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acids. On average, 100 g of legume seeds provides

23% of the nicotinic acid, 50% of the thiamin, 15%

of the riboflavin, 20% of the vitamin B

, 195% of the

folate, and 30% of the pantothenic acid requirement

of an adult (based on US RDAs). Among the legumes,

peanut is particularly rich in B vitamins. In general,

legume seeds are poor sources of fat-soluble vitamins

and vitamin C, but the leaves and immature seeds of

some legumes contain high amounts of carotene, vita-

min C, and vitamin E. For example, immature soy-

bean seed is a good source of vitamin C, whereas

mature soybean seed contains virtually no vitamin

C. (See Vitamins: Overview.)

Minerals

0025 Legumes are high in iron, with one serving of legume

providing around 2 mg of iron. This compares

favorably with the iron RDAs of 10 and 15 mg for

adult men and premonopausal women, respectively.

However, iron bioavailability from legumes is poor,

and thus their value as a source of iron is diminished.

In contrast, zinc bioavailability from legumes is

relatively good at around 25%. Also, many beans

are good sources of calcium, providing on average

50 mg of Ca per serving. Calcium bioavailability

from soyfoods is essentially equivalent to that from

milk, but the balance of calcium to phosphorus is

inadequate, since the ratio is 1:2, whereas the ideal

is 2:1. Legume seeds contain very low amounts of

sodium. (See Minerals – Dietary Importance.)

Bioactive Phytochemicals

0026 The correlation between diets rich in soybean and

reduced incidence of certain types of cancer and

cardiovascular disease has led to studies on identify-

ing the responsible substances.

Effects on Heart Disease

0027 Animal studies have shown clearly that soy protein

lowers serum cholesterol and triglycerides, but the

results in humans have not been consistent. However,

a metaanalysis of a number of human studies using

strict criteria, concluded that soy protein reduces

serum cholesterol and triglycerides. The US FDA ap-

proved a health claim that diets low in saturated fat

and cholesterol that include 25 g of soy protein may

reduce the risk of heart disease. The mechanism of

action remains to be elucidated.

Effects on Cancer

0028A number of soybean compounds are being studied

for their anticancer properties. These include phyto-

sterols, phytates, saponins, protease inhibitors, iso-

flavones, and a recently discovered unique peptide

termed lunasin.

0029Phytic acid may play a role in reducing cancer risk,

possibly because of its antioxidant effects. Specific-

ally, it has been suggested that phytic acid may lower

the risk of colon cancer and perhaps breast cancer.

0030Saponins have been shown to have anticancer

properties, as suggested by a recent rodent study

that a saponin-containing diet inhibited by about

two-thirds the development of preneoplastic lesions

in the colon. However, given that the human intake of

saponins is much less than the experimental amount,

it is not clear to what extent these results in rodents

are relevant to humans.

0031Bowman–Birk inhibitor (BBI) is a protease inhibi-

tor found in soybean and many other types of

legumes. BBI in the form of BBI concentrate (BBIC)

has shown the greatest suppression of carcinogenesis

in animal models so far. BBIC is now being tested in

human clinical trials. (See Trypsin Inhibitors.)

0032Isoflavones, which are found in plants but are

especially high in soybean, belong to the class of

phytoestrogens. Isoflavones offer potential alterna-

tive therapies for a range of hormone-dependent con-

ditions that include cancer, menopausal symptoms,

cardiovascular disease, and osteoporosis. Epidemi-

ological evidence and animal experiments strongly

suggest beneficial effects of isoflavones on human

health. Large-scale human clinical trials are needed

to confirm limited but promising pilot studies.

tbl0003 Table 3 Micronutrients contents per 100 g of edible legume seeds

Common names Ca (mg) Fe (mg) Zn (mg) Vitamin A (IU) Thiamin (mg) Folate (mg) Vitamin C (mg)

