It is well-known that the plant kingdom is the major source of remedies against illnesses. Over the centuries, new methods to extract bioactive compounds, prepare herbal drugs and administer them have been experimented with and invented. As with all medicinal products, plant-derived compounds have to be packed and presented in specific pharmaceutical forms in order to maximize the potential medicinal effects, maintain their activity, and prolong their shelf life.
Herbal medicines can be prepared and administered in different forms, such as powder or extracts. For a correct preparation of plant-derived pharmaceutical forms, it is necessary to know the qualitative-quantitative composition of the plant and the physical-chemical characteristics of the plant-derived active ingredients, such as solubility, dosage and form to maximize the therapeutic effects and the major drug bioavailability.
Moreover, it is necessary to know the phyto-complex due to the possible interference with the active ingredients and the product degradation. [1]
Harvesting time
Depending on which part of the plant contains the desired active ingredients, the harvesting time can change, as well as the post-harvesting procedures.
- Herbs are generally harvested when they are in the flowering stage, removing the excessively big branches and stems in addition to yellowed leaves. It should be noted that there are exceptions to this rule, and some herbs are collected before the flowering season.
- Inflorescences are generally picked when they are completely mature, but plants such as Chamomile or Arnica are commonly harvested before the blooming of the flowers. When picking wild species, it is better to pick the flowers once the dew has evaporated in order to avoid moisture in the final product.
- Fleshy fruits are picked when they are completely or almost ripened before the juice becomes too viscous to be extracted (i.e. raspberries, currants, blackberries..). Other fruits, such as bitter oranges, lemons, or quinces, are picked immaturely. Dehiscent fruits (poppy) have to be collected maturely but before they dry. Indehiscent fruits such as the ones of sunflowers have to be collected before complete ripening.
Post harvesting procedures
Once the desired parts of the plant are collected, a cleaning step follows, in which all the undesired plant parts are removed, as well as the damaged ones. Bulbs, roots, tubers and rhizomes are cleaned from the soil residues. The stalk and peduncle are removed from the leaves and the flowers, respectively.
Herbal drugs that are consumed fresh don’t need any other post-harvesting procedure and can be processed in the desired way, for instance, by making a tincture. [1]
Generally, herbal drugs have to be preserved for long periods in order to have them available all over the year. It should be noted that the most important thing is to get rid of the water present in the plant material; if not, the enzymatic activities promoting oxidation or hydrolysis can contribute to the morphological alteration of the plant, as well as the loss or damage of the active ingredients.
The drying process is essential in order to preserve the quality of the plant material and its constituents. The more rapid and homogeneous it is, the more effective it is. Moreover, in order to avoid the degradation of thermolabile compounds, it is necessary to operate at relatively low temperatures. Dryers are special well-aerated areas in which the plant material is collected and desiccated.
Often, air extractors or hygroscopic materials are placed in order to collect the humidity from the air, avoiding the formation and spread of mould and fungi. A common way to organize a drying room is to place overlapping racks at a suitable distance permitting the air flow and reducing the occupied space.
The plant material can be turned once in a while to permit a homogeneous drying. The main goal of the desiccation of leaves and flowers is to maintain the colours and to preserve the active compounds, including the volatile ones. Another important aspect is to protect the plant material of interest from the sunlight in order to preserve all the constituents and avoid ultraviolet (UV) induced degradation. [2]
Fleshy plant organs such as bulbs, roots, tubers, and rhizomes are generally put in heaters where the initial temperature of 20-25 °C can be raised up to 50-60 °C. With these temperatures, in addition to getting the plant material dehydrated, it is possible to get rid of some unwanted yeasts and non-spore-forming bacteria.
Notably, a proper drying process ensures the final herbal drug quality. For those plants containing sensitive compounds, it is possible to cool the plant material at -20 °C / -40 °C and put the iced plant under a deep vacuum, performing a freeze-drying process. [1]
Herbal drug stabilization and Sterilization
The drying process is not sufficient to stop the enzymatic processes leading to herbal drug degradation. In fact, enzymes remain in a state of quiescence till the right temperature and humidity are reached in order to continue the chemical and morphological product alteration. It is always recommended to store herbal drugs in dry and dark areas, in closed containers and in the presence of a hygroscopic substance to reduce the humidity that could reduce the shelf life of the product.
In order to preserve the therapeutic activity, herbal drugs can be stabilized and inactivate the enzymes contained in them irreversibly. This operation can be performed only if the plant material doesn’t contain thermolabile molecules. The fresh plant material can be put in an autoclave at 105-110 °C and exposed to water, ethanol and acetone vapours for a few minutes. In this way, it is possible to eliminate the unwanted enzymes and to keep the active ingredients as they are for a long time.
To get rid of the microorganisms present in the plant material and sterilize it from any organism, it is possible to expose it to ethylene oxide or gamma rays. However, there is a risk of reducing the active ingredient levels by doing so or altering the phytochemical composition of the medicinal plant.
Moreover, in the case of ethylene oxide, the herbal drug could retain a high concentration of the solvent, so it would be necessary to wait before consuming the herbal derivative. The most important aspect is to collect and process the plants following basic rules of hygiene and to be sure that no hepatotoxic and carcinogenic toxins, such as aflatoxins, are present in the final product. [1]
Wrap-up on Herbal Drug Preparation
It should be noticed that the progressive reduction of the active compounds present in plant materials can be only slowed down instead of completely stopping it. Nevertheless, with good preparation and preservation practices, it is possible to reduce the risk of drug colour, odour, flavour, and consistency changes. Keep in mind that to avoid altering your drug, you should keep it away from:
- Light
- Heat
- Oxygen
- Humidity
- Insects
- Mold
References:
[1] Volker Schulz, Rudolf Hänsel, Mark Blumenthal, V. E. Tyler, Rational Phytotherapy: A Reference Guide for Physicians and Pharmacists (2004) V Ed. Springer-Verlag, Berlin Heidelberg
[2] Rabinovich, L., & Kazlouskaya, V. (2018). Herbal sun protection agents: Human studies. Clinics in Dermatology, 36(3), 369–375.doi:10.1016/j.clindermatol.201
