It is impossible to establish the exact numbers of slaves that were loaded onto sailing vessels off the west coast of Africa and later sold in the New World. Records are incomplete and possibly fraudulent, reflecting attempts to get around the laws that belatedly tried to improve conditions aboard these transport ships by regulating the number of slaves that could be carried. As late as the 1820s more than five hundred human beings were being packed, on Brazilian slave ships, into an area less than nine hundred feet square and three feet high. Some historians calculate that upward of fifty million Africans were shipped to the Americas over the three and a half centuries of the slave trade. This figure does not include those who would have been killed in slaving raids, or those who died on the trip from the interior of the continent to the African coasts, or those who did not survive the horrors of the sea voyage that came to be known as the middle passage.
The middle passage refers to the second side of the trade triangle known as the Great Circuit. The first leg of this triangle was the trip from Europe to the coast of Africa, predominantly the west coast of Guinea, bringing manufactured goods to exchange for slaves. The third leg was the passage from the New World back to Europe. The slave ships by that point would have exchanged their human cargo for ore from mines and produce from the plantations, generally rum, cotton, and tobacco. Each leg of the triangle was hugely profitable, especially for Britain: by the end of the eighteenth century the value of British income derived from the West Indies was much greater than the value of income from trade with the rest of the world. Sugar and sugar products, in fact, were the source of the enormous increase in capital and the rapid economic expansion necessary to fuel the British and later the French Industrial Revolution of the late eighteenth and early nineteenth centuries.
SWEET CHEMISTRY
Glucose
Glucose is the most common of the simple sugars, which are sometimes called monosaccharides from the Latin word saccharum for sugar. The mono prefix refers to one unit, as opposed to the two-unit disaccharides or the many-unit polysaccharides. The structure of glucose can be drawn as a straight chain or as a slight adaptation of this chain, where each intersection of vertical and horizontal lines represents a carbon atom. A set of conventions that need not concern us give numbers to the carbon atoms, with carbon number 1 always drawn at the top. This is known as a Fischer projection formula, after Emil Fischer, a German chemist who in 1891 determined the actual structure of glucose and a number of other related sugars. Though the scientific tools and techniques available to Fischer at the time were very rudimentary, his results still stand today as one of the most elegant examples of chemical logic. He was awarded the 1902 Nobel Prize in chemistry for his work on sugars.
Fischer projection formula for glucose, showing the numbering of the carbon chain
Although we can still draw sugars such as glucose in this straight chain form, we now know that they normally exist in a different form—cyclic (ring) structures. The drawings of these cyclic structures are known as Haworth formulas, after Norman Haworth, the British chemist whose 1937 Nobel Prize recognized his work on vitamin C and on structures of carbohydrates (see Chapter 2). The six-membered ring of glucose consists of five carbon atoms and one oxygen atom. Its Haworth formula, shown below, indicates by number how each carbon atom corresponds to the carbon atom shown in the previous Fischer projection formula.
Haworth formula of glucose, showing all the hydrogen atoms
Haworth formula of glucose, without all the H atoms but showing carbon atoms numbered
There are actually two versions of glucose in the cyclic form, depending on whether the OH at carbon number 1 is above or below the ring. This might seem a very minor distinction, but it is worth noting as it has very important consequences for the structures of more complicated molecules that contain glucose units, such as complex carbohydrates. If the OH at carbon number 1 is below the ring it is known as alpha (α)-glucose. If it is above the ring, it is beta (β)-glucose.
When we use the word sugar, we are referring to sucrose. Sucrose is a disaccharide, so called because it is composed of two simple monosaccharide units: one a glucose unit and the other a fructose unit. Fructose, or fruit sugar, has the same formula as glucose; C6H12O6, and also the same number and type of atoms (six carbon, twelve hydrogen, and six oxygen) found in glucose. But fructose has a different structure. Its atoms are arranged in a different order. The chemical definition of this is that fructose and glucose are isomers. Isomers are compounds that have the same chemical formula (same number of each atom) but different arrangements of these atoms.
