Medicinal and Aromatic Plants—Uses and Functions

Abstract

Medicinal and aromatic plants contain the chemical constituents first used by humans as medicines for healing, as flavoring agents for food and drink, and as mental stimulants for mystic interactions with super natural gods. These plant materials continue to play positive roles in human life, as sources of modern pharmaceuticals to treat medical problems, as herbs and spices to tempt the palate, and in a multitude of other applications. Demand and trade for these plant materials initiated globalization that spread new ideas, new diseases, and new settlements along with native resentment. This chapter presents a detailed and thorough review of plant metabolites used in modern and conventional medicine and those providing spices and condiments. The taxonomy, biochemistry and extraction are described. Truly it may be said that medicinal and aromatic plants have changed the world and undoubtedly play an important role in the future as they have in the past.

Introduction: Special Plants

Medicinal and aromatic plants represent a broad group of plant materials valued primarily for their chemical constituents. Since ancient times, these plants have been used to help restore and maintain health, to preserve and flavor foods, to add color and aroma to daily life, and to serve as pathways to spiritual dreams and mystical adventures. While sometimes referred to as medicinal herbs , culinary herbs , potherbs , spices , seasonings , essential oil plants, or other terms to distinguish plant use, collectively these plants have been used for a variety of applications throughout recorded history, enhancing the quality of human life. Currently, an estimated 70,000 plant species are used in traditional medicine (Farnsworth and Soejarto 1991).

The discovery and initial uses of medicinal and aromatic plants by various groups of people are unknown, but possibly the plants were chosen for use as alternative food sources and then recognized for an attractive aroma or enhanced healing when applied to cuts and scrapes. Undoubtedly, these first trials with plants were tentative and perhaps came from watching the plant materials selected and eaten by animals and earlier humanoids. From this beginning, plant trade among groups would lead to trade among near-by countries and eventually to global trade of medicinal and aromatic plants . The activities of early humans in medicinal and aromatic plants continue today with plant exploration and botanical sourcing for new medicines , new aromas , and new uses. Global trade in medicinal and aromatic plants has become a US$ 30 to US$ 60 billion business (Anon 2008).

Although the populations of many countries remain dependent on traditional uses of medicinal and aromatic plants for health care , these plants are also recognized as major contributors to modern life, pharmaceuticals for human diseases, seasonings to flavor foods and beverages , and beauty through cosmetic goods and herbal gardens . Laboratory chemists work to modify plant constituents to improve bioactivity . In contemporary societies, individualsfrequently seek to develop and protect health through self-treatment with herbal materials as foods, food additives , and dietary supplements meant to improve the normal diet and reduce medical costs. Although known by several names, medicinal and aromatic plants continue to make useful and valuable contributions to world societies (Table 18.1).

Table 18.1 Terms used with medicinal and aromatic plants. (Source: American Botanical Council (2013))

Discovery: Historical Progress

To fully understand and appreciate medicinal and aromatic plants and their uses and functions, some history on their interactions with people is necessary (Table 18.2). The anthropological residues provide clues in the search for plant materials to be used as pharmaceuticals . The discovery of medicinal and aromatic plants that could produce delightful aromas , flavor foods, enhance health , and cure illnesses was an important development during the early history of humans. Such information would be incorporated into traditional rituals, stories, and myths that could transmit the learned uses of these plants to future generations. Medicine men and women with specialized knowledge and skills in the use of plant materials for healing would become significant and honored members of societies.

Table 18.2 Significant events in the history of medicinal plants and medicine. (Source: Mamedov and Craker (2012))

With the development of mnemonic symbols and subsequent writing skills, instructions on medicinal plants and their use was recorded in several locations, including Egypt , India , China , the Middle East, and the Americas . The oldest written document on the use of medicinal plants in the preparation of a pharmaceutical is an approximately 5,000 year old Sumerian clay tablet that refers to 250 plants. Early written instructions, which were frequently a mixture of plant material and mysticism, provide historical detail on plants and plant preparations used in the treatment of medical issues. The Ebers papyrus , dated 1,550 BC, but thought to be copied from even older written sources, is the most complete of the medical papyri. A recommended treatment for urinary problems in the Ebers papyrus includes the plants, carob (Ceratonia siliqua) , celery (Apium graveolens) , date (Phoenix dactylifera) , fig (Ficus carica) , grape (Vitis vinifera) , gum arabic (Senegalia senegal and Vachellia seyal) , and wheat (Triticum spp.) (Carpenter et al. 1998).

