Moisture Measurements in Agricultural Products
Moisture Content: Moisture content refers to the amount of water present in agricultural products and is usually expressed as a percentage by weight.
Two Common Methods of Representing Moisture Content:
i) Wet Basis (m): This method expresses moisture content as a percentage of the total weight (wet weight), which includes both the dry matter and the moisture.
M = Wm / Wm + Wd × 100m
Where:
- Wm = weight of moisture
- Wd = weight of dry matter
ii) Dry Basis (M): This method expresses moisture content as a percentage of the weight of dry matter.
M=Wm / Wd × 100
Where:
- Ww = wet weight of the sample
- Wd = dry weight of the sample
Comparison of Wet and Dry Basis:
- The wet basis is commonly used in the grain industry but can be misleading as the total mass changes with moisture loss.
- The dry basis is more accurate and widely used in scientific and technical calculations because it’s based on the dry material weight.
For example: If the moisture content of a grain is 25% on the wet basis, its dry basis moisture content will be:
M = 2575 × 100=33.3%.
Methods of Moisture Content Determination
Direct Methods: These methods involve the removal of moisture directly from the sample and measurement of the remaining dry matter.
- Air Oven Method: A small sample (2-3 g) is dried in an oven at 130°C for about 1 hour. The sample is then cooled in a desiccator and weighed to determine the loss in weight, which is the moisture content.
- Vacuum Oven Method: A sample is placed inside an airtight vacuum oven. The air pressure is reduced by a vacuum pump, and moisture evaporates at a lower temperature, preventing thermal degradation of the sample. The remaining dry mass is then weighed.
- Infrared Method: Infrared radiation from a halogen lamp is used to heat the sample. The sample’s weight is measured continuously during the drying process until it stabilizes, indicating the complete removal of moisture.
- Distillation Method: Involves heating a grain sample with mineral oil to release moisture, which condenses in a measuring cylinder for measurement.
Indirect Methods: These methods do not directly measure moisture but instead use other properties that correlate with moisture content.
- Electrical Resistance Method: This method measures the electrical resistance of the grain, which varies with its moisture content. Devices like the Universal Moisture Meter (USA) and Kett Moisture Meter (Japan) use this principle. The measurement time is quick (around 30 seconds).
- Dielectric Method: This method relies on the grain’s dielectric properties, which are influenced by moisture content. A sample is placed between condenser plates, and the capacitance is measured, which changes with the moisture content. Motomco Moisture Meter (USA) is an example. This method takes about 1 minute to measure moisture content.
