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"Sugar". In: Kirk-Othmer Encyclopedia of Chemical Technology
SUGAR
1. Introduction
The term sugar describes the chemical class of carbohydrates (qv) of the general
formula C
n
(H
2
O)
n
1
or (CH
2
O)
n
for monosaccharides. Colloquially, sugar is
the common name for sucrose, the solid crystalline sweetener for foods and
beverages. Sucrose, a disaccharide, is found in most plants, but is in sufficient
concentrations for commercial recovery only in sugarcane and sugarbeet plants.
Sucrose [57-50-1] (b-
D
-fructofuranosyl-a-
D
-glucopyranoside), C
12
H
22
O
11
,
formula weight 342.3, is a disaccharide composed of glucose and fructose resi-
dues joined by an a,b-glycosidic bond (Fig. 1).
The most common sugar in plants, sucrose is formed as a result of photo-
synthesis and occurs in abundance in sugarcane (Saccharum officinarum, a per-
ennial tropical grass) and sugarbeets (Beta vulgaris, a hearty biennial) in
amounts ranging from 12–15 and 13–20% by weight, respectively. Commercial
quantities are provided only from these two sources. Other sources include sor-
ghum (Sorghum vulgare), the sugar maple (Acer saccharum), and the date palm
(Phoenix dactylifera) (2). Sucrose is called maple, cane, beet, and more familiarly,
table sugar.
For centuries, sucrose has been valued for its pure, sweet taste. There are
indications of sugar production having occurred in what is now New Guinea (the
geographic origin of sugarcane) as early as 12,000
BC
(3). Cane production spread
to other parts of the Pacific and India by 6,000
BC
, and to China by ca 800
BC
.
Sugarcane reached present-day Iran by 500
AD
, Egypt and Spain in the eighth
century, and Sicily by 950
AD
(4). Knowledge of sugar spread throughout Europe
during the Crusades (3). In 1493, sugarcane reached the New World with
Columbus. The first sugar mill in the Western Hemisphere was built near
Santo Domingo in 1508 (2).
The sweetness of sugarbeets was recorded in 1590
AD
. Sugar was isolated
from beets by Marggraf in 1747 and the first beet sugar factory was established
in Silesia by 1802. The industry grew during the Napoleonic wars, and the year
1813 saw the presence of over 300 sugar mills in Germany, France, and Austria–
Hungary. The first American beet sugar mill was built in 1838.
In the initial stages of purification, sucrose is recovered in juice form by
crushing cane stalks or by extraction of sliced sugarbeets (cossettes) with hot
water. The resulting solutions are clarified with lime, then evaporated to thick
syrups from which sugar is recovered by crystallization. The final syrup obtained
after exhaustive crystallization of sucrose is known as molasses. Enhanced
recovery of sucrose from beet molasses is accomplished by ion-exclusion chroma-
tography, a process used in some sugar mills in the United States, Japan, Fin-
land, and Austria.
An in vitro enzymatic synthesis of sucrose was carried out in 1944 (5). A
successful chemical synthesis was performed by Lemieux and Huber (6) in
1953 from acetylated sugar precursors. However, the economics and chemical
complexities of both processes make them unlikely sources of supply.
The sucrose in cane sugar is identical to that in beet sugar; both white
refined products are 99.9% sucrose, with water as the principal nonsucrose com-
ponent. Trace components from the plant indicate the origin of the sugar.
1
Kirk-Othmer Encyclopedia of Chemical Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
2 SUGAR
Vol. 19
2. Physical Properties
2.1. Sucrose.
Physical properties of sucrose are summarized in Table 1.
Sucrose is one of the purest substances available in bulk quantities, with purities
averaging
99.96%. Water accounts for about half of the nonsugar impurities
(11).
Sucrose crystals are triboluminescent and emit light when they are frac-
tured. Aqueous sucrose solutions rotate polarized light in direct proportion to
sugar concentration. This property is utilized for quantitation, purity evalua-
tions during factory processing, and for setting the selling price of sugar. Sucrose
quantitation has also been performed by colorimetric methods. However, in
recent years, automated enzymatic analyzers and instrumental methods have
become increasingly popular, as they provide greater sensitivity and accuracy.