Soybean 277 15.7 4.89 24 0.874 375.1 6

Peanut 92 4.58 3.27 0 0.64 239.8 0

Pea 43 2.08 0.27 145 0.15 41.7 60

Lentil 51 9.02 3.61 39 0.475 432.8 6.2

Cowpea 110 8.27 3.37 50 0.853 632.6 1.5

Chickpea 105 6.24 3.43 67 0.477 556.6 4

Pigeon pea 130 5.23 2.76 28 0.643 456 0

Fava bean 37 1.55 1.00 333 0.133 148 3.7

Lupine 176 4.36 4.75 23 0.64 355 4.8

Winged bean 440 13.44 4.48 0 1.03 44.6 0

LEGUMES/Dietary Importance 3533

0033 Lunasin is a 43-amino acid peptide with a

polyaspartic acid carboxyl end, a cell adhesion motif

(-RGD- or arg–gly–asp), and a predicted conserved

helical region. The peptide was isolated and se-

quenced 14 years ago, and a few biological effects

were proposed, but none was proven until the lunasin

gene was isolated. Transfection of the lunasin gene

into mammalian cells leads to the arrest of cell div-

ision, enlargement of the cells, and eventually cell

death, resulting in cell lysis and chromosome frag-

mentation. The expressed lunasin peptide binds pref-

erentially to chromatin. This suggests the use of the

lunasin gene as a chemotherapeutic agent in conjunc-

tion with a specific delivery system that targets cancer

cells. While the lunasin gene is potentially a chemo-

therapeutic agent, the lunasin peptide has been shown

in in vitro experiments using mammalian cells to

suppress transformation of normal cells to cancer

cells caused by chemical carcinogens and oncogenes.

The first in vivo experiment using a mouse skin

cancer model shows that topical application of luna-

sin peptide suppresses skin tumorigenesis.

Anti-nutritional Factors

Amylase and Protease Inhibitors

0034 Protease inhibitors from legumes can interfere with

protein digestion, leading to growth retardation and,

in some species of animals, can cause pancreatic en-

largement and enhance chemically induced pancreatic

tumors. Two types of trypsin and chymotrypsin inhibi-

tors have been identified in soybean: Kunitz trypsin

inhibitor and BBI. Boiling dry beans generally reduces

the protease inhibitor content by 80–90%, and there is

little reason to think that the amount of trypsin inhibi-

tors obtained by eating commonly consumed beans

would exert any adverse effects in humans. (See Plant

Antinutritional Factors:Characteristics.)

Lectins

0035 Lectins, also known as hemagglutinins, are found in

many legume seeds. Lectins are toxic to animals by

causing lesions in the intestinal mucosa, malabsorp-

tion of nutrients, weight loss, and eventually death.

Symptoms of lectin toxicity in humans include nausea,

vomiting, diarrhea, and abdominal pain. Proper

cooking of beans until a soft texture is achieved elim-

inates lectin toxicity. (See Hemagglutinins (Haemag-

glutinins).)

Goitrogens

0036 Goitrogenic compounds are found in soybean and

peanut. Consumption of unheated soybean or peanut

can lead to enlargement of the thyroid gland in rat

and chicken. Iodine administration and heat treat-

ment are common ways of eliminating goitrogens.

Cyanogenic Glycosides

0037Cyanide poisoning from lima beans in humans has

been reported. However, once lima beans are ground

and cooked, the enzymes responsible for the liber-

ation of the cyanogenic compounds are generally

denatured.

See also: Bioavailability of Nutrients; Cancer: Diet in

Cancer Prevention; Diet in Cancer Treatment; Coronary

Heart Disease: Prevention; Ethnic Foods; Fermented

Foods: Soy (Soya) Sauce; Peas and Lentils; Plant

Antinutritional Factors: Characteristics; Protein:

Quality; Soy (Soya) Beans: Dietary Importance; Toxins

in Food – Naturally Occurring

Further Reading

Galvez AF and de Lumen BO (1999) A soybean cDNA

encoding a chromatin-binding peptide inhibits mitosis

of mammalian cells. Nature Biotechnology 17: 495–

500.

Galvez AF, Chen N, Macasieb J and de Lumen BO (2001)

Chempreventive property of a soybean peptide that

binds to deacetylated histones and inhibits histone acet-

ylation. Cancer Research 61: 7473–7478.

Kelloff GJ, Boone CW, Crowell JA et al. (1996) New agents

for cancer chemoprevention. Journal of Cellular Bio-

chemistry 26S: 1–28.