In India , the traditional medicine system, Ayurveda , is based on the Charaka Samhita (100 BC–100 AC), the Sushruta Samhita (300–400 AC), and the Bower manuscript (400–600 AC). Unani , a second traditional medicine system of India , is a Greco-Arab medicinal practice imported to India from the writings of Avicenna , a Persian physician that wrote a series of treatises on medicine. Ayurvedic medicine contains over 600 plant species for use in treatments and Unani medicine approximately 40 plant species, including the Damson tree (Terminalia spp.) , Malacca tree (Emblica officinalis) , Ajowan (Ptychotis ajowan) , and Senna (Cassia angustifolia) . Both the Ayurvedic and the Unani systems of medicine had spread to other parts of Asia by 1,300AC.

In China , the traditional medicine system originated primarily from a 2,000 year-old Chinese medical text, Huangdi Neijing , containing a series of chapters, which along with earlier writings and later revisions became the foundation for Traditional Chinese Medicine , a health system based on humoral balances. In the Chinese medicine system, good health requires equity among the natural forces associated with diet, lifestyle, emotions, and environment. Thus, good health requires a balance among natural forces, such as Yin-Yang (two opposites form the whole), Qi (life energy), and the Five Elements (interactions and relationships). Medicinal and aromatic plants , along with animal and other natural substances are used to bring the body into balance. The first recognized Chinese herbalist is Shennong , a legendary ruler of China of 5,000 years ago who taught the people the use of medicinal herbs (Anon 2013).

Early European medicine markets made use of spices that were undoubtedly part of the trade goods coming from Asia via routes that followed the Silk Road and via crossings at the Isthmus of Suez located between the Red Sea and the Mediterranean Sea . Although limited western trade with India and China occurred as early as 1,600 BCE, the conquests of Alexander the Great into Northern India by 325 BCE probably introduced other European countries to spices as flavorings and medicines.

A book by Marco Polo , The Travels of Marco Polo (Polo and Rossabi 2012), which detailed the riches in Asia he observed during travel (1271–1291) with his father and uncle, became popular reading in Europe , bringing an awareness of other civilizations and spices. Large demands for Asian spices occurred during the fourteenth to seventeenth centuries as possible preventatives or cures for the plagues that swept through Europe during this time period. The limited quantities of spices available via the overland Asian-European trade routes, especially after the Ottoman victory at Constantinople in 1453, promoted the search for a sea route to Asia . An ocean-going ship building and learning facility built in southern Portugal by Prince Henry the Navigator in 1418, trained sailors in ship building, navigation, and other trades necessary for an ocean voyage into unknown waters.

Sailing East for Portugal , as agreed upon in the Treaty of Tordesillas in 1494, Vasco da Gamma reached Calicut, India , opening a new trade route to Asian spices in 1498. Sailing West for Spain , Columbus reached America , a continent unknown to Europe , in 1492, but instead of the pepper (Piper nigrum) of Asia , he returned to Europe with samples of allspice (Pimenta dioica) , and red pepper (Capsicum spp.) (Fig. 18.1).

Fig. 18.1

Some early trade routes of spices along with spices frequently used in various regions. (Sources: Czarra (2009); Food University (2013); Keay (2007); Kew Gardens (2013); Spices & Herbs (2013))

Constituency: Chemical Extracts

Medicinal and aromatic plants are valued for their chemical constituents known as secondary metabolites . These secondary metabolites , which include essential oils , are organic compounds with no known necessity for the normal growth, development, or reproductive activity of an organism, but are bioactive, interacting with tissues and provide the aromas, tastes, and medicinal activity associated with such plants. For example, in sensing the aroma of aromatic plants , the volatile chemical constituents from the plants interact with sensory neurons within the olfactory epithelium system to produce nerve impulses that communicate the aroma signal along the olfactory nerve to the brain. Anticancer activity exhibited by the American Mayapple (Podophyllum peltatum), for example, contains constituents that interfere with DNA replication, thus stopping the rapid cell division associated with cancer (Chaurasia et al. 2012).