The sweet taste of sucrose is its most notable and important physical prop-
erty and is regarded as the standard against which other sweeteners (qv) are
rated. Sweetness is influenced by temperature, pH, sugar concentration, physical
properties of the food system, and other factors (12–14). The sweetening powers
of sucrose and other sweeteners are compared in Table 2. The sweetness thresh-
old for dissolved sucrose is
0.2–0.5% and its sweetness intensity is highest at
32–388C (13).
Fructose is sweeter than sucrose at low temperatures (
58C); at higher
temperatures, the reverse is true. At 408C, they have equal sweetness, the result
of a temperature-induced shift in the percentages of a- and b-fructose anomers.
The taste of sucrose is synergistic with high intensity sweeteners (eg, sucralose
and aspartame) and can be enhanced or prolonged by substances like glycerol
monostearate, lecithin, and maltol (13).
Sucrose has eight hydroxyl groups capable of hydrogen-bond formation. The
glucose and fructose residues in crystalline sucrose are nearly perpendicular to
each other and are held in this conformation by hydrogen bonds between the C
1
–
OH of fructose and the C
2
–oxygen of glucose, and C
6
–OH of fructose and the ring
oxygen of glucose (Fig. 1b). In aqueous sugar solutions, the latter bond is absent;
however, the overall conformation is essentially unchanged (1).
Elicitation of sweetness is explained by the AH–B–X Theory, or the Sweet-
ness Triangle (1,17,18), wherein AH and B represent a hydrogen donor and
acceptor, respectively, of sucrose. These interact with complementary regions
of taste-receptor proteins. An extended hydrophobic region (X) of sucrose docks
in a hydrophobic cleft of the receptor, facilitating optimal electrostatic interac-
tion and sensory stimulation. The C
2
- and C
3
-hydroxyls of glucose (AH and
B, respectively) and the back side of the fructose ring (X) are critical to this
interaction.
The hydroxyl groups of sucrose contribute to its very high water solubility.
This is enhanced by the presence of other dissolved solids and diminishes the
yield of crystals produced from sugar syrups. Sucrose forms sphenoidic monocli-
nic crystals (Fig. 2), the shapes of which are affected by syrup impurities like raf-
finose and dextran, which promote growth of the B- and C-axes, respectively,
resulting in formation of elongated, needle-like crystals (11,19,20). A secondary
consequence is a decrease in bulk density with its accompanying packaging
Vol. 19
SUGAR 3
problems (21). Impurities included within sucrose crystals impart color, raise ash
levels, and reduce sugar quality. Thus, nonsucrose impurities can seriously
impact the recovery and quality of sucrose. Sucrose also has moderate solubility
in pyridine, glycerol, methylpyrrolidinone, methylpiperazine, DMSO, and DMF,
and is slightly soluble in methanol and ethanol (10,21). The high dielectric con-
stant of sucrose (3.50–3.85) gives it substantial microwave-absorbing capacity
and makes it a valuable ingredient in formulating microwavable foods (12).
2.2. Cane Sugar.
Cane sugar is generally available in one of two forms:
crystalline solid or aqueous solution, and occasionally in an amorphous or micro-
crystalline glassy form. Microcrystalline is here defined as crystals too small to
show structure on x-ray diffraction. The melting point of sucrose (anhydrous) is
usually stated as 1868C, although, because this property depends on the purity of
the sucrose crystal, values up to 1928C have been reported. Sucrose crystallizes
as an anhydrous, monoclinic crystal, belonging to space group P2
1
(22).
The specific rotation in water is [a]
2
D
¼þ
66
:
5298 (26 g pure sucrose made to
100 cm
3
with water). This property is the basis for measurement of sucrose con-
centration in aqueous solution by polarimetry. 1008Z indicates 100% sucrose on
solids.
