Sugar Crops and Natural Sweeteners

views updated

SUGAR CROPS AND NATURAL SWEETENERS

SUGAR CROPS AND NATURAL SWEETENERS. Chemically, the substance in the breakfast sugar bowl comes to us unchanged from the living organism in which it was manufactured. The familiar crystals are virtually pure sucrose, an organic chemical belonging to a large family of compounds classified as sugars. These, in turn, are members of a still larger group—the carbohydrates—that also includes starch and cellulose. The breakdown of carbohydrates into monosaccharides, oligosaccharides, and polysaccharides points to the relationship.

Carbohydrates are a product of photosynthesis, the complex biochemical process whereby green plants use light energy to combine aerial carbon dioxide and hydrogen from soil water, forming rings of carbon atoms to which atoms of hydrogen and oxygen are attached, usually in the ratio in which these elements occur in water (hence the name), surplus oxygen being released as free gas. Stripped of all detail, the photosynthetic reaction that produces the basic monosaccharide glucose may be summarized by the equation:

Affinities

Various carbohydrates are made in the course of photosynthesis. Sugars are the simplest. Glucose, also called dextrose and grape sugar, is the most prevalent sugar in nature. It occurs in the free state in fruits, plant juices and honey, as well as in blood. Polymerization of glucose leads to starch and cellulose—the one a form of energy storage, the other the main structural material of plants—both nothing but chains of glucose units. Conversely, natural starches can be saccharified by acid hydrolysis and enzymes to yield different types of sweet syrups or crystalline dextrose. Another monosaccharide, fructose (also called levulose and fruit sugar), occurs in the free state notably in honey. Sharing the same chemical formula but differing in structure, glucose and fructose are in fact interconvertible by chemical and biological reactions.

Fructose and glucose combine to form the oligosaccharide — more precisely, disaccharide — sucrose, C₁₂H₂₂O₁₁, shedding a molecule of water in the process. Sucrose is found in the sap of many plants and like starch functions as a storage product. It is easily hydrolyzed by acid or enzyme to equimolar amounts of glucose and fructose, and the mixture is then called invert sugar.

A family trait of many mono-and oligosaccharides is their sweet taste. Fruits, young vegetables, the nectar of flowers, and the sap of certain plants and trees taste sweet because they contain saccharides. Sucrose is generally used as the standard for relative sweetness, but this also depends on concentration, temperature and other factors affecting the physiology of taste. Approximate relative sweetness values for the sugars mentioned are:

Fructose	120	Invert sugar	95
Sucrose	100	Glucose	65

Sucrose, almost all of which is obtained from sugar cane and sugar beet, is commercially by far the most important sweetener. Its origin in photosynthesis explains how two wholly dissimilar botanical sources can furnish a practically identical product. Sugar cane—currently providing roughly three-quarters of the world's sucrose supply—is a perennial monocotyledon, propagated from cuttings, except in the breeding of new varieties, and capable of giving repeated harvests. In contrast, sugar beet is a biennual dicotyledon, harvested in the first season and replanted annually from seed. Cane grows in the tropics and subtropics, beet in temperate climates. The cane's sucrose comes from the stalk, the beet's from its root. Sugar cane has been exploited industrially for more than two millennia, sugar beet for just two centuries.

The origin of sugars in photosynthesis also explains the exploitation of other, more or less important sweetener sources, some going back to ancient times, for example boiled-down grape juice, fig and date syrup, the sap of palms, the maple tree, and sweet sorghum, and the exudations from certain trees and shrubs. Like the sugar from cane and beet, honey—the first concentrated sweetener known to humans—is ultimately a product of photosynthesis. In essence, bees making honey and humans processing cane or beet to crystal sugar are doing the same thing—both extract dilute sugar solutions from plants and convert them into forms that are easier to handle and storable by evaporating unwanted water.

