Sugar is fundamental to the nutrition of plants and animals, and exists in one form or another in all living creatures. Sugars are Carbohydrates, i.e. their molecules consist of carbon, hydrogen, and oxygen. (These molecules are relatively small. The more complex carbohydrates such as starch and cellulose are mainly composed of many sugar molecules joined together.) There are many different chemical forms of sugar.
The simplest sugars are termed monosaccharides, ‘single sugars’. Of these, the fundamental one is dextrose (commonly called glucose), which occurs naturally in fruits and vegetables, and also in the blood of animals, where it provides a short-term store of energy. The digestion of all carbohydrates is essentially a process of reducing them to dextrose. That is why powdered glucose and glucose syrup, which consist mainly of dextrose, give quick energy. They need no digestion, but go straight into the blood.
Another common monosaccharide is fructose (sometimes called laevulose), also naturally present in plants, and abundant in honey. A third is galactose, one of the constituents of the sugar in milk.
All these monosaccharides have the same chemical formula. The difference between them is in the way the atoms are arranged. They have six atoms of carbon in each molecule and are therefore called hexoses. (Pentoses, with only five carbon atoms, also exist. Common ones are arabinose and xylose. They are widely found in plants, for example in pectin and other constituents of cell walls.)
Ordinary white sugar is a disaccharide, or ‘double sugar’. Each of its molecules consists of two single sugar molecules joined together: one of dextrose and one of fructose. The chemical name is sucrose. Refined white sugar is 99% pure sucrose. Other disaccharides are maltose, found in malt extract and consisting of two molecules of dextrose; and lactose or milk sugar, found in milk and made up of one dextrose and one galactose molecule.
Not all sugars taste equally sweet. Pure fructose is almost one and three-quarters times as sweet as ordinary sucrose; dextrose only three-quarters as sweet as sucrose; maltose one-third; and lactose one-sixth. Other sugars may be even less sweet and some actually taste bitter. But all the common sugars have the same food value. An ounce of any pure sugar yields 112 calories.
Differences in sweeteners result from the differences in arrangements of atoms, from which other consequences can flow. The atoms of natural sugars are all laid out in an asymmetrical manner, having a right-handed spiral twist. They are known as D or dextro (right) sugars as opposed to L or laevo (left) sugars. L sugars do not commonly occur in nature, since their shape makes them useless to plants and animals, and they pass straight through the digestive system unchanged, with a food value of nil. They can, however, be made in the laboratory by a difficult, expensive chemical process. This could turn out to be more than an academic exercise, since researchers have discovered that some L sugars taste sweet. If the new skills of genetic engineering can be used to breed bacteria or produce enzymes that will manufacture L sugars in quantity, the ideal sweetener is in prospect: something which is just like normal sugar but has no calories.
Incidentally, although dextrose is a D or dextro sugar, this is not the reason for its name. It was called dextrose to contrast it with laevulose (the alternative name for fructose). Nor should it be supposed that laevulose is an L or laevo sugar. It too is a D sugar. The explanation is that, quite apart from the L and D classification described above, sugars have been classified as ‘right’ and ‘left’ according to the direction in which a beam of polarized light is twisted when it is passed through a solution of each. This helps in the chemical analysis of sugar.
A wide variety of plants have been exploited for the sugar they contain. These may be present in stems or tubers where they function as a food store for the plant. Sugar is also formed in seeds when they germinate, to provide food for the young plant. Seeds are deliberately germinated to make malt.
One of the first sources of sugar to be exploited was honey. This comes indirectly from plants, by courtesy of the bees. Having extracted the sugary nectar from flowers, bees use enzymes in their saliva to split the sucrose into dextrose and fructose. Fructose is the most abundant sugar in honey, which is therefore considerably sweeter than a sucrose syrup of the same concentration. However, because it is an impure and variable food containing other flavourings, subjective assessment of its sweetness varies greatly. Some of the best and most flavourful honeys seem less sweet than plain sucrose.
Various kinds of manna containing sugars have been used since ancient times for sweetening purposes.
Another ancient sweetener which is still prepared in the Levant is dibs, a syrup made by boiling down raisins, sweet grapes, or locust beans.
In the Indian subcontinent and in SE Asia a crude, dark palm sugar called jaggery or gur is used.
