am j clin nutr 1984 selvendran 320 37

8/13/2019 Am J Clin Nutr 1984 Selvendran 320 37

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32 0

S pecia l A rticle

The p lan t cell w all as a source of d ietary fiber:chem istry and structure1Robert R Selvendran , PhD , C C hem , FRSC

Introduction

D efinition ofd ieta ryfiber and scope of the

review

The hypotheses of Burkitt and Trow ell

1-3 that m any diseases of the W esternw orld are associated w ith diets high in re-

fined carbohydrates and low in fiber, haveled to m any stud ies on the com position and

physio log ical effects o f d ietary fiber (D F ).D F w as defined by Trow el in 1 974 (4) as

that part of plant m aterial in our diet w hichis resistan t to digestion by secretions of the

hum an digestive tract. A s th is definition didno t include polysaccharides present in som e

food additives (such as plant gum s, algalpo lysaccharides, pectins, m odified cellu-

loses, and m odified starches) Trow el et a(5) ex tended the definition to include all the

po lysaccharides and lignin that are not d i-

gested by endogenous secretions of the hu-

m an digestive tract. A ccordingly , the term

D F now refers m ain ly to nonstarchy poly-

saccharides and lignin in the diet (6 ). H ence,plant cell w alls are the m ain source of D F,

and m ost of our D F intake com es from the

cell w alls in foods such as fru its, vegetab les,

and cereal products. The principal com po-nents of D F are com plex polysaccharides

som e ofw hich are associated w ith lignin and

pro teins. The am ounts of D F provided bycereal products depend on the type of cerealand particu larly on the exten t of its refine-m ent; h igh ex traction w heat products (eg ,

w holem eal bread, w holem eal breakfast ce-

reals) contain m ore D F than products from

low extraction w hite flours. A lthough m ost

ofthe D F constituents m ay surv ive digestionin the m outh , stom ach , and sm all intestine,som e of the constituen ts m ay be degradedby m icroorganism s of the hum an co lon (7 ,

8 ).The average intake of D F in the U nited

K ingdom is about 20 g /person /day and, ofthis, about a one-third com es from cerealsources. T here is considerable variation be-tw een ind ividuals; those w ith a h igh con-sum ption tend to obtain m ore from cerealfoods (9). A s yet there is no official recom -m endation on a desirable level o fintake, but30 g/person/day m igh t be recom m ended.The B ritish intake of D F is sm all com paredw ith that o fa rural A frican d iet, w hich m igh t

contain 100 to 170 g D F/day (9).H erein w e discuss the chem istry of cell

w alls from various tissues of edib le plants,som e polysaccharide food add itives, and D Fpreparations used in clinical feed ing trials.The m ain em phasis is to show how thisknow ledge enhances our understanding ofthe chem istry and analysis o f D F, and itspossib le fate in the hum an co lon.

The com ponents of D F

The m ain com ponents w hich m ake upD F are sum m arized in Table I; th is lists thetypes of polym er that can be obtained fromvarious types of plan t m aterial. The paren-

chym atous tissues are particu larly im portant

I From the A FR C Food Research Institute, Colney

Lane, N orw ich NR 4 7UA , U K.2 A uthor to whom requests for reprints should be

addressed.R eceived Septem ber 27, 1982.Accepted for publication Septem ber 20, 1983.

T he ne rie an Jou rna l o f c lin ic al N utrition 39: FEBRU A RY 1984, pp 320-337P ri nt ed i n U .S .A . C 1 9 8 4 American S o c i e l y tsr (linca1 N utrition


2/18

M ain com ponents of a m ixed diet Tissue types M a in c on st it u en t g roups of D F poly m ers

ruits and vegetables .

C e llu lo se, h em icellu lo ses (eg , x ylo glu -cans), pectic substances, and som e

M ainly parenchym atous w ith glycoproteins

som e lignified and cutinised C ellulose, hem icelluloses (eg, glucu-

tissues ronoxylans), lignin and som e gly-

coproteinsC utin and w axes

H em icelluloses (eg, arabinoxylans

and D -gIucans), cellulose, pro-teins, and phenolic esters

H em icelluloses (eg, glucuronoarabi-

n ox ylan s), cellu lo se, lig nin an dp hen olic esters, an d p ro tein s

S eeds other than cereals (eg, leg-

um inous seeds)

{arenchym a tous (pea cotyle-dons) and cells w ith thick-

ened endosperm w alls (guarendosperm )

cans), p ectic substances, and glyco-ellu lo se, hem icellu lo ses (eg , xyloglu -proteins

H e m icellu lo ses (m ain ly g alacto m a n-nans), and som e cellulose, pectic

substances, and (glyco)proteins

Seed husk of P lan iag o ovata (Is-paghula husk)

M ucilage ofepiderm a l cellsM ainly arabinogalacturonosyl-

rham no-xylan

G um s-gum arabic, alginates, can-a-Food additives g een an , guar g um , carbox ym ethyl-

cellu lo se, m o dified starch es, etc

C H E M IST R Y O F D IE T A R Y FIB ER 321

T A B L E I

C om ponents of D F

e r e l s

in connection w ith D F, because the w alls of

these tissues com prise the bulk of the D F

from fruits and vegetables, and the endo-

sperm of cereals. T he parenchym atous tis-

sues have (m ainly) thin prim ary cell w alls,

w hereas the lignified tissues have cell w alls

that have ceased to grow and have under-

gone secondary thickening. T he lignified tis-

sues are of greater im portance w ith som e

cereal products, eg, w heat bran and bran

based products.

In the developm ent of the prim ary cell

w all of dicotyledonous plants pectic sub-

stances (m ainly as their calcium salts) are

deposited first on the cell plate to form the

m iddle lam ella, w hich cem ents the cells to-

gether. H em icelluloses, additional pectic

substances, and glycoproteins are then de-

posited as an am orphous m atrix of m acro-

m o lecules closely associated w ith the cellu-lose m icrofibrils w hich are the m ain struc-

tural elem ents. T he m icrofibrils contain

highly ordered crystalline regions, in w hich

linear chains of cellulose m olecules are

highly packed, and less ordered am orphous

regions in w hich the cellulose chains are less

closely packed and in w hich other polysac-

charides m ay be found.

C ells in som e regions ofthe plant, vascular

tissues, becom e differentiated into special-

ized structures such as the xylem and

phloem bundles; these form the veins and

ribs of leaves, and continue into the petiolesand stem s. T he xylem cells becom e thick-

ened and hardened by lignification as theplant organ m atures. L ignification begins in

the prim ary w all region and then extends

outw ard to the m iddle lam ella and inw ard

into the developing secondary w all, the final

lignin concentration decreasing from the

outside to the inside ofthe w all. T he lignifiedw alls have cellulose m icrofibrils dispersed in

hem icelluloses and lignin. T he relevant in-

form ation on the biochem ical processes thattransform prim ary w alls into secondary

w alls is sum m arized by N orthcote (10).

T he m ain cell w all polym ers of parenchy-m atous tissues of dicotyledons are pectic

sub stan ces, h em icellulo ses (eg , x ylo glu can s),

and cellulose, w hereas those of lignified tis-

sues are lignin, hem icelluloses (eg, glucuron-

oxylans), and cellulose; usually different

types of hem icellulosic polysaccharides oc-


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322 S E LV E N D R A N

cur in the cell w alls ofthe tw o types of tissue.

In contrast to the cell w alls of parenchym a-

tous tissues of dicotyledons, those of cereal

grains (w h eat, barley, etc) contain very little

or no pectic substances (1 1-1 3). T he pri-m ary cell w alls of m ost cereal grains have

cellulose m icrofibrils, w hich are closely as-

sociated w ith glucom annan, and these fi-

brillar structures are em bedded in an am or-

phous m atrix of hem icelluloses, w hich con-

sist m ainly of arabinoxylans and/or - D-

glucans, som e of w hich are cross-linked by

phenolic esters and/or proteins. R ice endo-

sperm w alls seem to be interm ediate in char-

acter betw een these and parenchym a of di-

cotyledons in that they contain about 0%

of pectic substances and a substantialam ount of cellulose. T hese differences in

organization of the cell w all structure arereflected in their o verall co m p osition.