Among physical properties of cane sugar that are most important for its use
in foods are bulk density, dielectric constant, osmotic pressure, solubility, vapor
pressure, and other colligative properties, and viscosity (23). Bulk density,
important for cane sugar as an ingredient in dry mixes, is listed in Table 3, as
typical values for several types of sugars. Solubility of sucrose with other com-
mon sugars is shown in Figure 3 and in Table 4. Colligative properties vary
with concentration of sucrose in solution. The strong effect of cane sugar on freez-
ing point depression is widely used in frozen desserts; the reduction in vapor
pressure and increase in boiling point are essential for manufacture of hard
candy and other confectionery (22,24). The high osmotic pressure generated by
sucrose in solution (Table 5) (22) reduces the water activity and therefore the
equilibrium relative humidity, so that insufficient moisture remains to sustain
microorganisms, as in jams and preserves. Most common microorganisms
require at least 80% equilibrium relative humidity to grow; both crystalline
sugar, and concentrated solutions such as jams and preserves, are well under
70%. Dielectric constant values for sucrose in solution are shown in Table 6
(23); the high values make sugar an important ingredient for quick heating in
microwaveable foods. The viscosity of cane sugar solutions varies greatly with
degree of purity of the sugar; tables for sucrose are readily available (22-27).
3. Chemical Properties
3.1. Sucrose.
The carbonyl groups of fructose and glucose partake in
the glycosidic bond of sucrose, making the latter nonreducing. Therefore, sucrose
has no anomeric forms, cannot undergo mutarotation or osazone formation, and
is inert to alkaline copper reagents. The hydroxyl groups of sucrose are very
weakly acidic (1); the C
2
–OH of glucose is the most acidic. The pK
a
sofsucroseat
258Care pK
1
¼
12.02, pK
2
¼
12.56, and pK
3
¼
12.01 (28). The three primaryhydroxyl
groups of sucrose are most reactive (6–6
0
>
1); of the secondary hydroxyls, those
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Vol. 19
on C-2 and C-3
0
are more reactive (29). The hydroxyls dissociate in alkali to form
alcoholates called saccharates, and can be derivatized to produce valuable sucro-
chemicals.
Oxidation of Sucrose.
Sucrose can be oxidized by HNO
3
,KMnO
4
, and
peroxide. Under selected conditions using oxygen with palladium or platinum,
the 6- or 6
0
-hydroxyls can be oxidized to form sucronic acid derivatives (29).
Hydrolysis of Sucrose.
Sucrose can be enzymatically hydrolyzed to glu-
cose and fructose by invertase. During this reaction, the optical rotation falls
to a negative value owing to the large negative specific rotation of fructose.
The reversal is called inversion and the resulting glucose–fructose mixture is
called invert.
Sugar is destroyed by pH extremes, and inadequate pH control can cause
significant sucrose losses in sugar mills. Sucrose is one of the most acid-labile dis-
accharides known, and its hydrolysis to invert is readily catalyzed by heat and
low pH; prolonged exposure converts the monosaccharides to hydroxymethyl fur-
fural, which has applications for synthesis of glycols, ethers, polymers, and phar-
maceuticals (10). The molecular mechanism that occurs during acid hydrolysis
operates, albeit slowly, as high as pH 8.5 (12).
Alkaline Degradation.
At high pH, sucrose is relatively stable; however,
prolonged exposure to strong alkali and heat converts sucrose to a mixture of
organic acids (mainly lactate), ketones, and cyclic condensation products. The
mechanism of alkaline degradation is uncertain; however, initial formation of
glucose and fructose apparently does not occur (31). In aqueous solutions,
sucrose is most stable at
pH 9.0.
Thermal Degradation.
The heats of formation and combustion of sucrose
are
2.26MJ/mol (540 kcal/mol) and
5.79MJ/mol (
1384 kcal/mol), respec-
tively (32,33). At high temperatures (160–1868C), sucrose decomposes with char-
ring, emitting an odor of caramel. Thermolysis of crystalline sucrose at 1708C
yielded a complex mixture of products (34). The mixture contained several non-
reducing trisaccharides, including 6-kestose. Acid-catalyzed thermolysis causes
decomposition to glucose and fructofuranosyl cation. The latter reacts with
sucrose to form a complex mixture of products, including fructoglucan and sev-
eral kestoses (35). These substances are examples of fructooligosaccharides
(FOS) and are known to promote the growth of beneficial intestinal microorganisms.