Kelloff GJ, Sigman CC and Greenwald P (1996) Cancer

chemoprevention: Progress and promise. European

Journal of Cancer 35: 1755–1766.

Kennedy AR (1995) The evidence for soybean products as

cancer preventive agents. Journal of Nutrition 125:

733S–743S.

Kennedy AR (1998a) Chemopreventive agents: protease

inhibitors. Pharmacology and Therapeutics 78: 167–

209.

Kennedy AR (1998b) The Bowman–Birk inhibitor from

soybean as an anticarcinogenic agent. American Journal

of Clinical Nutrition 68: 1406S–1412S.

Messina MJ (1999) Legumes and soybeans: overview

of their nutritional profiles and health effects.

American Journal of Clinical Nutrition 70 (Supple-

ment): 439S–450S.

Messina M, Persky V, Setchell KDR and Barnes S (1994)

Soy intake and cancer risk: A review of in vitro and

in vivo data. Nutrition and Cancer 21: 113–131.

Moyad MA (1999) Soy, disease prevention, and prost-

ate cancer. Seminars in Urology and Oncology 17(2):

97–102.

Setchell KDR and Cassidy A (1999) Dietary isoflavones:

Biological effects and relevance to human health.

Journal of Nutrition 129: 758–767.

3534 LEGUMES/Dietary Importance

Lemons See Citrus Fruits: Types on the Market; Composition and Characterization; Oranges; Processed and

Derived Products of Oranges; Lemons; Grapefruits; Limes

Lentils See Peas and Lentils

Lettuce See Salad Crops: Leaf-types; Other Types of Salad Crops; Root, Bulb, and Tuber Crops

Leukotrienes See Prostaglandins and Leukotrienes

Levulose (Laevulose) See Fructose

LIGNIN

M T Holtzapple, Texas A&M University, College

Station, TX, USA

Introduction

0001 Lignin is the world’s most abundant noncarbohydrate

biological material. An estimated 35 000 million

tonnes is produced annually, a daily production of

about 16 kg per person.

0002 Lignin is derived from the Latin lignum, meaning

wood. It is an important component of wood and

other higher plants, serving as nature’s ‘glue.’ Not

only does it hold plant cell walls together, its hydro-

phobic properties prevent water loss from plant vas-

cular systems. Lignin is highly resistant to enzymatic

degradation, so it protects plants from invading

insects and microbial attack.

0003 Because lignin is nondigestible, it is an important

component of dietary fiber. Lignin resists decay and

thus forms a large fraction of soil humus. When sub-

jected to geological forces, lignin and other plant

components are transformed into lignite and, ultim-

ately, bituminous and anthracite coals. (See Dietary

Fiber: Properties and Sources.)

0004Pure lignin does not occur in nature because it

always accompanies plant polysaccharides as a com-

posite material called ‘lignocellulose.’ Lignin removal

is required to manufacture paper, so tree lignin –

particularly softwood lignin – is well studied. (See

Carbohydrates: Classification and Properties; Cellu-

lose; Hemicelluloses.)

Structure

Monomers

0005The three phenylpropylene lignin monomers –

trans-coniferyl alcohol, trans-sinapyl alcohol and

trans-p-coumaryl alcohol – are shown in Figure 1.

The monomers are essentially the same, differing

only in the number of methoxyl groups. In softwoods,

coniferyl alcohol is the dominant monomer (94%

coniferyl, 1% sinapyl, 5% coumaryl alcohols). In

hardwoods, both coniferyl and sinapyl alcohols are

LIGNIN 3535

present in significant amounts, whereas grasses have

large quantities of all three phenylpropylenes. The

guaiacyl, syringyl and p-hydroxy-phenyl aromatic

groups of the three monomers are also described in

Figure 1. The aromatic carbons are numbered

1 through 6, whereas the propylene carbons are

lettered a, b, and g, starting from the phenyl group.

Plants produce these phenylpropylene monomers by

deaminating tyrosine and phenylalanine, two aro-

matic amino acids. (See Amino Acids: Properties

and Occurrence.)

