There are several things to consider when harvesting plants for extraction. Firstly, the plant material harvested should come from healthy, disease-free plants. The plant material is typically air-dried in arid regions or oven-dried in humid regions (to avoid mold) to a moisture content of < 10%. The plant material is then ground or milled to a small particle size, typically 1-10 mm. This provides a larger surface area for extraction and a more exhaustive extraction in a shorter period of time.
For some types of extracts, the fresh plant material is extracted without drying. For example, volatile compounds (e. g., monoterpenes, sesquiterpenes) may evaporate off during the drying process. So, the fresh plant material is often steam distilled or extracted as soon as it is harvested. Certain highly sensitive compounds may be degraded during the drying process by heat, light, oxygen, or even enzymes within the plant material. These may also be extracted shortly after harvest in order to maintain biological activity of the final extract.
It is not uncommon for a desired constituent to be found at varying concentrations in different parts of the plant (Table 1). So, for instance, glaberdines, the skin lightening constituents from licorice, are found in higher concentration in the roots of the licorice
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Table 1 Concentration of Constituents in Different Parts of the Plant
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plant. For green tea catechins, the leaf is utilized. The beneficial constituents of Saint John’s Wort are highest in the flowers, although a combination of flowers and leaves are often used. The entire aerial, or above ground portion of the plant, may be used in other cases. Other aspects to consider are that constituent concentration can vary depending on the weather, time of year, elevation, soil conditions, fertilizer, age of plant, disease state, etc. (14).