3.2. Cane Sugar.
Among chemical properties of cane sugar that affect
daily use are color, flavor, sweetness, antioxidant properties, and reactions in
aqueous solution (23). The purity of cane sugar is generally assessed by its
color; lowest color sugars are highest purity sucrose with the lowest content of
color and flavor molecules, and other organic and inorganic components. Table
7 shows composition of cane sugar, beet sugar (qv), and other cane sugar pro-
ducts. Brown sugars and golden syrup are generally made from cane sugar, for
reasons of flavor.
Sucrose, traditionally cane sugar, is the standard for sweetness, and other
sweeteners are ranked against sucrose as 100% (see Table 2) (23). Reactions of
cane sugar in aqueous solution are important both in manufacturing (process is
almost entirely in solution) and in use as a food and in food processing (qv).
Hydrolysis of sucrose, called inversion, forms an equimolar mixture of
glucose and fructose, called invert sugar or invert, because of the change in
Vol. 19
SUGAR 5
the polarimetric measurement, or inversion from positive to negative, upon
hydrolysis. Hydrolysis is the initial step for most reactions of cane sugar in
food chemistry. It is depicted in Figure 4 (23). It occurs up to about pH 8;
above that, nucleophilic displacement of a proton is the initial reaction in sucrose
decomposition. Reactions after initial hydrolysis (inversion) include the follow-
ing. (1) Reactions in acidic medium which lead to formation of 5-hydroxymethyl
furfural (HMF). HMF rapidly decomposes into dark-colored compounds, with off-
flavors (22,23,36). (2) Reactions in alkaline medium, including lactic acid forma-
tion by chemical means (rather than by fermentation), and the rearrangement of
glucose to a mixture of mannose and fructose, which is often responsible for the
reported presence of fructose and mannose in products that in actuality contain
only glucose. An alkaline environment present during extraction or hydrolysis
procedures can cause the transformation of glucose to a mixture of mannose
and fructose by this mechanism (22,23,36). (3) Maillard reactions, ie, the reaction
of a reducing sugar with an a-amino group to form a condensation product that
can subsequently polymerize into dark-colored compounds. This is the basis of
the browning reaction observed during baking and cooking processes. Several
alternative pathways of color, or melanoidin, formation are possible after the
initial Maillard reaction (22,23). (4) Thermal degradation of sucrose and caramel
formation. The thermal decomposition of solid sucrose may be the exception to
the rule that the common decomposition-related reactions occur in water solu-
tion; however, moisture absorption by sucrose as it is heated can account for
some thermal degradation along pathways of solution reactions. Multiple reac-
tions, some anhydrous, some involving water, are involved in the formation of
the complex mixture known as caramel (22,23).
Color of cane sugar depends on its nonsucrose content; sucrose, glucose, and
fructose are white crystalline materials. Colorant compounds are in two classes:
one from the cane plant, including flavonoid and polyphenolic compounds, and
one from process-developed colorant, based on sucrose degradation products.
These degradation reactions occur in aqueous solution, in process, and in a rela-
tively slow manner in the syrup layer surrounding the sugar crystal. Reactions
in solution, included in those described above, that are responsible for color for-
mation include thermal degradation of sugars, with condensation at low pH and
caramel formation; alkaline degradation of fructose, with subsequent condensa-
tion; and Maillard reactions with primary amines and subsequent melanoidin
formation. Many of the colorant compounds are also responsible for the caramel,
butterscotch, and toasty flavors of brown cane sugar.
4. Production from Sugarcane
4.1. Cultivation, Harvesting, and Processing of Sugarcane.
Cane
sugar production is accomplished in one or two stages. At sugarcane factories,
located in cane-growing areas, harvested sugarcane is brought in, sugar-contain-
ing juice is extracted, and sugar crystallized from the concentrated juice. In the
single-stage process, the juice is purified and bleached for the manufacture of
plantation white (mill white, direct white) sugar, usually for local consumption.
In the two-stage process, partially purified, unbleached juice is crystallized into
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