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

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nutrients due to their disruption of the absorptive

area of the intestine. (See Carbohydrates: Digestion,

Absorption, and Metabolism.)

Detoxification

0011 The detoxification of plant lectins is usually achieved

by traditional methods of household cooking. There

are conditions where this type of processing is not

completely effective. When leguminous seeds are

used in the manufacture of animal feeds, cooking is

usually omitted, although steam pelleting of the final

mixed feed may have some effect in the degradation

of antinutritive factors. The time and temperature of

the feed during the pelleting process are clearly insuf-

ficient to degrade a significant proportion of the

hemagglutinins. As manufacturers do not want re-

duced food utilization occurring with their products,

they usually limit the amount of leguminous seed

protein incorporated in their mix.

0012 Lima bean lectin has been observed to be stable

between pH 5 and 7 at elevated temperatures. Add-

ition of N-acetyl-l-cystine (NAC) inactivated lectin at

a lower temperature than in its absence. This suggests

that NAC reacted with the disulfide linkage of the

lectin to bring about inactivation. NAC had no effect

on lectins extracted from soya bean flour; this sup-

ports the proposed role of NAC, as it is known that

soya bean lectins do not contain any disulfide bridges.

Addition of NAC prior to heatprocessing of some

legume flours may reduce the amount of heat process-

ing required and improve the overall quality of the

final product.

0013 Hemagglutinins are almost completely destroyed

by boiling for 60 min at atmospheric pressure. Auto-

claving at 105



C for 30 min is also effective. If leg-

uminous seeds are previously soaked for 18 h, the

hemagglutinins can be completely destroyed by

boiling for 2–5 min. Other traditional techniques

used to improve the nutritional value of legume

seeds involve soaking and germinating. It has been

shown that germination of lima beans for 6 days

results in a 50% reduction in activity. Soaking for

the same time results in an overall 34% reduction in

activity. It should be noted that soaking, though not

as effective as germination, would only be the first

step in preparation; normal cooking would destroy

further lectin in the beans.

0014 Dry heat treatment (roasting) to destroy hemagglu-

tinins is remarkably ineffective. For example, winged

beans (Psophocarpus tetragonolobus) heated at

100



C for 2 h show less than 5% reduction in lectin

activity. The generally low but quite variable levels of

hemagglutinins in this seed could be easily destroyed

by autoclaving at 120



C for 5 min.

0015The treatment of whole cow peas (Vigna unguicu-

lata) with microwaves (2400 MHz) for 6 min was

almost ineffective in destroying hemagglutinins,

while dry roasting (160



C for 50 min) led to between

28 and 47% reduction in lectin content. Treatment of

cow peas with mild alkali (0.5% sodium bicarbonate

for 12 h) led to an 80% reduction in hemagglutinin

content. (See Cooking: Domestic Use of Microwave

Ovens.)

0016Infrared heating of winged bean (Psophocarpus

tetragonolobus) for 60 s was remarkably effective

in reducing the hemagglutinating activity of this

seed, whereas oven and microwave heating had little

effect.

Conclusions

0017Extensive research has been carried out on the chem-

ical, physical, and biochemical properties of hemag-

glutinins, but the understanding of their toxicological

and nutritive properties is limited. These properties

are an important aspect of our knowledge as legumes

provide a vital source of protein in developing coun-

tries. The preparation of protein isolates from legum-

inous seeds and the use of these in quick cooking food

products need careful consideration of the possible

antinutritional effects of residual hemagglutinins. It is

possible that these toxic effects may be only partially

destroyed, and the resulting, slightly reduced food

utilization may be difficult to detect. It is not clear

what the long-term effect of consuming low levels of

these compounds will be.

0018Hemagglutinins occur in a wide range of legumin-

ous seeds. Lupins are the only common legume that

do not appear to contain any lectins. Legumes contain

a wide variety of toxins, including hemagglutinins,

that are heat-labile and should therefore present

no health hazard when properly cooked. Thus, it is

surprising that outbreaks of poisoning following the

consumption of incompletely cooked leguminous

seeds occur in an otherwise well-informed society.

See also: Beans; Carbohydrates: Digestion, Absorption,

and Metabolism; Cooking: Domestic Use of Microwave

Ovens; Legumes: Dietary Importance; Peas and Lentils;

Protein: Food Sources

Further Reading

Bender AE (1983) Haemagglutinins (lectins) in beans. Food

Chemistry 11: 309–320.

Boyd WC (1963) The lectins: Their present status. Vox

Sanguinis 8: 1–32.