Polymer Structure

000 6 The phenylpropylene monomers are extremely react-

ive because they contain two hydroxyl groups and

one double bond. Plant enzymes oxidatively dehydro-

genate the phenylpropylene, causing it to polymerize

randomly into the three-dimensional structure sche-

matically shown in Figure 2. During polymerization,

the reactive double bonds are lost, so phenylpropane

is the repeating unit. The polymer contains the ether

and carbon–carbon linkages shown in Table 1.

(Although this structure has been determined for

softwood lignin, it is probably representative of

other lignins as well.) The b-O-4 ether linkage is the

most common, so it is assigned a relative frequency of

1.00, whereas other linkages have a lower relative

frequency. Because of the reactive double bond, the

a and b carbons are the primary linkage sites between

monomers. The g carbon is rarely linked, except in

the circular (or multiple) bonds. About 70% of the

phenolic oxygens are involved in ether linkages,

whereas the remaining 30% are free. (See Phenolic

Compounds.)

0007The propane unit contains a variety of functional-

ities, including hydroxyl groups, ketones, aldehydes,

double bonds, and ether linkages. The b carbon is

frequently involved in an ether linkage. The adjacent

OCH

Guaiacyl

OCH

Syringyl

p-Hydroxyphenyl

Grass

Hardwood

Softwood

COH

OCH

COH

OCH

COH

trans-Coniferyl

alcohol

trans-Sinapyl

alcohol

trans-p-Coumaryl

alcohol

fig0001 Figure 1 Lignin monomers. Reproduced from Lignin. Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R,

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

3536 LIGNIN

a carbon generally has a hydroxyl group, or another

ether linkage. The g carbon normally retains its

hydroxyl functionality, although it is sometimes

converted to an aldehyde.

0008The phenylpropylene monomers not only cross-

link with other phenylpropylenes, but also covalently

bond to hemicellulose, primarily through the a

carbon. The exact nature of the carbohydrate bond

COH

OCH

Xylan

4−O− 5

α−6

β−1

α−carb

β−O − 4

β−O −4

β−5

4−O −5

1−O −4

α−O −4

β−O −4

5−5

1− 5

α−O−4

β−β

Glucomannan

fig0002 Figure 2 Schematic of softwood lignin structure. Reproduced from Lignin. Encyclopaedia of Food Science, Food Technology and Nutri-

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

tbl0001 Table 1 Lignin bonds

Ether Relative frequency Carbon^carbon Relativefrequency Circular Relativefrequency

b-O-4 100 b^b 038 D b^b, a-O-g, a-O-g 029

a-O-4 018 b –1 033 E a-O-4, b–5 022

a-carb — b–5 031 F a–6, b–b, g-O-g —

4-O-5 — 5–5

sim027

1-O-4 — a–6 —

1–5 —

LIGNIN 3537

is still being studied. It is thought that carbohydrate

linkages infrequently occur as periodic ‘spot welds’

among the intertwined hemicellulose/lignin matrix.

0009 Softwood lignin has a repeating nine-carbon phe-

nylpropane subunit with an approximate formula of

795

24

(OCH

)

092

and a molecular weight of

about 184. The polymer is believed to have an ap-

proximate molecular weight of over 10 000, which

corresponds to a degree of polymerization (DP) of 54.

It is difficult to know accurately the molecular weight

of native lignin because it is impossible to isolate it in

an unaltered state.

0010 Hardwood lignin has a repeating nine-carbon phe-

nylpropane subunit with an approximate formula of

749

253

(OCH

)

139

and a molecular weight of

about 200. It has more methoxyl groups than soft-

wood lignin because sinapyl alcohol is a more preva-

lent monomer in hardwoods. The polymer probably

has a molecular weight under 5000 (DP about 25).

Cellular Structure

0011 Plant cell walls are composed of microfibrils de-

posited in lamellar layers spaced approximately 7–

10 nm apart (Figure 3). According to Fengel’s model

of the microfibril, four 4 4 arrays of elementary

fibrils are embedded in a hemicellulose matrix. The

elementary fibrils are composed of cellulose which is

largely crystalline, except for the paracrystalline

edges and amorphous regions interspersed along the

length. These rigid cellulose elementary fibrils are

primarily responsible for structural load bearing.