Sugar Cane

Physical characteristics. Sugar cane is a giant perennial grass that tillers at the base to produce clumps of solid unbranched stems up to six centimeters in diameter and typically two to three meters long at maturity. It is grown for these thick stems which, stripped of tips and leaves, weigh between 500 and 2000 grams and, as a rule, contain 10–15 percent sucrose and 11–16 percent fiber. Each stem is divided into a number of joints comprising a node and an internode up to twenty-five centimeters in length. A node consists of a lateral bud in a leaf axil, a band containing root primordia, and a growth ring. Sword-shaped leaves, consisting of a sheath and a blade, are attached to the stem at the base of the nodes, alternating in two rows on opposite sides of the stem. The stems range in color from green or yellow to red, purple, violet, or striped, and have a hard wax-covered rind that reduces loss of water by evaporation. Depending on variety, plant age, and natural conditions, sugar cane may flower, producing plumelike panicles known as arrows or tassels that bear hundreds of small spikelets with inconspicuous flowerets. For sugar production, however, sugar cane is propagated vegetatively from stem cuttings or setts bearing at least one bud. Planted in moist soil, the bud develops into a primary stem, the basal buds of which form secondary stems, and so on, while rootlets sprout from the root primordia band on the sett. In time, the tillers throw out their own roots and, in favorable soil conditions, an established cane stool develops an elaborate root system of widely spreading superficial roots that absorb water and nutrients and buttress roots to provide stability, as well as a few vertical roots that may penetrate deep into the soil where moisture is available even during a severe drought.

Origin and geographical spread. All forms of sugar cane are species or hybrids of the genus Saccharum, a member of the large family of Gramineae in the tribe Andropogoneae, the sorghum tribe. Six Saccharum species are now recognized. Historically, sugar canes are derivatives of what is known as the Saccharum complex, which comprises the interbreeding genera of Saccharum, Erianthus, Sclerostachya, Narenga, and Miscanthus. The most primitive Saccharum species, S. spontaneum L., a highly variable and vigorous, but thin-stemmed, fibrous cane low in sugar, is believed to have originated in northern India. Subsequent modification, movement, and hybridization generated various Asian, Pacific, and African forms. Still being debated is the botanical lineage of Saccharum officinarum L., clones of which—so-called noble canes—furnished the raw material of plantation-based sugar industries from the latter part of the eighteenth century until the early 1900s. This species—thickstemmed, soft-rinded, rich in sugar, and originally selected for chewing—is generally thought to have evolved in eastern Indonesia or New Guinea from the wild cane Saccharum robustum Brandes and Jeswiet ex Grassl. An alternative theory has S. officinarum derived from the Chinese sugar cane, S. sinense Roxb. emend. Jeswiet, which was carried to the Philippines, making that area the likely site of initial hybridization and development of S. officinarum. In any event, S. officinarum probably became a cultivated food plant about 5000 or 6000 years ago. In the course of time, sugar cane of one form or another spread eastward across the Pacific and north-westward to India, and thence via Persia and the Mediterranean basin to the Atlantic seaboard, finally reaching the New World in 1493 on the second voyage of Columbus.

The first cultivars grown in the Western Hemisphere were male-sterile, and the possibility of deliberately breeding new varieties was not generally recognized until the late 1800s after the fertility of cane seed was definitely established simultaneously in Barbados and Java. Finding that adaptability to less-than-optimal ecological conditions and, above all, resistance to diseases was unobtainable within the genetic variability of S. officinarum, breeders eventually began to cross noble canes with other Saccharum species and cross the progeny back with noble canes, a process called nobilization. Today, most cane-growing countries in the world pursue their own breeding programs. In addition to disease resistance and high yields in the local conditions over several seasons, the aim is to obtain varieties tailored to modern production methods such as suitability for mechanized harvesting and herbicide tolerance.

Environment and cultivation. Sugar cane is grown in about eighty countries between roughly 30° latitude north and 30° latitude south. Often described as a tropical crop, much cane is actually grown in subtropical areas. The ten leading producers are India, Brazil, China, Thailand, Australia, Mexico, Cuba, the United States, Pakistan, and South Africa. Sugar cane is a very adaptable plant that can be grown successfully on a wide range of soils. It grows best under ample sunlight in moist hot climates where a period of heavy rainfall is followed by a cool and dry season to increase sugar content and facilitate harvesting, or with controlled irrigation. Depending on location and other factors, the crop is ready for harvesting ten to twenty-four months after planting. In a harvesting period lasting between three and eleven months, again depending on location, the stems are cut, topped, and stripped of leaves by hand or machine. Yields vary widely from less than forty metric tons to more than one hundred metric tons of cane per hectare, equivalent, after processing, to upwards of four metric tons of sugar per hectare. Cut cane deteriorates rapidly and must be processed promptly to avoid heavy loss of sucrose. After the first harvest, underground buds on the stool throw out new shoots, and the plant develops a new root system. This allows the production of second, third, or more crops, known as ratoons, in a similar or shorter growth period and at less cost. The complete crop cycle usually lasts three to ten years, the field being replanted when the yield drops below an economic level.

