Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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0008 There is confusion in the old literature with regard

to the use of the letters d-andl- and also d- and l-.

Both the upper-case and lower-case letters were used

in the past to describe the configuration as well as the

direction of optical rotation. The latter has also been

denoted less ambiguously as (dextro-) and (levo-). The

conventional notation currently in use is d- and l- for

configuration and (þ) and () for optical rotation.

Isomerism of Monosaccharides

0009 Isomers are substances that have identical molecular

formulas but differ in the way in which the atoms are

bonded to each other. Stereoisomers differ only in

the way the atoms are arranged in space as already

explained. The number of stereoisomers for any

monosaccharide is 2

, where n is the number of chiral

centers. Aldohexoses have four chiral centers and,

therefore, 2

¼16 isomers (Figure 3). There will be

eight d-isomers and eight l-isomers. Therefore, the

number of enantiomer pairs is 8 (2

n1

). Ketohexoses

have only three chiral centers and thus eight isomers

and four enantiomer pairs (Figure 4). It should be

noted that the specific rotation of a pair of enantio-

mers has the same numerical value and differs only in

sign. For example, the enantiomer of a d-(þ)-glucose

([a]

¼þ52.5



)isl-()-glucose ([a]

¼52.5



Biological systems can utilize either d-orl-molecules,

but not both. The human body utilizes only d-glu-

cose. By contrast, amino acids in the human diet

belong to the l-series, and their d-form is not metab-

olized. However, l-arabinose, l-fucose, and l-rham-

nose occur in plants, whereas their d-counterparts do

not.

Epimers

0010Two sugars differing in configuration at a single

asymmetric carbon atom are known as epimers. Glu-

cose and mannose are C2 epimers, ribose and xylose

D-(−)-Erythrulose

Dihydroxyacetone

D-(−)-Ribulose

D-(+)-Psicose D-(−)-Fructose D-(+)-Sorbose D-(−)-Tagatose

D-(+)-Xylulose

CHOH

CHO H

CHOH

CHO H

CHOH

CHO H

CH OH

fig0004 Figure 4 The D-series of ketoses derived from dihydroxyacetone. Sugars containing four, five, and six carbon atoms are known as

ketotetroses, ketopentoses, and ketohexoses, respectively. These Fischer projection formulae have the penultimate OH group (bold

characters) attached to the highest numbered chiral center, on the right-hand side of the projection formula.

866 CARBOHYDRATES/Classification and Properties

are C3 epimers, and gulose and galactose are also C3

epimers (Figure 3). d-Arabinose and l-xylose are C4

epimers, and so are d-glucose and d-galactose. In

all cases (and there are several more), changing the

position of H and OH on the same carbon atom

(epimeric) of one sugar gives rise to the other sugar,

i.e., its epimer. Epimers are also diastereoisomers, i.e.,

they have a similar structure but are not enantiomers

(chiral or mirror images).

Mutarotation and Cyclic Hemiacetals

0011 Glucose crystallized from methanol has a melting

point of 147



C. When dissolved in water, it has an

initial specific rotation of þ113



, which falls after

several hours to þ52.5



. Glucose crystallized from

water at a high temperature (> 50



C) has a melting

point of 150



C. Its initial specific rotation is þ19



and rises gradually to the value of þ52.5



. The

change in optical rotation on standing is called muta-

rotation. This is explained by the fact that glucose

gives rise to a cyclic ‘internal’ hemiacetal with the

formation of a bond between carbon atom 5 (carrying

an OH group) and carbon atom 1 (the aldehyde

carbonyl group), as shown in Figure 5.

0012 During ring closure, a new chiral center is formed

(carbon atom 1), and the hydroxyl group will assume

either the a- or the b-configuration (right or left

respectively of the projection formula). The new

chiral center is known as the anomeric carbon,

and the resulting two structures as the a-andb-

anomers. Anomers are not enantiomers and have

different specific rotations. The planar ring struc-

tures, termed Haworth projections, are perpendicular

to the plane of the paper with the thick lines directed

towards the observer. The cyclic structures are

then described as pyranoses (and their derivatives as

pyranosides) and furanoses (or furanosides) by refer-

ence to the heterocyclic compounds pyran and

furan. In aqueous solution, a dynamic equilibrium is

established between the a- and b-glucopyranose

structures. The equilibrium mixture of glucose at

room temperature consists of about 36% of the a-

anomer and 64% of the b-anomer. There is only a

negligible amount of the open chain form. There is

also a very small proportion (< 0.5%) of the a- and b-

anomers of glucofuranose rings. In general, equili-

brated solutions of most sugars contain different pro-

portions of the two furanose and the two pyranose

structures.