Golstein IJ, Monsigny M, Osawa T and Sharon N (1980)

What should be called a lectin? Nature 285: 66.

HEMAGGLUTININS (HAEMAGGLUTININS) 3059

Grant G, More LJ, McKenzie NH, Stewart JC and Pusztai A

(1983) A survey of the nutritional and haemagglutina-

tion properties of legume seeds generally available in the

U.K. British Journal of Nutrition 50: 207–214.

Hill GH (1977) The composition and nutritive value of

lupin seed. Nutrition Abstracts and Reviews (Series B)

42: 511–529.

Huprikar SV and Sohonie K (1965) Haemagglutinin in

Indian pulses. 2. Purification and properties of haemag-

glutinin from white pea. Pisum sativum. Enzymologia

28: 300–345.

Jaffe

WG (1960) Effect of feeding lectins and peas on the

intestinal and hepatic enzymes of albino rats.Arzneimittel

Forschung 10: 1012–1016. (Cited by Jindal et al.,1982.)

Jaffe

WG (1969) Haemagglutinins. In: Liener IF (ed.) Toxic

Constituents of Plant Food Stuffs, 1st edn, pp. 69–74.

New York: Academic Press.

Jaffe

WG (1980) Hemagglutinins (lectins). In: Liener IF

(ed.) Toxic Constituents of Plant Food Stuffs, 2nd edn,

pp. 73–102. New York: Academic Press.

Jaffe

WG and Vega Lette CL (1968) Heat-labile growth-

inhibiting factors in beans (Phaseolus vulgaris). Journal

of Nutrition 94: 203–210.

Jaffe

WG, Moreno R and Wallis V (1973) Amylase inhibi-

tors in legume seeds. Nutrition Reports International 7:

169–173.

Jindal S, Soni GL and Singh R (1982) Effect of feeding of

lectins from lentils and peas on the intestinal and hepatic

enzymes of albino rate. Journal of Plant Foods 4:

95–103.

Kadam SS, Smithard RR, Eyre MD and Armstrong DG

(1987) Effects of heat treatments of antinutritional

factors and quality of proteins in winged bean. Journal

of the Science of Food and Agriculture 39: 267–275.

Liener IE (1955) The photometric determination of the

hemagglutinating activity of soyin and crude soybean

extracts. Archives of Biochemistry and Biophysics 54:

223–231.

Liener IE (1962) Toxic factors in edible legumes and their

elimination. American Journal of Clinical Nutrition 11:

281–298.

Liener IE (1979) The nutritional significance of plant pro-

tease inhibitors. Proceedings of the Nutrition Society 38:

109–113.

Muzquiz M, Burbano C, Gorospe MJ and Rodenas I (1989)

A chemical study of Lupinus hispanicus seed – toxic and

anti-nutritional components. Journal of the Science of

Food and Agriculture 47: 205–214.

Ologhobo AD and Fetuga BL (1984) Haemagglutinin ex-

tracts from raw, sprouting and differentially processed

lima beans. Journal of Agricultural Science (Cambridge)

102: 241–244.

Pusztai A and Palmer R (1977) Nutritional evaluation of

kidney beans (Phaseolus vulgaris): the toxic principle.

Journal of the Science of Food and Agriculture 28:

620–623.

Reaidi GB, McPherson L and Bender AE (1981) Toxicity of

red kidney beans (Phaseolus vulgaris). Journal of the

Science of Food and Agriculture 32: 846–847.

Romestand B, Corbier F and Roch P (2002) Protease

inhibitors and haemagglutinins associated with resist-

ance to the protozoan parasite, Perkinsus Marinus,in

the Pacific oyster, Crassostrea gigas. Parasitology

125(4): 323–329.

Sales MG, Maia GA, Teles FFF et al. (1984) Cowpea solids

prepared by mild alkali treatment, dry roasting, and

microwave heating. Nutrition Reports International

30: 973–981.

Tan NH, Rahim ZHA, Khor HT and Wong KC (1983)

Winged bean (Psophocarpus tetragonolobus) tannin

level, phytate content, and hemagglutinating activity.

Journal of Agricultural Food Chemistry 31: 916–917.

Wallace JM and Friedman M (1985) Inactivation of

haemagglutinins in lima bean (Phaseolus lunatus) flour

by N-acetyl-l-cysteine, pH and heat. Nutrition Reports

International 32: 743–748.