Structural and economic aspects. As a perennial ratooning grass, sugar cane does not easily lend itself to crop rotation and is usually grown in monoculture. Extremely labor-intensive until mechanization, particularly at harvest time, sugar cane has been regarded as the archetypal plantation crop, produced in large enterprises employing many low-skilled workers under the supervision of a few skilled managers. Broadly speaking, this was true for the export-oriented sugar cane industries of the colonial period and lies at the heart of the historical association of sugar with slavery. However, the organizational structures of sugar cane agriculture have long exhibited great diversity, even in territories described as plantation economies, and range globally from small-holders with less than two hectares to miller-planter complexes in which a centralized management controls thousands of hectares as well as a factory.

Throughout the world, cane farming operations from soil preparation to harvesting and transport are increasingly mechanized. A bulky crop of low unit value, sugar cane presents formidable materials-handling problems. Until the 1960s, Hawaii, Louisiana, and Queensland were the only cane-growing areas to have mechanized both cutting and loading. Despite the arduous nature of the work, growers have tended to retain manual harvesting using heavy machete-type knives as long as economically feasible because mechanized harvesting systems entail extensive field reforms and modifications of transport equipment and factory reception facilities and because of the increased extraneous matter content in mechanically harvested cane, at least in the early years. Rising labor costs and better machines, however, have led to the progressive mechanization of harvesting operations in one country after another. In some areas, mechanization proceeded in stages, starting with the piling and loading of hand-cut cane or wholestalk harvesters; other areas have gone straight to combines that cut, chop into billets, clean, and load in a continuous operation.

Sugar Beet

Physical characteristics. The root of sugar beet, in which sucrose accumulates, consists, from the top down, of the epicotyl or crown, the hypocotyl or neck, and a swollen taproot. Roots vary greatly in size but average about 600 grams. The crown is the part above the lowest leaf scar; it is stem tissue with leaf buds and supports a rosette of leaves, the botanical engines that put the sucrose into the root. The hypocotyl, the region between crown and tap root and the thickest part of the root, extends from the lowest leaf scar to the uppermost lateral roots. Epicotyl and hypocotyl make up the part of the root that rises above ground. Depending on variety, plant population, and the various factors affecting growth, these two sections account for about 20 percent of the length and weight of the entire root. The white-fleshed tap root has two grooves on opposite sides from which lateral roots emerge. It tapers off to a tail less than one centimeter thick that with the hairlike ancillary roots often extends two meters or more deep into the soil.

The sucrose and nonsucrose constituents of sugar beet are not distributed uniformly in the root; sucrose content and purity are higher in the middle of the root than in the crown and tail. Hypocotyl and taproot together contain 14 to 20 percent sucrose. In harvesting, the beet is topped below the green leaf stalks of the epicotyl, a certain margin in the upper hypocotyl being allowed, while the tail usually breaks off in the lifting and subsequent handling of the root. A biennial, sugar beet flowers and bears seed in the second season, but is harvested for sugar in the first.

Origin and geographical spread. Sugar beet, Beta vulgaris L., is a member of the Chenopodiaceae or goose-foot family. Four distinct types of the species are cultivated: sugar beet, garden (red) beet, leaf beet and Swiss chard, and fodder beets. Sugar beet, the second major source of the world's sugar supply, is commercially by far the most important of the four types. Wild forms from which the crop could have been derived are widely distributed throughout the Mediterranean region and the Middle East. It is not known when the beet was taken into cultivation, but use of the plant medicinally and as a vegetable was already well established in Greek and Roman times. Cultivars with swollen roots, the result of human selection, are recorded in northern Europe from the sixteenth century onwards. The French agriculturist Olivier de Serres (1539–1619) compared the liquid from cooked red beet to sugar syrup. In 1747, Andreas Sigismund Marggraf (1709–1782), a member of the Berlin Academy of Sciences, reported having extracted from red and white beets a substance identical with cane sugar. Towards the end of the 1700s, another academician, Franz Carl Achard (1753–1821), began to grow white beets for sugar production, first near Berlin and then in Silesia where in 1802 he opened the first beet sugar factory. Achard's beets had a sugar content of up to 6 percent, but this was raised by simple mass selection to about 9 percent by the 1830s. Since then, thanks to advances in breeding methods, the sugar content in beets has roughly doubled. Average German beet yields per hectare have also more than doubled, so that a hectare of beets now produces approximately four times as much sugar as it did in the mid-1800s.