0013Fructose undergoes a similar rearrangement. Crys-

talline b-d-fructopyranose has an initial specific rota-

tion of 133.5



and undergoes rapid mutarotation to

92



. The equilibrated solution contains about 20%

of the fructofuranose form. The four ring forms of

fructose are shown in Figure 6.

Ring Conformations

0014So far, we have dealt with the spatial arrangement of

atoms in various forms of monosaccharides. This is

known as the configuration of molecules and involves

the breaking of chemical bonds when converting, for

example, from the d- to the l-form or from the a-to

the b-anomeric form. Although the Haworth ring

structure of sugars gives an exact view of the spatial

arrangement of the hydroxyl groups of a particular

sugar, it is misleading, because there is now ample

OH CH

Pyran

OH H

H H

HO H

-Glucose

Fischer projection

β-

D-Glucoseα-D-Glucose α-D-Glucopyranose β-D-Glucopyranose

Hemiacetal ring closure Haworth projection of the pyranose form

The ring is perpendicular to the plane of the paper and the

thick lines point towards the observer

fig0005 Figure 5 Cyclic hemiacetal formation between carbon 5 and carbon 1 of glucose. The anomeric carbon 1 is chiral, and two positions

are possible for the OH group, which can be below (a-form) or above (b-form) the pyranose ring, or right and left, respectively, on the

projection formula. The two forms are in equilibrium in aqueous solution. There is a small proportion (<0.5%) of each of the two

furanose forms and an almost negligible amount (0.003%) of the open-chain form.

CARBOHYDRATES/Classification and Properties 867

evidence that the ring is not planar. The bonds in a

planar ring are under considerable strain, which

could be relieved if the ring adopted a nonplanar

conformation to comply with the tetrahedral direc-

tion of valences. A change in the conformation of the

ring structure of a sugar does not involve the breaking

of bonds. It affects only bond angles and leads to an

energetically favorable arrangement of atoms. Theor-

etical and experimental investigations have thrown

considerable light on the conformation of the fura-

nose ring in particular, both in the solid state and

in solution. Complex formation, nuclear magnetic

resonance spectroscopy and X-ray crystallography

are the most useful techniques. Various theoretical

schemes and energy minimization studies have also

been useful in elucidating the conformation of carbo-

hydrates. The preferred conformation for glucose is

the so-called ‘chair form’ (Figure 7).

(a) (b)

fig0007 Figure 7 Cyclic hemiacetal form of glucose anomers. Chair [4C1] conformation of (a) a-D-Glucopyranose and (b) b-D-Glucopyranose.

Note that all OH groups are equatorial, with the exception of OH on the anomeric carbon atom 1 in (a). In aqueous solution anomer (b)

is more abundant (64%) than (a) (36%), because the all-equatorial conformation is thermodynamically favoured. The two forms are in

equilibrium; there is a negligible fraction of the acyclic (open chain) form in solution.

HOCH

Furan

HOH

D-Fructose

Fischer projection

α-

D-Fructose

β-

D-Fructose

α-

D-Fructose β-D-Fructose

Hemiacetal ring closure

α-

D-Fructofuranose β-D-Fructofuranose

α-

D-Fructopyranose β-D-Fructopyranose

Haworth projection of the furanose form

The ring is perpendicular to the plane of the paper and the thick lines point

towards the observer

Haworth projection of the pyranose form

The ring is perpendicular to the plane of the paper and the thick lines point

towards the observer

fig0006 Figure 6 Cyclic hemiacetal (hemiketal) formation between carbon 5 and carbon 2 of fructose leads to the furanose structure.

