Herbal Drugs: Their Collection, Preservation, and Preparation; Evaluation, Quality Control, and Standardization of Herbal Drugs

Abstract

The WHOs’ criteria for good herbal drug preparation include the identity of source plant, optimum time of harvest, post-harvest handling, cooking utensils etc. Natural drug products may be obtained from the wild or through cultivation, fermentation, cell or organ culture, microbial transformation as well as biologics. Right source of drug plant and harvest time are important factors for maximizing the yield of the desired phytochemical content. Collection schedule of different plant parts are different, e.g., roots and rhizomes at the end of the vegetation period, bark in the spring, leaves and herbs at bloom, flowers at anthesis or shortly after opening, and fruits and seeds after maturity or ripe. Hand collection is preferable for wild source. Post-harvest handling including garbling, washing, drying in air, oven drying, milling and re-milling, sieving, storage, labeling with the name of the plant, place and date of collection are important for standard herbal preparation. Collected plant material must be preserved to keep the active compounds unchanged during transport and storage. Drying, freeze-drying or lyophilization, stabilization, fermentation etc. are some of the common methods of preservation. Herbal preparations are made from herbal drugs, such as whole plant, plant parts, algae, fungi, lichen, exudates, in a crude state in dried or fresh form and extracts with the help of different processes such as infusion, decoction, maceration, distillation, expression, fractionation, purification, concentration, fermentation. These herbal preparations include whole plant or parts, comminuted or powdered herbal drugs, tinctures and extracts, fatty oils, essential oils, expressed juices and processed exudates of herbal materials. Herbal preparations are the basis for finished herbal products. Finished herbal products are medicinal products containing exclusively herbal drugs (active substances) and herbal drug preparations. They also include preparations made by steeping or heating herbal materials in alcoholic beverages and/or honey, or in other materials and may consist of herbal preparations made from one herb or more herbs (mixed herbal product). They may contain excipients in addition to the active ingredients or may contain natural organic or inorganic active ingredients not of plant origin (e.g., animal materials and mineral materials). Finished products or mixed products to which chemically defined active substances have been added, including synthetic compounds and/or isolated constituents from herbal materials, are not considered to be herbal. Herbal drugs are precisely defined by the botanical scientific name according to the binominal system. Herbal medicines include herbs, herbal materials, herbal preparations and finished herbal products. Storage of crude drugs in dry condition in airtight container placed in dry dark place is very important for stability and quality maintenance. Grinding of crude drugs by hammer-, knife- or tooth mill to a powder of suitable particle size is carried out for isolation of a pure compound or for manufacture of a simple preparation. Cold grinding is preferable for crude drugs containing heat labile compounds. Sifting to ensure particle size (course—2.00 mm to fine—0.18 mm) can be performed following the principles of sieving and blast sifting. Extracts are preparations of crude drugs containing all the constituents soluble in the extracting solvent. Extracts may be dry (when all solvent has been removed), soft or fluid (solvent prepared with mixtures of water and ethanol). Tinctures are prepared by extraction of the crude drug with five to ten parts of ethanol of varying concentration without concentration of the final product. For both extracts and tinctures the ratio drug to solvent should always be stated. The criteria for ideal solvent for a certain pharmacologically active constituent include high selectivity for the compound to be extracted, high capacity for extraction in terms of coefficient of saturation of the compound in the medium, nonreactive with the extracted compound or with other compounds in the plant material, low price, harmless to man and to the environment, completely volatile. Aliphatic alcohols (up to 3C) or mixtures of the alcohols with water, are the solvents with the greatest extractive power for almost all natural substances of low molecular weight such as alkaloids, saponins, and flavonoids. Ethyl alcoholis used for obtaining tinctures and fluid, soft and dry extracts. The ethanol–water mixture induces swelling of the plant particles and increases the porosity of the cell walls and thus facilitates the diffusion of extracted substances. For extraction of barks, roots, woody parts and seeds the ideal alcohol: water ratio is about 7:3 or 8:2. For leaves or aerial green parts the ratio 1:1 is usually preferred in order to avoid extraction of chlorophyll. Herbal internal preparations include infusions, decoctions, tinctures, macerations, percolation, digestion, inhalation of powdered plants, steam inhalation, aromatherapy, dry preparations etc. and washes, compresses, poultices, salves and balms are the main external preparations. Infusions are made with leaves and flowers, while decoctions are made with roots, bark, seeds, and berries. Infusion and decoction are preferred for water soluble chemicals (e.g., anti-inflammatory plant steroids) while tincture for alcohol soluble chemicals (anti-bacterial alkaloids). This may explain why a tea of the plant is used for arthritis while a tincture is traditionally used to treat various bacterial infections. Several problems influence the quality of herbal drugs. Quality control and the standardization of herbal medicines involve several steps. The source and quality of raw materials, good agricultural practices and manufacturing processes are certainly essential steps for the quality control of herbal medicines and play a pivotal role in guaranteeing the quality and stability of herbal preparations. For standard herbal drug production at industrial level, source herbal ingredients should be analyzed in detail in respect of quality, efficacy, performance and safety because drugs in commerce are frequently adulterated and do not comply with the standards prescribed for authentic drug. Quality refers to the status of a drug and is based on three important pharmacopoeial definitions such as identity, purity, and content of active constituents. Voucher specimens are reliable reference sources. Purity evaluation includes ash values, contaminants, heavy metals, microbial contamination, aflatoxins, radioactivity, and pesticide residues. Analytical methods, such as photometric analysis, TLC, HPLC, GC, MS or GS/MS, can be employed to establish the constant composition of herbal preparations. For content, sometimes markers can be used for control purposes because in most herbal drugs the active constituents are unknown. In other cases, where no active constituent or marker can be defined for the herbal drug, the percentage extractable matter with a solvent may be used as a form of assay. The choice of the extracting solvent depends on the nature of the compounds involved, e.g., hot water for herbal tea, steam distillation is suitable for essential oils. Complex nature of herbal drugs, unknown active principle, unavailability of selective analytical methods or reference compounds, chemical and natural variability in the plant materials as well as in source and quality of the raw material, methods of harvesting, drying, storage, transportation, and processing etc. are some of the problems that influence the quality of herbal drugs. Strict guidelines have to be followed for the successful production of a quality herbal drug. Standardization involves adjusting the herbal drug preparation to a defined content of a constituent or a group of substances with known therapeutic activity by adding excipients or by mixing herbal drugs or herbal drug preparations because botanical extracts made directly from crude plant material show substantial variation in composition, quality, and therapeutic effects. Standardized extracts are high-quality extracts containing consistent levels of specified compounds, and they are subjected to rigorous quality controls during all phases of the growing, harvesting, and manufacturing processes. Evaluation of crude drug means confirmation and determination of its identity, quality and purity and it can be done by several methods viz. organoleptic and microscopic evaluation; analytical determination of foreign matter, plant ash, heavy metals, microbial contaminants and aflatoxins, etc. Potentially hazardous contaminants and residues in herbal medicines may be grouped as chemical, biological, agrochemical residues, residual solvents, etc.