Carbohydrates 1 - Principle of food chemistry

Carbohydrates 1 - Principle of food chemistry

Carbohydrates occur in plant and animal

tissues as well as in microorganisms in many

different forms and levels In animal

organ-isms, the main sugar is glucose and the

stor-age carbohydrate is glycogen; in milk, the

main sugar is almost exclusively the

disac-charide lactose In plant organisms, a wide

variety of monosaccharides and

oligosaccha-rides occur, and the storage carbohydrate is

starch The structural polysaccharide of

plants is cellulose The gums are a varied

group of polysaccharides obtained from

plants, seaweeds, and microorganisms

Because of their useful physical properties,

the gums have found widespread application

in food processing The carbohydrates that

occur in a number of food products are listed

in Table 4-1

MONOSACCHARIDES

D-glucose is the most important

monosac-charide and is derived from the simplest

sugar, D-glyceraldehyde, which is classed as

an aldotriose The designation of aldose and

ketose sugars indicates the chemical

charac-ter of the reducing form of a sugar and can be

indicated by the simple or open-chain

for-mula of Fischer, as shown in Figure 4-1 This

type of formula shows the free aldehydegroup and four optically active secondaryhydroxyls Since the chemical reactions ofthe sugars do not correspond to this structure,

a ring configuration involving a hemiacetalbetween carbons 1 and 5 more accuratelyrepresents the structure of the monosaccha-rides The five-membered ring structure iscalled furanose; the six-membered ring, pyra-nose Such rings are heterocyclic because onemember is an oxygen atom When the reduc-ing group becomes involved in a hemiacetalring structure, carbon 1 becomes asymmetricand two isomers are possible; these are calledanomers

Most natural sugars are members of the Dseries The designation D or L refers to twoseries of sugars In the D series, the highestnumbered asymmetric carbon has the OHgroup directed to the right, in the Fischerprojection formula In the L series, thishydroxyl points to the left This originatesfrom the simplest sugars, D- and L-glyceral-dehyde (Figure 4-2)

After the introduction of the Fischer mulas came the use of the Haworth represen-tation, which was an attempt to give a moreaccurate spatial view of the molecule Be-cause the Haworth formula does not accountfor the actual bond angles, the modern con-

for-Carbohydrates

CHAPTER 4

Table 4-1 Carbohydrates in Some Foods and Food Products

14.517.38.4

9.78.718.617.122.126.36.6

82.365.5

4.918-2014-28

Mono- and Disaccharides (%)

glucose 1.17; fructose 6.04;

sucrose 3.78; mannose traceglucose 5.35; fructose 5.33;

sucrose 1.32; mannose 2.19glucose 2.09; fructose 2.40;

sucrose 1 03; mannose 0.07

glucose 0.85; fructose 0.85;

sucrose 4.25glucose 2.07; fructose 1.09;

sucrose 0.89sucrose 4-1 2

sucrose 12-17glucose 0.87; sucrose 2-3

glucose 1.5; fructose 1.18;

sucrose 0.42

glucose 28-35; fructose 34-41 ;sucrose 1-5

sucrose 58.2-65.5;

hexoses 0.0-7.9glucose 0.01lactose 4.9sucrose 18-20glucose + fructose 4-8;

sucrose 10-20

Polysaccharides (%)

starch 1 5;

cellulose 1 0cellulose 0.6cellulose 1 3

starch 7.8;

cellulose 1.0cellulose 0.71cellulose 2.4starch 14;

cellulose 0.5cellulose 0.7;

cellulose 60starch 14.65;

cellulose 0.7cellulose 0.9

glycogen 0.10

formational formulas (Figure 4-1) more

accurately represent the sugar molecule A

number of chair conformations of pyranose

sugars are possible (Shallenberger and Birch

1975) and the two most important ones for

glucose are shown in Figure 4—1 These are named the CI D and the IC D forms (also described as O-outside and O-inside, respec- tively) In the CID form of (3-D-gluco-pyra-

nose, all hydroxyls are in the equatorial

position, which represents the highest

ther-modynamic stability

The two possible anomeric forms of

monosaccharides are designated by Greek

letter prefix a or p In the oc-anomer the

hydroxyl group points to the right, according

to the Fischer projection formula; the

hydroxyl group points to the left in the

p-anomer In Figure 4-1 the structure marked

Cl D represents the oc-anomer, and 1C D

represents the p-anomer The anomeric

forms of the sugars are in tautomeric

equilib-rium in solution; and this causes the change

in optical rotation when a sugar is placed in

CHO CHO

HCOH HOCH

I I

Figure 4-2 Structure of D- and

L-Glyceralde-hyde Source: From R.S Shallenberger and G.G.

