Alcohol and other chemicals

Living plants are biochemical factories. In addition to cellulose (a plant’s main structural material used for making paper and rayon), there is a wide range of chemicals produced by various plants, from sugars and fats to complex compounds employed as drugs, dyes, and pigments. Some chemical substances, such as tannins, may be extracted directly from plants. Others, such as alcohols, are made by processing plant materials.

Ripe grapes contain glucose, a simple sugar that can be fermented in the wine-making process to produce ethyl alcohol.

Sugars and starch

The most common sugar in plants is glucose, a product of photosynthesis and the building block, or monomer, from which the natural polymer cellulose is made. Glucose is the principal sugar in grapes. Other simple sugars, known chemically as monosaccharides, that are derived from plants include fructose (in various fruits and in honey) and ribose (in the polymeric backbone of the nucleic acid DNA).

Disaccharides, formed of two monosaccharide units joined together, are more complex sugars. They include maltose (from malt, or sprouted barley) and sucrose (from sugar beet or sugar cane), the sugar used in cooking and for sweetening beverages. This sugar has been used for nearly 8,000 years. It was extracted from sugar cane first in India and much later (after about 300 B.C.) in the Middle East; it was introduced to the Americas by Europeans in 1493. Sugar was not obtained from beets until the eighteenth century. Maple syrup is a sugary sap extracted from various North American species of maple trees (Acer spp.).

The chief polysaccharides—polymers made of thousands of monosaccharide (glucose) units—are cellulose and starch. Cellulose and its uses are described in other articles in this section. Starch is a storage material in cereal grains, roots, and tubers such as potatoes. Apart from being the principal ingredient in various kinds of flours, it is also used to make adhesive paste and sizing for textile fabrics.

Tannins have long been used for curing animal hides to make leather, either in a traditional way as in Morocco (left) or in modern tanneries. A much more recently introduced plant product is soy oil, which is extracted from the seeds of the soybean plant (right) ami used in various foodstuffs.

Vegetable oils and fats

Various plants are grown for the oils contained in their seeds, from palms and olives to flax (the source of linseed oil), sunflower, and specially developed strains of safflower. Vegetable oils are used to make soap and in foodstuffs. One of the more recent additions to the list of oil-bearing plants is soybean (Glycine sp.). Since its potential was first realized in the 1930’s, annual output has increased more than 200 times, and soy now occupies 68 million acres (27.5 million hectares) of arable land in the United States. The food industry uses most of the oils extracted from soybeans in such products as candy, ice cream, margarine, mayonnaise, and sausages. Other soy oils are used to make glycerin, paints, and soaps.

Numerous other plants yield industrial oils. Citrus seeds, tung [Aleurites sp.), and linseed (from flax, Linum sp.) provide the bases for enamels, paints, and lacquers; rice bran produces a hard wax and an oil employed in rustproofing. Linseed and soy oils can be chemically processed to make vinyl ethers, which in turn are polymerized to make tough plastic film.

Safflower (Carthamus tinctorius), originally from India, has been genetically modified so that its seeds yield 40 per cent oil with a lin-oleic acid content of nearly 80 per cent, hence its popularity as a low-calorie, low-cholesterol ingredient in cooking and salad oils. Recently, new strains have been introduced with a high oleic acid content, making them competitive with olive oil.

Castor (Ricinus communis), originally an ornamental plant from Africa, is now widely grown for its oil-rich “beans.” Castor oil is a good lubricant and an ingredient in polyurethane foam plastics, printing inks, and adhesives. Genetic modification has succeeded in creating a uniformly short plant with tripled yields of oil.

Another new and successful product of plant breeding is a member of the Scandinavian mustard family, known as crambe [Crambe sp.). Its seeds contain large quantities of erucic acid, which also constitutes up to 50 per cent of rapeseed oil. The substance has found profitable application as a lubricant in the continuous casting method of making steel. It is also used in the processing of rubber and some plastics.

The seeds of a Mexican desert plant called jojoba (Simmondsia californica) yield a liquid wax which is used as a “tough” industrial lubricant in place of spermaceti oil from whales. It is also an ingredient of some cosmetics and shampoos.

The flowers of many plants contain oils—extracted by squeezing in presses, using solvents, or by careful steam-distillation—which are the chief aromatic ingredients of perfumes. Also called essential oils, they are the basis of flower-growing industries in several Mediterranean countries. Yields are predictably low—for example, 1.25 tons of rose petals are needed to produce 1 pound of rose oil— and the value of the oils is correspondingly high.

Grape-pickers in France work along rows of Pinot vines, the variety employed for making Champagne. Carbon dioxide gas, a byproduct of the fermentation process, is retained to give the wine its sparkling, effervescent quality.

Alcohols

Traditionally, alcohol (specifically ethyl alcohol, or ethanol) is made by fermenting sugars or starch, one of the earliest chemical processes to be discovered. In wine making, yeast is employed to ferment the sugars in fruit; in beer making, the source of the alcohol is the starch in cereals, such as barley. (Any sugary or starchy plant material, however, can be fermented to make alcohol). The fermentation process is accompanied by the evolution of carbon dioxide gas, which is what gives the effervescent quality to beer and some wines.

Most ordinary wines (as opposed to fortified wines such as port and sherry) contain 8 to 15 per cent alcohol, whereas beers seldom have more than 2 to 6 per cent. To extract concentrated alcohol from such sources they haye to be distilled (giving 95 per cent ethyl alcohol).

Ethyl alcohol manufactured for commercial purposes is usually “denatured” (to prevent misuse) by adding the highly poisonous substance methyl alcohol (methanol). Also called wood alcohol, it can be made by the destructive distillation of wood—that is, by heating pieces of wood strongly in the absence of air. Methyl alcohol can be added to gasoline to produce “gasohol,” a liquid fuel for motor vehicles. Like ethyl alcohol, methyl alcohol is also used as a solvent and as an important starting material in synthetic chemistry, particularly for making monomers for the production of plastics.