Positive effects from secondary metabolites are associated with pharmacological action beneficial to the consuming organism, while negative effects are associated with toxicity that harms those consuming the plant tissue. As with other drugs, the bioactivity exerted by secondary metabolites is directly related to absorption, distribution, metabolism, and excretion (ADME) of the metabolite. Beneficial effects from a secondary metabolite may, however, become toxic if too much is consumed.

While secondary metabolites may not be essential for bodily functions, they can enhance life through improvement in health, spirit, and aesthetics. The exact reason why some plants contain essential oils and other secondary metabolites is unknown, but the presumptions are that these chemical constituents could attract or repel insects and other life forms that benefit or stress the plant, respectively, or could act as storage centers for metabolic waste products. Secondary metabolites are used by humans to improve life through use in disease prevention and healing, food flavorings and preservation, beauty products and aesthetics , and recreational drugs and mysticism.

Secondary metabolites are extracted from plant tissue by distillation, expression, or solvents, depending on the constituents and plant material (Table 18.3; Fig. 18.2). Extracts (Table 18.4) are frequently grouped according to the extraction method, application, physical properties, chemical structure, or synthesis pathway of the extract (Figs. 18.3 and 18.4). Grouping of medicinal and aromatic plants by chemical properties would include alkaloids , anthraquinones , flavonoids , glycosides , lipids , phenylpropanoids , resins , saponins , and terpenoids (Figs. 18.5, 18.6, and 18.7). Variations in chemical structures are numerous, differing in chemical constituency, isomerism, and substituents.

Fig. 18.2

Preparing for distillation of essential oil in Nepal. (Source: Photo courtesy of Han Saran Karki)

Fig. 18.3

The mevalonic acid synthesis pathway. (Source: Samuelsson (1992))

Fig. 18.4

The shikimic acid synthesis pathway. (Source: Samuelsson (1992))

Fig. 18.5

Phenylpropanes. (Source: Jiangsu New Medical College (1978); Samuelsson (1992))

Fig. 18.6

Terpenoids. (Source: Jiangsu New Medical College (1978); Samuelsson (1992))

Fig. 18.7

Alkaloids. (Source: Jiangsu New Medical College (1978); Samuelsson (1992))

Table 18.3 Methods for extracting essential oil from plant tissues. (Source: Soysal and Öztekin (2007); Esoteric Oils (2013))

Table 18.4 Medicinal and aromatic plant extracts. (Source: Leung and Foster (1996))

Although a number of plants have been tested with some becoming new pharmaceuticals or innovative commercial products, many more plants have not yet been fully evaluated for bioactive properties. Thus, continued exploration, but not exploitation, of plants with health or other benefits is necessary. Over collection and habitat destruction of wild medicinal and aromatic plants used in commercial trade can lead to shortages and possible extinction of species. An estimated 400 species are currently at risk. These at risk species and other species that become endangered must be cultivated to ensure survival of the species and that an adequate supply of the plant is available for commercial use. Some future supplies of medicinal and aromatic plant bioactives may come from production of the chemical constituents in bioreactors (McCoy and O’Connor 2008), and modifications to plant genomes to produce variations in constituents (Runguphan and O’Connor 2009).

The World Health Organization has published quality control methods for plant materials (Anon 2003b) to help ensure public safety and conservation and protection of plants collected in the wild (Table 18.5). In the European Union herbal medicines are regulated under the European Directive on Traditional Herbal Medicinal Products and in the United States dietary supplements and herbal remedies are controlled by the Food and Drug Administration . These agencies, along with the processing and manufacturing companies are responsible for maintaining safe marketable medicinal and aromatic products . Plants and plant extracts go through a series of steps to determine the safety and biological activity of the chemical constituents (Fig. 18.8).

Fig. 18.8

Procedures to determine bioactivity and safety of chemical constituents in plants. (Modified from Fabricant and Farnsworth (2001))

Table 18.5 World Health Organization guidelines for good practices in production, collection, and manufacturing of medicinal and aromatic plants

Applications: Pharmaceutical

The use of medicinal and aromatic plants in medicine obviously began when humans first discovered that plants or parts of plants could be used to make ill people feel better. From this beginning of pharmacognosy (the study of medicines derived from natural sources), plant medicines have moved from crude preparations to an important source of pharmaceuticals in the treatment of several diseases. Today, the exploration of plant materials to develop new and effective medicines involves partnerships among plant explorers, ethnobotanists, physicians, medical chemists, and many others interested in developing better medicines.