However, sugar cane has become much more important than any of the ancient sources of sugar. A plant native to E. or S. Asia, it too has been exploited for millennia. At first it was used whole or only partly refined into a crude product, practices which still survive in Asia. But methods of refining have been developed which now make it possible to extract 99 per cent pure sucrose from sugar cane. The many forms which it takes in commerce are described below, while the botany and economic and technological history are in the section headed Sugar cane.
There are many kinds of cane sugar and syrup, differing in their purity and degree of refinement. The least pure is blackstrap molasses (from the Dutch word ‘stroop’, meaning syrup) or black treacle, which is a residue from cane refining. Next is ‘raw’ dark brown sugar. The darkest of ordinary western sugars is Barbados or muscovado. (This was originally made in Barbados but may now come from elsewhere. The name muscovado comes from the Spanish más acabado (more finished), because it has been separated from the molasses with which it was originally mixed.) The crudest sugar of all brown sugars is known as foot sugar or foots because in early processes it settled at the bottom of the barrel.
There are various partly refined, lighter brown sugars. Demerara, originally made in Demerara (Guyana), has relatively hard crystals. However, much modern brown sugar, including ‘London Demerara’, is made not by partial refining but by adding a little molasses to white sugar. As a result, the taste is relatively feeble. Pieces, yellows, sand, or scotch is a pale, soft, sticky sugar which is a by-product of a late stage of refining.
Light cane-sugar syrups are used for cooking in Britain (much as corn syrups and maple syrups are used in the USA). One such is a medium brown treacle made by partial refining of black treacle. However, the pale yellow golden syrup which emerges from another stage of white sugar production is now more popular. This is technically known as an invert sugar syrup. Inversion is a process by which sugar is heated or treated with acid (or both), to split some or all of the sucrose into dextrose and fructose. These single sugars are much less willing to crystallize than is sucrose, and the syrup will remain liquid for years. The inversion process incidentally gives golden syrup a mild but distinctive flavour.
All partly refined sugar products have special flavours due to residual plant substances from the original cane, or created by the manufacturing process. They are not interchangeable in the kitchen; substituting one for another in a recipe is likely to alter the flavour of the dish.
Fully refined white sugars have virtually no flavour apart from a sweet taste. The difference between them is one of crystal size.
Preserving sugar is the coarsest. The big crystals do not stick together when stirred into a liquid, so they actually dissolve faster than granulated sugar. This reduces the risk of caramelization and burning. Otherwise preserving sugar is exactly the same as any other white sugar.
Granulated sugar has medium-sized crystals, as does lump sugar, which is simply granulated sugar moistened with syrup and pressed into blocks. An American term for this, ‘loaf sugar’, is a misnomer, since real loaf sugar is the cone-shaped product of an obsolete refining process. It is seldom seen in the West, but occasionally encountered in oriental countries. There is also a Colombian type of loaf sugar called panela.
Caster sugar is so named because it is of the right fineness for use in a sugar caster or sprinkler. (The spelling ‘castor’ is now quite common.) This is the same as ‘superfine sugar’ in N. America.
Icing sugar or confectioner's or ‘powdered’ sugar is the finest of all, made by mechanically crushing crystals. A little starch may be added to keep it dry.
Candy sugar is a special type made by growing large crystals in a strong sugar solution which is allowed to evaporate. The crystals are often grown on strings. Coffee sugar, which is naturally white but sometimes artificially coloured, is a candy sugar.
Barley sugar is not made from barley, but from ordinary white sugar. (The name was originally applied to a sweet flavoured with barley water.) To make barley sugar, white sugar is melted without water. At 170 °C (320 °F) it liquefies. It is then allowed to cool and harden, which happens so quickly that it does not manage to form crystals. Instead it sets to a hard substance for which the scientific term is supercooled liquid. (Other common supercooled liquids are ordinary glass and asphalt.) Barley sugar is slightly brown as a result of the formation of caramel.
Sugar beet (see below) yields a white sugar which is (to scientists although not to marmalade-makers) indistinguishable from white cane sugar. However, the partly refined products which occur during the refining process have a foul smell, as anyone who has been downwind of a beet sugar refinery will know, and a correspondingly bad taste. Consequently there is no brown beet sugar or beet molasses. The waste products are used for animal feed.
In the USA corn syrup is an important sweetener. This is made by breaking down the long molecules of maize starch into individual sugar molecules.
The N. American maple also produces a syrup (see maple syrup), with a delicious flavour. See also birch sugar.