Birch, Sugar Chemistry, 1975, AVI Publishing

Co

solution Under normal conditions, it maytake several hours or longer before the equi-librium is established and the optical rotationreaches its equilibrium value At room tem-perature an aqueous solution of glucose canexist in four tautomeric forms (Angyal1984): P-furanoside—0.14 percent, acyclicaldehyde—0.0026 percent, p-pyranoside—

62 percent, and oc-pyranoside—38 percent(Figure 4-3) Fructose under the same con-ditions also exists in four tautomeric forms

as follows: oc-pyranoside—trace, side—75 percent, oc-furanoside—4 percent,and p-furanoside—21 percent (Figure 4-4)(Angyal 1976)

p-pyrano-When the monosaccharides becomeinvolved in condensation into di-, oligo-, andpolysaccharides, the conformation of thebond on the number 1 carbon becomes fixedand the different compounds have either anall-a or all-p structure at this position.Naturally occurring sugars are mostly hex-oses, but sugars with different numbers ofcarbons are also present in many products.There are also sugars with different func-

Figure 4-1 Methods of Representation of D-Glucose Source: From M.L Wolfrom, Physical and

Chemical Structures of Carbohydrates, in Symposium on Foods: Carbohydrates and Their Roles, H W.

Schultz, R.F Cain, and R.W Wrolstad, eds., 1969, AVI Publishing Co

GLUCOSE(deKtrose) Aldose (oldohexose)

Howorth

Conformotionol Glucopyronose

Glucose

Fischer Fischer

Figure 4-3 Tautomeric Forms of Glucose in Aqueous Solution at Room Temperature

Figure 4-4 Tautomeric Forms of Fructose in Aqueous Solution at Room Temperature

tional groups or substituents; these lead to

such diverse compounds as aldoses, ketoses,

amino sugars, deoxy sugars, sugar acids,

sugar alcohols, acetylated or methylated

sug-ars, anhydro sugsug-ars, oligo- and

polysaccha-rides, and glycosides Fructose is the most

widely occurring ketose and is shown in its

various representations in Figure 4-5 It is

the sweetest known sugar and occurs bound

to glucose in sucrose or common sugar Of

all the other possible hexoses only two occur

widely—D-mannose and D-galactose Their

formulas and relationship to D-glucose are

given in Figure 4-6

RELATED COMPOUNDS

Amino sugars usually contain

D-glu-cosamine (2-deoxy-2-amino glucose) They

occur as components of high molecular

weight compounds such as the chitin of

crus-taceans and mollusks, as well as in certain

mushrooms and in combination with the

ovomucin of egg white

Glycosides are sugars in which the gen of an anomeric hydroxy group has beenreplaced by an alkyl or aryl group to form amixed acetal Glycosides are hydrolyzed byacid or enzymes but are stable to alkali For-mation of the full acetal means that glyco-sides have no reducing power Hydrolysis ofglycosides yields sugar and the aglycone.Amygdalin is an example of one of the cyan-ogetic glycosides and is a component of bit-ter almonds The glycone moiety of thiscompound is gentiobiose, and completehydrolysis yields benzaldehyde, hydrocyanicacid, and glucose (Figure 4-7) Other impor-tant glycosides are the flavonone glycosides

hydro-of citrus rind, which include hesperidin andnaringin, and the mustard oil glycosides,such as sinigrin, which is a component ofmustard and horseradish Deoxy sugarsoccur as components of nucleotides; forexample, 2-deoxyribose constitutes part ofdeoxyribonucleic acid

Sugar alcohols occur in some fruits and areproduced industrially as food ingredients

Figure 4-5 Methods of Representation of D-Fructose Source: From M.L Wolfrom, Physical and

Chemical Structures of Carbohydrates, in Symposium on Foods: Carbohydrates and Their Roles,

