Sugar

I

Introduction

Sugar, term applied loosely to any of a number of chemical compounds in the carbohydrate group that are readily soluble in water; are colourless, odourless, and usually crystallizable; and are more or less sweet in taste. In general, all monosaccharides, disaccharides, and trisaccharides (see Carbohydrate) are termed sugars, as distinct from polysaccharides such as starch, cellulose, and glycogen. Sugars, which are widely distributed in nature, are manufactured by plants during the process of photosynthesis and are found in many animal tissues (see Sugar Metabolism). Ribose, a monosaccharide sugar containing five carbon atoms in its molecule, is a constituent of the nuclei of all animal cells; five-carbon sugars are known as pentoses. Trioses (three-carbon sugars), tetroses (four-carbon sugars), heptoses (seven-carbon sugars), octoses (eight-carbon sugars), and nonoses (nine-carbon sugars) are also found in nature, but the most widespread of the sugars are the hexose sugars, characterized by the presence of six carbon atoms in the molecule and by an empirical formula of C₆H₁₂O₆. The various hexoses, having the same empirical formula and molecular weight, are structural isomers of each other. Each hexose is known in a dextrorotatory and a laevorotatory form; in solution a dextrorotatory form will rotate the plane of a beam of polarized light to the right and a laevorotatory form will rotate it to the left, but all hexoses taken into the animal body are converted into dextrorotatory forms. The most important of the hexose sugars are glucose and galactose, which are aldehydes, and fructose, which is a ketone, similar to but less reactive than an aldehyde.

The disaccharide sugars, maltose, lactose, and sucrose, have the empirical formula C₁₂H₂₂O₁₁. When treated with acids or enzymes, the disaccharides combine with one molecule of water and split into two molecules of monosaccharide hexose sugar. Maltose, for example, splits into two molecules of glucose when treated; lactose splits into one molecule of glucose and one of galactose; sucrose splits into one molecule of glucose and one of fructose.

Most of the important sugars, with the exception of sucrose, reduce cupric oxide in alkaline solution to cuprous oxide. This reaction is used in qualitative tests of sugar in the urine and the blood, and in quantitative tests of sugar in the blood; such tests are important in the diagnosis and control of diabetes.

Among the commercially important sugars are glucose, lactose, and maltose, which are widely used in infant feeding; most important, however, is sucrose, also called saccharose or cane sugar. Sucrose is generally known as cane sugar even when its source is not the sugar cane. Sucrose is used as a sweetening agent for foods and in the manufacture of confectionery, cakes, puddings, preserves, soft and alcoholic beverages, and many other foods. As a basic foodstuff sucrose supplies approximately 13 per cent of all energy that is derived from foods. Its role and value in the human diet are controversial (see Nutrition, Human).

Sucrose is present in limited quantities in many plants, including various palms and the sugar maple, but the sugar beet and the sugar cane are the only commercially important sources. More than half of the world sugar supply is obtained from the sugar cane, which is grown in tropical and subtropical climates. The rest is supplied by the sugar beet, which is grown in temperate countries. Sugar beet is the chief source of sugar for most of Europe and is grown extensively in Russia, Ukraine, Germany, France, and Poland. The countries producing the largest amounts of sugar include Brazil, Cuba, Kazakhstan, Mexico, India, and Australia.

II

Sucrose from Sugar cane

After harvesting, the thick stems of the sugar cane are stripped of leaves. In the sugar factory the stems are crushed and shredded between toothed rollers. The juice of the crushed stems is extracted in mills consisting mainly of a system of rollers, often 9 or 12 in number, through which the shredded material passes. This process is called grinding. During grinding, hot water is sprayed over the crushed material to dissolve out some of the remaining sugar. The solid, pulpy material remaining after extraction of the juice is known as bagasse; it is dried and used as fuel. Lime is added to the raw juice drawn from the mill and the mixture is heated to boiling; during this heating, unwanted organic acids form insoluble compounds with the lime, which can be filtered off along with other solid impurities. Often the juice is treated with gaseous sulphur dioxide to bleach it and is then passed through filter presses. The resulting clear juice is then evaporated in a partial vacuum and heated until it forms a thick syrup containing many crystals of sugar. The dense mass of crystals and syrup is known as massecuite. The massecuite is placed in a centrifuge turning at a rate of 1,000 to 1,500 rpm; the centrifuge walls are pierced by small holes through which the syrup, called molasses, is forced out during centrifuging. The yellowish or brown sugar removed during the centrifuging process is called first sugar, or raw sugar. The first sugar is sprayed with water to remove any molasses that may have clung to the crystals, and is then moved to the refinery. The molasses may be boiled again and reevaporated in an attempt to crystallize out some of the rich sucrose content of this liquid; in modern cane-sugar manufacture, the syrup is usually crystallized only once. The molasses is a valuable by-product of the sugar industry, being used in the manufacture of ethanol and rum, as a table syrup and food flavouring, as food for farm animals, and in the manufacture of several processed tobaccos. At the refinery, the raw sugar is redissolved, decolourized, and recrystallized into crystals of desired size. Powdered, granulated, and lump sugar, as well as brown sugars, which contain some molasses, are produced in the refineries.

III

Sucrose from Sugar Beet

Sugar is manufactured from the roots of the sugar beet, the leaves and tops being removed after harvesting and used as stock feed. The roots are cut into cossettes, or chips, at the sugar factory, and the cossettes are crushed to remove the juice. The pulp remaining after the extraction of the juice is a rich food for domestic animals. After extraction lime is added to the juice, and the remainder of the process is similar to sugar production from sugar cane. Beet molasses is fed to livestock; no table molasses is made from beets because of difficulties in purification. The sugar that is produced from the sugar beet is identical to the sugar that is derived from the sugar cane.

IV

Products

Sugar is used not only as a constituent in home-produced and industrially produced foods, but also as the raw material from which fermentation produces ethanol, butanol, glycerine, citric acid, and levulinic acid. Sugar is an ingredient in some transparent soaps, and it can be converted to esters and ethers, some of which yield tough, insoluble, and infusible resins.