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

Подождите немного. Документ загружается.

considerable species differences in ability to digest

legume globulins. (See Protein: Quality.)

0008 Purified radiolabeled pea legumin and vicilin are

digested by rats almost as effectively as casein (97.4,

97.1, and 98.5%, respectively). Equally, isolated

lentil globulins are readily degraded (99%) by rats.

The globulins do not therefore appear to be inher-

ently resistant to proteolysis in vivo. Digestibility

values obtained with raw meals or crude protein

extracts are much lower. This is likely to be a result

of interference with proteolysis by the seed matrix or

seed components, such as protease inhibitors. This

would not be a problem with protein isolates

prepared from peas or lentils.

Utilization of Raw Legumes

0009 Weight gains and net protein utilization values

obtained with young rats fed raw peas or lentils as

the sole source of dietary protein (approximately

400 g per kilogram of diet) are low. This is mainly a

result of reduced food intake and increased fecal dry

matter and N and urinary N excretion. Supplemen-

tation of diets with methionine improves food intake,

N retention, and weight gain, but the overall nutri-

tional quality remains below that of high-quality

protein (Table 3).

0010 Inclusion of raw peas at 150–450 g per kilogram of

diet severely impairs food intake and growth by

weanling pigs. The effects are much less marked in

pigs over 5–6 weeks old, and supplementation of diets

with tryptophan or methionine greatly improves

performance. The maximal inclusion levels recom-

mended are 100 g per kilogram of diet for weaner

pigs, 200 g kg

1

for grower pigs and 350 g kg

1

for

finishers. The growth of chicks is severely retarded

by inclusion of up to 200 g of raw pea meal in their

diet. Supplementation with methionine, cystine, and

tryptophan improves performance but only to a

limited extent. Older birds appear more tolerant of

peas, and with laying hens, up to 330 g kg

1

can be

incorporated into the diet without any significant

adverse effects. Addition of supplementary methio-

nine and lipid to the diet allows this incorporation

to be increased to around 440 g per kilogram of diet.

Fecal dry matter and N outputs by human subjects are

increased by intake of 130 g of lentils per day, but

urinary N output is slightly decreased.

Possible Health Benefits

0011Peas are hypocholesterolemic for rats, pigs, and

humans. The mechanisms remain unclear but may be

linked to fiber and saponins in the seeds. With rats, the

effect may be partly a result of reduced lipid depos-

ition by the animals. Lentils are hypocholesterolemic

for rats but not apparently for pigs or humans, at least

at the dietary inclusion levels studied.

Bioactive Factors

0012Peas and lentils contain a number of potentially

bioactive components such as protease inhibitors

and lectins that may influence body metabolism in a

negative or positive manner (Table 4).

Trypsin/chymotrypsin Inhibitors

0013Peas and lentils contain Bowman–Birk-like trypsin/

chymotrypsin inhibitors that inhibit the activity of

these digestive enzymes in vitro and in vivo. These

inhibitors are of a relatively low molecular weight

(around 8–10 kDa) and are double-headed in that

they inhibit two enzyme molecules simultaneously.

The original Bowman–Birk inhibitors (BBI) were

isolated from soyabean and were found to inhibit

tbl0003 Table 3 Nutritive value of peas or lentils for rats after heat treatment and/or L-methionine supplementation

Peas Lentils

Raw Cooked Extruded Raw Cooked

Unsupplemented

N digestibility (%) 80–84 84–86 80–87 80–85 82–86

Net protein utilization (%) 50–60 44–54 60–66 27–33 36–40

Biological value (%) 60–75 55–65 75–83 32–40 43–47

Protein efficiency ratio 1.8–2.4 2.0–2.2 1.9–2.3 0.8–1.6 nd

Amino acid supplemented

Biological value (%) nd nd nd 53–61 69–71

Protein efficiency ratio 2.7–3.3 3.0–3.7 3.1–3.7 2.1–3.0 nd

nd, not determined.

Data from Savage (1988), Savage and Deo (1989, and references therein), and Alonso et al. (2000) Journal of Agricultural and Food Chemistry 48: 2286–2290,

Alonso et al. (2001) Nutrition Research 21: 1067–1077.

PEAS AND LENTILS 4435

only trypsin and chymotrypsin in combination. How-

ever, in other legume seeds, variants capable of in-

hibiting two molecules of trypsin or chymotrypsin are

also present. This heterogeneity in the reactivity pro-

files of BBI-like inhibitors makes it difficult to quan-

tify the amounts present in the seeds or seed products.

Therefore, for comparison purposes, the amount of

each enzyme inhibited by a known amount of product

is usually given. Alternatively, enzyme inhibitory

units are calculated from the same data.