Phytotherapy , the medical practice of using medicinal and aromatic plants and extracts to promote health , has been criticized by Western medicine due to the unregulated variations within alternative medicines that contrast with the standardized drugs and treatments of conventional medical therapies. The use of extracted constituents that have pharmacological benefits has led to an accepted status for natural product medications in most countries. A number of medications have originated with plants, but may be currently synthesized in chemical laboratories or modified to improve drug standardization, administration, and/or bioactivity (Table 18.6). Use of plants as a source of medicine is approached in different ways, depending on the society (traditional to modern) (Table 18.7).

Table 18.6 Some modern medicines originating from plants. (Source: MMS Gardens (2012))

Table 18.7 Approaches for use of medicinal plants. (Source: Modified from Hirani (2012))

The World Health Organization indicates plants are used as the primary healthcare for 80 % of the world’s population (Anon 2008). Most plant based healthcare systems occur in areas lacking modern facilities and pharmaceuticals or in locations that have established a traditional healthcare system based on herbal products (Table 18.8). As both traditional and modern health care become more available, the trend is for people to choose that medicine that will best return them to good health.

Table 18.8 Some traditional medicinal plants of Africa and Asia. (Source: Ayensu (1978); Kokwaro (2009); Simon et al. (2007); van Wyk et al. (2011); Wiart (2001, 2012))

The types of plant materials used in traditional medical systems throughout the world reflect the available plant material and the experience of generations of people that have tested various plant materials when ill. Traditional healers generally learn their trade by apprenticeships to established practitioners, by following healing practices used within a family group, or by personal experience from being a patient. Supernatural spirit powers are frequently invoked during the treatment process.

Although recognizing that traditional herbal medicines may sometimes be effective, conventional medicine practitioners generally reject the use of traditional medicines, primarily due to the lack of cleanliness and hygiene associated with the practitioner and plant materials, the lack of defined dosages, the lack of practitioner education, the mystification of the medicine and medical treatment, and the reluctance to share information on treatments except to those initiated into a code of secrecy. Plant materials used in treatments are frequently exposed to “unclean” environments (Fig. 18.9). Production, collection, and processing of medicinal and aromatic plants should be done under clean and sanitary conditions (Fig. 18.10).

Fig. 18.9

Supply of plant material for use in traditional medicine-herbal market in Nigeria. (Source: Herbal market in Nigeria. Photo courtesy of Dr. Ade-Ademilua, University of Lagos)

Fig. 18.10

Medicinal plant cultivation, harvesting, processing, and wild collection. (Source: All photographs courtesy of the authors, Maca photograph courtesy of Christopher S. Kilham, Medicine Hunter Incorporated)

By contrast, conventional medicines used in “modern” nations requires trained medical doctors and an associated professional staff, diagnostic equipment, and laboratory tests to confirm a patient’s health status , and uses manufactured pharmaceuticals in the form of pills, lotions, capsules, liquids, or injections to bring the body and/or mind to a “normal,” healthy state. These pharmaceuticals , chemical substances used for treatment, curing, or preventing a disease, are sometimes synthesized in a chemistry laboratory, but due to difficulty in chemical synthesis could also be extracted from medicinal plants. According to Taylor (2004), at least 120 current pharmaceuticals are sourced from plant materials (Table 18.10). Such plant based pharmaceutical drugs are used to treat a variety of medical problems, exemplifying the range of medicines extracted from plant tissues.

A cultural divide separates the traditional herbal healing (in which the medical practitioner brings the body into balance using natural products to modify the factors that caused the imbalance) to conventional healthcare (in which the medical practitioner determines the biological cause and treats the ailment with clinically approved, but expensive medicines). In the United States , an increasing number of healthcare consumers are beginning to seek natural herbal remedies along with a healthy approach to diet and lifestyle as an alternative to the high costs and side effects of chemically-based medicines (Dickinson et al. 2012). In the United Kingdom , 25 % of the population regularly uses herbal medicine (Behrens 2004). Herbal medicine, sometimes referred to as traditional medicine or complementary/alternative medicine (CAM), uses herbal materials, preparations, and finished herbal products that contain bioactive ingredients . Drug terms indicate the plant parts being used in herbal medicinal preparations (Table 18.9).