The hemicellulose is tightly associated with the cellu-

lose, particularly in the paracrystalline and amorph-

ous regions. Hemicellulose is noncrystalline, but is

largely oriented in the same direction as the cellulose

fibers. The microfibril is encrusted with a lignin layer

about 2 nm thick spanned by about two to four phe-

nylpropane units. Although the lignin may be par-

tially oriented, it is mainly amorphous. In effect, the

lignin acts as a ‘net’ holding the fibrous structure

together. This net-like form of lignin is dispersed

throughout the secondary plant cell wall and ac-

counts for about 20% of its mass.

0012In mature plant cells, the interstitial space between

the cells (i.e., middle lamella) is highly lignified. The

lignin layer is about 100 nm thick and forms a com-

pletely amorphous three-dimensional array. Although

the lignin concentration is very high in the middle

lamella, it is very thin and accounts for only about

20% of the total lignin.

7−10 nm

3−5 nm

~2 nm

Lignin

Hemicellulose

Cellulose

elementary

fibril

Microfibril

7−30 nm

fig0003 Figure 3 Lignin, cellulose, and hemicellulose composite structure. Reproduced from Lignin. Encyclopaedia of Food Science, Food

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

3538 LIGNIN

0013 The lignification process occurs late in plant cell

life after cell growth has ceased. The lignin is first

deposited within the primary wall at the cell corners.

Then lignification extends along the middle lamella

(completely covering the cell exterior) and into the

thick secondary wall. In oats, immature plants have a

very low lignin content (2% tops, 6% roots) during

their first 42 days. Then, they enter a period of rapid

lignification for about 20 days in which the lignin

content dramatically increases (8% tops, 10%

roots). At the end of 112 days, the lignin content

reaches its highest level (11% tops, 12% roots).

Properties

Physical

0014 Lignin is a noncrystalline solid with a density of about

1.3–1.4 g cm

3

and a refractive index of 1.6. Commer-

cially available lignins, isolated from paper pulp

manufacture, are brown powders. The brown color

probably comes from degradation reactions because

native lignin in some woods is very light in color.

0015 When heated, lignin does not melt, but softens at its

glass transition temperature and transforms from a

glassy solid to a rubbery plastic. Higher glass transi-

tion temperatures (about 193



C dry; 128



C moist)

occur with high-molecular-weight (85 000) lignin,

whereas lower glass transition temperatures (about

127



C dry; 72



C moist) occur with low-molecular-

weight (4300) lignin. Minor lignin components begin

to decompose at about 200



C, with major decompos-

ition occurring at about 300–400



C. The heat of

combustion is about 29.5 MJ kg

1

, which is 67%

greater than that of cellulose and hemicellulose. In

softwoods, lignin is only about 28% of the mass, but

produces approximately 40% of the combustion heat.

0016 No known solvents are capable of dissolving signifi-

cant amounts of lignin from intact plants as it is so

intimately associated with other cell wall components.

However, lignin isolated by extensive ball milling is

soluble in a number of solvents, including pyridine,

acetone, dioxane/water (9:1, v/v), 1,2-dichloroethane/

ethanol (2:1, v/v), 50% aqueous sodium thiocyanate,

and benzyl alcohol/dimethyl formamide. Lignin which

has been subjected to acidic treatment conditions is

soluble in aqueous sodium hydroxide.

Chemical

0017 Lignin bonds can be broken by a number of chemical

reagents, including acids, bases, halogens, nitrates,

oxidants, and reductants.

0018 Acidic conditions are often employed in wood

pulping. Even without acid addition, steaming

spontaneously generates mild acidic conditions (pH

3.6–4.0), primarily from acetic acid released from

hemicellulose acetyl groups. a Ether linkages are

more labile than b ether linkages. In acid, not only

are bonds broken, allowing the lignin to be solubil-

ized, but new bonds are also formed in undesirable

condensation reactions. A classic laboratory method

for identifying lignin is to reflux plants with ethanol/

hydrochloric acid and assay for the degradation prod-

ucts known as ‘Hibbert’s ketones.’