0014 Trypsin and chymotrypsin inhibiting activity in

peas and lentils varies greatly between cultivars

(Table 4). Assuming that the mix of inhibitor variants

leads to an average profile of one molecule trypsin

and one molecule chymotrypsin inhibited per mol-

ecule inhibitor, available data suggest that peas con-

tain 0.4–2.8 g of BBI-like inhibitor per kilogram and

lentils 0.15–1.4 g kg

1

. This is comparable with the

levels found in many other legumes. Soyabean has

3.2–3.7 g of BBI per kilogram. However, it also con-

tains 4.5–9.0 g per kilogram of Kunitz trypsin inhibi-

tors. Thus, the total trypsin inhibiting activity in

soyabean is far higher than that in peas or lentils.

0015 BBI are highly resistant to proteolysis in vitro.

Nonetheless, native BBI and BBI-type inhibitors

seem to be readily degraded in vivo, although

125

I-BBI seems to be highly resistant to proteolysis

in vivo. Native inhibitors have limited effects on di-

gestion and absorption of dietary protein and growth

of animals, even when they are included at high levels

(10 g kg

1

) in the diet. However, BBI or BBI-like

inhibitors interfere with cholecystokinin-mediated

control over pancreas function and trigger hyper-

secretion of pancreatic trypsin, chymotrypsin and a-

amylase in rats, chicks, quails, and humans. In rats,

chicks, and quails, this leads to pancreas enlargement,

mainly as a result of hyperplasia and hypertrophy of

the acinar cells and, in the long term, may lead to

tissue dysfunction and disease. The effects of the

inhibitors are, however, species-specific. Thus, they

induce pancreas growth in young rats, mice, ham-

sters, guinea-pigs, and chickens but have little or no

effect on the pancreas of young pigs, cattle, monkeys,

or dogs.

0016Pancreatic enlargement is evident in rats fed pea

diets, although the increase is usually much less than

observed in rats given an equivalent intake of raw

soyabean. Pancreatic growth was not apparent in

rats fed lentil meal, although an inhibitor-enriched

fraction of lentil meal did cause enlargement.

0017Consumption of soyabean BBI by experimental

animals appears to significantly reduce the incidence

and severity of liver, colon, and mammary cancers

that develop as a result of treatment with chemical

carcinogens or radiation. The mechanisms by which

this occurs, however, remain unclear. BBI acting

through localized effects on gut endocrine cells may

induce the release of a number of hormones, growth

factors, or peptides that interfere with tumor cell

metabolism. Alternatively, bioactive fragments of

BBI absorbed from the gut may be the main protective

agents. The BBI-like inhibitors of field bean have also

been shown to have cancer-preventing properties. It is

thus possible that pea and lentil inhibitors will have a

similar protective effect. (See Trypsin Inhibitors.)

a-Amylase Inhibitors

0018a-Amylase inhibitors inhibit the activity of salivary

and pancreatic amylase in vitro and in vivo. They can

impair the growth and metabolism of animals when

given at high levels in the diet but may have beneficial

uses in treatment of obesity or diabetes. They are

generally present in very high amounts in Phaseolus

species (kidney beans, 4.3 g of inhibitor per kilo-

gram). However, the levels in peas and lentils are

very low (Table 4) and unlikely to contribute sig-

nificantly to the effects of these legumes on me-

tabolism.

Lectins

0019Lectins are defined as carbohydrate-binding proteins/

glycoproteins other than enzymes that are present in

most plant materials. They are highly resistant to

proteolytic degradation in vivo and survive passage

tbl0004 Table 4 Bioactive factors present in raw legume seeds

Peas Lentils Soyabeans

Trypsin inhibitor (g kg

1

)

1–8 0.1–4 16.7–27.2

Chymotrypsin inhibitor (g kg

1

)

0.5–6 0.1–2 0.2–4.9

a-Amylase inhibitor (g kg

1

)

<0.1 <0.1 <0.1

Lectin (U  10

6

1

)

5.1–15.1 1.8–7.7 50–350

Lectin (g kg

1

)

0.3–0.7 *0.8 2–4

Phytic acid (g kg

1

) 2.2–8.2 1.2–8.1 10.0–14.7

Tannin (g kg

1

) 0.2–5.0 0.6–2.7 0.2–0.9

Phenolics (mg kg

1

) 13–27 10–30 8–15

Saponin (g kg

1

) 1.1–2.5 1.1–4.6 2.2–6.7

Trypsin and chymotrypsin inhibitor is given as the number of grams of

enzyme that would be inhibited per kilogram of meal.

a-Amylase inhibitor is expressed as inhibitor equivalents using pure

kidney bean a-amylase inhibitor as standard.

Lectin levels are given as hemagglutinating units (U) per kilogram of

meal based on the ability to agglutinate rabbit blood cells or as g kg

1

based on recoveries obtained during purification of lectin.