T he constituents ofcell w alls therefore fall

into three groups: T he fibrillar and the m a-

trix polysaccharides, w hich are form ed si-m ultaneously, and the encrusting substances

(lignins), w h ich are form ed during secondary

thickening of specialized cells 1 0 , 14-16).T he fibrillar polysaccharides, w hich are the

basic structural units of m icrofibrils, are

m ade up m ainly ofcellulose. T he a-cellulosefraction isolated from m ost plant tissues,

how ever, usually contains sm all but signifi-

cant am ounts of nonglucan polysaccharides

(and glycoproteins) associated w ith it (17-20). T his association of cellulose w ith other

polysacchandes m ight occur by adsorption

during the preparation of the m aterial, or it

m ight form part ofthe essential organization

ofthe polysaccharides w ithin the w all. If the

latter is true, then the associated polysac-

charides probably serve as linking com -

pounds for the entanglem ent ofthe cellulose

m icro fib rils w ith the m atrix polysaccharides.

T he m atrix polysaccharides are m ade up

of linearly orientated polym ers, w hich arepresent at all stages of the developm ent of

the w all, and also of highly branched poly-

saccharides that are deposited at particularstages of grow th. T hese polysaccharidesm ay, at the surface of the m icrofibri , be

incorporated into its structure. T here are

tw o m ajor fractions in the m atrix polysac-

charides. 1) T he pectic substances, w hich bydefinition are those polysaccharides that are

solubilized from the cell w all by aqueous

solutions of chelating agents such as ethyle-

nediam inetetra-acetate or am m onium oxa-

late. T he solvent action of the chelating

agents depends on their ability to com binew ith C a and M g Som e insoluble pectic

m aterial is alw ays found in close association

w ith oth er cell w all con stitu en ts, p articu larly

the a-cellulose fraction. T he soluble pectic

substances com prise the m ethyl ester, pec-

tin, the deesterified pectic acid and its salts,

pectates, and certain neu tral polysaccharides

lacking the rham nogalacturonan backbone,

such as arabinans, galactans, and arabino-

galactans. A large proportion of the neutral

polysaccharides m ay be breakdow n products

of the m ore com plex acidic pectic polysac-

charides. 2) T he hem icelluloses, w hich are

those polysaccharides solubilized by alkali

from the depectinated (and delignified) cell

w alls. R ecent w ork suggests that som e of the

alkali-soluble po lym ers are po ly sacch arid e-

protein-polyphenol com p lexes. In addition

to polysaccharides, sm all am ounts of glyco-

proteins, both hydroxyproline-rich and hy-

droxyproline-poor, are also present, and the

associated sugars are m ainly arabinose and

galacto se (21).W ater is also an im portant com ponent of

the cell w all, and is present in varying

am ounts, high in the prim ary cell w alls of

m ost tissues, except m ature dry seeds, butlow in secondary w alls. T he am ount of w ater

w ithin the w all m atrix can be partly con-

trolled by the deposition of m atrix polysac-

charides w hich form close, interm o lecular

associations, or by the deposition of a hy-

drophobic filler such as lignin. D uring sec-

ondary thickening, the space occupied by

w ater in the w all becom es progressively filled

by lignin-polysaccharide com plexes. T he

overall com positions (% w /w ) of cell w alls

from parenchym atous and lignified tissues

ofm ature runner bean pods are as follow s-

prim ary cell w alls: w ater 70% , cellulose10% , pectic substances 12% , hem ice luloses

6% , and glycoproteins 2% ; secondary cell

w alls: w ater 1 5 , cellulose 35 , lignin 18% ,hem icelluloses 25 , pectic substances 5 ,

and proteins 2% (Se vendran R R , unpub-

lished results).


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C H E M IST R Y O F D IE T A R Y FIB E R 3 2 3

T he chem istry of cell w alls from

angiosperm s

B ecause the nature of the carbohydrate

polym ers associated w ith various types oftissue from dicotyledonous and m onocoty-

ledonous plants are different, the polym ers

present in these species w ill be discussed

separately, w ith the m ain em phasis on thecell w alls of edible plant organs. A s several

review s dealing w ith the chem istry of cell

w all polysaccharides are available (6, 14-16,

22, 23) the detailed chem istry ofthe individ-

ual polysaccharides w ill not be described.

H ow ever, attention w ill be draw n to certain

structural features of the polym ers w hich

m ay be of interest, in the context of the

analysis of D F, or its m ode of action in the

hum an alim entary tract. For convenience,the relationship of the operationally defined

groups of polysaccharides to structural fam -

ilies, and the m ain sugars and glycosidic

linkages present in the m ajor polysaccha-

rides are tabulated. W here necessary, suita-

ble exam ples from edible plant organs are

given to illustrate certain special aspects of

cell w all and D F com position.

C ell w a lls ofdicotyledonous plants

Prim ary cell w alls. In addition to the po-lysaccharide and glycoprotein com p onents,

the cell w all m aterial (C W M ) of parenchy-

m atous tissues of runner beans, cabbage,apples, and carrot contains about 5 to 10%of polyphenolic com pounds in the form

of polysaccharide-protein-polyphenol com -

plexes (Selvendran R R , O N eill M A , Stevens

B JH , unpublished results; 24). T he relative

proportions ofthe different types ofcell w all

polym ers vary w ith the type and m aturity of

the tissue. T he m ajor structural features of

the cell w all polym ers are sum m arized in

T able 2.

Som e indication of the nature andam ount of polysaccharides present in the

cell w alls can be obtained from their overall

carbohydrate com positions (T able 3). In thisconnection, see also the tables given in R ef-

erences 17 to 19, 24, 5 and 33, w hichdescribe the results of purifying cell w alls

and of chem ical fractionation studies. T he

cell w alls of the products listed in T able 3

contain m ainly cellulose and pectic acid as

w ell as other pectic substances, and sm all

am ounts of xyloglucans and w all glycopro-

teins, and are devoid ofxylans. T he first tw ocom ponents can be estim ated from the val-

ues for glucose and uronic acid 35 , 36); thism ethod has the advantage that it m easures

the total pectic acid: chelating agent-solu-

ble and insoluble pectic acid. T he validity of

the m easurem ents depends on the follow ing:

1) the bulk of the glucose released on Sae-m an-hydrolysis arises from cellulose; 2) alarge proportion (- 0 to 95 ) ofthe uronicacid arises from the galacturonic acid of

pectic substances; 3) the pectic substances of

potatoes are rich in galactose, w hereas those

of apples and cabbage are rich in arabinose;

4) prim ary cell w alls have little or no (glu-

curono)-xylans w hich are the predom inant

hem icelluloses of secondary w alls; m uch ofthe xylose of prim ary w alls arises from xy-

loglucans.

Secondary cell w alls.T he differentiationof the prim ary w all into secondary w all in-volves considerable thickening of the w all

and profound changes in its chem ical com -

position. D uring the grow th ofthe secondary

w all, a-cellulose, hem icelluloses, and lignin

are deposited, and the thickening results

m ore from the a-cellulose than the hem icel-

lulose deposition (10). L ignin occurs to the

extent of 1 5 to 35 of m ost supportingtissues of higher plants and seem s to form

covalent linkages w ith the hem icelluloses(37), cem enting together the w all polym ers

into a unified rigid m atrix and stratifying

the w all. T he hem icelluloses are m ainly

(- 90% ) glucurono- and 4-0-m ethylglucu-

rono-xylans, w hich have -(l 4)-linked D-

X ylp residues w ith side chains of D -glucu-

ronic acid or 4-0-m ethyl-D -glucuronic acid

linked to C -2 of about 10% of the X ylp

resid ues; abo ut 50 of the xylose residueshave an acetyl group, linked m ostly to C -3.