HEMICELLULOSES

M T Holtzapple, Texas A&M University, College

Station, TX, USA

Introduction

0001 Hemicellulose is the world’s second most abundant

carbohydrate (after cellulose). An estimated 45 000

million tonnes are produced annually, a daily produc-

tion of about 20 kg per person. (See Carbohydrates:

Classification and Properties.)

0002 Hemicellulose comprises 20–30% of plant cell

walls, which have a composite structure analogous

to fiberglass resins. Cellulose provides cell rigidity

and strength (like the glass fibers) and is embedded

in a hemicellulose matrix encrusted with lignin which

bonds the entire structure together. (See Cellulose;

Lignin.)

Definition

0003Hemicellulose is defined as the cell wall polysac-

charides extractable in dilute (10+8%) sodium

hydroxide solutions. It includes all the cell wall poly-

saccharides except cellulose and pectin. The term

‘hemicellulose’ was first used by Schulze in 1891,

3060 HEMICELLULOSES

who believed these extractable polysaccharides to be

cellulose precursors. This is now known to be abso-

lutely false, but the name has remained. Staudinger

has proposed the name ‘polyoses’ to prevent this er-

roneous association, but it has not gained widespread

acceptance.

0004 Hemicellulose is quite distinct from cellulose.

Hemicellulose polymers are short, with a degree of

polymerization (DP) of 50–200. In contrast, cellulose

polymers are much longer, with a DP of 500–15 000.

Cellulose is a linear polymer without attached side

groups, whereas some hemicelluloses are Y-branched

and most have attached side groups. Cellulose is

a homopolymer of glucose, whereas hemicellulose

generally is a heteropolymer composed of many

sugars and modified sugars.

Structure

Monomers

0005 Hemicellulose monomers are shown in Figure 1.

Three monomers are hexoses (glucose, galactose,

and mannose) and two are pentoses (xylose and

arabinose). Some of the sugars are acetylated

(glucose, mannose, and xylose). Glucose appears

as its uronic acid and also as the methylated uronic

acid. Most of the hemicellulose sugars appear as

d enantiomers except for three l enantiomers

(arabinose, fucose, and rhamnose). Galactose is

primarily a d enantiomer, although the l enantiomer

is occasionally present. Two 6-deoxy sugars (fucose,

rhamnose) may be found in small quantities. (See

Galactose.)

0006 Linear free-aldehyde sugars are unstable and cyclize

into a ring structure (Figure 1). In reality, the ring

resembles a garden chair, but simpler Haworth pro-

jections are commonly used. The ring can have six

members (pyranose) or five members (furanose). In

hemicellulose, arabinose appears as both a pyranose

and a furanose, whereas the others appear only as

pyranoses. When the linear sugar cyclizes, C1 be-

comes chiral. Its axial hydroxyl groups are designated

a and its equatorial hydroxyl groups are designated b.

Both a and b sugars appear in hemicellulose, although

the b forms are more common.

0007 Hemicellulose can be classified into three families

(xylans, mannans, and galactans), named here

according to the backbone polymer. (Note: the

naming conventions for hemicellulose vary widely.)

Xylan

0008 Xylan is known by many names – including glucur-

onoxylan, araboxylan, glucuronoarabinoxylan, and

l-arabino(4-O-methyl-d-glucurono)xylan – which

reflects its heterogeneous nature. Figure 2a shows

the backbone polymer is b-1,4-linked xylose. Numer-

ous side groups can attach to the backbone, as

shown in Figure 2b. Generally, 10–20% of the

xyloses have side groups (i.e., n ¼ 4–9), although

30–40% is sometimes observed (i.e., n ¼ 1.5–2.5)

in cereal flours. Most of the side-group attachments

are made through arabinose, which may occur

singly or with additional attached groups (xylose,

galactose, or 4-O-methyl-d-glucuronic acid). Galact-

ose is generally found in the xylans of annual plants.

Glucuronic acid and its methylated form may

also attach directly to the backbone at xylose C2,

although C3 attachment is sometimes observed.

Often, the xylose backbone is partially acetylated,

particularly in the C2 position, although C3 acetyla-

tion is found. Xylans are generally linear, with a DP of

50–200. In some xylans, there are one to two Y

branches in a single molecule.

0009The following generalizations may be made about

xylans from different plants:

0010monocotyledons – arabinose side groups attached

along with glucuronic acid (or its methylated

form);

0011dicotyledons (including hardwoods) – 4-O-methyl-

glucuronic acid attached every 10th xylose;

0012softwoods – 4-O-methylglucuronic acid attached

every sixth xylose with small amounts of arabinose

side groups.