Data from Trowbridge (1974) Journal of Biological Chemistry 249: 6004–

6012, Savage (1988), Savage and Deo (1989), Liener (1991) Journal of the

American Oil Chemists Society 56: 121–129, Anderson and Wolf (1995)

Journal of Nutrition 125: 581–588, Huisman and Jansman (1991) Nutrition

Abstracts and Reviews (Series B) 61: 902–921, Alonso et al. (1998) Food

Chemistry 63: 505–512, Alonso et al. (2000) Journal of Agricultural and Food

Chemistry 48: 2286–2290, Alonso et al. (2001) Nutrition Research 21: 1067–

1077, Cuadrado et al. (1999), Grant (1999), and Grant et al. (1999).

4436 PEAS AND LENTILS

through the gastrointestinal tract. If appropriate

carbohydrate receptors are present on gut epithelial

cells, lectins bind to them and may be taken up sys-

temically. As a result, lectins can potentially interfere

with and modify many aspects of gut and systemic

metabolism. Individual lectins vary greatly in their

effects in vivo, but most species appear to be respon-

sive to dietary lectins. (See Hemagglutinins (Haemag-

glutinins).)

0020 Lectins can be separated into eight general categor-

ies on the basis of their carbohydrate-binding

specificity: complex, fucose, galactose, N-acetyl-

glucosamine, mannose, mannose/glucose, mannose/

maltose, and sialic acid. Some, such as soyabean

agglutinin (galactose-specific), alter gut and pancreas

metabolism in rats, in particular causing rapid

growth of these tissues, without affecting systemic

systems. A few, including kidney bean lectin (complex

specificity), have additional effects on systemic hor-

mone balance and lipid and muscle metabolism and

can be very deleterious if consumed in high amounts.

Others, such as pea and lentil lectins (glucose/

mannose-specific) appear to have little or no effect

on the body metabolism of rats.

0021 The sensitivity of animals to lectins may, however,

vary with species, age, period of exposure, gastro-

intestinal bacteria, diet composition, and dietary

history. Thus, the glucose/mannose-specific jack

bean lectin (Con A) has no effect on mature germ-

free rats but has limited effects (causes small intestine

and pancreas enlargement) in specific pathogen-free

rats. It is, however, highly deleterious to rats carrying

a salmonella infection, to suckling guinea-pigs, and to

quails. The glycoconjugates expressed on the gut sur-

face of very young animals differ greatly from those in

mature counterparts; in particular, a high proportion

of mannose residues are present. This is also evident

in rats with a pathogen infection. In these circum-

stances, lectins that would not normally affect the

gut may be able to bind to it and elicit changes in

body metabolism.

0022 The levels of lectins in peas and lentil are low

compared with that in soyabean (Table 4) and very

low by comparison with kidney bean (15–30 g kg

1

Furthermore, in studies with mature rats, these lectins

have no significant effects on metabolism. This would

suggest that they are unlikely to cause problems.

However, in view of the data with Con A and the

number of factors that influence the sensitivity of an

animal to lectin, one cannot exclude the possibility of

specific circumstances where these dietary lectins

have profound effects.

0023 Young chicks do not do well when raw peas are

added to their diet in low amounts but will tolerate

quite high dietary inclusions if they are a few weeks

old. During this period, the gut develops from its very

immature form at hatching to its adult form. The gut

may be susceptible to the action of pea lectin at the

early stages of the maturation period.

Antigenic Proteins

0024Native 11S globulins (glycinin) and 7S globulins (con-

glycinin) of soyabean induce very adverse immune

reactions in preruminant calves and newly weaned

piglets, leading to gut damage, scouring, and poor

performance. Pea globulins partially survive gut pas-

sage and can trigger some immune responses in pre-

ruminant calves. However, the degree to which this

occurs is very much lower than that observed in

soyabean-fed animals.

0025Lentils have been linked to allergy problems in a

small number of pediatric patients. A number of

possible allergens have been identified, including

subunits of vicilin. The incidence of intolerance to

pea proteins seems to be low.

Phytate

0026Phytic acid is often the main reserve of phosphorus in

legumes. However, it also chelates with minerals and

metals, such as calcium, magnesium, zinc, and iron,

forming insoluble salts that are not readily absorbed

by animals or humans. In particular, it can severely

impair availability of zinc and iron. Phytate can

also complex with proteins and may thereby reduce

digestibility or enzyme activity.

0027Mineral uptake by pigs and chickens fed with soya-

bean-based diets is slightly impaired. Addition of

phytase, a phytate-degrading enzyme, to the diet

appears to counteract this effect, leading to an

improvement in mineral uptake and better overall

performance by the animals. However, the efficacy

of this treatment can be very variable.

0028Phytic acid levels in peas and lentils are lower than

those in kidney beans (11–17 g kg

1

) and soyabean

(Table 4). None the less, they can still affect mineral

metabolism, since iron absorption from a pea protein-

based infant formula is significantly enhanced after

enzymatic degradation of phytate.