Environment and cultivation. Sugar beet is currently grown in nearly fifty countries, all save Chile in the northern hemisphere and most enjoying moderate summer temperatures and at least 250 millimeters of rainfall during the growing season except in areas where irrigation is available. Beet is successfully cultivated in many soils, but a deep loam, moist yet well-drained, is best. Member countries of the European Union, the United States, Turkey, Poland, Ukraine, Russia, and China are the leading producers. Sown in spring, the crop is lifted before the first frosts are expected. Different from cane, harvested beet can be stored for months under suitable conditions without intolerable loss.

Structural and economic aspects. Unlike cane, beet is grown in rotation with other crops and is closely integrated in the farming systems practiced in the regions where it is cultivated. As a root crop, it improves soil conditions. The beet tops can be used for fodder or, when plowed in, provide organic manure. The beet pulp remaining after sugar extraction is also used for animal feed, in contrast to bagasse, the fibrous residue of cane processing, which mainly ends up as factory boiler fuel. Like cane, beet was formerly an extremely labor-intensive crop. In the case of beet, however, not harvesting but thinning out the seedlings after emergence to create space between plants was the operation most resistant to the reduction of labor requirements. Until the advent of monogerm varieties with single flowers at each inflorescence node, sugar beet bore clusters of flowers which gave rise to multigerm seed balls. Even where animal-or tractor-drawn implements replaced the hand hoe in blocking out excess seedling clusters, the final singling to one plant per clump had to be done manually. Labor shortages in the United States during World War II spurred the introduction of mechanically segmented seed (followed after the war by decorticated seed) with a large proportion of single germs, better suited for precision drilling but at the risk of poorer germination and an uneconomic plant population in the field. The same drawbacks adhered to the seed of early genetically monogerm varieties, and the total elimination of manual labor in sugar beet agriculture did not become possible until the arrival of improved forms since the late 1960s.

Other Natural Sweetener Sources

Polysaccharide-bearing plants. In 1811, a Russian-German chemist, K. S. Kirchhof, working in St. Petersburg, discovered that adding diluted sulfuric acid to cooked potato starch produced a sweet syrup containing glucose. A year later, the first starch sugar factory was established in Germany. While the technological details would vary depending on the source of starch—cereals, roots and tubers, or stem pith, the door was opened to obtaining sweeteners from a wide range of plants. Syrup from corn (maize), rather than potatoes, has been made in the United States since the mid-1800s. Elsewhere, the raw materials could be sweet potatoes, tapioca (cassava, manioc), rice, wheat, and sago. But while such starch-based syrups could compete to some extent with cane and beet sugar in processed foods and drinks, their application was constrained by the fact that glucose is markedly less sweet than sucrose. This disadvantage was overcome in the second half of the 1960s and early 1970s by a new process of continuous enzymatic isomerization of glucose to fructose. (In essence, a glucose solution is passed through columns or beds containing immobilized enzyme, which changes the atomic arrangement of glucose into that of fructose.) The result is high-fructose corn syrup (HFCS), called isoglucose in Europe. Unlike the older glucose syrups, an equilibrium fructose–glucose syrup compares in sweetness and in other respects to invert sugar syrup made from sucrose. This greatly widened the possibilities of using starch sweeteners in industries such as the soft-drink industry, that were major users of sucrose. In the second half of the 1970s, further technological advances brought onto the market second–generation syrups with higher fructose contents, which deliver more sweetness with fewer calories, thanks to the fact that fructose is sweeter than sucrose. Since 1985, consumption of HFCS, glucose syrup, and dextrose, on a comparable dry basis, has exceeded that of beet and cane sugar in the United States, but this is so far the only country in which sucrose is no longer the leading sweetener.

Certain plants lay down fructose polymers as energy reserves in place of or in addition to starch. Inulin—found in, among other plants, chicory and Jerusalem artichoke—belongs to this category of polysaccharides. Inulin syrup is produced on an industrial scale in the European Union and falls under the EU's sugar and sweetener market regime.

