Some common unsaturated fatty acids are:
The monounsaturated, eighteen-carbon oleic acid is the major fatty acid of olive oil. Although oleic acid is found in other oils and also in many fats, olive oil is the most important source of this fatty acid. Olive oil contains a larger proportion of monounsaturated fatty acid than any other oil. The percentage of oleic acid in olive oil varies from about 55 to 85 percent, depending on the variety and the growing conditions, cooler areas producing a higher oleic acid content than warmer areas. There is now convincing evidence that a diet with a high proportion of saturated fat can contribute to the development of heart disease. The incidence of heart disease is lower in the Mediterranean region, where a lot of olive oil—and oleic acid—is consumed. Saturated fats are known to increase serum cholesterol concentrations, whereas polyunsaturated fats and oils lower these levels. Monounsaturated fatty acids, like oleic acid, have a neutral effect on the serum cholesterol level (the level of cholesterol in the blood).
The relationship between heart disease and fatty acids also involves another factor: the ratio of high-density lipoprotein (known as HDL) to low-density lipoprotein (known as LDL). A lipoprotein is a water-insoluble accumulation of cholesterol, protein, and triglycerides. High-density lipoproteins—often called the “good” lipoproteins—transport cholesterol from cells that have accumulated too much of this compound back to the liver for disposal. This prevents excess cholesterol from depositing on artery walls. The “bad” lipoproteins, LDLs, transport cholesterol from the liver or small intestine to newly formed or growing cells. While this is a necessary function, too much cholesterol in the bloodstream ultimately ends up as deposits of plaque on the artery walls, leading to narrowing of the arteries. If the coronary arteries leading to the heart muscles become clogged, the resultant decreased blood flow can cause chest pain and heart attacks.
It is the ratio of HDL to LDL, as well as the total cholesterol level, that is important in determining the risk of heart disease. Although polyunsaturated triglycerides have the positive effect of reducing serum cholesterol levels, they also lower the HDL:LDL ratio, a negative effect. Monounsaturated triglycerides like olive oil, while not reducing serum cholesterol levels, increase the HDL:LDL ratio, that is, the proportion of good lipoprotein to bad lipoprotein. Among the saturated fatty acids, palmitic (C16) and lauric (C12) acids raise LDL levels appreciably. The so-called tropical oils—coconut, palm, and palm kernel—which have high proportions of these fatty acids, are particularly suspect in heart disease because they increase both serum cholesterol and LDL levels.
Although the healthy properties of olive oil were prized by ancient Mediterranean societies and were considered to account for longevity, there was no knowledge of the chemistry behind these beliefs. In fact, in times when the main dietary problem would have been simply to obtain enough calories, serum cholesterol levels and HDL:LDL ratios would have been irrelevant. For centuries, for the vast majority of the population of northern Europe, where the main source of triglycerides in the diet was animal fat and life expectancy was less than forty years, hardening of the arteries was not a problem. Only with increased life expectancy and the higher intake of saturated fatty acids accompanying economic prosperity has coronary heart disease become a major cause of death.
Another aspect of the chemistry of olive oil also accounts for its importance in the ancient world. As the number of carbon-to-carbon double bonds in a fatty acid increases, so does the tendency for the oil to oxidize—become rancid. The proportion of polyunsaturated fatty acids in olive oil is much lower than in other oils, usually less than 10 percent, giving olive oil a longer shelf life than almost any other oil. As well, olive oil contains small amounts of polyphenols and of vitamins E and K, antioxidant molecules that play a critical role as natural preservatives. The traditional cold-press method of extracting oil from olives helps retain these antioxidant molecules, which can be destroyed by high temperatures.
Today one method of improving the stability and increasing the shelf life of oils is the elimination of some of the double bonds by hydrogenation, a process of adding hydrogen atoms to the double bonds of unsaturated fatty acids. The result is also a more solid triglyceride; this is the method used to convert oils into butter substitutes like margarine. But the process of hydrogenation also changes the remaining double bonds from the cis configuration to the trans configuration, where the carbon atoms of the chain are on opposite sides of the double bond.