0029Pea and lentil phytate can clearly have adverse

effects on mineral uptake and body metabolism.

However, in many cases, their impact is likely to be

minimal because the mineral content of the diets is

well above the requirements. Pea and lentil phytate,

however, may cause significant problems if mineral

intake, particularly of zinc and iron, is close to or

below requirements.

0030Dietary phytic acid may have health-promoting

properties. It can inhibit a-amylase, limit carbohy-

drate digestion, and lower blood glucose. There

PEAS AND LENTILS 4437

are also indications that it is hypocholesterolemic and

protective against colon cancer. However, the amounts

required are quite high. It is unclear whether a normal

physiological intake of peas or lentils would provide

sufficient phytate to have a significant health-protect-

ive effect. (See Phytic Acid: Nutritional Impact.)

Tannins

0031 Tannins are present in a wide array of plant crops.

Legume condensed tannins are oligomers of variously

substituted flavan-3-ols, and their antinutritional

effects have recently been comprehensively reviewed.

These compounds can reduce enzyme activity in the

gut, impair gut morphology, lower nutrient (protein,

carbohydrate and lipid) digestion and absorption,

reduce mineral uptake, and greatly stimulate excre-

tion of endogenous N. Thus, at high dietary intakes,

they can adversely affect animal performance.

0032 In contrast to the antinutritional effects, dietary

flavan-3-ols have also been suggested to have import-

ant roles in disease prevention, particularly of cardio-

vascular diseases and some forms of cancer. They may

act as antioxidant and free radical scavengers, inhibit

tumor initiation and promotion, and have antibacter-

ial and angioprotective properties.

0033 The tannin contents of peas and lentils tend to be

higher than that in soyabean (Table 4). However, the

levels are comparable with those in many other

legume seeds. Lentils contain moderate amounts of

catechins and proanthocyanidin dimers and trimers,

whereas pea samples tend to have low amounts. The

compounds may have health-protective effects. How-

ever, since questions still remain as to how effectively

these compounds or products derived from them are

absorbed from the gut and distributed throughout the

body, it is difficult to assess whether a normal intake

of peas or lentils will provide sufficient flavanols to

have a significant beneficial effect. (See Tannins and

Polyphenols.)

Saponins

0034 Triterpenoid saponins, found in leguminous plants,

are composed of a triterpene aglycone linked to one,

two, or three saccharide chains of varying size and

complexity. The levels in peas and lentils are similar

to, or slightly lower than, those in soyabean (Table 4)

and other legume seeds. They may reduce weight gain

if consumed at very high levels. However, at the levels

present in peas, lentils, and soyabeans, they are con-

sidered to have no significant antinutritional effects.

Saponins, however, may have beneficial effects. They

have been found to be hypocholesterolemic in a

number of species, owing in part to their ability to

facilitate adsorption of bile acids to dietary fiber.

Evidence of their efficacy in humans is not conclusive.

It has also been suggested that saponins may

have anticarcinogenic or antioxidant properties. (See

Saponins.)

Fiber

0035Peas and lentils contain high levels of crude and total

dietary fiber (Table 1). The fibers have a high binding

capacity for bile acids in vitro and also promote fer-

mentation in the hind-gut and production of butyrate

in vivo. As with other dietary fibers, pea and lentil

fibers are generally considered to be beneficial. How-

ever, the specific effects of these dietary fibers on

metabolism remain unclear. Inclusion of pea or lentil

fibers in diets appears to lower postprandial blood

triglyceride levels without affecting cholesterol con-

centrations. However, other studies suggest that pea

fibers can influence cholesterol levels in humans.

Equally, studies show that pea fibers reduce or delay

starch absorption and reduce the blood glucose peak,

whereas others show no effect on blood glucose. This

variability may be a result of differences in the test

meals used or the nutritional/health status of the sub-

jects. Pea fibers are used in the treatment of hyper-

cholesterolemic patients. However, they are not used

alone but given as part of a mixture of dietary fibers.

(See Dietary Fiber: Properties and Sources.)

Starches

0036Legume starches in general tend to be less digestible

or more slowly digested than corn starches. This slow

release means that their glycemic index is low. They

can thus be useful in diets for those with impaired

carbohydrate tolerance. Lentils have been found to

have beneficial effects on blood glucose profiles of

diabetic patients. This is due, at least in part, to the

slow release property of the constituent starch com-

bined possibly with the effects of the fiber. (See

Starch: Structure, Properties, and Determination.)

Vitamins and Minerals

0037Peas and lentils contain significant amounts of im-

portant vitamins and minerals. The levels of most are

fairly similar in both seeds. However, lentils appear to

contain higher levels of ascorbic acid, folic acid and

vitamin B

than peas but have considerably less

vitamin A. (See Vitamins: Overview.)

Effects of Processing

0038Various processing methods are routinely used in

the preparation of peas and lentils for consump-

tion. These include dehulling, soaking, germination,

cooking, autoclaving, extrusion, and fermentation.