0019Basic conditions are also employed in some pulping

reactions. Ether linkages are broken with the simul-

taneous formation of phenolic hydroxyl groups. In

addition, new bonds are again formed in undesirable

condensation reactions. Lignin methoxyl groups gen-

erally resist alkaline cleavage at temperatures below

200



0020Oxidation of lignin readily occurs using peracetic

acid, chlorine, nitric acid, chlorine dioxide, sodium

hypochlorite, sodium chlorite, hydrogen peroxide,

and ozone. These oxidants selectively attack lignin

(primarily the aromatic ring) while leaving carbohy-

drates largely unaffected. Thus, they are useful as

bleaching agents in the production of high-quality

pulp, or in the laboratory preparation of holocellu-

lose (i.e., cellulose þhemicellulose). Hypochlorite

oxidation is thought to be initiated at a free phenolic

hydroxyl group and adjacent molecules are then

attacked in a ‘peeling’ reaction analogous to alkaline

degradation of cellulose and hemicellulose. Biological

enzymatic cleavage of lignin occurs by oxidation.

Irradiation at short wavelengths (< 385 nm) photoox-

idizes lignin, resulting in darkening, a phenomenon

readily exhibited when a newspaper is left in the sun.

0021Reduction of lignin occurs with hydrogen gas using

Raney nickel catalyst in a mildly acidic or basic aque-

ous solvent at 160–170



C. The product is ‘hydrol

lignin,’ a mixture of low-molecular-weight lignin

fragments. The yield of hydrol lignin is higher under

alkaline conditions.

Occurrence

0022The lignin content in vegetables and fruits is low

(generally < 5%), in agricultural residues it is medium

(7–13%), in hardwoods it is high (16–24%), and in

softwood it is extremely high (26–30%). ‘Compres-

sion wood’ (i.e., from the underside of a horizontal

branch) is higher in lignin, whereas ‘tension wood’

(i.e., from the topside) is lower in lignin.

Chemical Pulping

0023Chemical pulping removes lignin from lignocellulose.

The resulting cellulose fibers may be used for paper

production, or ‘chemical cellulose’ for manufacturing

LIGNIN 3539

cellulose derivatives such as carboxymethyl cellulose.

Pulping conditions tend to be so harsh that hemi-

cellulose is hydrolyzed and removed from the fiber.

Softwoods are generally preferred for paper manufac-

turing because of their long fibers, but hardwoods

and agricultural residues (e.g., bagasse) may also be

pulped. Pulping necessarily requires that the lignin be

cleaved into smaller fragments which may be washed

away from the fiber. Because lignin is insoluble in

water it must be converted into water-soluble deriva-

tives. Advanced organosolv pulping uses an organic

solvent in which the lignin is soluble. Pulping is

unable to remove all lignin, so high-quality pulp is

subsequently bleached with oxidants to remove re-

sidual lignin and color. The two major chemical

pulping technologies are the kraft and sulfite pro-

cesses, with kraft dominating about 96% of the

market.

0024 Kraft pulping is an alkaline process wherein lignin

is removed by the action of sodium hydroxide and

sodium sulfide. The pulping digester operates at

about 175



C for 2–5h. Kraft pulping has replaced

the older soda process, in which sodium hydroxide

was the primary pulping chemical. The addition of

sodium sulfide in kraft pulping promotes ether cleav-

age and inhibits undesirable condensation reactions.

The simplified chemistry can be represented as shown

in eqn (1).

(1)

Sodium lignate

lig

OH +

NaOH

lig

−

+ H

NaOH/Na

lig

O R

lig

+ H

2 R

lig

Some sulfur is also incorporated into the sodium

lignate. The number-average molecular weight is

about 1600 (DP about 8) for softwood sodium lig-

nates and 1050 (DP about 5) for hardwood sodium

lignate. The solubilized sodium lignate and spent

chemicals are present in aqueous solution as ‘black

liquor.’ The black liquor is concentrated and burned

to regenerate the pulping chemicals. Because unre-

covered sodium and sulfur are replenished by adding

sodium sulfate, the process is often called ‘sulfate

pulping.’ Sulfate is not the active pulping chemical,

however, because organic carbon reduces it to sodium

sulfide when the black liquor is burned. The kraft

process produces high yields of strong fibers, hence

the name ‘kraft,’ meaning strong in German.