T he carbohydrate com positions and hg-

nm content of the cell w all preparations

from hignified tissues (parchm ent layers and

strings) of m ature runner bean pods areshow n in T able also included in T able 4are the carbohydrate com positions of the

C W M from beesw ing w heat bran and som e

hem icelluloses (20). T he carbohydrate com -

positions ofthe C W M show that the hignified

tissu es are rich in cellulo se an d acid ic-x ylan s


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T A B L E 2

C lassification of som e polysaccharides from prim a ry cell w alls ofedible dicotyledons

C ellulose

Polysacchande M onosaccharides

an d

R epresentative S o u r c e Structural features R eferencesM a jor M in or

type

o-G lc 1 4, 2 2

H em icelluloses

X y l o g l u c a n s

Pectic substancesPectin

A rabinan

G alactan

A rabinoga-lactan-l

W all glycopro-

teinsH ydroxy-

proline-rich

glycopro-

teins

A ll prim ary L inear chains of -(Iw alls 4 )-lin ked o-G lcp resi-

dues

A ll prim ary c-G lc o-X yl, -( 1 - 4)-linked r-G lcp 22, 23, 18,w alls n-G al,

L-Fuc,L-A ra

residues containingct-o-X yl p resi#{243}uess

side chains on C -6 of

at least half the G Icresidues, w ith som e of

the X y l residues being

further substitutedw ith fl-o-G al p, a -L-

A rafor L -Fuc p resi-

dues

25

A ll prim ary o-G al A L -R ha, a - (1 - 4)-linked o-G al 22, 23, 26w alls L-A ra,

o-G al

pA (or m e thyl esteri-

fled G a l pA ) resid ues

c on ta in in g l - p2 -

linked $-i-R hap; gal-actan an d arab in an


R ha p.Im m ature i-A ra ( 1 -* 5)-linked i-A raf 22, 27, 28

cabbage residues having

leaves, branch points through

and coty- C -3, as w ell as C -3

ledons of and C -2 (doublym ustard branched).

and soy-beans

Potato tu- D -G al M ainly -( 1 - 4)-linked 29bers o-G al p residues, w ith

som e(l -+4,6)-linkedresidues

Soybean n-G al L -A ra fl-( 1 -+ 4)-linked r-G al p 30

cotyledon residues containing

a-L-A rafll 5)-L -A raf-( l-+ ) resid ues as


som e of the G al resi-

dues

R unner L -A ra o-G al H ydroxyproline tetra- 21, 31, 32

beans and tri-arabino-furano

sides; a-D-Gal p -serine

324 S E LV E N D R A N

and contain very sm all am ounts of pecticsubstances. T hese inferences are corrobo-rated by the carbohydrate com positions ofth e po lysaccharid e fraction s.

Cel l wal l s ofseeds T he seeds of dicotyle-donous plants can be classified into those

w hich are free of an endosperm , referred to

as nonendosperm ic (eg, bean, pea, etc) andthose w hich have an endosperm , referred toas endosperm ic (as in certain legum inousspecies, eg, guar, locust bean, etc). T he for-m er types of seeds usually have starch as the


6/18

C H E M IST R Y O F D IE T A R Y FIB E R 3 2 5

T A B L E 3C arbohydrate com positions of the C W M and som e C W polysaccharides from

edible plants (v alu es are g anh ydrosugar/m g dry preparation)

parenchym atous tissues of som e

P otatoes ( 8) Apples

Sugars

CW M O x-soI4 XG a-CcItt C W M O x -soi-t

C ab bage 33R un ner b ean s 32.

34

C W M O x-so l.t -C W M H P-G PI6-D eoxyhexose 20.4 14.2 1 1.5 19.6 26.5 29.6 42.9 19.3 8.2

Ar a b i n os e 4 6 . 1 3 9 . 4 6 7 . 4 1 2 . 5 1 2 3 1 5 9 1 2 5 1 7 9 3 5 . 1 3 8 7X ylose 16.5 190 9.3 33.3 12.3 44.9 7.7 17.3 11.1

M annose T race 14.1 20.3 43.0 3.0 24.7 2.7 15.7G alactose 284 192 75.3 32.6 56.8 36.5 67.2 54.9 75 70.4Gl uc os e l i 3 1 5 T r a c e 4 3 0 6 9 0 2 2 7 1 0 . 2 3 5 5 1 1 . 6 3 1 4 9 . 3Ur oni c a c i d* * 2 7 0 7 2 0 1 9 0 3 2 8 7 0 7 2 7 9 6 7 2 3 6 7 7 5

* Stevens B JH , Selvendran R R , unpublished results.

t Oxa late-so luble pectic substances.: :Purified xyloglucan.

a-Cellulose residue.

It P urified h yd ro xy pro lin e-rich g ly co pro tein fractio n. { 8 2 }he bulk (90% ) ofthe glucose from the C W M arises from cellulose.** T he bulk 95 ) ofthe uronic acid arises from pectins.Potatoes (var D esiree): A pples (var C oxs O range Pippin); C abbage (var D ecem a); R unner beans (var Stream -

l i n e ) .

TABLE 4

C om positions ofC W M , hem icelluloses and a-cellulose from lignifled tissues (values are gig anhydrosugar/m g drypreparation)

R unner beans B eesw ing w heat bran (20)

Sugar Stringst Parchm entt Parchm ent Parchm ent C W M I K O H - I I

C W M C W M I M K O H- s o tt (a-Cellulose solt

6-D eoxyhexose 5.2 7.1 7.0 2.1

A rabinose 6.7 11.2 3.8 16.4 216 258 280 73Xy l o s e 2 5 0 2 4 0 8 1 0 7 . 2 1 6 8 2 0 4 2 7 0 5 3M annose 9.1 6.2 T race 3.1 T race

G alactose 4.2 16.0 T race T race 12 39 15.0

G lucose 43#{216}** 420** 6.7 920** 204** 90 8.7 657**U ronicacid 40 80 94 40 56 60 20

* S e lve n dra n R R unpub lished resu lts.

t Strings and parchm ent layers of m ature runner bean pods.j: P olysaccharides solubilized from the holocellulose w ith lM K O H .

a-C ellulose from the holocellulose (delignified C W M ). { 8 2 }olysaccharides solubilized from the oxalate-extracted C W M by IM K O H .

IIO xalate-soluble polysaccharides, w hich represent only about 1.6% of the am ount solubilized by IM K O H .** T he bulk (-95% ) ofthe glucose arises from cellulose.

T he K lason lignin content (% w /w ) of the C W M from strings, parchm ent layers, and beesw ing w heat bran are

17.2 , 16.9 , and 12.0 , respectively.

m ain storage polysaccharide, and their

C W M is derived m ainly from the tissues of

the cotyledons and there is som e contribu-

tion from the testa (w hich are usually higni-

fled). T he cell w all polysaccharides of thecotyledons are sim ilar to those of parenchy-

m atous tissues and are m ainly pectic sub-

stances, cellulose, and hem icehluloses (eg,

xyloglucans). T he notable difference is that

arabinans either free or linked to rham no-

galacturonans tend to predom inate in the

seeds. In this respect the pectic substances of

the cotyledons resem ble those of im m ature

cabb ag e leav es.