Hemiacetal formation between carbons 6 and 1 leads to the pyranose structure. The anomeric carbon atoms 2 and 1 are chiral, and two

configurations are possible for the OH group. There are about 76% of the b-pyranose form, 20% of the b-furanose form, about 4% of

the a-furanose form, and a negligible amount of the open-chain form.

868 CARBOHYDRATES/Classification and Properties

Glycosides

0015 When an aldehyde or a ketone reacts with an alcohol

in the presence of acid, a hemiacetal is formed first,

and then an acetal with the addition of a second

molecule of alcohol. The reaction is shown in Figure 8.

0016 We have already seen that the cyclic forms of

monosaccharides are hemiacetals. The hydroxyl

group attached to the anomeric carbon atom can

react with an alcohol (or the OH of another sugar).

The resulting product is known as a ‘glycoside’ (from

Greek glykys ¼sweet; gluco- is used specifically for

glucose). For example, glucose (a-pyranose form)

reacts with ethanol and gives rise to ethyl a-d-gluco-

pyranoside, where the ethyl part of the molecule is

known as the aglycone (see Figure 9).

0017 There are many naturally occurring glycosides.

Salicin is an example where the aglycone is o-(hydro-

xymethyl) phenol. Amygralin is another glycoside,

which occurs in the kernels of bitter almonds,

peaches, and apricots. Its composition is [(6-O-b-

d-glucopyranosyl-b-d-glucopyranosyl)oxy]benzene-

acetonitrile. It is hydrolyzed by b-glucosidase (the

enzyme emulsin). Glycosides linked in the a-position

are hydrolyzed by a-glucosidase, of which yeast is a

principal source. Anthocyanins, the coloring matter

of flowers and fruits, occur in nature as glycosides,

which are hydrolyzed by acid to yield anthocyanidins

and a number of sugars. Of course, oligo- and poly-

saccharides are all glycosides where the aglycone is

any other sugar.

Representative Monosaccharides

0018 Both glucose (See Glucose: Properties and Analysis)

and fructose (See Fructose) occur widely in plants,

particularly in fruits. They are also found in

honey (See Honey). Glucose is an important item

of commerce as crystalline dextrose monohydrate

O), or as a component of glucose

syrup and high fructose syrup (See Syrups). Other

monosaccharides occur as components of polysac-

charides rather than as free sugars (see below).

The pentoses d-ribose and d-2-deoxyribose are com-

ponents of nucleotides.

Disaccharides

Nomenclature

0019Disaccharides can be reducing (having a free

carbonyl group) or nonreducing. In the latter case,

the two component monosaccharides are linked at

their respective anomeric centers, and therefore,

the carbonyl group is not available for reaction.

Disaccharides are named as glycosides where the

aglycone is another monosaccharide. Reducing disac-

charides are named as substituted monosaccharides

(Figure 10).

0020Sucrose (saccharose) is by far the most important

disaccharide (See Sucrose: Properties and Determin-

ation; Dietary Importance; Sugar: Sugarcane; Sugar-

beet; Palms and Maples; Refining of Sugarbeet and

Sugarcane). Lactose (See Lactose) occurs in the milk

of mammals but very rarely in the plant kingdom.

Trehalose (a-d-glucopyranosyl a-d-glucopyranoside)

(nonreducing) occurs in mushrooms and other fungi.

Maltose is formed during the mashing of malt

(See Malt: Malt Types and Products; Chemistry of

Malting) in brewing and serves as a substrate for

yeast in alcoholic fermentation. It is also a component

of high-maltose syrup. Cellobiose (b-d-glucopyrano-

syl-(1!4)-d-glucose) (reducing) is formed by the en-

zymatic hydrolysis of cellulose.

Oligosaccharides

0021The so-called raffinose family of oligosaccharides

comprises raffinose (trisaccharide), stachyose (tetra-

saccharide), and verbascose (pentasaccharide), all of

which occur in the seeds of legumes, as well as in

different parts of plants. Verbascose (Figure 11) has

three molecules of a-d-galactose attached to sucrose;

stachyose has two, and raffinose one. They are all

nonreducing. Invertase releases the fructose moiety

and gives rise to reducing saccharides. Treatment of

raffinose with invertase (b-fructosidase) gives fruc-

tose and melibiose, whereas a-galactosidase (from

green coffee beans) gives sucrose and galactose. Lac-

tase (b-galactosidase) has no effect.