H.W Schultz, R.F Cain, and R.W Wrolstad, eds., 1969, AVI Publishing Co

They can be made by reduction of free

sug-ars with sodium amalgam and lithium

alumi-num hydride or by catalytic hydrogenation

The resulting compounds are sweet as

sug-ars, but are only slowly absorbed and can,

therefore, be used as sweeteners in diabetic

foods Reduction of glucose yields glucitol

(Figure 4-8), which has the trivial name

sor-bitol Another commercially produced sugar

alcohol is xylitol, a five-carbon compound,which is also used for diabetic foods (Figure4-8) Pentitols and hexitols are widely dis-tributed in many foods, especially fruits andvegetables (Washiittl et al 1973), as is indi-cated in Table 4-2

Anhydro sugars occur as components ofseaweed polysaccharides such as alginateand agar Sugar acids occur in the pectic sub-

Figure 4-6 Relationship of D-Aldehyde Sugars Source: From M.L Wolfrom, Physical and Chemical Structures of Carbohydrates, in Symposium on Foods: Carbohydrates and Their Roles, H.W Schultz,

R.F Cain, and R.W Wrolstad, eds., 1969, AVI Publishing Co

Figure 4-7 Hydrolysis of the Glycoside Amygdalin

Benzaldehyde o-Glucose

Amygdalin Gentiobiose

stances When some of the carboxyl groups

are esterified with methanol, the compounds

are known as pectins By far the largest

group of saccharides occurs as oligo- and

polysaccharides

OLIGOSACCHARIDES

Polymers of monosaccharides may be

either of the homo- or hetero-type When the

number of units in a glycosidic chain is in

the range of 2 to 10, the resulting compound

is an oligosaccharide More than 10 units are

usually considered to constitute a

Figure 4-8 Structure of Sorbitol and Xylitol

charide The number of possible charides is very large, but only a few arefound in large quantities in foods; these arelisted in Table 4-3 They are composed ofthe monosaccharides D-glucose, D-galac-tose, and D-fructose, and they are closelyrelated to one another, as shown in Figure4-9

oligosac-Sucrose or ordinary sugar occurs in dant quantities in many plants and is com-mercially obtained from sugar cane or sugarbeets Since the reducing groups of themonosaccharides are linked in the glycosidicbond, this constitutes one of the few nonre-ducing disaccharides Sucrose, therefore,does not reduce Fehling solution or formosazones and it does not undergo mutarota-tion in solution Because of the unique car-bonyl-to-carbonyl linkage, sucrose is highlylabile in acid medium, and acid hydrolysis ismore rapid than with other oligosaccharides.The structure of sucrose is shown in Figure4-10 When sucrose is heated to 21O0C, par-tial decomposition takes place and caramel isformed An important reaction of sucrose,

abun-Table 4-2 Occurrence of Sugar-Alcohols in Some Foods (Expressed as mg/100g of Dry Food)

Source: From J Washiittl, R Reiderer, and E Bancher, A Qualitative and Quantitative Study of Sugar-Alcohols in Several Foods: A Research Role, J Food ScL, Vol 38, pp 1262-1263,1973.

300128

Mannitol

4050476

which it has in common with other sugars, is

the formation of insoluble compounds with

calcium hydroxide This reaction results in

the formation of tricalcium compounds

C12H22O11-S Ca(OH)2 and is useful for

recovering sucrose from molasses When thecalcium saccharate is treated with CO2, thesugar is liberated

Hydrolysis of sucrose results in the tion of equal quantities of D-glucose and D-

forma-MANNlNOTRlOSE GALACTOBIOSE

Figure 4-9 Composition of Some Major Oligosaccharides Occurring in Foods Source: From R.S.

Shallenberger and G.G Birch, Sugar Chemistry, 1975, AVI Publishing Co.

Table 4-3 Common Oligosaccharides Occurring in Foods

(1 -»6)-O-a-D-glucopyranosyl-(1 -»2)-p-D-fructofuranoside][O-a-D-galactopyranosyl-(1^6)-O-a-D-galactopyranosyl-(1 -»6)-O-cc-D-galactopyranosyl-(1 ->6)-O-oc-D-glucopyrano-syl-(1 ->2)-p-D-fructofuranoside]

[O-a-D-galactopyranosyl-(1^6)-O-a-D-galactopyranosyl-Source: From R.S Shallenberger and G G Birch, Sugar Chemistry, 1975, AVI Publishing Co.