0025 Sulfite pulping is an acidic process that produces

lower yields than the kraft process, and the fibers are

also weaker. Its advantage is that a greater percentage

of lignin is removed, making the resulting fibers

more suitable for high-quality paper and ‘chemical

cellulose.’ The wood is treated with magnesium

bisulfite and excess sulfur dioxide for 6–12 h at

about 175



C. The simplified chemistry can be repre-

sented as shown in eqn (2).

lig

O R

lig

+ Mg(HSO

)

2 R

lig

−

+ H

Magnesium lignosulfonate

(2)

The magnesium lignosulfonate number-average

molecular weight is typically 10 000–40 000 (DP

45–180), although some fractions may be as high as

22000 000 (DP 100 000). The magnesium lignosulfo-

nate and spent chemicals are dissolved in an aqueous

‘weak red liquor’ which contains about 2–3% sugar

from the degraded hemicellulose. These sugars may

be fed to Candida utilis (also known as Torulopsis

utilis) to make single-cell protein. The spent liquor is

concentrated and burned to produce magnesium

oxide and sulfur dioxide, which are reacted together

to remake magnesium bisulfite.

0026Organosolvpulpingheatslignocellulosewithwater/

solvent/catalyst for about 2 h. Many solvents have

been tried, including mono- and polyalcohols, alde-

hydes, ketones, thio compounds, organic acids and

bases, and dimethyl sulfoxide. Triethylene glycol is a

superior solvent, but its high boiling point (190



makes it difficult to recover. Phenol and butanol are

excellent solvents, but ethanol is preferred because of

its low cost and low toxicity. Various catalysts have

been employed, including hydrochloric acid, sulfuric

acid, ferric chloride, ammonia, aluminum salts, and

urea. If the temperature is raised above 180



C, self-

generated acids serve as a catalyst. The simplified

reaction shown in eqn (3) occurs.

(3)

lig

O R

lig

+ C

lig

= O + HO + R

lig

+ R

lig

O C

Catalyst

Note that some ethanol solvent is incorporated into

the solubilized lignin. Double-bonded carbons have

also been detected in the propane side chain. The

organosolv lignin number-average molecular weight

is about 1000 (DP about 5). Of the three pulping

technologies, the organosolv process produces lignins

most similar to native lignin. Organosolv pulping has

been practiced on an industrial scale in Canada.

Uses

0027In the USA, less than about 2% of the lignin produced

by pulping is used for nonfuel applications. Thus, it

represents an enormous, underutilized resource. Over

3700 literature references (mostly patents) have ex-

plored potential uses, as described in Table 2. The

3540 LIGNIN

applications most relevant to food technology are

binding agents in animal feed pellets and vanillin

production.

0028 Vanillin is one of the most popular food flavors. It

is found throughout the plant world, but generally in

low concentrations. The orchid vanilla bean has an

unusually high concentration (< 5%) and serves as a

natural source. Vanillin can also be manufactured

from lignosulfonate by alkaline hydrolysis or oxida-

tion. The yields are low (< 10%), so other uses must

be found for the unconverted lignin. (See Flavor (Fla-

vour) Compounds: Production Methods.)

0029 Dietary fiber contains lignin because it is very non-

digestible. Lignin adsorbs bile salts and reduces their

intestinal uptake, thus possibly lowering blood chol-

esterol levels.

Lignin Isolation

0030 Milled wood lignin is isolated from plants by breaking

the lignin–carbohydrate complex through extensive

ball milling or vibratory milling for 2–28 days. The

lignin is extracted by dioxane/water (9:1, v/v) with

subsequent purifications in solvents. Approximately

half the lignin can be isolated by this process. Its prop-

erties are very similar to native lignin.

0031Enzyme lignin is prepared by ball milling the

sample for 5–8 h to expose the carbohydrates to

enzymatic attack by extracellular enzyme prepara-

tions, such as cellulase/hemicellulase isolated from

Trichoderma reesei. The residue consists primarily

of lignin with some unreacted carbohydrates, which

can be removed with further ball milling and water

extraction. Because the enzymes selectively dissolve

polysaccharides, the lignin is similar to native lignin.

0032Brauns lignin is prepared by extracting finely

ground plants with 95% ethanol followed by subse-

quent solvent purification steps. The chemistry of

Brauns lignin is very similar to that of native lignin;

however, the molecular weight is lower (generally

< 1000). Less than 10% of plant lignin can be isolated

by this technique.