0013One of the few examples of homopolymer xylan is

produced by esparto grass (Figure 2c) that contains

Y-branched xylose backbone without attached side

groups. (Note: esparto grass also makes some hetero-

polymer xylan with attached arabinose.)

0014Wheat flour xylan (Figure 2d) has irregularly

attached arabinose on about 30–40% of the xylose

backbone. ‘Open regions,’ consisting of about five

sequential bare xyloses, occur about every 20–25

xyloses. Wheat bran xylan is similar, except it has

more side groups (65% of xylose has attached arabi-

nose) with glucuronic acid on every seven to eight

xyloses. Barley husk xylan (Figure 2e) also has at-

tached side groups of glucuronic acid and xyloarabi-

nose. The xyloarabinose side group also occurs in

corn cob xylan and perennial rye grass. Maize seed

coat xylan (Figure 2f) has many side groups, includ-

ing arabinose, xylose, galactose, and glucuronic acid.

(See Barley.)

0015Hardwood xylan (Figure 2g) has 4-O-methyl-

glucuronic acid every 10th xylose. Of these 10

xyloses, approximately 3.5–7 are acetylated primar-

ily at the C3 position, although C2 acetylation is also

found. Softwood xylan (Figure 2h) has more side

groups because 4-O-methylglucuronic acid occurs

HEMICELLULOSES 3061

HEXOESE

Hexose

Acetylated

hexoses

Hexose uronic

acid

Methylated hexose

uronic acid

COH

G G

HOH

COH

HOH HOH

OH OH

COH

HOH

OH OH

HOH

Acetyl-

D-xylose

O C CH

COH

HOH

COH

HOH

O C CH

COH

HOH

Gu Gum

D-Glucose

(pyranose)

Acetyl-

D-glucose

D-Glucuronic acid

4-O -Methyl-

D-glucuronic acid

D-Mannose

(pyranose)

L9L

OCCH

Acetyl-D-mannose

-Galactose

(pyranose)

L-Galactose

(pyranose)

L-Fucose

(pyranose)

L-Rhamnose

(pyranose)

6-Deoxy sugar

Pentose

Acetylated

pentose

PENTOSES

D-Xylose

(pyranose)

Ap Af

L-Arabinose

(pyranose)

L-Arabinose

(furanose)

COH

D-Xylose

fig0001 Figure 1 Hemicellulose monomers. Reproduced from Hemicelluloses, Encyclopaedia of Food Science, Food Technology and Nutrition,

Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

3062 HEMICELLULOSES

HOH

ββββ

βββ

ββ

41 1

Gum

(a)

(b)

(c)

(d)

(e)

Q = Gum

ββ

ββββ

βββ

Af GuAf

HEMICELLULOSES 3063

ββ

βββ

ββ

Af Af

Af X X

Gum

LXL

ββ β β

Gum Gum

Af Af

ββββββ

ββ

(f)

(g)

(h)

(i)

(j)

fig0002 Figure 2 Xylan structures: (a) polymer backbone; (b) general structure; (c) esparto grass; (d) wheat flour; (e) barley husk; (f) maize

seed coat; (g) hardwood; (h) softwood; (i) cress seed; (j) runner bean. Reproduced from Hemicelluloses, Encyclopaedia of Food Science,

Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

3064 HEMICELLULOSES

every sixth xylose, and it also has arabinose side

groups.

0016 Some unusual xylose-containing hemicelluloses

have nonxylose backbones. Cress seed hemicellulose

(Figure 2i) has an arabinose backbone with attached

xyloarabinose side-groups. Runner bean hemicellu-

lose (Figure 2j) has a glucose backbone similar to

cellulose with side groups containing xylose, galact-

ose, arabinose, and fucose. (See Legumes: Legumes in

the Diet.)

Mannans

0017 Mannans generally occur with other sugars, includ-

ing galactose and glucose. Four distinct groups

(Table 1) are generally recognized depending on the

relative amounts of the additional sugars. In some

mannans, particularly from softwoods, the sugars are

acetylated.

0018 Mannan (Figure 3a) is a relatively pure (> 95%)

mannose polymer found in the food reserves of

some seeds. The most famous mannan source is vege-

table ivory (i.e., tagua palm seed), a hard crystalline

material that may be fashioned into buttons. It is not

completely pure, because traces of galactose have

been isolated with the mannose. Mannan also occurs

in coffee beans and orchid tubers.