4438 PEAS AND LENTILS

0039 In general, dehulling or soaking seems to have

limited effects on the levels of bioactive factors or

the nutritional quality of the seeds. Dehulling of lentil

increases the solubility of lentil proteins, and soaking

slightly reduces trypsin inhibitor and phytate content.

Germination lowers phytate and slightly reduces

trypsin inhibitor, tannin and polyphenol levels in

peas, and lowers trypsin inhibitor and phytate in

lentil. Nonetheless, it may also increase levels of non-

protein amino acids. Natural fermentation also re-

duces the levels of bioactive compounds and slightly

improves the starch and protein availability from

legumes. The efficacy of this treatment is, however,

extremely variable, partly because the environmental

factors responsible are unknown.

0040 Cooking, autoclaving, or extrusion treatment of

peas fully inactivates trypsin inhibitors and lectins,

has limited effects on phytate and polyphenols, but

greatly reduces tannin concentrations. Starch digest-

ibility is significantly increased, but protein digestibil-

ity is improved only to a limited extent (Table 3). The

effects of cooking on lentils appear similar, with the

exception that the reduction in tannins and poly-

phenols may be more pronounced than that observed

with peas. Prolonged thermal treatment (cooking,

autoclaving, or extrusion) can adversely affect

nutrient availability from peas and lentils.

0041 Thermal treatment of peas and lentils clearly im-

proves the nutritional value of the constituent pro-

teins (Table 3). With rats, this is evident only after

supplementation of the diet with methionine to bring

the sulfur amino acid content up to the requirements

for rats. The human need for sulfur amino acids is

much lower than that of rats and could be met from

the levels present in peas and possibly lentils (Table 2).

For humans, the biological value of cooked peas or

lentils alone may thus be close to that found for the

supplemented diets in rats.

0042 Pea-based diets are hypocholesterolemic for rats.

This property is retained even after extrusion treat-

ment of the seeds. Some lentil proteins retain their

antigenicity, even after cooking.

Digestibility of Heat-treated Globulins

0043 The moderate digestibility of raw legume proteins has

mainly been attributed to the presence of antinutri-

tional factors in the seeds and the refractory nature of

the proteins. However, although heat-treatment abol-

ishes the activity of the main antinutritional factors, it

appears to have remarkably little effect on the digest-

ibility of pea or lentil proteins. In addition, pea and

lentil globulins have a very high digestibility in vivo,

once isolated from the seeds. The proteins are thus

not inherently indigestible.

0044It is possible that interactions between nonprotein

components of the seed matrix and the constituent

proteins may be preventing digestion of a proportion

of the proteins. Heating may not abolish these inter-

actions. Alternatively, globulin-like proteins from

some heat-treated legumes have been shown to be less

digestible than counterparts from raw seeds. This may

be due to the formation of large protein multimers that

are not readily accessible to digestive enzymes. Any

beneficial effects of heat treatment of seeds on protein

availability may be negated, in part, by the develop-

ment of these poorly digestible multimers of the

globulin proteins. This merits further study.

Summary

0045Peas and lentils are potentially good sources of diet-

ary protein and carbohydrate and micronutrients.

They contain a range of bioactive compounds that

may be detrimental to consumers. However, some of

the bioactive compounds may also be beneficial for

health. Thermal treatment greatly improves the nutri-

tional quality of the seeds. Both raw and processed

seeds have hypocholesterolemic properties.

See also: Carbohydrates: Interactions with Other Food

Components; Cholesterol: Factors Determining Blood

Cholesterol Levels; Dietary Fiber: Physiological Effects;

Effects of Fiber on Absorption; Extrusion Cooking:

Principles and Practice; Chemical and Nutritional

Changes; Food Intolerance: Types; Hemagglutinins

(Haemagglutinins); Legumes: Legumes in the Diet;

Dietary Importance; Phytic Acid: Properties and

Determination; Nutritional Impact; Plant Antinutritional

Factors: Detoxification; Protein: Interactions and

Reactions Involved in Food Processing; Trypsin

Inhibitors

Further Reading

Bardocz S, Hajos G and Pusztai A (eds) (1999) Cost 98 –

Effects of Antinutrients on the Nutritional Value of

Legume Diets, vol VI. Luxemburg: Office for Official

Publications of the European Communities.

Carbonaro M, Grant G, Cappelloni M and Pusztai A

(2000) Perspectives into factors limiting in vivo digestion

of legume proteins: antinutritional compounds or

storage proteins? Journal of Agricultural and Food

Chemistry 48: 742–749.

Castell AG, Guenter W and Igbasan FA (1996) Nutritive

value of peas for nonruminant diets. Animal Feed

Science Technology 60: 209–227.