Fischer projection formulas of the isomers glucose and fructose, showing the different order of hydrogen and oxygen atoms at C#1 and C#2. Fructose has no H atoms at C#2.
Fructose exists mainly in the cyclic form, but it looks a bit different from glucose since fructose forms a five-membered ring, shown below as a Haworth formula, rather than the six-membered ring of glucose. As with glucose, there are α and β forms of fructose, but as it is carbon number 2 that joins the ring oxygen in fructose, it is around this carbon atom that we designate OH below the ring as α and OH above the ring as β.
Sucrose contains equal amounts of glucose and fructose but not as a mixture of two different molecules. In the sucrose molecule one glucose and one fructose are joined together through the removal of a molecule of water (H2O) between the OH at carbon number 1 of α-glucose and the OH on carbon number 2 of β-fructose.
Removal of a molecule of H2O between glucose and fructose forms sucrose. The fructose molecule has been turned 180° and inverted in these diagrams.
Structure of the sucrose molecule
Fructose is largely found in fruit but also in honey, which is about 38 percent fructose and 31 percent glucose, with another 10 percent of other sugars including sucrose. The remainder is mainly water. Fructose is sweeter than sucrose or glucose, so because of its fructose component, honey is sweeter than sugar. Maple syrup is approximately 62 percent sucrose with only 1 percent of each of fructose and glucose.
Lactose, also called milk sugar, is a disaccharide formed from one unit of glucose and one unit of another monosaccharide, galactose. Galactose is an isomer of glucose; the only difference is that in galactose the OH group at carbon number 4 is above the ring and not below the ring as it is in glucose.
β-galactose with arrow showing C#4 OH above the ring compared to β-glucose where the C#4 OH is below the ring. These two molecules combine to form lactose.
Galactose on the left is joined through C#1 to C#4 of glucose on the right.
Again, having an OH above or below the ring may seem like a very minor difference, but for those people who suffer from lactose intolerance, it is not. To digest lactose and other disaccharides or larger sugars, we need specific enzymes that initially break down these complex molecules into simpler monosaccharides. In the case of lactose, the enzyme is called lactase and is present in only small amounts in some adults. (Children generally produce greater amounts of lactase than adults.) Insufficient lactase makes the digestion of milk and milk products difficult and causes the symptoms associated with lactose intolerance: abdominal bloating, cramps, and diarrhea. Lactose intolerance is an inherited trait, easily treated with over-the-counter preparations of the lactase enzyme. Adults and children (but not babies) from certain ethnic groups, such as some African tribes, are missing the lactase enzyme completely. For these people, powdered milk and other milk products, often found in food aid programs, are indigestible and even harmful.
The brain of a normal healthy mammal uses only glucose for fuel. Brain cells are dependent on a minute-to-minute supply from the bloodstream, as there are essentially no fuel reserves or storage in the brain. If blood glucose level falls to 50 percent of the normal level, some symptoms of brain dysfunction appear. At 25 percent of the normal level, possibly from an overdose of insulin—the hormone that maintains the level of glucose in the blood—a coma may result.
SWEET TASTE
What makes all these sugars so appealing is that they taste sweet, and humans like sweetness. Sweetness is one of the four principal tastes; the other three are sourness, bitterness, and saltiness. Achieving the ability to distinguish among these tastes was an important evolutionary step. Sweetness generally implies “good to eat.” A sweet taste indicates that fruit is ripe, whereas sour tells us there are still lots of acids present, and the unripe fruit may cause a stomachache. A bitter taste in plants often indicates the presence of a type of compound known as an alkaloid. Alkaloids are often poisonous, sometimes in only very small amounts, so the ability to detect traces of an alkaloid is a distinct advantage. It has even been suggested that the extinction of the dinosaurs might have been due to their inability to detect the poisonous alkaloids found in some of the flowering plants that evolved toward the end of the Cretaceous period, about the time the dinosaurs disappeared, although this is not the generally accepted theory of dinosaur extinction.
