- The optimum way to dry herbal material is in a kiln, where heat and humidity can be controlled. The drier can be a closed de-humidification system or simple flow-through hot-air drying system. Other more sophisticated (and expensive) drying systems are available.
- Hot air should delivered through a heat exchange system. Herbal material should not come in contact with hot exhaust gases.
- Herbal material must be uniformly dried to evenly remove moisture and prevent mould formation. A common way of ensuring uniform drying is to thinly and evenly spread plant material on racking, which allows for the even circulation of drying air through the plant material.
- Final moisture content in herbal material should be no more than 10-12%.
- Ideally, herb crops should be dried in a system where there is minimal potential for mould growth. This will require adequate ventilation
Echinacea crop, Tasmania (photo: Tim Groom and control of heat and humidity.
- Most herb crops will be optimally dried at less than 60°C to avoid a change in the colour or odour of the herb.
- Dried medicinal plant materials should be inspected, sieved, cut or winnowed to remove discoloured, mouldy materials, soil, stones or other foreign matter that can not have been detected during the primary processing.
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