B y contrast to nonendosperm ic seeds, all

the endosperm ic legum inous seeds containgalactom annans, w hich are located in the

endosperm cell w alls (38). T he galactom an-

nans are deposited on the cell w alls of the

endosperm during seed developm ent and

are later m obilized during germ ination of

the seeds. T he galactom annans are essen-


7/18

32 6 SELVENDRAN

tiahly linear molecules but are highly substi-tuted on C-6 of the f3-( 1- 4)-hinked-D-M anp residues with single Gal p residues, whichconfers on them properties which are quite

different from those of unbranched, cellu-lose-like, water-insoluble mannans and glu-comannans. The galactomannans are hydro-phihic and are usually obtained from thecrushed seeds (or endosperms) by hot waterextraction (39). The interactions of galacto-mannans with water and other polysacchar-ides form the basis of the widespread indus-trial uses of galactomannans from guar andlocust bean seeds (40). Because guar gumlowers blood glucose level (41), guar-en-riched breads have been tested on diabetics(42), and the results are encouraging. Guarappears to slow glucose absorption in the

small intestine by interacting with intestinalmucosa (43).

The mucilages ofthe seeds ofthe plantainfamily (Plantinaginaceae) are localized inthe thick mucilage cells which form theouter epiderm ofthe spermoderm. The mu-cilage from the seed husk ofP la n ta go ova taForsk (usually referred to as Ispaghula husk),has important physiological effects on largebowel action and is widely used for treatinglarge bowel disorders such as diverticulardisease (44). The husk contains much poly-saccharide which forms a gel in water, re-taming many times its own weight of water

45 , 46). This property of the husk, coupledwith the fact that the constituent polysac-charide has a highly substituted xylan back-bone (and is therefore not readily degradedby microorganisms of the human colon) isprobably one ofthe main factors responsiblefor its laxative action. The structural featuresof the above polysacchande, and those ofguar gum, are given in Table S along withthose of other food gums. For a detailedaccount of the chemistry and properties ofguar gum see References 47 and 48.

The carbohydrate compositions of thestarch-free alcohol-insoluble residues (A IR)

ofthe hulls and cotyledons ofpeas, and thoseof guar seeds and guar seed splits (whichcontain mainly the endosperm) and Ispa-ghula husk are shown in Table 6; also in-cluded in Table 6 are the D F content of theproducts. Selvendran and D uPont (36) givedetails of the method of preparation and

analysis of the A IR. The results of suchanalyses show that 1) the cell walls of coty-ledons are rich in arabinose-containing pec-tic substances, pectic acid, and cellulose; 2)

the cell walls ofpea hulls are rich in cellulose,pectic acid, (acidic) xylans, and hignin; thislast inference is based on the positive stain-i ng reaction w ith phlorogluci nol/H C 1; 3) thecell walls of guar seed splits are very rich ingalactomannans and contain small amountsof pectic substances and cellulose; and 4)Ispaghula mucilage is very rich in arabinox-ylans.

C ell wa lls o fmonocotyledonous pla nts

The cell walls of certain organs of mono-cotyledonous plants, particularly those ofcereal grains, are an important source of DF.

Cereal grains are used for the production offlour required for bread, cakes, biscuits, etc.Products derived from grains are also com-monly used as breakfast foods in the formof bran based products, flakes, or porridge.A ll cereals have endospermous seeds; theendosperm ofwheat, for example, representsabout 80 to 85 of the grain, and is thesource of white flour, which is usually pre-ferred for bread-making. To obtain whiteflour, almost pure endosperm must be sep-arated and ground. Thus the milling processfor wheat is a complex operation, involvingseparation into three main fractions: bran or

pericarp, with the attached testa and aleu-rone layer (in all about 12 to 17%), theendosperm itself(about 80 to85 ), and theembryo or wheat germ (about 3%). Theremoval of the bran may result in a loss ofup to 50% of the total D F content of thegrain.

T he cell wall polysaccharides of parenchy-matous tissues of cereals are mainly arabi-noxylans and 3-i-glucans, but they all con-tam small but significant amounts of cellu-lose, usually associated with glucomannan( 1 1-1 3). The relative amounts of the firsttwo polysaccharides could vary considerably

among cereals. Thus wheat endosperm wallsare very rich in arabinoxylans but poor inf glucans, whereas those of barley are richin fl-glucans but relatively poor in arabinox-ylans. The polymers present in the primarycell walls of wheat and barley together withthose of lignifled tissues of wheat, and the


8/18

. M onosaccharidesRepresentative S ource S tructu ral features

type ofgum M ajor M inor

Endospermof guars e e d s

o-Man,

o-Gal

o-X y l, L -A ra i-R ha, D -G al Aucilages

A rabic E x udate ofA cacia sen-

egal

K appa C arrageenan G igartinaspecies

o-GIc

* T he U S N ational Form ulary includesP lantago psylluim , P lan ta go arenaria, an d P lantago o vala under the

nam e Plantago seed. In the B ritish Pharm aceutical C odex the seeds of the f irst tw o speciesare included as Psy lliumB PC , w hile those ofP ovata form Ispaghula B PC . R eference: T ex tbook of Pharm acognosy . 9th ed , T heaseE v ans, PubI B ailliere, T indall C assell, L ondon.

parenchy m atous tissues of certain m onoco-ty ledons w hich are rich in pectic substancesw ill be d iscussed brief ly .

P rim ary cell w allsfrom the endosperm and

aleu ro ne la yer. T he m ost notable dif ferencesbetw een the cell w alls of the endosperm (andaleurone lay er) of cereals and those of par-enchy m atous tissues ofd ico ty ledons are thatunlik e the latter, the form er are v irtually f ree

of pectins and pectic substances, and theam ount of cellulose is v ery low . T hese dif -ferences are reflected in the carbohy dratecom positions of the cell w alls and som e cellw all po ly saccharide f ractions, f rom the en-dosperm and aleurone lay er, w hich are giv enin T able 7. T he arab inose and x y lose arise

f rom arabinox y lans and the bulk of the glu-cose f rom the cell w alls of aleurone lay erarises f rom m ix ed-link age f l-g lucans and notf rom cellulose. T he arab inox y lans of w heatand barley endosperm cell w alls hav e ahighly branched f l-( 1 4)-link ed D -X ylpback bone, containing A raf residues as sidechains on C -3 , as w ell as C -3 and C -2; som eside chains, how ev er, contain (1- 5)-l inked

A raf residues as w ell (1 3 , 5 1 , 52). T he m ix edlink age 13-g lucans are linear m olecules andcontain f l 1 - 3)- and f l 1 - 4)-link ed I)-G lc p residues (51).

D espite the relativ ely low lev els of cellu-lose, chem ical ex traction stud ies, coupledw ith electron m icroscopy , hav e show n that

C H E M IS T R Y O F D IE T A R Y FIB E R 3 2 7

T A B L E 5C lassif ication of som e f ood gum s

Guar

M ucilagecells ofP lantago

ovata*Forsk

n-G al, L -A ra i-G lc pA ,L - R h a

A lginate L am inaria D -M annu-species ronic acid,

L-gulu-ronic acid

C arbox y m ethy l-cellu- C elluloselose

3 6 - A n h y -d r o - o - Ga lp . - o - Ga l psulphatedat 0-4

H ig hl y s ub st itu te d 1 - 4) -link ed o-M an p residues hav inga-D -G al p residues as sidechains on C -6

H ig hly b ranched arabinox ylan ,the x y lan back -bone hav ingboth (1 - 4)- and (1- 3)-link-ages; side chains of a-L A raf(m ostly ), /3-D -X yl p and G alpA -( 1- 2)-a-i-R hap-( 1-* ,link ed to C -3 (m ostly ) and C -2of X y l residues

- (1 - 3)-link ed o-G al p residuessom e ofw hich are substitutedat C -6 w ith side chains bearing 1 - 6)-link ed G al p residues;

side chains on the latter includeterm inal L -A raf , L -R ha p and4-0-M e-f i-o-G lc pA

L inear chains of f l-(l-. 4)-linkedD -m annuronic acid anda- I -. 4 )-lin ke d L -g ulu ro nicacid

C onsists of alternating sequenceof0-4 substituted 3, 6-anhy dro-a-D -G al p and 0-3 substituted-c-G al p w hich is sulphated at0- 4

- (l - 4)-link ed D -G lc p residues,w ith a m ax im um substitutionofO .95 carbox y m ethy l (-C H 2C O O H ) group per glucose unit


9/18

TA B L E 7Carbohydrate compositions of the CW M and some hemicelluloses from the endospermwheat and barley (values are monosaccharide composition % w/w)

and aleurone layers of

Wh e a t endosperm Barley endosperns W heat aleurone Barle aleurone

Sugars (I I ) (12) (13) (13)

WM H C WM WM W M

A rabinose 34.0 34.5 9 11 17 20 23 27X ylose 53.5 63.5 10 14 48 60 44 54M annose 7.0 2 2 0 1 2Galactose 2.5 3 2 2 1G l u c o s e 3 . 0 7 9 t 7 5 t 3 1 t l 8 t 2 9 t 1 7 t

T he major hemicellulosic fraction of the CW M.

t T he bulk (>95%) of the glucose arises from (i-D -glucans.W heat endosperm and aleurone (var Insignia): Barley endosperm (var Julia); Barley aleurone (var Clipper).