OCH

OH H

Ethyl α-

D-glucopyranoside

Aglycone

fig0009 Figure 9 Formation of a ‘glycoside’ by the addition of ethanol

and elimination of water.

OR⬙

⬙

⬘

Hemiacetal

⬙

Acetal

+ +

fig0008 Figure 8 Addition of an alcohol to an aldehyde gives rise to a

hemiacetal. Addition of a second molecule of alcohol gives an

acetal (with the elimination of water).

CARBOHYDRATES/Classification and Properties 869

HOCH

Verbascose

(non-reducing

pentasaccharide)

α-galactosidase

β-fructosidase

Sucrose

Raffinose

Stachyose

fig0011 Figure 11 O-a-D-Galactopyranosyl-(1!6)-[O-a-D-galactopyranosyl-(1!6)]

-O-a-D-glucopyranosyl-(1!2)b-D-frutofuranoside. Arrows

indicate the points of hydrolysis by enzymes.

OH CH

H,OH

HOCH

H,OH

HHO

Maltose (reducing)

4-O-(α-D-glucopyransyl)-D-glucopyranose

or O-α-

D-glucopyranosyl-(1

4)-α-D-glucopyranoside

Lactose (reducing)

4-O-(β-

D-galactopyranosyl)-D-glucopyranose

Sucrose (non-reducing)

α-

D-glucopyranosyl-β-D-fructofuranoside

or (β-D-fructopyranosyl)-α-D-glucopyranoside

or O-α-

D-glucopyranosyl-(1

2)-β-D-fructopyranoside

fig0010 Figure 10 Formulae and nomenclature of three common disaccharides. Sucrose does not have a free carbonyl group. The OH group

on the anomeric carbon atom of maltose and lactose can acquire either the a- or the b-configuration. Both sugars mutarotate when

dissolved in water, and equilibrium is established after several hours. A few drops of ammonia accelerate the rate of mutarotation, and

a constant specific rotation is rapidly attained.

870 CARBOHYDRATES/Classification and Properties

Polysaccharides

Nomenclature

0022 Polysaccharides are also known as ‘glycans.’

Depending on the monomeric sugar, they are

described as glucans, fructans, galactans, mannans,

xylans, etc., where the first four letters refer to glucose,

fructose, galactose, mannose, and xylose, respect-

ively. Polysaccharides consisting of a single type

of monomer are known as ‘homopolysaccharides.’

When two or more types of sugar monomers are

involved, they are known as ‘heteropolysaccharides.’

The properties of polysaccharides are discussed

in Carbohydrates: Interactions with Other Food

Components.

Representative Polysaccharides

0023 Cellulose (See Cereals: Contribution to the Diet) is

the most abundant polysaccharide in nature. It con-

sists of b-linked glucose units in the form of long

chains, which associate parallel to each other by

hydrogen bonding to form fibers, which are insoluble

in water. Cellulose is a component of plant cell walls

and occurs also in leaves and stems, often associated

with lignin and xylans. The textural characteristics of

leafy vegetables depend, in part, on cellulose. The

related b-glucans (water-soluble) of malted barley

cause filtration problems in brewing.

0024Starch (See Starch: Structure, Properties, and

Determination; Sources and Processing; Functional

Properties; Modified Starches; Resistant Starch) is

the energy reserve polysaccharide of plants. It

occurs in the form of microscopic granules (about

5–50 nm in diameter) in cereal grains, in roots,

tubers, stems, and, to a lesser extent, in some fruit

and leaves. The granules are insoluble in water at

ambient temperature but absorb water and swell

when heated to their gelatinization temperature

(c.50–70



C depending on the origin). Starch consists

of two polymers: essentially linear amylose with an

average degree of polymerization (DP) of c. 1000,

and branched amylopectin as shown in Figure 12.