Table 18.9 Drug terms used with traditional natural product medicines. (Source: Samuelsson (1992))

Pharmaceuticals originating from plants arise from literature reports or with a study of plant materials used in traditional medicines by plant explorers. These medicine hunters visit native villages to observe plant use, identify the plant material, and source plant samples for analyses in chemical laboratories. Plant constituents are subsequently bio-assayed and those with positive indications are tested in clinical trials. The plant constituent to be used as a pharmaceutical may be modified to increase solubility, selectivity, and effectiveness (Lipinski 2004). If the chemical cannot be synthesized in the laboratory, an adequate supply of the plant material must be sourced through collection or cultivation.

Natural products remain an important source for developing medicines in the foreseeable future. Of the 877 new medicines developed during 1981–2002, 52 were from natural products , 237 were derivatives of natural products , and 140 were synthetics modeled after a natural product (Newman et al. 2003). Of the drugs considered essential by the World Health Organisation (Anon 2011), at least 10 % are exclusively from plant material. In the United States , interest in natural products decreased with the development of vaccinations and antibiotics and many in the American medical establishment instructed patients that natural products were ineffective for healing (Craker and Gardner 2006). This negative attitude by American medical personnel began to change after the Dietary Supplement Health and Education Act of 1994 was passed, and the medical doctors began to understand natural product supplements could help individuals remain healthy.

Table 18.10 Examples of current pharmaceuticals extracted from plants. (Source: Li and Adair (1994); Rajwar et al. (2011); Taylor (2004))

In addition to personal use of traditional medicines, farmers and others are using natural products to treat livestock and other domestic animals (Table 18.11). These ethnoveterinary practices frequently include holistic approaches (physical exercise and touch therapy) along with the use of medicinal plants. In Nigeria , 92 plant species are reportedly used in the treatment of animals (Nwude and Ibrahim 1980), and many plants are used for animal care in Asia , Africa , Central Europe and South America . As with using plants for personal healthcare, availability, low cost, effectiveness, cultural appropriateness, and ease preparation make ethnoveterinary practices attractive to farmers and to animal owners interested in a natural approach to animal health.

Table 18.11 Plant materials used in veterinary practice. (Source: Day (2012))

Applications: Herbs and Spices

Herbs and spices are terms used to distinguish two types of aromatic plants based on their use and preparation. The herbs are generally fresh, but sometimes dry, leafy tissues are used in small portions as a flavoring for foods and as a garnish. The spices are dried flowers, seeds, roots, or bark used in small portions for flavoring, coloring, and preserving food. The separation of aromatic plants into herbs and spices , however, is not clearly defined and some plants, such as dill (Anethum graveolens) fit into both categories as dill weed (a herb) and dill seed (a spice).

The herbs and spices used in various cuisines are territorial, a taste and aroma gained as a child when introduced to foods in the home. As individuals and groups migrate from their home country to another, they generally favor the taste and aroma of food from the area in which they came. Thus, these migrations bring new foods and flavors to areas to which they immigrate and introduce these foods and flavors to others, making these new tastes and aromas available as alternatives to the usual cuisine. This observation is particularly apparent in countries, such as the United States , to which a number of immigrants have arrived from several countries.

Immigrants to the United States from Mexico have brought traditional foods flavored with chili peppers , oregano , cumin , Mexican oregano , and epazote . Immigrants to the United States from China have brought traditional foods flavored with ginger , star anise , fennel seed, Szechuan pepper , cassia , and cloves . Immigrants to the United States from Mediterranean countries have brought traditional foods flavored with basil , bay leaves, caraway seeds, cardamom , chervil , cilantro , parsley , rosemary , saffron , tarragon , and other herbs and spices . The popularity and use of various herbs and spices are associated with familiar foods and culture and generally defined by location (Table 18.12).