3 28 SELVENDRAN

T A BL E 6Carbohydrate compositions of the starch-dpg anhydrosugar/mg dry preparation)

epleted alcohol -insoluble resi dues of someissues of seed s (values are

Sugar Pea hull Pea tiourl G uar seedG u a r s e e d

splitsi s p a g h u l a

mucilage

i s p a g h u t a

mucilage

5 0)

6 - D e o x y h e x o s e 1 6 . 3 8 . 5 8 .9 5 32.9A rabinose 27.2 132 35.9 18.9 222 234X ylose 118 11.7 54.2 3.6 627 626M annose 3.2 4.9 326 506 21.9 7.4Galactose 11.3 27.8 198 301 74.9 29.5G l u c o s e 5 7 0 5 8 . 1 1 5 6 2 2 . 5 1 4 . 1 2 7 . 3

U r o n i c a c i d 1 5 4 6 0 . 2 7 8 4 2 . 3

D F (%w/w) (73) ( I I .6) (65) (88) (94)* Selvendran RR, D uPont M S. unpublished results.

t T he pea flour was derived mainly from the cotyledons, which contain much intracellular proteins.: 1 :M ucilage from the seed husk of Ispaghula; the sugar values are from Englyst et al (50) and these values

compare well with our data, given in column 5. T he bulk (-90%) ofthe glucose arises from cellulose.

II T he D F content of the products, based on their content of nonstarch polysaccharides only, is given on a dryw ei ght b asi s.

the endosperm cell walls consist of a micro-fibrillar phase embedded in an amorphousmatrix (1 1); in this respect they are similarto most primary plant cell walls. The micro-fibrils probably consist of glucomannans inclose association with cellulose (1 1). Thepredominant polysaccharides of wheat en-dosperm cell walls are arabinoxylans (88%),of which one-third are soluble in water; al-kahine agents are needed to dissolve the re-maining two-thirds. The cellulose content ofthe walls is about 2% and it is notable thatthe /3-glucan content is only about 1 The

latter finding is in striking contrast with theunusually high levels of mixed-linkage fi-glucans in barley (12, 5 1) and rye grass en-dosperm walls (53); the bulk of the glucoseof barley endosperm walls arises from j3-glucans (Table 7). The requirement of alka-line conditions to solubilize the bulk of the

arabinoxylans suggests the possible involve-ment of phenohic ester (or phenohic) cross-linkages in the wall (1 2). Some supportingevidence for the occurrence of phenolic estercross-linkages is provided by the work ofN eukom and colleagues 5 4 55 . They haveshown that a diferuhic acid-arabinoxylancomplex was formed when a solution of awater-soluble arabinoxylan (containing fer-uhic acid) from wheat flour was oxidized withH2O2 in the presence of peroxidase 55 .

The work of Ballance and M anners (51)has shown that about 70% of the barley

endosperm walls is composed of mixed-link-age 3-glucans. The ratio of(l -+ 3 ) - t o 1 -*4)-linkages in the fl-glucans is about 3:7.The 3-glucans of barley seem to be proteo-glycans (polysaccharide-protein complexes)(5 1 56, 57 ; the available evidence to thepresent time has been summarized in Ref-


10/18

C H E M IST R Y O F D IE T A R Y FIB ER 329

erence 16. C hem ical fractionation suggests

the involvem ent of phenolic acids (esters)and proteins in cross-linking som e of the

w all polym ers.

T he w alls of w heat and barley aleuronecells are com posed m ainly of fl-glucans and

arabinoxylans, and sm all am ounts of cellu-

lose, glucom annan, and phenohics 1 3). T hebulk -95 ) ofthe glucose ofthe w alls, fromboth species, arises from m ixed-linkage -

glucans. T he xylose and arabinose of the

w alls arise from arabinoxylans, w hich are

differentially soluble in a range of neutral

and alk aline solv en ts.

C ells w alls of lign ified tissues of w hea t.B eesw ing w heat bran consists m ainly of the

outer coating ofthe w heat grain and is about

three cells thick, containing the epiderm is,

hypoderm is, and cross cells, w hich are hg-nifled. T he studies of various groups of

w orkers have show n that the C W M of

beesw ing bran consists of hem icelluloses,

cellulose, lignin and phenohic esters, and

proteins (20, 58). T he K lason lignin and

protein contents ofthe C W M are I2 and 6%

(w /w ), respectively (20). T he hem icelluloses

are highly branched glucurono- and 4-0-

m ethylglucurono-arabinoxylans; som e of

the x ose residues of the backbone appear

to be substituted in quadruphicate. In con-

trast to arabinoxylans of endosperm cellw alls, w hich are neutral, those of hignifled

tissues are acidic, due to the presence ofglucuronic acid and 4-0-M e glucuronic acid;

the bulk of the uronic acids are directly

linked to C -2 of the 1 4)-linked X yl presidues. M ost of the (acidic) arabinoxylans

can be solubihized only w ith alkali, suggest-

ing the occurrence of phenolic ester cross-

linkages in the w alls; indeed, ferulic and p-coum aric acids have been detected in the

alkaline extracts (20, 59). T he (acidic) ara-binoxylans are linked to other m acrom ole-

cules such as hignin or proteins, or both. T his

i s i n f er r e d 1) from the occurrence of ligninand proteins in the C W M , as w ell as in the

isolated polysaccharide fractions, and 2) be-cause the constituent (anhydro) sugars ac-

count for only 50 to 60% of the dry w eight

of the polysaccharide fractions (20).

T he carbohydrate com positions of the

C W M and alkali-soluble hem icellulose frac-

tion of beesw ing bran are show n in T able 4.

T he arabinose and xylose ofthe C W M arise

from acidic arabinoxylans and m ost (90% )

of the glucose arises from cellulose. T heseresults coupled w ith those of T able 7 clearly

show that the cell w alls ofthe hignified tissuesofw heat grain are quite different from those

of the parenchym atous tissues.