Amylopectin consists of a-(1 !4)-linked glucose

units (95%) and a-(1 !6)-linked branched points

(5%). The DP is 10

–10

. Glycogen (See Glycogen)

is the reserve carbohydrate of the animal kingdom.

It resembles amylopectin in structure but has more

extensive multiple branching.

0025Dextran (See Dextran) is a polymer of dextrose

synthesized from sucrose by Leuconostoc dextrani-

cum or L. mesenteroides. It consists mainly of

a-d-(1 !6) linked glucose units to which are

attached a-(1 !3) and some a-(1 !2) glucan

branches. Low-molecular-weight products are water-

soluble, whereas high-molecular-weight products

are insoluble. Dextrins (See Dextrins) are products

of partial acid hydrolysis of starch (very low acid at

H, OH

O O

Amylose: α-(1

4)-glucan: average n = ca. 1000. The linear molecule may carry a few occasional

moderately long chains linked α-(1

60).

Amylopectin: α-(1 6) branching points. For exterior

chains a = ca. 12−23. For interior chains b = ca. 20−30.

Both a and b can vary according to the botanical origin.

fig0012 Figure 12 Basic structure of amylose and amylopectin.

CARBOHYDRATES/Classification and Properties 871

high temperature). They are water-soluble and avail-

able in several grades. They are used mainly as

adhesives, although food-grade dextrins are also

known. Nonstarch, noncellulosic polysaccharides

(formerly known as hemicelluloses) occur in cereals,

roots, fruits, leaves, and stems of plants. They are

the main components of dietary fiber (See Dietary

Fiber: Properties and Sources; Determination;

Physiological Effects; Effects of Fiber on Absorption;

Bran and may consist of pentosans (composite

polymers of pentoses), b-glucans, and pectic sub-

stances. Some are insoluble, whereas some are soluble

in water. Rhamnogalacturonans, arabinogalactans,

glucuronoxylans, and xyloglucans occur mainly in

fruits and vegetables, whereas glucuronoarabinoxy-

lans occur in cereals. Inulin (Figure 13) is a polymer

of fructose, having one molecule of glucose at

the start of the chain. It is soluble in water and is

not hydrolyzed by invertase. Polysaccharides from

various sources used as food additives are discussed

in Carbohydrates: Interactions with Other Food

Components.

Glycoproteins and Glycolipids

0026 Some proteins may contain small amounts of carbo-

hydrates. Casein contains 1% galactose, 1.2% galac-

tosamine, and 2.4% acetyl neuramic acid. Egg white

contains considerable amounts of carbohydrate

(2–30% in the various fractions), the main sugars

being galactose, mannose, and glucosamine. Glycoli-

pids are found in various membranes in both plants

and animals. The carbohydrate content is relatively

minor.

Physical Properties

0027Most sugars are soluble in water and form

syrups at high concentrations. Crystallization (See

Crystallization: Basic Principles) occurs in saturated

solutions. They are mainly used for their sweetness.

Sucrose and fructose at a concentration of > 65% (w/

w) have a preservative effect because of high osmotic

pressure (See Water Activity: Principles and Measure-

ment; Effect on Food Stability) that prevents the

proliferation of microbes (See Carbohydrates: Inter-

actions with Other Food Components). They also

impart desirable textural characteristics to a wide

range of foods like cakes, biscuits, and confectionery

products. Polysaccharides are mostly used in foods

to impart viscosity and induce gel formation (See

Carbohydrates: Interactions with Other Food Com-

ponents). Table 1 lists some of the properties of

sugars.

Chemical Properties

0028Sugars undergo a variety of reactions and give rise to

many derivatives. These are not all of immediate

interest to the food scientist. Relevant interactions

are discussed in Carbohydrates: Interactions with

Other Food Components.

Reduction

0029Monosaccharides are easily reduced to alditols

(known also as ‘polyols’) by sodium borohydride

(NaBH

) in weak alkaline solution. Large-scale

reduction is carried out by catalytic hydrogenation.

