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

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Important Biochemical Composition

Acid Content

0010 Glycolic, malic, ascorbic, and citric acids were the

major organic acids detected in guavas. Also present

is fumaric acid. Total acid level calculated as citric

acid ranges from 0.2 to 1.1% fresh weight. The level

of ascorbic acid is quite substantial, those of malic

and glycolic acids are relatively low, while fumaric

acid is only found in trace amounts. The presence of

these acids in guava is responsible for the tart flavor

and also for its relatively low pH of 3.2–4.1. (See

Acesulfame/Acesulphame.)

Flavor

0011 Flavor and aroma vary widely among the different

cultivars. There are low-acid, sweet types, bland types

that are low in both sugars and acidity, and high-acid

types. The musky aroma is more pronounced in the

fully ripened, low-acid, sweet types. The flavor of

guava is determined by the types and amounts of

sugars, acids, phenolics, and volatile compounds

that are present in the fruit. The proportions of

these chemical constituents may vary with the fruit

age and with the cultivar. Depending on the cultivar,

these flavor compounds may accumulate to different

extent during ripening and thus may result in guava

fruits having a distinctive aroma and taste.

0012 The volatile flavor constituents of guava have been

isolated and separated. There are 22 compounds

in Hawaiian guavas, of which methyl benzoate,

hexanol, p-phenylethyl acetate, methyl cinnamate,

cinnamyl acetate, and b-ionone are believed to play

predominant roles in the flavor and odor of the fruit.

More recently, 154 compounds were identified, of

which 116 were described in guava for the first

time. Later on, 122 volatile compounds in guava

were identified. Among the identified compounds

were 13 aldehydes, 17 ketones, 31 alcohols, 10

acids, 28 esters, 10 hydrocarbons, and 13 miscellan-

eous compounds.

Tannins

0013 Tannins or polyphenols are another group of com-

pounds that have been considered in many fruits and

lately in guavas. The function of these compounds in

plants is unknown, but their ability to combine with

proteins has often caused problems for food scientists.

Their role in the browning of fruit and fruit products,

and a cause of cloudiness in many fruit juices, has long

been recognized. Tannins have also been associated

with astringency in many fruits, especially in less ripe

ones. Changes in polyphenols and polyphenoloxidase

activity have been reported during fruit development

in guavas. There were decreases in seven categories of

phenolics (total phenols, nontannin phenols, tannin

phenols, hydrolyzable tannins, simple phenolics, non-

tannin flavans, and condensed tannins) during

ripening; insoluble leucoanthocyanins were the

major components. At the mature stage, polypheno-

lase activity was barely detectable in both pink and

white-fleshed guavas. However, at the ripe and fully

ripe stages, polyphenolase activity increased dramat-

ically, and this change was stated to be responsible for

the decrease in polyphenolics during ripening and the

subsequent decline in astringency. (See Dietary Refer-

ence Values; Tannins and Polyphenols.)

Pectin

0014Several workers demonstrated the potential of guava

as a source of pectin. Pectic substances are contained

in the intercellular spaces of cell walls where they

function as bonding and lining materials. The signifi-

cance of pectin is in contributing to the rigidity of

plant tissues. The pectin content in guavas is highest

in the mature stage. The value decreases in the ripe and

overripe stage. The methoxyl content is an important

factor in evaluating the gelling property of pectin. The

values obtained from fresh white and pink cultivars

were 3.23% and 3.03% respectively. The equivalent

weight of guava pectin varied from 2181 to 3173,

which is considerably higher than that reported for

mangoes. The presence of pectin in guava impart a

viscous property to the guava pure

e or juice. The

pectic substances have been shown to combine with

protein or polyphenolic compounds to form a cloud

suspension. During the extraction of juice from fruit,

pectin is released from the cell wall and since the pH of

the juice is lower than the isoelectric point of protein,

the positively charged protein is surrounded by the

negatively charged pectin. Due to the interruption of

the negative charge of the outer pectin molecule, the

cloud suspended in the juice is more stable. The pectin

contains neutral sugars, rhaminose, arabinose glu-

cose, and fructose in branched chains that can bind

to the protein with a hydrogen bond and make the

cloud more stable. The content in guava pure

e was

found to be between 5 and 8%.