P rim ary cell w alls rich in pectic sub-stances. In contrast to cereal grains, the cellw alls of parenchym atous tissues of certain

m onocotyledonous plants such as onion

bulbs, decutinized leeks, and asparagus

shoots (pith region of young shoots) are richin pectic substances, and their overall com -

positions (% w /w dry C W M ) are as follow s:

onions (var G iant Fen G lobe)-rha 1 . 1 , ara1.5, xyl 1.3, m an 0.7, gal 13.5, glc 3 1. 2, a nduronic acid (ua) 32.5; asparagus (var C ono-

vors C olossal)-rha 1 .9, ara 3.7, xyl 4.0,m an 1.8, gal 7.9, glc 39.7, and ua 33.0; leeks

(var M usselburgh)-rha 1 .8, ara 4.2, xyl 4.2,m an 1.5, gal 1.6, glc 22.0, and ua 24.2

(Selvendran R R , unpublished results). T hew alls of the m esophyll cells of grass leaves,

how ever, are poor in pectic substances (60);

the com position of the C W M (% w /w ) is as

follow s: rha 1 .0, ara 1 3.9, xyl 1 7.6, gal 4.0,

glc 57.2, and ua 6.5. In leeks there w ould besom e contribution to the C W M from vas-

cuhar tissues. T he C W M w as prepared bysequentially extracting the fresh tissues w ith1 % (w /w ) aqueous sodium deoxycholate and

phenol/acetic acid/w ater (1 7). A s the starch

content of the tissues w as low , extractionw ith 90% aqueous dim ethylsulphoxide w as

om itted. A lthough the uronic acids w ere not

rigorously identified, galacturonic acid (ofpectic substances) probably m akes a m ajor

contribution to the total uronic acid content.T he overall carbohydrate com positions of

the above cell w all preparations are com pa-rable w ith those of parenchym atous tissues

ofdicotyledons (cfT able 3), and are in strik-ing contrast w ith those of cereals (cf T able7). Fractionation studies on the cell w allpreparations have show n that they containm ainly pectic substances, cellulose, and

hem icelluloses. T he occurrence of pecticsubstances in onions has been recently re-

ported (61).

Lignins

L ignin is an am orphous, high m olecularw eight, arom atic polym er com posed of


11/18

330 SELVENDRAN

phenylpropane residues. It is form ed at the

sites of hignification by the enzym atic dehy-

drogenation and subsequent polym erization

of coniferyl, sinapyl, and p-coum aryl alco-

hols; the m onom eric units derived fromthese alcohols are called guaiacyl (3-m eth-

oxy-4-hydroxyphenylpropane), syringylpro-

pane (3,5-dim ethoxy-4-hydroxyphenylpro-

pane), and p-coum aryl (4-hydroxyphenyl-

propane) residues, respectively (62). Phylo-

genetic origin determ ines the relative pro-

portions of the phenylpropane residues in

the lignins. T he lignins can be divided into

three broad classes, w hich are called soft-w ood (gym nosperm ), hardw ood (dicotyle-

donous angiosperm ), and grass (m onocoty-

ledonous angiosperm ) hignins. O f these the

softw ood hignins contain m ainly guaiacyl

residues, w ith sm all am o unts of p-coum aryland syringylpropane residues; the hardw ood

hignins contain about equal am ounts of

guaiacyl and syringylpropane residues, w ith

only m inor am ounts ofp-coum aryl residues;

and the grass hignins are com posed of ap-

proxim ately equal am ounts of all three res-

idues. H ow ever, grass cells w alls contain in

addition som e esters of p-coum aric and fer-

uhic acids in the bound form , and these esters

are not incorporated into the hignin polym eritself. T he angiosperm hignins, w hich are the

ones of interest in the D F context, dem on-strate considerable variation from species to

spec i e s ( 6 3 ) .L ignification serves tw o m ain functions.

It cem ents and anchors the cellulose m icro-

fibrils and other m atrix polysaccharides, and

because the lignin-polysaccharide com plexes

are hard, they stiffen the w alls, thus pre-venting biochem ical degradation and phys-

ical dam age to the w alls. T hese properties ofhignifled w alls are im portant in the D F con-

text, because they m inim ize the bacterial

degradation ofthe w alls in the hum an colon.

T he im plications of these properties are dis-

cussed later.

The skins and protective coverings of thewall

T he outer w alls of the epiderm al cells of

leaves, fruits, and m any other aerial organs

of plants are covered w ith a protective layer

ofw axes and cutin. G enerally the cutin pen-

etrates and is interm ingled w ith the outer

polysacchandes (eg, pectin and cellulose) of

the w all. U nderground organs, eg, tubers,

and healed w ound surfaces of plants are

protected by another type of lipid-derived

polym eric m aterial, called suberin, w hich isrelated to cutin. B oth cutin and suberin are

em bedded in and overlaid w ith a com plex

m ixture of relatively nonpolar lipids, w hich

are collectively called w axes. C utin is hydro-

phobic and contains a num ber of hydroxy

m onocarboxyhic acids w ith 16 and/or 18

carbon atom s in linear chain w ith hydroxyl

groups at C -9, C -8, or C -7. T he w axes are

com plex m ixtures, of w hich the com m on

constituents are long chain hydrocarbons,

alcohols, aldehydes, ketones, fatty acids and

hydroxy-fatty acids, and esters. T he length

ofthe above com pounds usually range from

Cl 6 to C 34 (64, 65).A m ong other functions, the cuticle pro-

vides the first potential barrier to attack by

fungi, insects, or other pathogens. B ecause

cutin and w axes are resistant to bacterial

degradation, cutinized tissues m ay serve an

im portant role in restricting the access of

intestinal bacteria (and bacterial enzym es)

to the cell w all polysaccharides of som e veg-

etables and fruits. T his phenom enon m ay

be particularly significant in the case of leafyvegetables, w hich have spongy parenchym acells sandw iched betw een cutinized layers.

T he chem istry of som e food additives-

gum s

T he technical definition of a gum is a

polym eric m aterial that can be dissolved or

dispersed in w ater to give a thickening and/

or a gelling effect (66). A m ong the leading

m aterials, in decreasing order of use in food,

are pectins, gum arabic, alginates, guar gum ,

carboxym ethylcellulose, carrageenan, locust

bean gum , and m odified starches. T hese

gum s have valuable properties and m anycom m ercial foods contain sm all am ounts of

them . A s they are not hydrolyzed by the

hum an digestive enzym es they are classifiedunder the term D F, as defined earlier. W ith

the exception of m odified starches and (pos-

sibly) gum arabic all the polym ers listed

above are derived from plant cell w alls. For

exam ple, the cell w all polysaccharides and

m ucilaginous m aterial of som e sea w eeds


12/18

C H E M IS T RY OF DI ETARY FI BER 3 3 1

(certain species of brow n algae) are im por-

tant sources of alginates and carrageenans.T he m odified starches used in the food in-

dustry are prepared by either slightly cross-

linking the starch or by esterifying w ith ace-tyl groups; the levels of substituents range

fro m 0 .03 to 0 .0 6 acetyl g rou ps/anh ydro glu-

cose residue. T he m ajor structural featuresof som e of the gum s listed above are show n

in Table 5; for convenience the gum s areclassified into different groups. For m ore

details on the structural characteristics of

gum s see R eferences 6, 66, and 67.

T he D F content of som e plant foods

T he D F content of som e vegetables, fruit

and seed products, and of som e cereal prod-

ucts, are given in T ables 8 and 9, respec-tively. A lso included in T ables 8 and 9 arethe m ain types of tissues com prising the

products. A s the m ajor types of cell w all

polym ers present in m ost of the products

have been discussed earlier, attention w ill be

draw n only to som e of the distinctive fea-

tures. A dditional inform ation on the D F

content of a range of plant foods can be

obtained from R eferences 68 to 75. In R ef-

erence 75 the various m ethods available forthe analysis of D F are discussed critically

and objectively.

Interestingly, despite the high starch con-

tent of potatoes, their D F content is com -

parable w ith that ofapples and cabbage. T he

D F content of m ature runner bean pods is

considerably higher than that of the aboveproducts, because it contains a significant

am ount of hignified tissues; this is reflected

in the relatively high level of xylose derived

from acidic-xylans. P rocessed starch-richproducts, such as potato pow der, contain a

significant but variable level of m odified or

resistant starch. T he m odified starch is not

fully degradable by treatm ent w ith a-am y-

lase and puhlulanase. It is form ed w hen ge-latinized starch is cooled and dried and

could arise from strong hydrogen bond for-

m ation betw een O H groups on adjacent

starch m olecules, or by elim ination of w ater

under heat treatm ent, or both. T he m ain D Fpolym ers of soya flour are pectic substances

(arabinogalactans, pectins, etc), hem icellu-

loses (xyloglucans), and cellulose, w hereas

those of soya bran, w hich contains m ainly

the hignified hulls, are cellulose, hem icellu-

loses (acidic xylans), and pectic substances.

