Why Moisture Matters
Thursday 9th May 2013
Moisture content is critical from harvest, through storage, to final sale of cereals and oilseeds. If moisture is too high, there is a risk of quality reduction, or even crop loss in store. On the other hand, excessive drying is wasteful and can lead to reduced returns.
Balancing these opposing risks is not easy due to the variable nature of grains within a bulk and the inherent difficulties of measuring grain moisture accurately. Cereals and oilseeds which are too moist in storage can be subject to:
- Mould growth and mycotoxin production.
- Mite infestations, especially in rapeseed.
- Heating due to moulds and mites.
Grain which is over-dried before, or during, storage can result in:
- Splitting and cracking.
- Impaired quality, particularly in rapeseed.
- Wasteful energy use.
- Grain contracts specify moisture content which, if not met, can result in penalties.
Assessing moisture content on the farm
Grain moisture meters and probes, available for farm use, measure an electrical property related to moisture content, rather than grain moisture itself. Meters and probes rely on an inbuilt calibration between moisture and either electrical capacitance or resistance.
Meters and probes are calibrated against oven-based moisture determinations. All probes and capacitance meters use whole grain samples. Most resistance meters use a ground sample.
Meters are calibrated to operate most accurately within a specific moisture range, typically 11–20%. The accuracy of this method is +/- 0.5% at best.
Causes of variation:
- Inadequate grinding, if required
- Temperature differences between grain and meter/probe
- Poor maintenance
- Damaged probe or measuring cell
- Depth of probe insertion
- Grain moisture outside range of meter/probe
- Contaminants, e.g. soil, screenings
- Out of calibration.
Measuring moisture content in the laboratory
The most accurate method of measuring grain moisture content is the standard oven-based test, which uses an ISO-specified protocol to dry a prepared sample of ground grain in a special laboratory oven (ISO712). The weight lost during drying is used to calculate the moisture of the sample. The accuracy of this method is within 0.15%.
Causes of variation:
- Unrepresentative grain sample
- Inadequate grinding
- Incorrect oven drying time
- Uneven oven temperature.