Trans-fatty acids are known to elevate LDL levels but not as much as saturated fatty acids.
TRADE IN OLIVE OIL
The natural preservatives present as antioxidants in olives would have been of paramount importance to the oil traders of ancient Greece. This civilization was a loose association of city-states, with a common language, a common culture, and a common agricultural economic base: wheat, barley, grapes, figs, and olives. For centuries the land around the shores of the Mediterranean Sea was more wooded than it is now; the soil was more fertile, and more water was available from springs. As the population grew, cultivation of crops spread from the original small valleys up the sides of coastal mountains. With its ability to grow on steep and stony ground and withstand drought, the olive tree became increasingly more important. Its oil was even more valuable as an export commodity, for in the sixth century B.C., along with the strict laws against uncontrolled felling of olive trees, Solon of Athens also mandated olive oil as the only agricultural product that could be exported. As a result, coastal forests were cut down and more olive trees were planted. Where grain crops once grew, olive groves thrived.
The economic value of olive oil was readily apparent. City-states became centers of commerce. Large ships, powered by sail or oars and built to carry hundreds of amphorae of oil, traded throughout the whole Mediterranean Sea, returning with metals, spices, fabrics, and other goods available from far-flung ports. Colonization followed trade, and by the end of the sixth century B.C. the Hellenic world had expanded well beyond the Aegean: to Italy, Sicily, France, and the Balearic Islands in the west, around the Black Sea to the east, and even to coastal Libya on the southern shores of the Mediterranean.
But Solon’s method of enhancing the production of olive oil had environmental consequences that are still apparent in Greece today. The woodlands that were destroyed and the grains that were no longer planted had fibrous root systems that had drawn water from near the surface and had served to hold the surrounding earth together. The long taproot of the olive tree drew water from layers deep below the surface and had no binding effect on the topsoil. Gradually springs dried up, soil washed away, and the land eroded. Fields that once grew cereals and slopes that bore vines could no longer support these crops. Livestock became scarce. Greece was awash in olive oil, but more and more other foodstuffs had to be imported—a significant factor in governing a large empire. Many reasons have been given for the decline of classical Greece: internal strife among warring city-states, decades of war, lack of effective leadership, the collapse of religious traditions, attacks from outside. Maybe we can add another: the loss of valuable agricultural land to the demands of the olive oil trade.
SOAP FROM OLIVE OIL
Olive oil may have been a factor in the collapse of classical Greece, but around the eighth century A.D. the introduction of a product from olive oil, soap, may have had even more important consequences for European society. Today soap is such a common item that we don’t recognize what a significant role it has played in human civilization. Try to imagine, for a moment, life without soap—or detergents, shampoos, laundry powders, and similar products. We take for granted the cleaning ability of soap, yet without it the megacities of the present day would hardly be possible. Dirt and disease would make living hazardous under such conditions and maybe not even viable. The filth and squalor of medieval towns, which had far fewer inhabitants than today’s big cities, cannot be blamed entirely on lack of soap, but without this essential compound maintaining cleanliness
would have been extremely difficult.
For centuries humankind has made use of the cleansing power of some plants. Such plants contain saponins, glycosidic (sugar-containing) compounds such as those from which Russell Marker extracted the sapogenins that became the basis for birth control pills, and the cardiac glycosides like digoxin and other molecules used by herbalists and supposed witches.
Sarsasaponin, the saponin from the sarsaparilla plant
Plant names like soapwort, soapberry, soap lily, soap bark, soapweed, and soaproot give a clue to the properties of the diverse range of saponin-bearing plants. These include members of the lily family, bracken, campions, yuccas, rues, wattles, and the genus Sapindus. Saponin extracts from some of these plants are still used today to wash delicate fabrics or as hair shampoos; they create a very fine lather and have a very gentle cleansing effect.