0019 Glucomannan is a heteropolymer of random-

sequence mannose and glucose. Hardwood gluco-

mannan (Figure 3b) has no side groups. Its

mannose-to-glucose ratio is 2:1 (common) to 1:1

(infrequent).

0020 Softwood glucomannan (Figure 3c) has side groups

with a mannose-to-glucose ratio of 3:1 (common) to

4:1 (infrequent). A galactose residue is attached to

approximately every 15th or 30th mannose unit at

C6. Some galactose may also be attached to glucose at

C3. Approximately 25% of the backbone sugars are

acetylated. Glucomannans also occur in the food

reserves of some plants, such as iris seeds (mannose-

to-glucose ratio 1:1), lily bulbs, bluebell seeds, and

orchids.

0021Galactomannan is common in the seeds of legumes

and carob trees. The most famous form is the food

thickener guaran (or guar gum), shown in Figure 3d.

Galactose side groups are attached to every other

mannose unit. (See Gums: Properties of Individual

Gums.)

0022Galactoglucomannan is similar to glucomannan,

except it has about 10 times more galactose and a

lower DP. The backbone from western hemlock

galactoglucomannan (Figure 3e) has randomly

ordered mannose and glucose. Galactose side groups

are attached to either mannose or glucose. Approxi-

mately 25% of the backbone is acetylated.

Galactan

0023Figure 4a shows the b-1,3-linked galactose backbone

of galactan. Arabinose is a common side group, so the

term ‘arabinogalactan’ is sometimes used. Larch-

wood has an unusually high galactan content (10–

15% typical, but as high as 25–30%), so it is well

studied. Larch galactan (Figure 4b) has side groups of

galactose, arabinose, and small amounts of glucuro-

nic acid. The arabinose-to-galactose ratio ranges

from 1:4 to 1:8, with 1:6 most common. Maritime

pine galactan (Figure 4c) contains a small amount of

xylose. Tobacco (Figure 4d) and maple sap galactans

contain some rhamnose.

Cellular Structure

0024Hemicellulose is a major component of plant cell

walls, which are well described in the Cellulose art-

icle. Figure 5 shows the distribution of lignin, cellu-

lose, and various hemicelluloses in the cell wall layers

for both softwoods and hardwoods. (The distribution

in herbaceous crops would be similar to hardwoods.)

Hemicellulose is the dominant carbohydrate in the

compound middle lamella, whereas cellulose is more

plentiful in the secondary layers (S1, S2, and S3). In

hardwoods (and herbaceous crops), glucuronoxylan

is the primary hemicellulose, whereas glucomannan is

more prevalent in softwood.

Properties

Physical

0025Because hemicellulose is so heterogeneous, its phys-

ical properties are not well studied. The higher heat of

combustion for xylan is 17.8 MJ kg

1

. Hemicellulose

begins to decompose above 200



C, which makes it

one of the least thermally stable components of the

cell wall.

0026In the cell wall, most hemicellulose is noncrystal-

line. (An exception to this is the mannan from

tbl0001 Table 1 Sugar and acetyl ratios in various mannans

Mannose Glucose Galactose Acetyl

Mannan > 19 1

Glucomannan

Hardwood 1–2

Softwood 3

–4 1 01–021

Iris 1 1

Galactomannan

Legume 1–5 1

Galactoglucomannan

Softwood 3 1 1 1

More common.

HEMICELLULOSES 3065

OH OH

HOH

COH

11 111

ββ β ββ

ββ

ββ β β

αα

ββ

ββββ

ββ

(a)

(b)

(c)

(d)

(e)

fig0003 Figure 3 Mannan structures: (a) ivory nut mannan; (b) hardwood glucomannan; (c) softwood glucomannan; (d) guaran; (e) western

hemlock. Reproduced from Hemicelluloses, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and

Sadler MJ (eds), 1993, Academic Press.

3066 HEMICELLULOSES

ivory nuts.) Isolated 4-O-methylglucuronoxylan is

crystalline with a repeating length of 1.48 nm. Adja-

cent xyloses are rotated 120



with respect to their

neighbors in a left-handed helix.

Chemical

0027 Only a few hemicelluloses are water-soluble, such as

galactan (from larch) and galactomannan (from

guar gum). Most hemicelluloses are soluble in dilute

(10 + 8%) sodium hydroxide, a property used to

define hemicellulose. Alkaline extraction deacetylates

hemicellulose, which can be avoided by using

dimethyl sulphoxide. The glucomannan cis-hydroxyl

groups (C2 and C3) impede alkaline solubility, so

sodium borate is sometimes added to improve dissol-

ution. A fraction of ivory nut mannan (mannan B)

forms microfibrils similar to cellulose. It is insoluble

in alkali but, like cellulose, it is soluble in cuprammo-

nium hydroxide solutions.