Humans do not seem to have an inborn liking for bitterness. In fact, their preference is probably just the opposite. Bitterness invokes a response involving secretion of extra saliva. This is a useful reaction to something poisonous in the mouth, allowing one to spit it out as completely as possible. Many people do, however, learn to appreciate, if not like, the bitter taste. Caffeine in tea and coffee and quinine in tonic water are examples of this phenomenon, although many of us still rely on having sugar in these drinks. The term bittersweet, connoting pleasure mixed with sadness, conveys our ambivalence about bitter tastes.
Our sense of taste is located in the taste buds, specialized groups of cells found mainly on the tongue. Not all parts of the tongue detect taste the same way or to the same degree. The front tip of the tongue is the most sensitive to sweetness, while sourness is detected most strongly on the sides of the tongue toward the back. You can test this easily for yourself by touching a sugar solution to the side of the tongue and then to the tip of the tongue. The tip of the tongue will definitely detect the sweet sensation more strongly. If you try the same thing with lemon juice, the result will be even more obvious. Lemon juice on the very tip of the tongue does not seem very sour, but put a freshly cut slice of lemon on the side of the tongue, and you will discover where the sourness reception area is the strongest. You can continue this experiment: bitterness is detected most strongly on the middle of the tongue but back from the tip, and the salty sensation is greatest just to each side of the tip.
Sweetness has been investigated far more than any of the other tastes, no doubt because, as in the days of the slave trade, it is still big business. The relationship between chemical structure and sweetness is complicated. One simple model, known as the A-H,B Model, suggests that a sweet taste depends on an arrangement of a group of atoms within a molecule. These atoms (A and B in the diagram) have a particular geometry, allowing atom B to be attracted to the hydrogen atom attached to atom A. This results in the short-term binding of the sweet molecule to a protein molecule of a taste receptor, causing a generation of a signal (transmitted through nerves) informing the brain, “This is sweet.” A and B are usually oxygen or nitrogen atoms, although one of them may also be a sulfur atom.
The A-H,B Model of Sweetness
There are many sweet compounds other than sugar, and not all of them are good to eat. Ethylene glycol, for example, is the major component of antifreeze used in car radiators. The solubility and flexibility of the ethylene glycol molecule, as well as the distance between its oxygen atoms (similar to the distance between oxygen atoms in sugars), account for its sweet taste. But it is very poisonous. A dose of as little as one tablespoon can be lethal for humans or family pets.
Interestingly, it is not ethylene glycol but what the body turns it into that is the toxic agent. Oxidation of ethylene glycol by enzymes in the body produces oxalic acid.
Oxalic acid occurs naturally in a number of plants, including some that we eat, such as rhubarb and spinach. We usually consume these foods in moderate amounts, and our kidneys can cope with the traces of oxalic acid from such sources. But if ethylene glycol is swallowed, the sudden appearance of a large amount of oxalic acid can cause kidney damage and death. Eating spinach salad and rhubarb pie at the same meal will not hurt you. It would probably be difficult to consume enough spinach and rhubarb to do any harm, except perhaps if you are prone to kidney stones, which build up over some years. Kidney stones consist mainly of calcium oxalate, the insoluble calcium salt of oxalic acid; those prone to kidney stones are often advised to avoid foods high in oxalates. For the rest of us, moderation is the best advice.
A compound that has a very similar structure to ethylene glycol and also tastes sweet is glycerol, but glycerol in moderate amounts is safe to consume. It is used as an additive in many prepared foods because of its viscosity and high water solubility. The term food additive has had a bad press in recent years, implying that food additives are essentially nonorganic, unhealthy, and unnatural. Glycerol is definitely organic, is nontoxic, and occurs naturally in products such as wine.
Glycerol
When you swirl a glass of wine, the “legs” that form on the glass are due to the presence of glycerol increasing the viscosity and smoothness characteristic of good vintages.