The sugar palms are a source of palm sugar, such as the jaggery already mentioned, in regions where they grow. Sugar is made by boiling down the sap. Sweet varieties of sorghum, an important grain, are used to makesorghum syrup.
As a foodstuff, sugar has attracted much criticism. This applies particularly to refined white sugar. Unrefined brown sugar is sometimes portrayed, in contrast, as healthful because of the nutrients which it retains. It is true that people who eat a lot of white sugar tend to have an unhealthy diet, may be obese, and are exposed to the risk of heart disease. This, however, is not the fault of the sugar, which contains no harmful substances; it is the fault of people who eat too much of it. Since sugar provides energy but nothing else (no vitamins, no protein, no essential fatty acids, virtually no minerals) it follows that a diet which is high in sugar will risk being short of necessary nutrients.
Brown sugar contains small traces of protein and common minerals which in a normal diet are freely supplied by other foods. There seems to be no good ground for regarding it as a ‘health food’. There is, however, a reason for preferring it to white sugar; it has a more interesting flavour.
See also confectionery; jam; jelly; preservation; sugar boiling.
The second most important source of sugar in the world, sugar beets, are certain cultivars of the plant now classified as Beta vulgaris, Crassa group. This group also contains the far more numerous cultivars of beetroot, the familiar crimson vegetable.
The root of the plant was originally small and disagreeable in flavour, but with a noticeably sweet taste. As early as 1590 the French botanist Olivier de Serres managed to extract a sugar syrup from it. In those days cane sugar was still very expensive, so his discovery might have been exploited, but nothing came of it at the time.
In 1747 the German chemist Marggraf extracted sugar from beet, and observed that the root contained up to 6.3 per cent sugar in dry matter. Again, interest was slight. Later, however, one of Marggraf's students, Karl Franz Achard, was backed by Frederick the Great of Prussia in research which eventually led to the setting up of a small beet sugar refinery in Silesia in 1800.
The Napoleonic wars cut off supplies of cane sugar to France. In 1812, under direct orders from Napoleon, beet was cultivated and refined on a large scale for the first time. After the war, renewed imports of cane caused the industry to collapse, but it was revived and, after various vicissitudes, established itself and grew in France, Germany, and Britain. Much of the sugar eaten in northern countries now comes from beet.
Selective breeding has increased the sugar content of beet to 20 per cent of which almost all is extractable. The modern sugar beet has a large, white root of a broad cone shape.
Only fully refined white sugar can be made from beet. Unlike sugar cane, which yields a range of agreeable semi-refined brown sugars and syrups, beet gives a malodorous crude extract. The smell which emanates from a beet refinery is notoriously disagreeable. Residue from the process is fed to uncomplaining farm animals.
To extract sugar from beets, they are washed, sliced, and boiled in successive changes of water, each stage yielding progressively less sugar. The sugary liquid is then treated successively with lime and carbon dioxide gas. The latter causes the lime to precipitate (solidify) as chalk, physically entangling much of the impurities as it falls out of solution. The chalk is then filtered out, after which the later stages of refining are exactly as for white cane sugar.
It is sometimes said that beet sugar ‘is not the same’ as cane sugar, for example in making jams and marmalades. Scientists retort that it is precisely the same, meaning that the chemist can detect no difference. However, the fact remains that the two sugars have different origins, and have undergone different initial refining processes. If, then, a difference in their performance is perceptible to ladies with expertise in making jam and marmalade, this should surely not occasion surprise, still less result in the ladies being denounced as ignorant; it should cause the chemists to reflect, humbly, that they are not omniscient in these matters.
Saccharum officinarum, the source of most of the world's sugar, is the descendant of a now extinct wild plant which probably grew in New Guinea. (A reference is made under Papua New Guinea to the consumption there of not only sugar cane but also the enclosed inflorescences of S. edule, locally called pitpit.)
The sugar cane is a giant grass looking rather like bamboo; but its stems, instead of being hollow, are filled with a sappy pulp. Nearly 90 per cent of the weight of cane is juice, and this juice contains up to 17 per cent sucrose (common sugar) and small amounts of two other sugars, dextrose and fructose. Sugar-cane juice is pleasantly sweet, although lacking in flavour. It is used as a soft drink in sugar-growing countries, and raw cane is chewed as a sweet.