Alditols are linear molecules, do not form rings, and

do not exist as anomeric forms. These are important

properties used advantageously in sugar analysis by

gas chromatography, since each alditol gives rise to a

single peak on a chromatogram. Tetritols, pentitols,

and hexitols have 2, 3, and 4 chiral centers, respect-

ively. In general, they have a low specific rotation,

and some are optically inactive (meso-forms); the

latter are also known as internally compensated

molecules, because they have a plane of symmetry

(Figure 14).

0030It is noteworthy that when d-glucitol is rotated 180



it is identical to d-gulitol, which is also identical to l-

glucitol, the latter being the enantiomer (mirror image)

of d-glucitol. l-Glucitol does not occur in nature, but

d-glucitol (sorbitol) is fairly common (plums, berries).

On hydrogenation of fructose, a new chiral center is

created, and thus two alditols are produced, namely

d-glucitol (75%) and d-mannitol (25%) (epimers).

Galactose gives rise to d-galactitol (dulcitol) and

maltose to maltitol. Lactitol is obtained by the hydro-

genation of lactose. The free carbonyl end of oligo- and

HOCH

HHO

HOCH

HHO

Inulin

(non-reducing)

n = ca. 30−60

fig0013 Figure 13 Structure of inulin. This polysaccharide is soluble in

water and is not hydrolyzed by b-fructosidase.

872 CARBOHYDRATES/Classification and Properties

tbl0001 Table 1 Some properties of carbohydrates

Commonly occurring

forms

Molecular

weight

Specific rotation

[a]p

Meltingpoint

(



Solubility (grams per

milliliter of water

)

Sweetness

b,c

Sucrose ¼10 0

Hydrolyzing

enzymes

Occurrence

Pentoses C

L-Arabinose 150.13 þ173



!þ105.1



157–160 1 As arabinan in plant cell walls

D-Xylose 150.13 þ92



!þ18.6



153–154 1.25 67–70 As xylan in plant cell walls

Hexoses C

a-D-Glucose (dextrose) 180.16 þ112.2



!þ52.7



146 0.91 70 Free in fruits

D-Glucose monohydrate 198.18 þ102.0



!þ47.9



83 1

D-Galactose 180.16 þ150.7



!þ80.2



167 2 63 As galactan in plant cell walls

D-Galactose 180.16 þ52.8



!þ80.2



167 0.6

D-Mannose 180.16 þ29.3



!þ14.2



133 Soluble 60 As mannan in plant cell walls and

nuts

D-Mannose 180.16 17.0



!þ14.2



132

2.5

D-Fructose (levulose) 180.16 132



!92



103–105

4 114–150 As fructan and free in fruits

Deoxyaldohexoses C

a-D-Fucose 164.16 þ127



!76.0



144 Soluble Plant cell walls

L-Fucose 164.16 124



!75.6



140 Soluble

L-Rhamnose 164.16 7.7



!þ8.9



82–92 Soluble 33 Plant cell walls

Uronic acids C

a-D-Galacturonic acid 194.14 þ98.0



!þ50.9



159 Soluble As pectin in fruits and plants

D-Glucuronic acid 194.14 þ11.7



!þ36.3



165 Soluble Plants

Disaccharides C

Sucrose

342.30 þ66.5



160–186

2 100 b-Fructosidase Sugar cane, sugar beet, fruits

Maltose 342.30 þ111.7



!þ130.4



102–103 Soluble 40–46 a-Glucosidase Free in malt extract

Lactose monohydrate 360.32 þ92.6



!þ52.3



201–202 0.2 39 b-Galactosidase Free in milk

Lactulose 342.30 51.4



169 3.2 60 Low concentration in UHT milk

Trehalose dihydrate

378.34 þ178



96.5–97.5 Soluble Trehalase Free in mushrooms

Tr i s a c c h a r i d e

Raffinose pentahydrate

504.44 anh þ105.2



80 0.14 22 a -Galactosidase Leguminous seeds

Sugar alcohols

Xylitol 152.15 Meso-form 93–94.5 2 90–102 Metabolic intermediate

D-Glucitol (sorbitol) 182.17 2.0



110–112 4.9 51–60 Free in some fruits and berries

Mannitol 182.17 Inactive 166–168 0.2 50–69 Free in plant exudates

Lactitol 344.32 þ14.0



146 1.4 36 Reduction of lactose

Polysaccharides (C

)

Amylose (162.14)

þ200



(CaCl

solution) ‘Soluble’ (retrogrades) 0 a- and b-amylase,

amyloglucosidase

c. 30% of starch in cereal grains,

tubers, roots, and stems

Amylopectin (162.14)

þ200



(CaCl

solution) ‘Soluble’

(retrogrades)

0 As above and isoamylase

and pullulanase

c. 70% of starch (as above)

Cellulose (162.14)

Insoluble Cellulase (b-glucanase) Plant cell walls, wood, etc.