SWEET NOTHING
There are numerous other nonsugars that taste sweet, and some of these compounds are the basis for the billion-dollar artificial sweetener industry. As well as having a chemical structure that in some way mimics the geometry of sugars, allowing it to fit and bind to the sweetness receptor, an artificial sweetener needs to be water soluble and nontoxic and, often, not metabolized in the human body. These substances are usually hundreds of times sweeter than sugar.
The first of the modern artificial sweeteners to be developed was saccharin. Saccharin is a fine powder. Those who work with it sometimes detect a sweet taste if they accidentally touch their fingers to their mouth. It is so sweet that only a very small amount triggers the sweetness response. This is evidently what happened in 1879, when a chemistry student at Johns Hopkins University in Baltimore noticed an unusual sweetness in the bread he was eating. He returned to his laboratory bench to systematically taste the compounds that he had been using in that day’s experiments—a risky but common practice with new molecules in those days—and discovered that saccharin was intensely sweet.
Saccharin has no calorific value, and it did not take long (1885) for this combination of sweetness and no calories to be commercially exploited. Originally intended as a replacement for sugar in the diet of diabetic patients, it quickly became an accepted sugar substitute for the general population. Concern about possible toxicity and the problem of a metallic aftertaste led to the development of other artificial sweeteners, such as cyclamate and aspartame. As you can see, the structures of these are all quite different and are very different from sugars, yet they all have the appropriate atoms, along with the specific atomic position, geometry, and flexibility that is necessary for sweetness.
No artificial sweetener is completely free of problems. Some decompose on heating and so can be used only in soft drinks or cold foods; some are not particularly soluble; and others have a detectable side taste along with their sweetness. Aspartame, although synthetic, is composed of two naturally occurring amino acids. It is metabolized by the body, but as it is over two hundred times sweeter than glucose, a lot less is needed to produce a satisfactory level of sweetness. Those with the inherited condition known as PKU (phenylketonuria), an inability to metabolize the amino acid phenylalanine, one of the breakdown products of aspartame, are told to avoid this particular artificial sweetener.
A new sweetener that was approved by the U.S. Food and Drug Administration in 1998 approaches the problem of creating artificial sweetness in a different way. Sucralose has a very similar structure to that of sucrose except for two factors. The glucose unit, on the left-hand side in the diagram, is replaced by galactose, the same unit as in lactose. Three chlorine atoms (Cl) replac
e three of the OH groups: one on the galactose unit and the other two on the right-hand fructose unit, as indicated. The three chlorine atoms do not affect the sweetness of this sugar, but they do stop the body from metabolizing it. Hence sucralose is a noncalorific sugar.
Sucralose structure, showing the three Cl atoms (arrows) replacing three OHs
Natural nonsugar sweeteners are now being sought from plant sources containing “high-potency sweeteners”—compounds that can be as much as a thousand times sweeter than sucrose. For centuries indigenous people have known about plants that have a sweet taste; the South American herb Stevia rebaudiana; roots of the licorice plant Glycyrrhiza glabra; Lippia dulcis, a Mexican member of the verbena family; and rhizomes from Selliguea feei, a fern from West Java, are examples. Sweet compounds from natural sources have shown potential for commercial application, but problems with small concentrations, toxicity, low water solubility, unacceptable aftertaste, stability, and variable quality still need to be overcome.
While saccharin has been used for more than a hundred years, it was not the first substance to be used as an artificial sweetener. That distinction probably belongs to lead acetate, Pb(C2H3O2)2, which was used to sweeten wine in the days of the Roman Empire. Lead acetate, known as sugar of lead, would sweeten a vintage without causing further fermentation, which would have occurred with the addition of sweeteners like honey. Lead salts are known to be sweet, and many are insoluble, but all are poisonous. Lead acetate is very soluble, and its toxicity was obviously not known to the Romans. This should give us pause to think, if we long for the good old days when food and drink were uncontaminated with additives.
Penny le Couteur & Jay Burreson Page 6