The process of making soap was most probably an accidental discovery. Those cooking over wood fires might have noticed that fats and oils that dripped from the food into the ashes produced a substance that formed a foamy lather in water. It would not have taken long to realize that this substance was a useful cleaning agent and that it could be deliberately manufactured using fats or oils and wood ash. Such discoveries no doubt occurred in many parts of the world, as there is evidence of soap production from many civilizations. Clay cylinders containing a type of soap and instructions for its manufacture have been found in excavations from Babylonian times, nearly five thousand years ago. Egyptian records dating from 1500 B.C. show that soaps were made from fats and wood ash, and through the centuries there are references to the use of soap in the textile and dyeing industries. The Gauls are known to have used a soap made from goat fat and potash, to brighten or redden their hair. Another use of this soap was as a type of pomade to stiffen hair—an early hair gel. The Celts have also been credited with the discovery of soap making and for using it to bathe and to wash clothes.
Roman legend attributes the discovery of soap making to women washing clothes in the Tiber River downstream from the temple on Mount Sapo. Fats from animals sacrificed at the temple combined with ashes from sacrificial fires. When it was raining, these wastes would run down the hill and enter the Tiber as a soapy steam, which could be used by the washerwomen of Rome. The chemical term for the reaction that occurs when triglycerides of fats and oils react with alkalis—from ashes—is saponification, the word derived from the name of Mount Sapo, as is the word for soap in a number of languages.
Although soap was manufactured in Roman times, it was mainly used for washing clothes. As with the ancient Greeks, personal hygiene for most Romans usually involved rubbing the body with a mixture of olive oil and sand, which was then removed with a scraper made especially for this purpose and known as a strigil. Grease, dirt, and dead skin were removed by this method. Soap gradually came to be used for bathing during the later centuries of Roman times. Soap and soap making would have been associated with the public baths, a common feature of Roman cities that spread throughout the Roman Empire. With the decline of Rome, soap making and soap using appears to have also declined in western Europe, although it was still made and used in the Byzantine Empire and the Arab world.
In Spain and France during the eighth century there was a revival of the art of soap making, using olive oil. The resulting soap, known as “castile” after a region of Spain, was of very high quality, pure, white, and shiny. Castile soap was exported to other parts of Europe, and by the thirteenth century Spain and southern France had become famous for this luxury item. The soaps of northern Europe were based on animal fat or fish oils; the soaps they produced were of poor quality and were used mainly for washing fabric.
The chemical reaction for making soap—saponification—breaks a triglyceride into its component fatty acids and glycerol through the use of an alkali, or base, such as potassium hydroxide (KOH) or sodium hydroxide (NaOH).
The saponification reaction of a triglyceride molecule of oleic acid, forming glycerol and three molecules of soap
Potassium soaps are soft; those made with sodium are hard. Originally most soaps would have been potassium soaps, as wood ash from burning timber and peat was the most readily available source of alkali. Potash (literally, the ashes from a fire pot) is potassium carbonate (K2CO3 ), and in water forms a mildly alkaline solution. Where soda ash (sodium carbonate, Na2CO3) was available, hard soaps were produced. A major source of income in some coastal regions—Scotland and Ireland in particular—was collecting kelp and other seaweeds, which were burned to make soda ash. Soda ash dissolved in water also produces an alkaline solution.
In Europe the practice of bathing declined along with the Roman Empire, although public baths still existed and were used in many towns until late in the Middle Ages. During the plague years, starting in the fourteenth century, city authorities began closing public baths, fearing that they contributed to the spread of the Black Death. By the sixteenth century bathing had become not only unfashionable but was even considered dangerous or sinful. Those who could afford it covered body odors with liberal applications of scents and perfumes. Few homes had baths. A once-a-year bath was the norm; the stench of unwashed bodies must have been dreadful. Soap, however, was still in demand during these centuries. The rich had their clothes and linens laundered. Soap was used to clean pots and pans, dishes and cutlery, floors and counters. Hands and possibly faces were washed with soap. It was washing the whole body that was frowned upon, particularly naked bathing.