Palms. Various species of palms have for centuries been tapped—notably in southern and southeastern Asia—for their sweet sap, which is drunk fresh or fermented. Alternatively, the sap has been boiled down until it sets to a solid mass of fudgelike consistency, called gur or jaggery, or is distilled to produce arrack. Among palm species exploited in these ways are the palmyra (Borassus flabellifer ), the toddy fishtail or jaggery palm (Caryota urens ), the coconut palm (Cocos nucifera ), the nipa palm (Nipa fruticans ), and a wild date palm (Phoenix sylvestris ) related to the palm of commercial date production (P. dactylifera ), the fruits of which are also a source of sugar.

Maples. Several species of the maple family, as well as birch and elm, can be tapped to make syrup and sugar, but the main source is the sugar maple, Acer saccharum. Maple syrup has been made in North America since before the arrival of the first European settlers. It was an important sweetener in the northern United States and Canada until overtaken by beet and cane sugar. A peculiarity of the maple sap run is that it takes place when the tree is still dormant. The sap that will make syrup differs from that circulating in the growing tree. Its flow is triggered by a thaw following a hard frost. The mechanics of the run are believed to involve changes in osmotic, water and gas pressures caused by the translocation of sugar stored in trunk xylem tissue the previous summer as the trunk warms up on a sunny late-winter day. Fresh sap contains up to 3 percent sucrose. Evaporation by boiling in open pans, which also adds color and the characteristic maple syrup flavor, raises the sucrose content to around 62 percent and reduces the water to 35 percent in the final product. At higher concentrations, the sugar crystallizes when the syrup cools. Thirty to 40 liters of sap make one liter of syrup.

Production methods have improved over the years. The Indian technique of cutting a wide gash in the trunk was hard on the tree. The colonists introduced the practice of making a small hole with an auger—now done with power drills—and fitting a spout from which a bucket was suspended. In recent times, plastic tubing has been adapted to collect the sap from an entire stand of trees, and central evaporator plants, with instruments to monitor boiling temperature and syrup concentration, now serve whole producer communities.

Sorghum. A near relative of sugar cane, sweet sorghum or sorgo (Sorghum vulgare ), a native of Africa, was, for a while in the second half of the eighteenth century, thought to have the potential for becoming a mainstream source of sugar in the United States. Although it could not compete against the growing availability of beet and cane sugar from about 1880 onwards, sorghum syrup is still produced on a small scale.

Mahua or Mowrah Tree. Several trees of the genus Madhuca or Bassia (Sapotaceae) bear sweet fleshy edible flowers. Those of M. indica , also named M. or B. latifolia, were mentioned in Indian medical writings as a source of sugar as early as the third to seventh centuries A.D.

Manna. The word "manna" has various meanings. The biblical manna may have been wind-borne edible lichens, Lecanora (Sphaerothallia ) esculenta or other species of the same genus. Two Middle Eastern shrubs, Alhagi maurorum and A. pseudalhagi, exude a sweet resin that hardens and can be collected by shaking the bushes over a cloth spread on the ground. Insect punctures in the stem of the French tamarisk, Tamarix gallica, of the same region produce drops of a honeylike exudation called manna. The word is also used for the incrustations formed by the sap that flows when incisions are made in the bark of the Sicilian flowering ash, Fraxinus ornus L. (Oleaceae). Numerous other sources of resinous mannas are listed in herbalist and pharmacological literature. Although not in all cases, the principal chemical constituent of such mannas is mannitol, also called mannite, a colourless sweet-tasting crystalline alcohol.

Stevia. The leaves of Stevia rebaudiana Bertoni, a wild shrub of the Compositae family that is native to Paraguay, contain a complex mixture of sweet diterpene glycosides. Stevioside, a high-intensity sweetener 250 to 300 times sweeter than sucrose, is extracted from the leaves and used as a sweetener in South America and Asia. Because of unresolved toxicological concerns, Stevia and stevio-side cannot be sold as food or food ingredients in the European Union.

Protein sweetener sources. A number of plants yield taste-modifying proteins that function as natural sweeteners of very high intensity (thousands of times sweeter than sucrose). Some are believed to have been used for centuries by indigenous peoples to improve flavor and suppress bitterness in food and drink. The most widely known, thaumatin, is contained in the aril of the seed of Thaumatococcus daniellii Benth. (Marantaceae), a West African shrub. Monellin from the berries of the West African Dioscoreophyllum cumminsii Diels (Menispermaceae) is another example.

See also Candy and Confections ; Syrups .