Tannins

Since the first animal skins were made into leather, tannins have been used to soften and cure hides. They are complex chemicals called polyphenols, found mainly in the bark and heartwood of hardwoods, such as oaks (Quer-cus spp.) and chestnuts (Castanea spp.l, as well as in the Australian acacia (Acacia sp.) and South American quebracho trees (Aspido-sperma spp.). Mangrove bark, coniferous trees, the nuts of the betel palm, and the leaves of the tea plant are also rich sources of tannins, whose natural function in the plants prevents decay. They are usually extracted by boiling the vegetable material in water.

In addition to their use in leather making, tannins have many other applications; they are used, for example, in the manufacture of dyes and inks and as coagulants in the early stages of rubber processing.

An unusual liquid wax, incorporated into some cosmetic preparations, is obtained from the crushed “beans” of the Mexican jojoba plant. Jojoba oil is also used as a lubricant for machinery.

Drugs from plants

Many drugs occur in the leaves, fruits, seeds, and roots of various plants, and although most can be made synthetically in the laboratory, such is their complexity that it is often more economical to use the natural sources.

Most plant drugs are alkaloids—nitrogen-containing compounds that have an alkaline reaction. Those derived from leaves include atropine, from belladonna (Atropa belladonna)-, digitalis, from foxgloves (Digitalis spp.); cocaine, from the South American coca shrub (Erythroxylon coca)-, and strychnine, from the evergreen Strychnos nux-vomica. People in southeastern Asia have traditionally chewed the leaves of betel-pepper (Piper betie), now the source of a counterirritant drug, and the addictive nature of cigarette smoking has been attributed—at least in part—to the presence of the alkaloid nicotine in the leaves of tobacco (Nicotiana tabacum).

In the case of certain trees it is the bark that is the source of drugs; for instance, the anti-malarial drug quinine comes from the cinchona tree (Cinchona officinalis), originally from South America but now also cultivated in Java, and pseudopelletierine, an antihelminthic drug, comes from the pomegranate tree (Punica granatum). In other cases it is the plant’s roots that provide the drug; reserpine, for example, which is used for treating high blood pressure, occurs in the roots of the tropical rauwolfia (Rauwolfia serpentina), and ginseng is extracted from the roots of the Chinese panax (Panax ginseng).

The best-known plant whose seeds yield drugs is the poppy (Papaver somniferum), source of opium and its morphine derivatives. Like all alkaloids, the opium compounds are highly poisonous, although the most toxic is probably ricin, extracted from the seeds of a castor plant (Ricinus sanguineus), which has been considered as a possible agent for chemical warfare.

Brewing is a fermentation process (using yeast) that produces beer containing ethyl alcohol derived mainly from the starch in malt, which is made by soaking grains of barley and allowing them to germinate. Some sugar is also added to the mixture (called wort), along with hops for flavor.

Dyes and pigments

Before the development of the synthetic dyestuffs industry, plants were a major source of colorants for dyes, inks, and paints. Dye plants were grown as farm and plantation crops, and included woad (Isatis tinctorial and indigo (In-digofera sp.), whose leaves yield blue dyes, and madder (Rubia tinctorum), which contains alizarin and various other red dyes in its roots.

The leaves of henna (Lawsonia inermis) are still used for dyeing hair and by many Asian peoples for coloring their nails reddish-brown and for decorating the skin. Another red pigment with cosmetic uses (it is employed as rouge) is made from safflower petals. The similarly named saffron, or autumn crocus (Colchi-cum autumnale), yields a yellow pigment employed for dyeing textiles and coloring food.

The chemical indicator litmus—which has a blue color in alkaline solutions and a red color in acid ones—is prepared from various species of lichens, particularly those belonging to the genus Variolaria. Lichens also provide soft colors used to dye wool in northern countries.

Sugar cane, resembling bundles of dried sticks, is a difficult crop to handle and requires special machines, although much of the actual cutting is still done by hand. It is now grown in most tropical countries as a source of table sugar and molasses; part of this crop, from Barbados, will be fermented to make rum.

Chemicals from wood pulp

The castor oil plant has creamy-yellow flowers that give way to prickly capsules containing a large seed or “bean.” The seeds are the source of castor oil, used as a lubricant and starting material for making plastics; they also contain the poisonous substance ricin.

The liquors that remain after the various processes for preparing wood pulp in papermaking—once discarded as waste—are a rich source of chemicals. In the United States, for example, more than 80 per cent of the annual production of turpentine and rosin is obtained from so-called “black liquor,” a by-product of the sulfate process for papermaking. Turpentine is condensed from the vapors and purified by distillation, and the concentrated liquor used as a lubricant and source of rosin, for sizing paper, and as a drying agent in paints and varnishes.

Another product of pulp manufacturing is lignin, which may be burned as a fuel (usually for making steam at the paper mill) or added to clay slurries in the ceramics industry (to reduce their viscosity) or to mud slurries employed as lubricants when drilling oil wells. Lignin can also be hydrogenated—that is, it can be reacted with hydrogen at high temperature and pressure in the presence of a catalyst—to give high-molecular-weight cyclic alcohols used as plasticizers (softeners) for plastics. Vanillin, a flavoring agent that tastes of vanilla, can also be manufactured synthetically from lignin.

An Indian woman’s hands display intricate patterns drawn on using the vegetable dye henna, made from the dried leaves of a plant widely grown in India and northern Africa. Sometimes the color is made purple by adding the blue dye indigo.