Commercial soap making began in England in the fourteenth century. As in most northern European countries, soap was made mainly from cattle fat or tallow, whose fatty acid content is approximately 48 percent oleic acid. Human fat has about 46 percent oleic acid; these two fats contain some of the highest percentages of oleic acid in the animal world. By comparison, the fatty acids in butter are about 27 percent oleic acid and in whale blubber about 35 percent. In 1628, when Charles I ascended to the throne of England, soap making was an important industry. Desperate for a source of revenue—Parliament refused to approve his proposals for increased taxation—Charles sold monopoly rights to the production of soap. Other soap makers, incensed at the loss of their livelihood, threw their support behind Parliament. Thus it has been said that soap was one of the causes of the English Civil War of 1642-1652, the execution of Charles I, and the establishment of the only republic in English history. This claim seems somewhat far-fetched, as the support of soap makers can hardly have been a crucial factor; disagreements on policies of taxation, religion, and foreign policy, the major issues between the king and Parliament, are more likely causes. In any event, the overthrow of the king was of little advantage to soap makers, since the Puritan regime that followed considered toiletries frivolous, and the Puritan leader, Oliver Cromwell, Lord Protector of England, imposed heavy taxes on soap.
Soap can, however, be considered responsible for the reduction in infant mortality in England that became evident in the later part of the nineteenth century. From the start of the Industrial Revolution in the late eighteenth century, people flocked to towns seeking work in factories. Slum housing conditions followed this rapid growth of the urban population. In rural communities, soap making was mainly a domestic craft; scraps of tallow and other fats saved from the butchering of farm animals cooked up with last night’s ashes would produce a coarse but affordable soap. City dwellers had no comparable source of fat. Beef tallow had to be purchased and was too valuable a food to be used to make soap. Wood ashes were also less obtainable. Coal was the fuel of the urban poor, and the small amounts of coal ash available were not a good source of the alkali needed to saponify fat. Even if the ingredients were on hand, the living quarters of many factory workers had, at best, only rudimentary kitchen facilities and little space or equipment for soap making. Thus soap was no longer made at home. It had to be purchased and was generally beyond the means of factory workers. Standards of hygiene, not high to start with, fell even lower, and filthy living conditions con
tributed to a high infant death rate.
At the end of the eighteenth century, though, a French chemist, Nicolas Leblanc, discovered an efficient method of making soda ash from common salt. The reduced cost of this alkali, an increased availability of fat, and finally in 1853 the removal of all taxes on soap lowered the price so that widespread use was possible. The decline in infant mortality dating from about this time has been attributed to the simple but effective cleansing power of soap and water.
Soap molecules clean because one end of the molecule has a charge and dissolves in water, whereas the other end is not soluble in water but does dissolve in substances such as grease, oil, and fat. The structure of the soap molecule is
A molecule of sodium stearate—a soap molecule from beef tallow
and can also be represented as
The next diagram shows the carbon chain end of many of these molecules penetrating a grease particle and forming a cluster known as a micelle. Soap micelles, with the negatively charged ends of the soap molecules on the outside, repel each other and are washed away by water, taking the grease particle with them.
A soap micelle in water. The charged ends of the soap molecules remain in the water; the carbon chain ends are embedded in the grease.
Although soap had been made for thousands of years and commercially manufactured for hundreds of years, the chemical principles of its formation were not understood for a very long time. Soap could be made from what seemed like a wide variety of different substances—olive oil, tallow, palm oil, whale oil, pork fat—and as the chemical structures of these products were not known until the early nineteenth century, the essential similarity of their triglyceride structures was not realized. It was well into the nineteenth century before soap chemistry was appreciated. By then social changes in attitudes toward bathing, the gradually increasing prosperity of the working classes, and an understanding of the relationship between disease and cleanliness meant that soap had become an essential item of everyday living. Fine toilet soaps made from different fats and oils challenged the long-established supremacy of castile soap made from olive oil, but it was castile soap—and hence olive oil—that had been mainly responsible for maintaining some degree of personal hygiene for almost a millennium.
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