Stone Cells

0015Guava contains a considerable amount of stone cells

which contribute to an undesirable gritty texture to

the processed pure

e. The number of stone cells that

is tolerable varies from region to region. In Brazil,

the presence of stone cells is considered a positive

quality factor. According to a recent report, there

are two types of stone cells in guava: an irregular-

shaped type, abundant under the epidermis, and a

smooth type found in the core region of the fruit.

2988 GUAVAS

Stone cells are developed from parenchyma cells by

secondary thickening of the wall. Cell differentiation

occurs when fruits are about 14 weeks old. As the

fruits increase in maturity, cell wall thickening appears

to be more prominent. The stone cells are found in

clusters, each measuring approximately 300 mmin

length. The presence of stone cells in the flesh of fruits

has no specific function. In leaves or other parts of the

plant, stone cells may help to give rigidity to the plant

part or may act as a protective covering, as in the testa

of seeds. The composition of stone cells is reported to

be as follows: 0.92% fat; 1.05% ash; 1.50% protein;

37.1% lignin; 53.9% cellulose; 5.49% soluble carbo-

hydrates.

Harvesting

0016 Guavas produce varying amount of fruit throughout

the year in the tropics. Fruit yields depend upon culti-

var potential, plant density, weather conditions, and

all the factors involved in management. On the

average the yield ranges from 25 to 40 t ha

1

year

1

Guava fruit generally takes about 17–20 weeks from

fruit-set to reach maturity. The recommended opti-

mum stage for harvesting is about 2–3 weeks before

attaining full growth, because guava fruits will

experience changes associated with ripening such as

skin yellowing, and decrease in tissue firmness and in

detachment force while they are still actively grow-

ing. It was reported that the Ka Hua Kula guava

entered the ripening phase when the fruit was only

half the maximum size while in Allahabad Safeda

ripening changes occurred towards the end of the

growing period. For green ripe guava cultivars, fruit

size, moisture content, or chemical attributes such

as total soluble solids, titratable acidity, sugar and

tannin contents, may also be adopted as additional

maturity indices.

0017 Guava fruit must be harvested with great care

because of their soft and thin skin. Harvesting is

normally done manually to avoid physical injuries,

although tests with machine harvesting have been

conducted in some countries. Subsequent handling

and transportation of the fruit also need extra precau-

tions to reduce bruising. Dessert fruit are hand-

harvested mature green and carefully handled to

avoid injury, graded to size, and packed carefully in

cartons for shipment. Processing fruit should be

picked at the firm yellow to half-ripe stage. Overripe

fruit and those severely infected with fruit flies and

diseases should be destroyed rather than left to fall

and rot in the field. When only fully ripe fruits are

harvested on a 3-day cycle, losses between 35 and

40% can occur. Harvesting fruits showing some

yellow to the half-ripe stage allows the interval be-

tween harvest cycles to be lengthened to about 3 days

and this would minimize losses. Fruits are harvested

into plastic buckets or packing bags worn by the

harvester with bottom delivery. The fruits are trans-

ferred into larger bins or small wooden boxes and

placed in the shade until delivered to processing plant.

Storage and Handling

0018The common guava (P. guajava) is a climacteric fruit.

If full-size pale-green fruits are stored at 20



C, the

peak in carbon dioxide and ethylene production

occurs about 5–6 days after harvest. Biochemical

and color changes accompany the ripening process.

Storage of two processing guava cultivars at 20



showed that the fruit developed eating ripe quality

after 5 days. After 7 days at 20



C, the skin was

discolored and some rotting occurred. Storage at

0–10



C extended the postharvest life for about 2

weeks. Storage at 0



C reduced rotting, but caused

chilling injury to the pulp, resulting in darkened

flesh. The optimum storage temperature was 5



since pulp injury occurred least at that temperature.