fructose Since the specific rotation of

sucrose is +66.5°, of D-glucose +52.2°, and

of D-fructose -93°, the resulting invert sugar

has a specific rotation of -20.4° The name

invert sugar refers to the inversion of the

direction of rotation

Sucrose is highly soluble over a wide

temperature range, as is indicated in Figure

4-11 This property makes sucrose an

excellent ingredient for syrups and other

sugar-containing foods

The characteristic carbohydrate of milk is

lactose or milk sugar With a few minor

exceptions, lactose is the only sugar in the

milk of all species and does not occur

else-where Lactose is the major constituent of

the dry matter of cow's milk, as it represents

close to 50 percent of the total solids The

lactose content of cow's milk ranges from

4.4 to 5.2 percent, with an average of 4.8

percent expressed as anhydrous lactose Thelactose content of human milk is higher,about 7.0 percent

Lactose is a disaccharide of D-glucose andD-galactose and is designated as 4-0-p-D-galactopyranosyl-D-glucopyranose (Figure4-10) It is hydrolyzed by the enzyme (3-D-galactosidase (lactase) and by dilute solu-tions of strong acids Organic acids such ascitric acid, which easily hydrolyze sucrose,are unable to hydrolyze lactose This differ-ence is the basis of the determination of thetwo sugars in mixtures

Maltose is glucopyranose It is the major end product ofthe enzymic degradation of starch and glyco-gen by p-amylase and has a characteristicflavor of malt Maltose is a reducing disac-charide, shows mutarotation, is fermentable,and is easily soluble in water

4-a-D-glucopyranosyl-f5-D-Lactose CellobTose

Maltose Sucrose

Figure 4-10 Structure of Some Important Disaccharides

TEMPERATURE Figure 4-11 Approximate Solubility of Some

Sugars at Different Temperatures Source: From

R.S Shallenberger and G.G Birch, Sugar

Chemistry, 1975, AVI Publishing Co.

Cellobiose is

4-p-D-glucopyranosyl-p-D-glucopyranose, a reducing disaccharide

resulting from partial hydrolysis of cellulose

Legumes contain several

oligosaccha-rides, including raffmose and stachyose

These sugars are poorly absorbed when

ingested, which results in their fermentation

in the large intestine This leads to gas

pro-duction and flatulence, which present a

bar-rier to wider food use of such legumes

deMan et al (1975 and 1987) analyzed a

large number of soybean varieties and found

an average content of 1.21 percent stachyose,

0.38 percent raffinose, 3.47 percent sucrose,

and very small amounts of melibose In soy

milk, total reducing sugars after inversion

amounted to 11.1 percent calculated on dry

basis

Cow's milk contains traces of

oligosaccha-rides other than lactose They are made up of

two, three, or four units of lactose, glucose,

galactose, neuraminic acid, mannose, and

acetyl glucosamine Human milk contains

about 1 g/L of these oligosaccharides, whichare referred to as the bifidus factor The oli-gosaccharides have a beneficial effect on theintestinal flora of infants

Fructooligosaccharides (FOSs) are mers of sucrose where an additional one,two, or three fructose units have been added

oligo-by a p-(2-l)-glucosidic linkage to the tose unit of sucrose The resulting FOSs,therefore, contain two, three, or four fruc-tose units The FOSs occur naturally ascomponents of edible plants includingbanana, tomato, and onion (Spiegel et al.1994) FOSs are also manufactured com-mercially by the action of a fungal enzyme

fruc-from Aspergillus niger,

p-fructofuranosi-dase, on sucrose The three possible FOSsare !^(l-p-fructofuranosyl)^ sucrose oli-gomers with abbreviated and commonnames as follows: GF2 (1-kestose), GF3

(nystose), and GF4 (lFnystose) The commercially manufacturedproduct is a mixture of all three FOSs withsucrose, glucose, and fructose FOSs arenondigestible by humans and are suggested

-p-fructofuranosyl-to have some dietary fiber-like function

Chemical Reactions

Mutarotation

When a crystalline reducing sugar is placed

in water, an equilibrium is establishedbetween isomers, as is evidenced by a rela-tively slow change in specific rotation thateventually reaches the final equilibrium val-

ue The working hypothesis for the rence of mutarotation has been described byShallenberger and Birch (1975) It is assumedthat five structural isomers are possible forany given reducing sugar (Figure 4-12), withpyranose and furanose ring structures beinggenerated from a central straight-chain inter-