Caygill JC and Mueller-Harvey I (eds) (1999) Secondary

Plant Products. Antinutritional and Beneficial Actions

in Animal Feeding. Nottingham, UK: Nottingham

University Press.

PEAS AND LENTILS 4439

Jansman AJM, Hill GD, Huisman J and van der Poel AFB

(eds) (1998) Recent Advances of Research in Anti-

nutritional Factors in Legume Seeds and Rape Seeds.

Wageningen, The Netherlands: Wageningen Pers.

Martinez San Ireneo M, Ibanez Sandin MD and Fernandez-

Caldas E (2000) Hypersensitivity to members of the

botanical order Fabales (legumes). Journal of Investiga-

tional Allergology and Clinical Immunology 10: 187–

199.

Rowland I (1999) Optimal nutrition: fibre and phytochem-

icals. Proceedings of the Nutrition Society 58: 415–419.

Sanchez-Monge R, Pascual CY, Diaz-Perales A et al. (2000)

Isolation and characterisation of relevant allergens from

boiled lentils. Journal of Allergy and Clinical Immun-

ology 106: 955–961.

Santos-Buelga C and Scalbert A (2000) Proanthocyanidins

and tannin-like compounds. Nature, occurrence, dietary

intake and effects on nutrition and health (review).

Journal of the Science of Food and Agriculture 80:

1094–1117.

Savage GP (1988) The composition and nutritive value of

lentils (Lens culinaris). Nutrition Abstracts and Reviews

(Series A) 58: 319–343.

Savage GP and Deo S (1989) The nutritional value of

peas (Pisum sativum). A literature review. Nutrition

Abstracts and Reviews (Series A) 59: 65–88.

Urbano G, Lopez-Jurado M, Aranda P et al. (2000) The

role of phytic acid in legumes: antinutrient or beneficial

function? Journal of Physiology and Biochemistry 56:

283–294.

van der Poel AFB, Huisman J and Saini HS (eds) (1993)

Recent Advances of Research in Antinutritional Factors

in Legume Seeds. Wageningen, The Netherlands:

Wageningen Pers.

Pecans See Walnuts and Pecans

PECTIN

Contents

Properties and Determination

Food Use

Properties and Determination

L Flutto, Danisco, New Century, KS, USA

Introduction

0001 Pectin is a high-molecular-weight carbohydrate poly-

mer which is present in virtually all plants where it

contributes to the cell structure. The term pectin

covers a number of polymers which vary according

to their molecular weight, chemical configuration,

and content of neutral sugars, and different plant

types produce pectin with different functional prop-

erties. The word ‘pectin’ comes from the Greek word

pektos which means firm and hard, reflecting pectin’s

ability to form gels.

0002 The gelling properties of pectin have been known

for centuries, but the isolation of commercial pectin

only started at the beginning of the twentieth century.

In this document we highlight the chemistry, origin

and production, and the functional properties of

pectin.

Chemistry

The Homogalacturonic Acid Backbone

0003Pectin consists of a chain of galacturonic acid units

which are linked by a-1,4 glycosidic bonds. The

galacturonic acid chain is partly esterified as methyl

esters. Pectin molecules can have a molecular weight

of more than 200 000, corresponding to a degree of

polymerization up to 1000 units (Figure 1).

0004Though the esters are the most significant compon-

ents on the galacturonic acid backbone, other chem-

icals, such as acetyl, can be important in specific

pectin types. Commercial pectin can also be partly

4440 PECTIN/Properties and Determination

amidated with ammonia to form galacturonamide

units in the molecular chain.

0005 Some neutral sugars are also included in the homo-

galacturonic backbone. This is the case for rhamnose

but also specifically in apple pectins for xylose.

0006 The percentage of galacturonic acid of the whole

molecule is defined as the galacturonic acid content

(% GA), which is set at a minimum of 65% in the

definition of pectin as a food additive.

0007 The percentage of esterified or amidated galacturo-

nic acid units of the total number of galacturonic acid

units in the molecule are respectively defined as the

degree of esterification (DE) and the degree of amida-

tion (DA) (Table 1). Regulations limit the DA to a

maximum of 25%.

Esterification Pattern

0008 In addition to the number of components on the

backbone, their position is of significant importance

to the functional properties of pectin. The distribu-

tion of esters is of special importance as it affects the

local electrostatic charge density of the polymer and

so its interaction with other charged molecules,

whether ions such as calcium, proteins, or other

pectin molecules.

0009 In apple pectin subjected to a mild extraction pro-

cess, the ester distribution is reported to be almost

random, while citrus pectin tends to have a somewhat

blockwise distribution.

Neutral Sugars

0010 Pectin always contains varying amounts of neutral

sugars such as d-galactose, l-rhamnose, l-arabinose,

and d-xylose. Some of these neutral sugars are con-

stituents of side chains to the galacturonan backbone.