Several trials have been carried out to prolong the

storage life of guavas. Recent experiments indicate

that the storage life of mature fruit may be extended

up to 1 week at room temperature by using modified

atmospheres. Waxing accompanied with low-

temperature storage (8–10



C) increased storage life

to 21 days. The use of carnauba-paraffin emulsion

imparted a gloss to the fruits that is normally lost

during storage. Fully ripe guava should be processed

without delay, but if necessary, they can be held for

about a week at 2–7



C with only a small loss in

vitamin C content. Storage at 0



C for up to 2 weeks

is used commercially in Hawaii.

0019Guava fruit is highly perishable; therefore, it is

important to select the right type of fruit and the

right packaging material for transport. Shallow

wooden boxes should be used to protect the fruit

from external hazards and from internal crushing of

lower fruits if containers are too deep. For long-

distance shipments, soft filling or cushioning mater-

ials should also be used. Good ventilation is import-

ant to prevent build-up of heat and humidity, which

promotes microbial spoilage. If ripe fruits need to be

shipped, it might be better to hold them in a cool or

shady area and transport them during the cooler night

temperatures. The best practice would be to ship

unripe fruit and ripen it under controlled conditions

at the processing plant.

Diseases

0020Some diseases and causal organisms are specific to

certain countries and others are widespread where

GUAVAS 2989

guavas are grown (Table 4). Anthracnose is wide-

spread and is considered an important disease in

most countries. Algal spots are very common but

are not usually serious, except they are of concern in

fruits for dessert. Other types of fruit rots are attrib-

uted to a number of organisms. Guignardia fruit rot

becomes serious in Hawaii when fruit is left to over-

ripen on the tree or on the ground. Wilting of guava

trees is reported from South Africa and India and

attributed to different organisms.

0021 Mucor fruit rot first appears as a water-soaked

area and later becomes covered with yellowish,

fuzzy mycelia and fruiting bodies. Infection rate can

be as high as 80–90% and, as a wound parasite, it is

commonly associated with fruit fly oviposition

wounds. Culture control is possible by removing

fallen fruit from the field at 2–4 days’ intervals.

Low-acid sweet cultivars are more tolerant to this

disease than acid types. Blossom end rot of fruit

appears to be widespread. In Hawaii, no organism

has been isolated and fungicidal sprays have been

ineffective. Calcium application to guavas largely al-

leviates this disease.

Pests

0022The types of pests that are common to all guava

growing areas are as indicated in Table 5. Report of

work done in Hawaii found a list of approximately

45 species of insects and six species of mites attacking

guavas. Included among them are species of aphids,

thrips, scales, mealy bugs, beetles, moth larvae, false

spider, eriophyid, and spider mites. There are several

species of parasitic wasps and predators that keep

scale insects and mealy bugs under reasonable con-

trol. Fruit flies cause serious damage and fruit rot

within a day or two upon ripening. Fruit bagging

along with thinning 30–40 days after anthesis can

significantly reduce the problem and produce high-

quality blemish-free dessert fruit.

