**2. Aromatic plants and herbal extracts**

Aromatic plants and herbal extracts have been used for thousands of years as incense, perfumes, and cosmetics. After the discovery of plants' medicinal properties, natural flora became a valuable source of health improvement in the ancient civilizations.

Literature dating from around 2000 BC lists over 700 substances, including ginger, myrrh, and coriander, used for therapeutic purposes in India. Chinese ancient herbal tradition was recorded in Yellow Emperor's *Book of Internal Medicine*, dating more than 2000 BC. However, the most famous and richest associations concerning the first aromatic materials are those surrounding the ancient Egyptian civilization. Through thousands of recipes, Egyptian papyruses showed that, for example, coriander and castor oils were used as cosmetics, for medicinal applications, and as preservatives [10].

Therapeutic uses of herbal plants were described by several scholars, during Greek and Roman period, namely Hippocrates, Galen, Dioscorides, and many others [11]. Romanians are known for their use of medicinal herbs since very long; thus, Herodotus was the first to mention the method of distillation of turpentine, in about 425 BC. These great Graeco-Roman works were translated into Persian, Arabic, and other languages, and at the end of Byzantine Empire, their knowledge was passed to the Arab world. However, in 1975, archeological expedition in Pakistan found perfectly preserved perfume containers and terracotta apparatus similar to distillation apparatus, from about 3000 BC. This discovery suggests that the Arabs simply revived or improved upon a process that had been known for over 4000 years. In 19th century, herbal products were introduced by Romanian pharmacopoeia, whereas in 1904 the first institute of medicinal herbs was established in Cluj city [10].

#### **2.1. History and definition of bioactive compounds**

The use of aromatic plants and herbal extracts in the past demonstrates the history of bioactive compounds usage. Typically, bioactive compounds of plants are produced as secondary metabolites [12]. Traditionally, secondary plant metabolites have been defined as all com‐ pounds synthesized by the plant that do not appear to be essential for plant growth and development and/or those compounds without an obvious function [13]. In different words, secondary metabolites are those metabolites that are often produced in a phase of subsequent to growth, have no function in growth (although they may have survival function), have unusual chemical structures, and are often formed as mixtures of closely related members of a chemical family [14]. They are not universally synthesized in all plants. In contrast, primary metabolites are produced by all plants, usually are part of essential metabolic processes of growth and development, such as proteins, carbohydrates, lipids, and amino acids.

In recent years, as interest as well as investigation of secondary metabolites grow, this artificial and naive definition is changing. Natural functions of many secondary metabolites are unknown because they have never been investigated; this lack of evidence or knowledge is then interpreted as lack of function [15]. Recently, it was discovered that they can help plant to increase their overall ability to survive and overcome local challenges by allowing them to interact with their surroundings [16, 17]. Thus, floral species synthesize aroma to attract insect for their pollination and fertilization while synthesis of toxic chemical has evolved to patho‐ gens as well as to herbivores for suppressing the growth of neighboring plants [18].

Among secondary metabolites, some of these substances have effect on biological systems, which are considered as bioactive. Thus, bioactive compounds in plants can be defined as follows: secondary plant metabolites eliciting pharmacological or toxicological effects in humans and animals [19]. Bioactive compounds of plants could be divided into three main categories: (1) terpenes and terpenoids (approximately 25,000 types), (2) alkaloids (approxi‐ mately 12,000 types), and (3) phenolic compounds (approximately 8000 types) [13].

#### *2.1.1. EOs as bioactive compounds*

EOs are natural, volatile, complex plant compounds, oily or lipid-like in nature, and frequently characterized by strong fragrance [20, 21]. They have been known as herbal extracts to mankind for hundreds of years, even millenniums, but there are different opinions about historical origin of EO production [22]. According to some authors, China has been the cradle of hydrodistillation [23], whereas others claim, as already mentioned, that Arabs invented this process about 3000 BC. Also, there are other opinions that distillation as a method of producing EOs was first used in the East (Egypt, India, and Persia) more than 2000 years ago and improved in the 9th century by the Arabs [23, 24]. However, Villanova (ca. 1235–1311), a Catalan physician, was the first scientist who performed distillation of EOs and left authentic written account [23].

By the 13th century, EOs were being made by pharmacies and their pharmacological effects were described in pharmacopoeias [24], but their use does not appear to have been widespread in Europe until the 16th century when they were traded in the City of London [20]. In "Grete Herbal," published in 1526, some of the illustrations of the retorts and stills used for the extraction of volatile oils are presented [25].

While knowledge of the science of EOs did not increase during 17th century when pharmacies generally used 15–20 different oils, 18th century brought about only small progress in the design of equipment and in refinements of the techniques used [23]. At the end of the 18th century, the use of tea tree oil for medical purposes has been documented in Australia [26]. In 19th century, the first movable apparatus appeared, cooling methods were improved, and double-wall distillation plants appeared. The medical properties and application of increasing number of new EOs were analyzed and recorded by the pharmacists, and the first experimental measurement of the bactericidal properties of EOs were performed [23]. With the scientific revolution in this century, chemists were able to identify for the first time the various constit‐ uents of EOs, and the most important investigation was performed by O. Wallach, an assistant of Kekule [27]. In 1876, Haarman and Reimer started the first production of synthetic aroma chemicals: vanillin, coumarin, anisaldehyde, heliotropin, and terpineol. These researches, partly, laid the ground for the development of the EOs' synthetic counterparts and the growth of the modern drug industry. By the middle of 20th century, the use of EOs in medicine gradually became secondary to their use for flavor and aroma in perfumes, cosmetics, and foodstuffs [28].

#### *2.1.2. Taxonomy of EO-producing plants*

EOs are complex mixture of volatile compounds produced by living organisms, isolated by steam distillation, hydrodistillation, or solvent extraction from a whole plant or plant part of known taxonomic origin [29]. Around 3000 EOs have been produced from around 2000 plant species that belong to various genera distributed to around 60 families. It is well known that all plants possess the ability to produce volatile compounds, however, quite often only in traces. Only 300 EOs are important from the commercial point of view, and "EO-bearing plants" in particular are those plant species delivering an EO of commercial interest [20]. Some of those plants have specialized anatomic structures (secretory cells, glandular trichomes, and cavities/ducts), which leads to accumulation of volatiles and higher concentration in the plant [30]. Others, such as rose (*Rosa* spp.), jasmine (*Jasminum sambac*), or tuberose (*Polyanthes tuberosa*), produce and emit the volatiles by the epidermal layers of their petals [31, 32]. Their EO yield is exceptionally small so special techniques have to be applied to recover volatile fragrance compound.

The term "EO" groups together a wide range of chemical compounds on the basis of their historic use and method of isolation and belies the variety and complexity of compounds found within them [33]. Some plant families are particularly well known for their oil-bearing species and ability to produce EOs of medicinal and industrial value. These include *Alliaceae, Apiaceae, Asteraceae, Cupressaceae, Lamiaceae, Lauraceae, Myrtaceae, Piperaceae, Poaceae, Rosaceae, Rutaceae, Santalaceae, Zingiberaceae*, and *Zygophyllaceae* [30, 34]. Those plant families are not restricted to one specialized taxonomic group but are distributed among all plant classes. Gymnosperms, for example, the families Cupressaceae (cedarwood, cedar leaf, juniper oil, etc.) and Pinaceae (pine and fir oils, etc.), produce EOs with significant biological activities [34] as well as angiosperms (*Magnoliopsida*, *Rosospida*, and *Liliospida* [35–37]).

#### *2.1.3. Composition of EOs*

EOs as herbal extracts are not simple compounds or even simple mixtures of several individual compounds. They may comprise up to approximately 100 components, but usually between 20 and 60 components [38–40]. Major components can constitute up to 85% of the EO, whereas other components are present only as a trace [24, 41]. Numerous publications have presented data on the composition of the various EOs [20, 21, 42–48].

The major volatile constituents are hydrocarbons (e.g. pinene, limonene, and bisabolene), alcohols (e.g. linalol and santalol), acids (e.g. benzoic acid and geranic acid), aldehydes (e.g. citral), cyclic aldehydes (e.g. cuminal), ketones (e.g. camphor), lactones (e.g. bergaptene), phenols (e.g. eugenol), phenolic ethers (e.g. anethole), oxides (e.g. 1,8 cineole), and esters (e.g. geranyl acetate) [45]. They are defined as substances composed of isoprene (2-methylbuta‐ diene) hydrocarbons joined together in a repetitive head-to-tail manner (known as the isoprene rule) [46, 49]. Leopold Ruzicka, recipient of the 1939 Nobel Prize in Chemistry, proposed biogenetic isoprene rule [50], which emphasizes the single biochemical origin of terpenes.

The composition of EOs are influenced by many factors such as plant ecotype or variety, genetic variation, plant nutrition, application of fertilizers, geographic location of plants, surrounding climate, seasonal variation, stress during growth or maturity, and postharvest handling, and also by isolation technique applied [51–53].

The major components of EOs isolated from aromatic herbs examined in this chapter are presented in Table 1.


**Table 1.** Major components of selected EOs

#### *2.1.4. Economic importance of EOs*

Use of herbal extracts, particularly EOs, for perfumery, additives in food/confectionary as well as for pharmaceuticals and cosmetics is a growing market trend. The huge production of EOs (>70,000 tons per annum) with estimated market value of more than 700 million \$US indicates that production and consumption of EOs is increasing all over the world [45]. This production is achieved mainly by major cultivators and producers like the United States, Brazil, India, and China; although some other countries are important contributors of EOs; for example, vetiver/ khus, clove, lemongrass, basil, and celery oils are mainly produced in India, whereas Spain and France are major producers of rosemary oil [52, 60].

Today, aromatherapy and use of herbal extracts as "natural" products are fast developing segment of the industry, and this is a return to what was common practice in ancient and medieval times [64].

The largest world consumer of EOs is the flavor industry, especially for soft drinks. However, this is limited to a few EOs, mainly citrus (orange, lemon, grapefruit, and lime), ginger, cinnamon, clove, and peppermint. Similar oils are used in confectionery, bakery, desserts, and dairy products, besides some fruity products and spices. Alcoholic beverage industry is the one of the largest consumers of various EOs as well as dairy, desserts, sweet bakery, confec‐ tionery, and cream manufacturers. The fast food and processed food industries use EOs with spicy and herbal flavors: oregano, basil, fennel, pepper, dill, etc [65].