0033Enzymatically liberated lignin results when pure

cultures of brown rot fungi selectively digest plant

carbohydrates, leaving residual lignin that may be

subsequently extracted with 95% ethanol. It is simi-

lar to Brauns lignin, but the yields are substantially

higher.

tbl0002 Table 2 Potential uses for lignin

Use Kraft

sodiumlignate

Sulfite

ligno-sulfonate

Organosolv

lignin

Polymer applications

Phenol–formaldehyde resin extender for wood adhesives þþ þ

Flame-retardant wood glue þþ þ

Asphalt extender þþ þ

Engineering plastics þ þ

Electrically conducting polymers þ 

Elastic polyurethanes þ þ

Carbon black extender in rubber þ þ

Slow release of chemicals (herbicides, pesticides, pharmaceuticals) þþ þ

Animal fodder pelletizing þ 

Storage battery plate þþ 

Road dust suppression þ 

Ore and mold binder þþ þ

Cement additive þþ 

Liquid additive

Dispersant þþ 

Surfactant þ þ

Tertiary oil recovery þ 

Flocculant þ 

Sequestering agent þþ 

Corrosion inhibitor þ 

Diesel fuel additive  þ

Drilling muds þþ 

Chemical production

Activated carbon þ 

Vanillin þ 

Dimethyl sulfide þþ 

Low-molecular-weight chemicals (e.g., carbon monoxide, hydrogen, methane,

acetylene, ethylene, phenol)

þþ þ

þ, reference found; , no reference found.

LIGNIN 3541

0034 Klason lignin is produced when plant matter is

contacted with 72% sulfuric acid to dissolve polysac-

charides. The residue contains most of the original

lignin, so this procedure is often used to quantify

lignin. However, the lignin has been highly modified

by condensation reactions, making it unsuitable for

studying lignin chemistry.

Lignin Analysis

Gravimetric Methods

0035 Two approaches to gravimetric lignin measurement

are taken: (1) the polysaccharides are removed and

the residual lignin weighed, or (2) the lignin is re-

moved and the residual polysaccharides weighed.

0036 The first approach is used when preparing Klason

lignin. Raw plant matter or acid detergent fiber

(ADF) is soaked in 72% sulfuric acid, which hydro-

lyzes the polysaccharides. The residue is primarily

lignin which is then filtered, dried, and weighed.

Details may be found in the Technical Association

of the Pulp and Paper Institute (TAPPI) methods

T13 or T222, or the American Society for Testing

and Materials (ASTM) method D1106. Approxi-

mately 0.2–0.5% of softwood lignin and 3–5% of

hardwood lignin is soluble in concentrated acid, so

this method (and all other methods) does not indicate

‘true’ lignin content.

0037 The second approach uses either potassium per-

manganate or acidified triethylene glycol to remove

lignin from ADF or uses chlorine compounds to pre-

pare hollocellulose.

0038 The most popular approaches for determining lignin

content are the Klason and permanganate methods

mentioned above. These two methods were compared

using 75 different materials with lignin contents

ranging from 1 to 50%. Although the two measure-

ments were well correlated, the Klason lignin was

about 19% less than the permanganate lignin. Sample

handling can also affect the measured lignin content

because heating may produce ‘artifact lignin’ by pro-

moting Maillard reactions with proteins and sugars.

Thus, it is difficult to measure the ‘true’ lignin con-

tent, so researchers must be satisfied with an oper-

ational definition of lignin. Some feel that the

permanganate assay is the most accurate. (See

Browning: Nonenzymatic.)

Lignin Indices

0039Some assays for lignin measurement do not give the

lignin content directly, but use arbitrary measures

which correlate with lignin. Examples are the k

number (TAPPI standard T236), which measures

potassium permanganate depletion, the chlorine

number (TAPPI standard T202), which measures

chlorine depletion, and the methanol number which

measures lignin methoxyl groups.

See also: Amino Acids: Properties and Occurrence;

Browning: Nonenzymatic; Carbohydrates:

Classification and Properties; Cellulose; Dietary Fiber:

Properties and Sources; Flavor (Flavour) Compounds:

Production Methods; Phenolic Compounds