Seed Moisture Content: basis, measurement and target weight

The moisture content of a seed is the share of water in its weight, just as it lies on the balance. The quantity comes from seed testing, which determines it by fixed rules and gives it in percent. For the grower it decides how long stored seed stays viable, because moist seed ages faster than dry seed. A percentage only works as a target with a clear basis and a known method of measurement. Once you have determined the value of a batch, you can dry it to any target with the balance alone.

What does the percentage refer to?

Seed banks relate moisture content to fresh weight, meaning the seed including its water. 100 g of seed at 8 % holds 8 g of water and 92 g of dry matter, the weight without water. Drying technology often relates it to dry matter instead, and the same seed then carries a higher figure. In this way 8 % on fresh weight becomes 8.7 % on dry matter.

This lexicon always uses fresh weight, as is usual in seed testing. Some hemp measurement series also give their values on dry matter, and without conversion such seeds look moister than they are.

  • Convert dry-matter values to fresh weight with value ÷ (100 + value) × 100, so 6 % becomes 5.7 %.

How is moisture content determined in a drying oven?

It is determined with the oven method: a small sample dries out completely, and its weight loss is the water. For Cannabis sativa the high-temperature method of seed testing applies, with unground seeds. The sample stays small, and the rest of the batch never goes into the oven.

Oil seeds such as linseed are otherwise tested at 103 °C, because high heat drives volatile substances out of oil-rich seeds. Cannabis is listed under the high-temperature method despite its oil content. The difference between 103 °C and 130 °C has not been measured for hemp. Several hemp measurement series instead dried at 130 °C for three hours or at 103 °C for five or 17 hours. Values from different series can therefore be shifted against each other by an unknown amount.

  1. Weigh two separate samples of 0.5–1.0 g each or at least ten seeds.
  2. Dry both unground for one hour at 130–133 °C.
  3. Let the samples cool for 45 minutes in a desiccator, a tightly closing vessel with desiccant, and weigh them.
  4. Moisture content = weight loss ÷ fresh weight of the sample × 100.
  5. The two samples may differ by no more than 0.2 percentage points.

How do I dry to a target without measuring again?

Once the starting value is known, a target weight steers the drying, and from then on you only weigh. It comes from this formula: target weight = starting weight × (100 − starting moisture) ÷ (100 − target moisture).

An example: 20 g of seed at 9 % moisture content is to reach 6 %. By calculation it is at the target at 19.36 g and loses 0.64 g on the way. That is 3.2 % of the starting weight, and a kitchen scale with 1 g resolution cannot show this path. A balance with 0.01 g resolution is needed. The moisture levels of a storage trial on hemp were set with the same calculation.

The calculation depends entirely on the starting value, because without it you weigh precisely against an unknown target. The target range, the calculator and the criterion for a stable weight are in the article on drying seed.

  1. Before fine drying, measure a sample in the drying oven or bring the batch to equilibrium and estimate via the humidity.
  2. Weigh the whole batch and calculate the target weight.
  3. Compare at every weighing: once the batch reaches the target weight, it is done.

What do rapid moisture meters and hygrometers show?

Rapid moisture meters show a moisture content in seconds, but must be calibrated against the oven method for each species. They work more reliably between 6 % and 25 % than below, yet the storage target reaches down to 5 %.

Without sacrificing a sample, a hygrometer does the job, a small instrument for humidity. In a sealed jar it shows the humidity at which the seeds are in equilibrium and give off no more water. What this equilibrium involves is explained in the article on equilibrium moisture content. How to set up the measuring jar and translate the reading is covered in the article on drying seed.

  • In the target range of 5–7 %, use a hygrometer in the measuring jar instead of a rapid moisture meter.

Sources