HOH

COH

L L Af L

LL L

LAfGu

LAfAf

LAf

βββββ

ββββ

βββββ

LAf

ββ

RAf

Gu X

6 6

(a)

(b)

(c)

(d)

11111

111111

1111

fig0004 Figure 4 Galactan structures: (a) galactan backbone; (b) larch; (c) maritime pine; (d) tobacco. Reproduced from Hemicelluloses,

Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

HEMICELLULOSES 3067

0028 Dilute acid readily hydrolyzes hemicellulose. The

xylose released from xylan degrades to furfural,

which further degrades to resins if the hydrolysis

conditions are severe.

0029 Alkaline degradation occurs from the polymer ‘re-

ducing end’, i.e., the end with the free (or unlinked)

C1 hydroxyl group. In xylan, successive terminal

xyloses ‘peel off’ and degrade to the saccharinic

acid, 2-hydroxymethyl-2,4-dihydroxybutyric acid.

The reaction proceeds more readily in calcium hy-

droxide than sodium hydroxide. This reaction occurs

slowly at the usual cold extraction temperatures. At

higher temperatures, 4-O-methylglucuronic acid loses

both its methyl and uronic acid functional groups.

Alkaline degradation of hemicellulose can be limited

by prereducing with borohydride.

0030 Hemicellulose is much more resistant to oxidation

than lignin – a fact exploited in its isolation from

natural materials as ‘holocellulose.’ However, slight

changes do occur. Preparing holocellulose using

chlorous acid (acidified sodium chlorite) causes de-

polymerization, oxidation of 2,3-hydroxyl groups,

and oxidization of reducing ends to aldonic acids.

Occurrence

0031 A detailed list of the hemicellulose content in various

foods and agricultural products is presented in the

Cellulose article.

0032 Table 2 lists the sugar distribution in various plants

determined by acid hydrolyzing the polysaccharides

and chromatographically measuring the resulting

sugars. Most of the glucose is derived from cellulose,

although some comes from hemicellulose mannans.

The softwoods have a much higher mannose content

because much of their hemicellulose contains man-

nans rather than xylans. The hemicellulose compos-

ition of hardwoods and agricultural residues is

similar.

Uses

0033Furfural can be made by heating xylan in the presence

of 12% hydrochloric or sulfuric acid. Oat hulls and

corn cobs are traditional sources. Furfural may be

used as a solvent in petroleum refining, in manufac-

turing furfural–phenol plastics (Durite), as a solvent

for cellulose nitrate and cellulose acetate, in the

manufacture of insecticides, and as a nylon precursor.

(See Oats.)

0034Xylitol is a sugar alcohol formed by reducing

xylose. It is as sweet as sucrose, but is not cariogenic.

Its endothermic heat of solution elicits a cool sensa-

tion to the mouth, so it has been incorporated into

chewing gum. Because it is completely metabolized,

it is not a low-calorie sweetener. It has no insulin

requirement and has been used for intravenous

infusion. (See Sugar Alcohols.)

0035Xylose is used as a media ingredient to produce

xylose/glucose isomerase which is used in the produc-

tion of high-fructose corn syrups.

tbl0002 Table 2 Composition of some hemicellulose-containing materials (g per 100 g dry matter)

Hexosans Pentosans

Cellulose

Anhydroglucose

Hemicellulose

Lignin Anhydromannose Anhydrogalactose Anhydroxylose Anhydroarabinose

Softwood

Balsam fir 294 46.8 12.4 1.0 4.8 0.5

Jack pine 286456106147114

White spruce 271465116126816

Hardwood

Aspen 163573230816004

White birch 189447150624605

Red maple 240466350617305

Agricultural residues

Barley straw 138375126 171 15.0 4.0

Corn stover 151351025 075 13028

Cotton gin trash 17618019014020

Rice hulls 194325270112326

Rice straw 99369160413040

Sorghum straw 145325080215030

Wheat straw 145329072 216 16921

Timell TE (1957) Carbohydrate composition of ten North American species of wood. TA P P I 40: 568–572.

Totals are less than 100% since they do not include extractives, ash, protein, or free sugars. From Wiley DF (1985) Enzymatic hydrolysis of cellulose. PhD

dissertation, University of California.

3068 HEMICELLULOSES