T he values for the D F content of cereal

products, given in T able 9, w ere obtained

from the results of E nglyst et a 50), w ho

have used an alternative m ethod to isolatethe D F-fractions. T he D F of w hite flour is

m ainly derived from the prim ary cell w alls

T ABL E. 8C arbohydrate com positions of the sanhydr sugar/m g dry preparation)

tarch-deplet ed alcohol-in soluble residu es of som e p1 ant foods (val ues are

Prxlucts Potatoes A p p Ies C ab b ageR u n n e r

t ,e an s 3 6)Po ta to t

p ow der 36)Soyab r a n

Soya*

flour

Tissie types M ainly P M ainly PM ainly P M a in ly P + m e + L and M ainly P M ainly L M ainly P

Sug ars L an d C som eC

6 - D eoy h e x os e 1 0 . 2 2 3 . 0 1 7 . 5 1 1 . 4 5 . 8 1 2 . 6 1 0 . 1A rabinose 37.7 128 46.5 29.4 20.6 60.8 37.4Xy l o s e 1 1 . 5 4 8 . 7 3 0 . 7 6 8 . 4 4 . 9 1 0 7 1 4 . 1M anncse 9.8 18.5 15.3 15.1 4.2 54.8 10.6

G alactose 204 57. 1 30.3 7 1.4 100 23.5 66.2G lucose 2 4 0 3 0 0 2 7 9 3 2 3 2 1 0 4 7 2 4 5 . 7(glucose) (43.8) (20.2) (19.1) (37.3) (130) (17.0) (5.1)Ur on i c a c i d 1 5 9 3 3 1 2 3 5 2 2 0 1 2 8 1 6 8 7 0 . 3

P w /w )II (1.8) (1.9) (2.2) (4.0) (9.5) (79.5) (14.0)* Selvendran R R, D uPont M S. unpublished results; the products w ere analyzed essentially by our m ethods

described in R eferences 36 and 75. T issue types: P. parenchy m ato us; L , lign ified; C , cutinized .

t C om m ercial C adbury Schw eppes potato pow der, contains m odified starch. W ith the exception of potato pow der, the bulk (>90% ) ofthe glucose arises from cellulose; the glucose values

in parentheses are from direct lM H 2 504 hydrolysis.

II F values based on the content of non-starch polysaccharides only; for potatoes, apples, cabbage, and runnerbeans e values are expressed on a fresh w eight basis and for the other products on a dry w eight basis.


13/18

33 2 SELVENDRAN

T A B L E 9

Carbohydrate compositions of non100 g dry matter basis)

starch polysa ccharides in some plant f oods ( 50) (values are expresse d on a gJ

Products W hi te f lourPea r l

barleyR ye

flourW h o l ew h e a t

flourW h o l e w h e a t

breadtW h e a t

bran productC o r n

fiakest

Tissue typcs, M ainly P M ainly PM ainly P+ some I

M ainly P+ some L

M ainly P+ some L

M ainly L+ some P

P + L

S u g a r s

A rabinose 0.91 1.18 3.63 2.67 2.62 5.87 0.11X ylose 1.38 1.62 5.76 4.32 4.17 9.68 0.14M annose 0.07 0.34 0.33 0.15 0.21 0.25Galactose 0. 1 8 0.07 0.3 1 0.29 0.30 0.50Glucosell 0.66 4.52 3.67 2.56 2.36 6.79 0.35(glucose) (0.19) (0.36) (1.49) (1.64) (1.61) (4.67) (0.24)U ronic acid 0.06 0.20 0.28 0.25 0.74 0.05D F(% w/w) 3.20 7.79 13.9 10.27 9.91 23.83 0.65* A ll the results are from Reference 50.

t T hese products contain a significant amount of starch resistant to enzymic hydrolysis; the resistant starchvalues for wholewheat bread and cornflakes are 0.84 and 2.92 g/lOO g dry weight respectively, and these figuresare not included in the values for D F.

T he D F content of this wheat bran product (A ll-Bran, K elloggs Company, U K ) is somewhat low, most

commercial wheat bran samples have D F values between 40-50g/lOOg dry matter (Selvendran RR, D uPont M S.unpu bl ished resu lts) .

T issue types: P, parenchymatous; L , lignified. Glucose released by a modified Saeman-hydrolysis procedure; the glucose values in parentheses are from

cell ul ose onl y.

which are rich in neutral arabinoxylans butrelatively poor in /3-glucans. In contrast, themajor D F-polymers of pearl barley are /3-glucans. The rye flour used for making ryebiscuits (ryvita) is relatively rich in /3-glu-cans, but contains significant amounts ofcellulose and (acidic) arabinoxylans derivedfrom the hignified bran layers of the grain.The main D F polymers ofwhole wheat flourare neutral arabinoxylans, acidic arabinox-ylans, and cellulose. The last two are derivedmainly from the hignified bran layers. Inaddition to the above polymers, whole wheatbread contains some modified starch whichis resistant to enzymic degradation. Themain D F polymers of wheat bran are acidicarabinoxylans and cellulose. W heat bran fi-ber also contains small but significant levelsof neutral arabinoxylans and /3-glucanswhich are derived mainly from the endo-sperm and aleurone layers, respectively. In

addition to cellulose and acidic arabinoxy-lans, cornflakes contain a very significantamount ofresistant starch. A measure of theresistant starch can be obtained by dis-solving it in alkali (for details see References50 and 75) and measuring the amount ofenzymica ly degradable starch solubilized.

A comparison of the compositions of someD F preparations used in clinical feedingtrails

In order to test the effect of the type ofD F on fecal weight and transit time, Cum-mings et a (76) fed concentrated fiber prep-arations from wheat bran, cabbage, carrot,apple, and guar gum to healthy volunteers.

The fibre was concentrated by preparing theA IR of the products. These were fed to thevolunteers who were on a metabolically con-trolled diet, the composition of which wassimilar to a British diet and contained 22 gof D F/day. Fiber from each of the materials(20 g/day) was added to the controlled dietsof separate groups and the changes in fecalweight and transit time were recorded (fordetails see Reference 76). Their resultsshowed that 1 fecal weight increased by127% on bran and 20% on guar gum, withcabbage, carrot, and apple producing inter-mediate changes (69-40% respectively); 2

the mean transit time was shortened by theaddition of fiber to the diet, with bran pro-ducing the greatest change; 3 these differ-ences are related to the pentose content ofthe fiber, cereal fiber (wheat bran) beingmuch more effective than vegetable (andfruit) fiber, and 4) individuals on a standard


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C H E M I S T R Y O F D I E T A R Y F I B E R 33 3

fiber intake showed widely differing colonicresponses. In view of these findings, the de-tailed chemical compositions of the abovefiber preparations were studied by us, with

a view to explaining the observed physiohog-ical effects, and some of the results havealready been published (19, 28, 77, 78).Herein, the compositions of the above D Fpreparations are discussed briefly, and a pos-sible mechanism for the mode of action ofDF in the human colon is postulated.

C arbohydra te com positions ofthe D Fpreparat ions

The A IR of wheat bran, cabbage, carrot,and apple were rendered free ofstarch, usingthe procedure of Selvendran and D uPont(36), and the carbohydrate compositions

and hignin content ofthe starch-free residueswere determined. The results of these anal-yses are given in Table 10; also included inTable 10 is the carbohydrate composition ofguar gum. Compared with the other fiberprepaiations wheat bran is clearly richer inarabirtose and xylose. The pentose contentof the samples decreased in the order wheatbran, cabbage, apple, carrot, and guar gum.The hexose contents of the vegetable fibersare, however, higher than those of wheatbran. A nother feature is the relatively highlevel of uronic acid-containing polymers invegetable fiber relative to that in bran. This

is mainly because the former are rich inpectic substances, whereas the latter has verylittle (ifany) pectic material. Guar gum con-tains mainly galactomannans, which areneutral pohysaccharides, and as expected

T A BLE 10

gave mainly mannose and galactose on hy-d r o l y s i s .