0023Thrips can cause silvering of leaves and scarifica-

tion of the fruit. When young developing fruits are

severely damaged, they often fail to develop and

become mummified. Natural enemies can keep thrips

under fair control, although outbreaks do occur,

especially during the fruiting season. Skin of scarred

fruit becomes russeted by disruptions of the

tbl0004 Table 4 Some important diseases of guava fruit

Commonname Organism Parts affected Region

Anthracnose Coiletotrichum Gloeosporioides Fruit Worldwide

Glomereila fruit rot Glomereila cingulata (perfect stage) Fruit Puerto Rico

Blossom end rot Botrytis cinerea Fruit South Africa

Phomopsis psidii Fruit India, South Africa

Physiological Fruit Hawaii, Australia

Fruit canker Pestalotia Fruit Australia

Fruit rot Macrophomina spp. Fruit Caribbean

Guava fruit rot Rhizopus stolonifer Fruit Hawaii

Guava wilt Gliocladium spp. Root South Africa, Australia

Fusarium wilt Fusarium solani Root India

Guignardia rot Guignardia spp. Fruit Hawaii

Mucor rot Mucor hiemalis Fruit Hawaii

Algal spots Cephaleuros virescens Leaf, fruit Florida, Hawaii

Adapted from Nakasone H and Paull R (1998) Guava. In: Tr o p i c a l Fru i t s , pp. 149–172. Wallingford, Oxon: CAB International.

tbl0005 Table 5 Some important insect and nematode pests of guava

Commonname Organism Parts affected Region

Mediterranean fruit fly Ceratitis capitata Fruit Hawaii, South Africa

Natal fruit fly Ceratitis rosa Fruit South Africa

Oriental fruit fly Dacus dorsalis Fruit Hawaii

Melon fly Dacus cucurbitace Fruit Hawaii

Caribbean fruit fly Anastrepha striata Fruit Caribbean, American tropics

No common name Monolepta australis Leaf Australia

Red-banded thrips Selenothrips Leaf, fruit Universal

Bark-eating caterpillar Indarbela quadrinotata Bark India

Root-knot nematode Meliodogyne incognita Root Australia

Meliodogyne aruenaria Root Caribbean

Meliodogyne acrita Root Caribbean

Adapted from Nakasone H and Paull R (1998) Guava. In: Tr o p i c a l Fru i t s , pp. 149–172. Wallingford, Oxon: CAB International.

2990 GUAVAS

epidermal layer and corky tissue development in the

subepidermal area. This corky tissue must be

screened out during the processing; otherwise the

pure

e from russeted fruit appears muddy pink in

color. Several sweet cultivars such as Allahabad

Safeda, Ruby  Supreme and Lucknow-49 have a

higher degree of resistance to thrips than others.

This resistance increases somewhat with increasing

foliar levels of nitrogen and potassium.

Industrial Use

0024 Guava is one of the easiest fruits to process since it

does not show many problems of physical or bio-

chemical nature in relation to texture and shape of

fruit or pulp browning during processing. However,

compared to other fruits, guavas have found very

limited use in the fruit-processing industry. Guava

pure

e, which is also known as guava pulp, is a liquid

product prepared by pure

eing or pulping whole

guavas. A guava-pure

eing process, which was de-

veloped in Hawaii, and is in commercial use today is

similar to that shown in Figure 2. Fruits for process-

ing are placed in a dump tank which serves to soak

and wash the fruit. The fruit which then floats is

picked up on to a moving conveyer belt; it is inspected

and sorted for decay, insect damage, and foreign ma-

terials such as leaves and dirt. Clean fruits are then

passed through a chopper or slicer to be broken up,

and are then fed into a pulper. The pulper removes

the seeds and fibrous tissues and forces the remainder

of the product through a perforated stainless-steel

screen. The pure

ed material coming from the pulper

is next passed through a finisher which removes the

stone cells from the fruit. The finisher is equipped

with a perforated screen of approximately 0.051-

cm-sized holes. Alternatively, the guava pure

e can be

passed through a mustard mill which effectively

grinds up the stone cells so that smooth guava pure

is obtained. However, the incorporation of an exces-

sive amount of milled stone cells into the pure

e can

discolor it. Other methods of removing stone cells

include the use of a finer screen in the finisher, or

using a centrifuge. The average yield of pure

e is ap-

proximately 85%. The prepared pure

e may be used

directly for preparation of various guava beverages,

or frozen for export. The pure

e is most commonly

manufactured into nectars, various juice drink

blends, syrups, icecream toppings, jams, and jellies.

See also: Acesulfame/Acesulphame; Dietary Reference

Values; Sensory Evaluation: Aroma; Taste; Tannins

and Polyphenols

Further Reading

Chyan C, Chen S and Wu C (1992) