Basil is an herbaceous member of the mint family, Labiatiaceae, that originated in India and has been used for about 5,000 years. The most common type of basil in cultivation is Ocimum basilicum, a heterogeneous species with a number of different cultivars. Even the same cultivar may vary greatly in morphology and also in chemical composition. There is a rich history surrounding basil and a great deal of mythology. Its use once limited to royalty, the name derives from the Greek basilikos, meaning “royal.” Now, basil is grown all over the world for its culinary use as a spice. Essential oil is also produced from the leaves and the flowers for flavoring and industrial purposes, and there has been a great deal of study on the antimicrobial properties of the herb and its oil.
There is a large amount of tradition about basil on all of the continents on which it has been grown for long periods. One common thread is consideration of the herb as an enticement to love, either as a part of sacred ceremonies such as weddings or as an ingredient in aphrodisiac dishes. Holy basil, Ocimum sanctum, has long been used in sacred Hindu ceremonies and is part of both traditional Hindu weddings and funerals. In some parts of Europe, acceptance of a sprig of basil by a man indicates that he will love the woman who gave it to him forever or even formal acceptance of a marriage proposal.
Alternatively, in Greece, basil has been considered a symbol of hatred and sorrow. The plant was vilified by the influential Greek philosopher Chrysippus in 200 BCE as causing one to become dim-witted. The ancient Romans cursed basil as they planted it, believing this would cause it to grow better.
As with many plants, the assignment of taxonomic systems to Ocimum species has been an active and contentious area. The genus was named by Swedish naturalist Carl Linneaus in 1753 and enumerated into five species. In the past 250 years, there have been a number of taxonomic systems proposed, greatly expanding the species number of Ocimum. These systems were based on the structure of the plants, particularly those of the flower parts. The most recent system in common use is that of British plant taxonomist Alan Paton and his fellow researchers from 1999, in which Ocimum basilicum is placed within the Ocimum section of this genus.
The morphology of the plants can vary greatly, however, given the crossbreeding between various cultivars and species. The science of chemistry was in its infancy when the original Ocimum taxonomic schemes were proposed. Since then, the advent of analytical chemistry has enabled the study of the numerous aromatic compounds found in all species of Ocimum, particularly those of the essential oils of these plants. This has enabled researchers to classify types of basil based on their chemical profile—a field called chemotyping. One problem with this avenue of research is that the particular chemicals in a given plant can vary widely depending on the environmental conditions, seasons, and soil and region in which the plant is grown.
Other methods of taxonomic classification of basils include examining the geographic origin of the plants and whether or not they can be crossed with one another. With the discovery of DNA and cell nuclei, karyotyping became a viable method of analysis. This technique involves determining the number and appearance of chromosomes per cell in an organism. One problem with using this technique in basil is that researchers have found the chromosome count to vary widely within the same species.
The advent of molecular biological techniques, such as the polymerase chain reaction (PCR), have ushered in a new era in plant taxonomy. A class of techniques based on PCR amplification of small amounts of DNA takes advantage of minor changes in the structure of genes. This enables genetic fingerprinting to be performed in plants and gives a much more rigorous analysis of genetic relatedness. Croatian scientist Klaudija Carović-Stanko and coworkers published such research in 2010 comparing 28 accessions of basil, including 22 of Ocimum basilicum. In addition to PCR techniques that had been previously used in studies of basil taxonomy, along with chromosome counting, they also examined nuclear DNA content.
Of the two PCR methods used, AFLP (amplified fragment length polymorphism) was successful in separating all of the accessions from one another. The commonly used technique RAPD (random amplified polymorphic DNA) did not distinguish between all of the accessions. In tandem, both of these techniques enabled the researchers to successfully classify all of the plants tested.
The researchers found a high level of genetic diversity among the basil accessions, which adds credence to the taxonomic system of Paton with its large number of species. They were able to clearly separate taxa, with a grouping of Ocimum basilicum type species and one of Ocimum americanum. All of the Ocimum basilicum cultivars and varieties fit within that species’ group. Surprisingly, Ocimum minimum fit within the group also and could be a subspecies or variety of Ocimum basilicum. An additional surprise was that two Russian accessions of Ocimum basilicum ssp. purpurascens were found to be genetically identical to the Ocimum americanum group, which has a different number of chromosomes.
The current classification of basil includes several varieties and a very large number of cultivars. The primary variety of Ocimum basilicum in cultivation is basilicum, in particular cultivars Sweet Basil and Genovese. Also notable are varieties purpurascens Benth. (purple basil), and difforme Benth.
Ocimum basilicum is an herbaceous plant originally from tropical areas such as southern Asia, including India, and Africa. In such climates, it can grow as a perennial. In much of the world, it is cultivated as an annual, however, since the plant cannot tolerate frost. Basil grows best in long days with full sun and requires consistent moisture.
Being a member of the mint family, basil has square stems. The leaves are dotted with glands containing oil comprising a number of aromatic secondary metabolites. The composition of the oil varies greatly between different cultivars. It is the oil that gives basil its distinctive, slightly mint-like smell.
Basil leaves are formed opposite to one another and are velvet green or purple, depending on the cultivar. The plants grow to about one-and-seven-tenths to four feet in height and produce white flowers in a terminal spike. Normally when cultivated as an herb, the flowers are cropped off, so the plant will keep producing leaves. The flowers set seeds that are also rich in oils. The seeds produced by a particular plant can vary greatly in their morphological characteristics and oil production. This lack of genetic uniformity can complicate the growth of cultivars from seeds. All of the members of the Ocimum basilicum group have 48 chromosomes.
Basil is used around the world as a culinary ingredient. Dried and fresh basil leaves are among the most popular herbs in Italian, Mexican, Greek, and French cuisines. The flavoring is added to vinegars, pickled vegetables, and salads. One common association of basil is as an ingredient in pesto, the classic Italian dish that uses this herb as a primary ingredient. Basil is also an important herb in Asian cuisines. The plant is versatile for so many cuisines partly because its taste varies over different regions. This is due to variation in the types of chemicals found in the plants in various parts of the world.
The use of dried or fresh basil can greatly affect its utility as a spice. The process of drying causes the plant to lose a high percentage of its oil content. Thus the resulting herb is less aromatic and flavorful. Drying can be highly important for storage, however, to keep pathogens from growing on the herb. Also, the tissue turns black fairly quickly after being harvested if it is not rapidly used or stored in a special manner. Once dried, the herb keeps its essential oils for several years if stored properly. Frozen basil is becoming a more popular commodity, since it can be used in the same manner as fresh basil, which is subject to seasonal availability in cooler climates.
The essential oil can be isolated from the leaves of the plants or from the flowers. The flowers contain a smaller percentage of oil, but it is considered superior to that from the leaves. Basil oil is an important industrial item and is used primarily in the food industry and in the making of perfumes. Sweet basil grown from seed is the primary source of essential oil for industrial purposes. Because of the variability of oil composition in plants grown from seed, along with other factors, the chemotype is determined before the plant is used for this purpose.
The food industry uses basil oil in a range of products, from candies to meat products and liqueurs. It dissolves in fatty acids and can be added to perfumes, shampoos, soaps, and dental products. The essential oils are produced by the distillation of flowering plants grown in favorable climates where it is economically feasible to produce them on a large scale.
Basil has been used in traditional medicine for millennia to treat a number of conditions. Its oil has long been considered to be antimicrobial. Research has shown promise with some Ocimum basilicum oils and is being vigorously pursued.
Basil contains a number of low-molecular-weight, volatile, aromatic compounds in its essential oil that give the characteristic aroma and flavoring to the plant. The herb contains 0.5–1.5 percent of essential oils that vary in their composition. Most of the compounds distilled from the essential oil have oxygen groups on them. Studies have found up to 54 different chemicals in the distilled oil of Ocimum basilicum. Prominent compounds include estragole, linalool, cardinol, ocimene, 1,8-cineole, and bornyl acetate. Seeds are the source of a nonvolatile oil known as “fixed oil” that is rich in fatty acids. This oil can be obtained by cold pressing. It is rich in polyunsaturated acids, with approximately 50 percent of the oil being composed of linoleic acid and around 22 percent composed linolenic acid. The monounsaturated fatty acid oleic acid is the next most prominent fatty acid at 8–15 percent of the total. Found in lesser quantities are the saturated fatty acids palmitic acid and stearic acid, while free fatty acids were found to comprise only 0.5 percent of the total.
The whole plant contains up to 10 percent of tannins. It also contains the phytosterol ß-sitosterol. Several phenolic compounds have been isolated, both unconjugated and with sugars attached (glycosides). The quercetin glycosides rutin and isoquercetin have been isolated from Ocimum basilicum. In addition, several flavonoids and flavones have been identified. The polyphenols caffeic acid and rosmarinic acid have both been isolated, as has p-coumaric acid—an important intermediary compound in phenolic compound biosynthesis.
The analytical techniques of secondary plant compound isolation are constantly being refined and improved, so it is highly likely that additional compounds will be discovered in this prolific producer of secondary metabolites.
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