Sugar
Sugar,
term applied loosely to any of a number of chemical compounds in the
carbohydrate group that are readily soluble in water; are colorless, odorless,
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
levorotatory form; in solution a dextrorotatory form will rotate the plane of a
beam of polarized light to the right and a levorotatory 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 (see Chemical Reaction) 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 sugarcane. Sucrose
is used as a sweetening agent for foods and in the manufacture of candies,
cakes, puddings, preserves, soft and alcoholic beverages, and many other foods.
As a basic foodstuff sucrose supplies approximately 13 percent 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 (see Maple), but
the sugar beet and the sugarcane are the only commercially important sources.
More than half of the world sugar supply is obtained from the sugarcane, which
is grown in tropical and subtropical climates. The rest is supplied by the sugar
beet, which is grown in temperate countries. Sugar beets are the chief source of
sugar for most of Europe and are grown extensively in Russia, Ukraine, Germany,
France, and Poland. The northwestern U.S. is also a center of sugar beet
production; sugarcane is grown in the U.S. in Hawaii, Louisiana, and Florida; it
is also grown in Puerto Rico. The countries producing the largest amounts of
sugar include Brazil, Cuba, Kazakhstan, Mexico, India, and Australia.
II SUCROSE
FROM SUGARCANE
After harvesting, the thick stems of the
sugarcane 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 sulfur 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 1000 to 1500 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 ethyl alcohol and rum, as a table
syrup and food flavoring, as food for farm animals, and in the manufacture of
several processed tobaccos. At the refinery, the raw sugar is redissolved,
decolorized, 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 BEETS
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 sugarcane. 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
sugarcane.
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 ethyl alcohol, butyl alcohol, 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.
Further Reading