Table 18.12 Some herbs and spices used in Asian dishes. (Sources: Kingsely (2009); Miyano and Miyano (2011))

Whether used by the pinch or by the bunch, herbs and spices can be used to infuse a meat or vegetable dish with unparalleled aroma and flavor (Fig. 18.11). In heated dishes, the aromatic oils within the herbs and spices diffuse from the plant tissue into the liquid in the cooking dish, flavoring the goods being prepared. In salads and other cool dishes, herbs and spices are usually added using a salad dressing or vinegar containing extracts of these plants. Recipes for using herbs and spices exist in several languages in numerous cookbooks for preparation of local dishes.

Fig. 18.11

Some culinary herbs and their uses. (Source: Authors)

Herbs and spices are a large global business with a complex market chain utilizing established brokers, importers, exporters, processors, and marketers (Fig. 18.12). As the largest herb and spice consumer in the world (Buzzanell et al. 1995), the United States imports over 40 separate spices from over 50 countries (Table 18.13). The end uses for these herbs and spices vary with about 40 % going to retail outlets and most of the rest being used in industrial food preparation and food service. Most herbs and spices are cultivated, as opposed to being collected in the wild because of the large quantities needed for world markets that could not be supplied by wild collection and by the need to provide authenticated, clean plant material to the market. Consumer demand is moving towards organically produced herbs and spices under fair trade conditions. A number of herb gardens have been established to familiarize the public with medicinal and aromatic plants (Fig. 18.13) (Table 18.14).

Fig. 18.12

Some spices of the modern world. (Source: Upper row photographs by the authors, bottom row photos courtesy of Ákos Mathe, West Hungarian University)

Fig. 18.13

Herb gardens. (Source: Authors)

Table 18.13 The production and use of some spices. (Sources: Kingsely (2009); Miyano and Miyano (2011))

Table 18.14 Some plant materials used in perfumes. (Sources: Bukisa (2013); Ellena (2011); Kole et al. (2005))

Conclusions

The market for medicinal and aromatic plants can be expected to continue into the future, serving as the primary medicine for most people in the world and as an alternative medicine for those individuals accustomed to more conventional medicine. The continued exploration for new medicines among the numerous plant species will most likely discover new chemical constituents that can be used to treat human and animal diseases. The challenge will be to protect and conserve medicinal and aromatic plants and plant habitats from poachers and land developers.

The exact level of the global market for medicinal and aromatic plants is difficult to discern due to the large number of plants, the multitude of countries importing and exporting, and the increasing number of sales outlets, including sales to mass markets via the internet. Over the past 20 years, consumers previously unfamiliar with medicinal and aromatic plants have become more aware of the many contributions these plants make to medicine, food products, cosmetics , and other goods used daily. The acceptance of natural medicines by the conventional medicine system in the United States has encouraged many previously cautious people into trying medicinal plants. The immigration of individuals and families from other countries and the expansion of global travel have introduced several people to new cuisines.

The value of medicinal plant extracts has encouraged scientists and businesses to invest time and money into developing alternative methods for producing these extracts. Chemists are beginning to synthesize constituents in some plant extracts that can perhaps replace the need for limited plant materials or can make changes in a constituent that will enhance bioactivity. Plant scientists and bioengineers are attempting to produce extracts via cell and microorganism cultures. In the future, geneticists may make modifications in a plant genome to enhance extract production or enable other plants to produce bioactive constituents. The success of these alternative production methods may limit the need for cultivation and wild harvesting of some plant materials.

Medicinal and aromatic plants have not cured all the diseases, but the interest in new medicines has led to the development of standardized screening processes for bioactivity and has advanced the use and science of medicinal and aromatic plant materials and their extracts. Scientific investigations on medicinal and aromatic plants are conducted in many countries with results presented at professional conferences and published in professional journals available to the public. Numerous books, websites, and blogs publicize the benefits of natural products. Medicinal and aromatic plant material can be observed in private and public display gardens.

Progress in good practices associated with crop production, wild plant collecting, and processing practices provide businesses and consumers with quality plant materials. Various conservation practices, laws, and trade restrictions help protect endangered species and ecosystems, although the destruction of plant habitats remains a serious issue for survival for some species. Medicinal and aromatic plants have played an important role in human life and will undoubted continue to be important for the foreseeable future.