However, 1,2-linked l-rhamnose is present in the

main polygalacturonic chain where it forms ‘kinks’

in the molecular chain. Xylose is also a very import-

ant neutral sugar in apple pectin where it is attached

in the homogalacturonic backbone.

0011It is well documented that neutral sugar side chains

are concentrated in relatively short segments of the

galacturonic backbone, described as ‘hairy regions.’

The part of the molecule free of side chains is

described as the ‘smooth region.’ (Figure 2).

Chemical and Physical Properties

0012Solubility Pectin is soluble in water, but insoluble

in most organic solvents. The solubilization rate in

water is related to the degree of polymerization and

the number and distribution of methyl-ester groups.

The pH, temperature, and ionic strength of the solu-

tion are of great importance to the rate of pectin

dissolution. The calcium content of the water used

to dissolve the pectin is of special relevance as it is

common that a high-water hardness will translate

into an incomplete dissolution of pectin.

0013Rheology of pectin solutions Pectin solutions are

viscous but pectin is not an especially effective visco-

sifier compared to other gums such as guar gum.

0014Dilute pectin solutions are almost Newtonian, and

they are only slightly affected by the presence of

calcium. However, solutions with more than 1%

pectin exhibit pseudoplastic behavior and are strongly

affected by calcium. There is a continuum in texture

starting with water through thixotropic solutions

with yield value to stiff gels depending on the pectin

type and concentration, level of calcium, and pH.

0015Stability High-ester pectins are stable at pH levels of

2.5–4.5. Above a pH level of 4.5, b-elimination will

occur, depolymerizing the galacturonic acid chain.

This mechanism requires an esterified carboxyl

group next to the glycosidic bond to be cleaved

(Figure 3), so low-ester pectins are more stable at

higher pH values.

0016The pectin molecular structure is quite resistant

to heat. At pH around 3.5 pectin is only marginally

depolymerized athigh temperatures. The heat-stability

COOH

COOCH

COOH

COOCH

fig0001 Figure 1 Pectin consists of long sequences of anhydro galacturonic acid completely or partly esterified with methanol.

tbl0001 Table 1 Classification of pectins

DE > 50% DE < 50%

Low-ester pectin (LE)

High-ester pectin (HE) No amidation DA < 25%

Conventional LE Amidated LE

DE, degree of esterification; DA, degree of amidation.

PECTIN/Properties and Determination 4441

of pectin is greatly improved when the water activity

of the system is lowered through the addition of

sugar.

Origin and Production

0017 Pectin is a natural component of plants, predomin-

antly in the form of pectic substance or protopectin,

which is not soluble in water. The pectic substance is

an essential part of the plant cell wall structure, acting

as a cement for the cellulosic network and a hydrating

agent. The exact nature of the pectic substance is not

completely understood. It is, however, generally rec-

ognized that it is a complex structure in which pectin

is attached to other cell wall components such as

cellulose, hemicellulose, and proteins by covalent

bonds, hydrogen bonds, and/or ionic interactions.

In the plant the residual carboxyl groups are partly

neutralized with cations of calcium, potassium, and

magnesium which are present in the plant tissues.

0018Pectin is today commercially produced mostly

from apple pomace and citrus peels by an extraction

process followed by separation, purification, isol-

ation, and then drying, milling, and standardization

(Figure 4).

Functional Properties

0019Pectins are used in a broad variety of food and

pharmaceutical products. Their principal properties

are gel formation with both high- and low-ester

pectins, viscosity build-up, and protein stabilization

with high-ester pectins. As methods develop for

obtaining a better understanding of the pectin mol-

ecular structure, it is likely pectin will be attributed

new functional properties in the future.

fig0003 Figure 3 b-elimination mechanism.

COOCH

COOH

Galacturonic acid

Methylated galacturonic acid

Rhamnose

Hairy region

Smooth region,

homogalacturonan

Arabinose or galactose

Rhamnogalacturonan

Xylose (high content in apple pectin)

Acetyl group (high content in sugar

beet pectin)

fig0002 Figure 2 Primary and secondary structure of pectin.

4442 PECTIN/Properties and Determination

Gelling of High-Ester Pectins

0020 The ability of pectin to form gels in specific condi-

tions has long been used in the production of jams

and preserves. The separation of high- and low-ester

pectins in accordance with the 50% esterification rule

proves somewhat arbitrary when dealing with gelling

mechanisms. It is clear that the dominant factor in the

gelling of pectin is highly dependent on its degree of

esterification but the complex network structure is

most often the result of a combination of several

mechanisms. The statement that high-ester pectins

will gel with sugars and acid, while low-ester pectins

gel with calcium, is certainly valid but certainly cal-

cium can alter the gelling of high-ester pectins, just as

pH and soluble solids will affect low-ester pectin

gelling.

