Sugar.pdf

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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.

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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

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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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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

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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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