Not surprisingly, cane was cultivated from an early date in many parts of Asia including India and China. The earliest known reference to it is in a love poem in an Indian sacred work, the Atharra-veda, where sugar cane is used as a symbol of sweetness and attractiveness. Herodotus, the Greek historian (5th century bc), knew of the plant, and in 327 bc Alexander the Great sent some back to Europe from India.
The fact that the juice could be boiled down to make solid sugar was discovered in early times. The first surviving account of solid sugar comes from a Persian tablet of 510 bc, which describes it as coming from the Indus Valley. This early product would have resembled the modern Indian raw, dark brown sugar called gur or jaggery.
During the 7th century ad the Persians improved the refining process, introducing the use of lime (see below) and other refining agents. Their product, an almost white loaf sugar, was soon being exported to the West, where hitherto the only sweeteners had been honey and occasionally manna. The exotic delicacy fetched high prices. Soon sugar cane was being cultivated in many regions to the east and south of the Mediterranean. For a while it was even grown in Spain, and Sicily, too far north for it to thrive. The Venetians set up an import and export trade supplying N. Europe.
Cane cultivation spread westwards to the Canaries and Madeira, and in 1493 Columbus took some plants to the Caribbean. It grew so well in that climate that it soon became the chief crop of the whole region. Mintz (1985), an anthropologist with much Caribbean experience, has written on the production of sugar in the region and, more generally, on the whole ramified process by which sugar was transformed from being a costly spice or condiment into a major foodstuff of the western world.
As cultivation spread and sugar became more plentiful, the price fell. By the beginning of the 19th century white cane sugar was no longer a luxury. During the Napoleonic wars the blockade of France stimulated the introduction of a rival plant, the sugar beet. Throughout the century refining methods for both became more advanced. Cane and beet sugar are now among the cheapest of foods.
A primitive and ancient process is still used to produce ‘raw’ sugar such as Indian gur or jaggery and Mexican piloncillo. The crop is burnt to remove the leaves, and then cut down close to the ground since the bottoms of the stems are richest in sugar. The stems are shredded and crushed in simple ox-driven machinery to press out the juice, which is then concentrated by being boiled in shallow pans. Lime is added to make the proteins in the juice coagulate and collect on their surface other impurities. These form a dirty scum which is skimmed off. Further boiling removes so much water that the sugar begins to crystallize. As it does so it is scooped out and set to drain. The solid crystals are moist, dark brown sugar, tasting strongly of molasses.
The earliest white sugar was made by boiling raw sugar with lime water and bullock's blood. The blood coagulated, absorbing more impurities and in doing so removing most of the brown colour. The scum was repeatedly skimmed off and the partly purified liquid filtered, boiled to concentrate it, and poured into conical moulds to crystallize and solidify. The conical loaves were then broken up, redissolved and repurified, this time with egg white. Finally, the ‘double refined’ sugar was reformed into conical loaves, and sold thus. Before use, the loaf sugar had to be smashed up with hammers, an exhausting process. The first kinds of lump sugar (which is still called loaf sugar in the USA) were made by sawing up loaves.
Modern sugar production is in its early stages a mechanized version of the primitive process. The cane may be harvested mechanically. The stems are smashed in hammer mills and the juice washed out in a diffuser, where hot juice is constantly circulated through the shredded cane. Heating with lime follows, and the partly purified liquid is evaporated in a series of vacuum pans. (The use of a partial vacuum makes it boil at a lower temperature, saving energy.) The concentrated juice is encouraged to crystallize by ‘seeding’ it with ‘magma’, a mixture of crystal and syrup. The result, known as ‘massecuite’, is a thick sludge of sugar and syrup. It is centrifuged to extract the syrup, which is dark blackstrap molasses and useful in its own right as a foodstuff, and for making rum, industrial alcohol, and citric acid.
The raw sugar is then purged of the last traces of molasses by washing it with clean syrup and centrifuging it again. It is mixed with hot water and partly decolorized with phosphoric acid, then neutralized with lime and further heated to coagulate a few remaining impurities. The liquid is again filtered, then run through beds of charcoal which absorb the last traces of colour. After a final filtration it is vacuum evaporated, ‘seeded’ with fondant sugar (whose tiny crystals make effective starters for crystal growth), centrifuged, and dried with hot air.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
Mintz, Sidney W. (1985), Sweetness and Power, London: Viking.