C hem icalfractionation ofthe D Fpreparat ions

In order to obtain further information onthe constituent polymers, the D F sampleswere first purified to free them of intracel-lular polymers (eg, proteins and starch). Thepurified cell wall preparations were thenfractionated by sequential extraction withaqueous inorganic solvents, and the isolatedpolymers were further fractionated on ion-exchange columns, and analyzed for theconstituent sugars (and amino acids) andglycosidic linkages. The results of purifyingthe D F-preparations and fractionating theCW M are given in Table 1 1 . During the

purification stages appreciable amounts ofpectic substances were solubihized from thecabbage, and carrot, and apple preparations,but only a relatively small amount of arabi-noxylan was solubihized from wheat bran.This trend was further reflected in the hotwater and 0.5% (w/w) oxalate-soluble frac-tions. H ow ever, the solubihity characteristicsof cabbage and carrot pectins were impairedas a result of the drying process during thepreparation of the A IR; some of the pectinswere therefore extracted with 1M KOH,resulting in a higher yield in this fraction.This was shown by fractionating the CW M

isolated from fresh cabbage (33). The cab-bage pectic substances are rich in arabinans,a small proportion of which are free, but thebulk are linked to rhamnogalacturonans; thecarrot pectic substances, on the other hand,

Carbohydrate compositions of the D F preparations used in the feeding trials (76, 78) (values areganhydrosugar/mg dry preparation)

Sugar W heat bran Cabbage Carrot A pple Guar g um

6-Deoyhexose 5.5 29.1 27.6 28.7 3.7A r a b i n o s e 1 3 9 100 6 5 . 1 6 0 . 2 2 1 . 8X y l o s c 2 1 0 3 3 . 7 1 0 . 6 4 9 . 1 2. 9M annse 12.3 25.9 19.9 18.3 516Galaciose 15.1 45.1 108 48.3 298

Glucose 179 221 241 303 32.2U r o n i c i d 5 8 . 6 3 3 4 3 5 1 3 9 1

L i g n i r 8 5 3 9 3 3 3 5

w/w)t (58.0) (60.4) (64.8) (80.0) (90.2)* T he lignin content of the preparations was determined by the acetyl bromide method (72). and is expressed

as pg Iignin/mg dry preparation. 1 he D F content ofthe preparations (ie. the alcohol-insoluble residues) as eaten are given within brackets, and

include the values for lignin.


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3 3 4 S E L V E N D R A N

T A BL E 11Y ields of material solubilized from the D F-preparations during purification and subsequent fractionation (78)[yield (mg/g recovered material)]

Fraction W heat bran Cabbage Carrot A pple

N o n s t a r c h y P S s o l u b i l i z e d t 2 4 . 2 2 3 3 3 0 1 1 6 7H ot water and Oxalate-soluble 8.8 68 93 106Chlorite/acetic acid-soluble 811 M K OH -soluble 408 168 92 514 M K O H - s o l u b l e 1 0 3 9 5 6 6 5 3

a-Cellulose 375 436 448 623

* T hese preparations were not treated with chlorite/acetic acid because they contained only very small amounts

oflignin. T he a-cellulose from these preparations contained small but significant amounts of pectic material.

t N onstarchy polysaccharides solubilized from the D F preparations during the purification stages-thesepolysaccharides are mainly pectic substances.

are rich in acidic-arabinogalactans (StevensBJH , Selvendran RR, unpublished results).The bulk ofthe pentosans ofwheat bran canbe solubihized only with alkali, and have

been found to be mainly (acidic) arabinox-ylans. For a detailed analysis of fiber fromcabbage and wheat bran see References 19and 77, respectively. The results of analysisof fiber from carrot and apple are in prepa-ration for publication. For an account of thecomposition and properties ofapple cell wallpolysaccharides see References 80 to 83.

P ossible m ode ofaction ofthe D Fpreparat ions

The above studies show that the compo-sitions and solubility characteristics of veg-etable D F polymers are very different from

those of wheat bran. W hereas the formercontain mainly pectic substances, cellulose,and hemicelluloses (eg, xyloglucans), the hat-ter contain mainly acidic arabinoxylans, cel-huhose, some /3-D -glucans, hignin, and phe-nolic esters. The chemical properties of thesepolymers (water-binding capacity, solubility,degree of substitution of the glycan back-bone, presence or absence of galacturonicacid, association with lignin, cross-linkagesby phenohic esters/acids, and proteins) mayhave an important bearing on the mode andextent ofdegradation ofthe parent D F prep-arations by bacteria in the large intestine. I t

has been shown that bacteria in the humancolon can degrade some of the componentsof D F, such as pectins, arabinogahactans,xyhans, gahactomannans, and some arabi-noxylans (8, 84-86), although these poly-mers, particularly those insoluble in buffer,are much less accessible when present as part

of the cell wall matrix. In a continuation ofthe earlier work of Cummings et a (76),Stephen and Cummings (7) have reportedthat 36% of the wheat bran fiber was de-

graded in the gut compared with 92% of thecabbage fiber. The increase in fecal weight(127%) which was observed with bran wasmainly due to incompletely degraded fiberand water held by it. Supporting evidence isprovided by scanning electron microscopestudies, which show that the cell wall struc-tures of the hignified tissues of wheat branare only slightly altered during passagethrough the gut (87, 88), or during incuba-tion of the CW M from wheat bran withmixed populations of human fecal bacteriafor 24 to 72 h at 37#{ 176} CStevens BJH , Selven-dran RR, unpublished results). The latter

studies also showed that on prolonged ex-posure to fecal bacteria (72 h), the relativelythick aleurone cell walls, which are not hg-nified but have phenolic ester cross-linkages,underwent significant breakdown. Thiscould be due to the structural characteristicsof the acidic arabinoxyhans of bran and theprotective action of hignin and phenohic es-ters (16, 77, 89, 90). The residual polymersbind water and appear to make a majorcontribution to the observed increase in fecalweight, compared with the bacterial mass.H owever, with cabbage fiber the smaller in-crease in fecal weight (69%) could be largely

accounted for by the extrabacterial mass andits associated water with some contributionfrom partly degraded fibrous elements,which come mainly from cutinized and hg-nified tissues. This is because fiber rich inpectic substances but poor in hignin andphenohic esters, such as those from cabbage


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C H E M IST R Y O F D IE TA R Y FIB E R 3 3 5

(carrot and apple), stim ulates m icrobial

grow th.. because very little fiber survives

digestion by bacteria (7, 9 1). H ow ever, the

structural and solubility characteristics of

pectins could be im p ortant in determ ining

the m ode and extent ofdegradation of pectic

sub stan ces by b acteria.W ith guar gum the contribution from un-

degraded fibrous tissues to the net increase

in fecal w eight w ould be m inim al. H ow ever,

since relatively few strains of colonic bacte-

ria can degrade the highly branched galac-

tom annan of the gum (85, 92, 93), the in-

crease in fecal w eight could be due to partly

degraded gum as w ell as increased bacterialm ass and the w ater associated w ith these

com ponents. C learly m ore w ork is required

on the m ode and extent of degradation of

D F polym ers by the m icroorganism s of the

hum an colon, in order to increase our un-

derstanding of the physiological effects of

D F.

T he author thanks M iss Susan D uPont and M r B JH

Stevens for som e of the unpublished results and for

th eir help in preparing the m anuscrip t.

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