. How dry the seed ends up is set by the air around it. The balance shows when it has got there, and the drying calculator below works out the target weight.

How dry does seed have to be for storage?

For storage over several years, hemp seed belongs at 5–7 % moisture content, and the middle of that range is 6 %. Two storage trials on hemp support this range. At a room temperature of 21 °C, seed at 6 % moisture content still germinated at 89 % after two years. Seed at 14 % moisture content no longer germinated at all after three months at the same temperature.

In the freezer at −20 °C, seed between 5 % and 12 % moisture content germinated equally well after three years. Storing it dry still pays off there, because only dry seed survives a failure of the cooling.

No trial on hemp found harm from a moisture content below 6 %, but hemp has not been tested below 5 %. According to the seed banks, the best moisture content for cold storage lies somewhat higher than for warm storage. For the freezer, the evidence therefore favours the moist end of the target range.

Which humidity gives hemp which moisture content?

Seeds give water to the air until they are in equilibrium with it. After that, even long periods of waiting change nothing. The moisture content in this state is called the equilibrium moisture content, and the article on equilibrium moisture content explains it. For hemp at 20 °C this overview applies:

Humidity around the seedMoisture content in the seedRating
15 %4.2 %drier than needed
25 %5.5 %within the target range
30 %6.1 %within the target range
50 %8.2 %too moist

The values come from an estimating equation that has been checked against measurements on hemp. How well it fits is covered in the article on the sorption isotherm.

A hygrometer reading of 21–38 % humidity at the seed means the seed is within the target range. Ordinary room air at around 50 % is too moist and only brings the seed to about 8 %. This pre-drying is still good enough as a starting value for weighing. The values apply at 20 °C, and the drying calculator below handles other temperatures.

How do I dry below the humidity of the room?

Seed only gets drier in a sealed container with silica gel or over a saturated salt solution. Silica gel is a desiccant and draws water out of the air. It easily dries too far, since even a tenth of the seed weight pulls a batch at 6 % below 5 %.

A saturated salt solution holds the air in a closed container at a fixed humidity and therefore cannot overdry. Over magnesium chloride it is 33.07 % at 20 °C, and hemp settles there at a calculated 6.1–6.4 %. Reaching equilibrium takes several weeks, but nobody has to stand by.

  1. Weigh the seed in paper bags, bag and seed together.
  2. With silica gel: put the bags and the gel in a sealed box and take the seed out at the target weight.
  3. With salt: mix 250 g magnesium chloride hexahydrate, sold as “47 % MgCl₂”, with 50 ml water into a slurry.
  4. Add salt as soon as no crystals are visible in the slurry.
  5. Place the solution and the seed in separate compartments of a sealed glass box, without the seed touching the solution.

How do I know drying is finished?

When the weight stays the same on two days in a row, the seed is in equilibrium with the air. One quiet day is not enough, because water from inside the seed only moves out slowly. The differences are small, and so you need a precision balance with 0.01 g resolution. For a target below the equilibrium of the room air, you weigh against a target weight from the drying calculator. The starting value for it comes from a small sample that is dried out completely in an oven. How this works for hemp is covered in the article on seed moisture content.

The hygrometer is an additional indicator, and the balance remains the measure. For this, a sample of the seed goes into a sealed jar without silica gel. After a few hours the air in the jar has adjusted to the seed, and the hygrometer shows their state. The table above turns the reading into a moisture content. Next to active silica gel, the device instead measures the air that the gel is drying.

Cheap hygrometers read 1.4 to 3.6 points too high in one test. Dial hygrometers are too inaccurate for this purpose. Any device can be checked over the magnesium chloride solution from the section above, since its humidity is known.

  • Weigh the empty bag and enter its weight in the calculator as packaging.
  • Weigh at the same time every day and note the figure.
  • Fill the jar at least one fifth full, with seed and jar at the same temperature.
  • Read only after a few hours and note the temperature as you do.
  • Note the hygrometer’s deviation over the salt solution as a correction value.

Drying calculator

Once the weight no longer changes on two days in a row, the seed stands in equilibrium with the air around it. That air's humidity and temperature then give its moisture content, and from that the weight it still has to lose to reach a target.

What was measured

Starting point

Where it should end up

The target range for hemp is 5 % to 7 %. In two storage trials, seed at 5 % or 6 % moisture content kept its germination at least as well as moister seed. The range applies to fridge and freezer alike, and for the freezer the evidence leans against the lower end. Below 4 %, drying can do harm in some species.

All target levels

The row for the selected target moisture is highlighted.

Target moistureGrossNetStill to lose

Where do I stand now

The moisture content is calculated from an estimating equation with an oil content of 32 %, not measured. Measured hemp isotherms range from about 20 % to 94 % humidity. In the dry range the calculation is close to the measurements, at 94 % almost three percentage points too high. The target weight is as accurate as the starting moisture content and the balance.

How long does drying take?

There is no fixed duration, so the balance decides. In a laboratory trial, hemp seed that was already pre-dried took two weeks spread out thinly to reach about 5 %. The last percentage points take longest, because the seed releases its water only slowly at the end.

Drying hemp seed with heat has not been tested. In soybean and peanut the seed coat cracked when the seeds dried out too fast. Seed banks dry at 5–20 °C and so without heat.

  • Dry at room temperature and do not speed it up with heat.

Sources