From Merck Index, 12th edn. (1996).

From Birch GG and Parker KJ (eds) (1982) Nutritive Sweeteners. London: Applied Science.

Approximate relative sweetness varies with temperature, concentration, and pH.

Decomposes.

Nonreducing.

Debranching enzymes.

polysaccharides can be reduced to -OH (alditol), as in

hydrogenated glucose syrup, which is claimed to be less

harmful to teeth and does not participate in the Mail-

lard reaction. Xylitol and sorbitol are also considered

as noncariogenic. They all taste sweet. Sorbitol, man-

nitol, and xylitol are absorbed by passive diffusion in

the digestive system and subsequently metabolized.

However, large doses have a laxative effect.

Oxidation

0031 The aldehyde group of monosaccharides is easily oxi-

dized (Figure 15) and gives aldonic acids. This occurs

when the familiar Fehling’s reagent is used for the

detection of reducing sugars, whereby blue cupric

ions in solution are reduced to insoluble red cuprous

oxide. Strong oxidizing agents (e.g., nitric acid) oxi-

dize both ends of the alsose molecule and give rise to

aldaric acid. For example, glucose is oxidized by

bromine water to gluconic acid, which is isolated as

the d-lactone. Nitric acid gives rise to glucaric (sac-

charic) acid, which forms a dilactone. A third type of

acid is obtained when the aldehyde group is protected

prior to oxidation. The OH group of carbon atom 6 is

oxidized to give uronic acid. Uronic acids (See Uronic

Acids) frequently occur in natural polymers (See

Pectin: Properties and Determination; Food Use),

and are discussed Carbohydrates: Interactions with

Other Food Components.

Esterification

0032The hydroxyl groups of sugars are relatively easily

esterified by anhydrides of organic acids. In sugar

analysis, monosaccharides are firstly reduced to aldi-

tols and subsequently fully esterified with acetic an-

hydride; the resulting alditol acetates are then

identified and quantified by gas chromatography. In

biological systems, d-ribose esterified with phos-

phoric acid is the carbohydrate moiety of ribonucleo-

tides and deoxyribose in deoxyribonucleotides.

Alkaline Rearrangement

0033In the presence of alkali, the sugars glucose, mannose,

and fructose are interconvertible. This is known as

the ‘Lobry de Bruyn–Alberda van Ekenstein re-

arrangement.’ The transformation is believed to

occur by a 1,2-enolization reaction. However, pro-

longed contact with alkali leads to degradation of

sugars.

Reaction with Amino Acids, Peptides, and Proteins

0034Reducing sugars readily interact with amino acids

and give rise to Maillard reaction products, which

lead to progressive browning and aroma formation.

The color and aroma of dark beer, baked goods,

toasted bread, and grilled foods are, at least in part,

due to this reaction, which is initiated when the car-

bonyl group of a sugar reacts with an amino group

(See Browning: Nonenzymatic). The initial products

of the reaction are N-glycosylamines or N-fructosy-

lamines, which give rise to intermediate products and

final heterocyclization and polymerization.

See also: Carbohydrates: Interactions with Other Food

Components Cereals: Contribution to the Diet; Dietary

Fiber: Properties and Sources; Determination;

Physiological Effects; Effects of Fiber on Absorption;

Bran; Fructose; Glucose: Properties and Analysis;

Glycogen; Honey; Lactose; Malt: Malt Types and

Products; Chemistry of Malting; Starch: Structure,

Properties, and Determination; Sources and Processing;

Functional Properties; Modified Starches; Resistant

Starch