Gelling Mechanism

0021 It is generally accepted that a high-ester pectin gel is

formed by the cross-linking of the polymer in junction

zones, in which mainly hydrogen bonds but also

hydrophobic attractions between the methyl-ester

groups play a part. Calcium bridges may also partici-

pate, especially if the esters are distributed in blocks,

leaving large parts of the molecule as free acids.

0022 Gelling will occur upon cooling of a media where

favorable conditions are met. Cooling is necessary to

decrease molecular movement and permit the forma-

tion of intermolecular interactions.

0023 As the pectin chains carry negative charges they

will tend to repel each other. This repulsion will

depend on the charge density of the molecule which

can be directly correlated to the pH of the medium

and the frequency of free galacturonic acids of the

polymer. The higher the pH and the lower the degree

of esterification, the higher the charge density and

hence the stronger the repulsion.

0024This repulsion, but more importantly the impossi-

bility of forming hydrogen bonds between ionized

pectin chains, are the reasons why a low pH is re-

quired for high-ester pectin gelling. At a low pH,

typically below 3.6, the repulsion is low enough for

the distance between the pectin chains to be reduced

sufficiently and hydrogen bonding can occur. In order

to achieve sufficient hydrophobic interactions to sta-

bilize the molecular network, the water activity of the

system also has to be decreased. Sugars are usually

added for this purpose.

0025So, typically, high-ester pectins will only form gels

when the pH is below 3.6 and soluble solids are above

55% (Figure 5).

0026Upon storage of a cooled gel, it is typical that the

texture will still develop into a stronger final gel. This

corresponds to a slow reorganization of the network

involving the creation of new junction zones or an

enlargement of the existing junctions between the

pectin molecules.

Parameters Influencing Gelling

0027The gelling and final gel structure of high-ester

pectins is influenced by a great number of parameters,

the main ones being the pectin concentration, the

degree of esterification, molecular weight, acetylation

and branching of the pectin molecule and the pH,

ionic strength, water activity, sugar type, and cooling

rate of the gelling medium.

0028Pectin concentration The concentration of high-

ester pectin will increase the final gel strength of the

system due to the increase in the number of junction

zones, increasing the number of chains with elastic

activity. An increase in molecular weight would have

the same effect. In addition to this expected effect, the

gelation rate is also increased with increasing pectin

concentration and a power law can be calculated

between the two parameters.

0029Degree of esterification The degree of esterification

of the galacturonic acids affects both the charge dens-

ity of the polymer and the number of sites for hydro-

phobic interaction. As pectin molecules with a high

degree of esterification are less charged, they can

form gels at a higher pH and will also start gelling

at a higher temperature.

Extraction

Separation

Purification

Liquid concentration

Precipitation

Drying and grinding

Standardization

LC pectin

HE pectin

LA pectin

Amidation

Ammonia

Peel or pomace

Water

Acid

(added before or

after precipitation)

Alcohol

fig0004 Figure 4 Pectin manufacture flow sheet. HE, high-ester; LC,

low ester conventional; LA, low ester amidated.

PECTIN/Properties and Determination 4443

0030 This last effect forms the basis for the classification

of high-ester pectins into rapid-set, medium-rapid-

set, slow-set, and extra-slow-set pectins as the degree

of esterification is decreased from more than 70% to

50% (Figure 6).

0031 The distribution of the ester groups on the back-

bone will also affect pectin gelation, as a marked

blockwise distribution of esters will result in a signifi-

cant contribution of calcium gelling. This contribu-

tion of calcium gelling will significantly increase the

gelling temperature of the pectin (Figure 7).

0032 Acetylation and branching Acetylation sharply

decreases the gelling ability of pectin as the size of

the acetyl groups does not allow the pectin chains

to come close enough for interaction between the

molecules.

0033The effect of neutral sugars on high-ester pectin

gelling can be twofold and would need to be studied

more extensively. Neutral sugars present on the pectin

molecule could result in steric hindrance of inter-

molecular interaction and thus decrease the ability

of the pectin to form gels. However, they could also

participate in gelling through hydrophobic inter-

action and contribute to an increased cohesion of

the gel.

0034pH and ionic strength of the gelling system The

lower the pH, the lower the repulsion between the

pectin molecules and, thus, the easier it will be for

them to interact. This means that a low pH will lead

to faster gelling in high-ester pectins. However, below

a critical level, the gel strength will be reduced as the

gelling is too fast to obtain a well-organized polymer

3.0

100

120

3.2 3.4

Jelly pH

% obtainable firmness

3.6 3.8

4.0

DE = 63

DE = 73

DE = 81

fig0006 Figure 6 High-ester pectin: the degree of esterification (DE) determines the optimal pH for the gelation.

Independent molecules in solution

Hydrogen bonding

Gel structure

Junction zone

Sucrose

Hydrophobic

interaction

fig0005 Figure 5 High-ester pectin gels through hydrogen bonding and hydrophobic interactions in an acidic water and sugar matrix.

4444 PECTIN/Properties and Determination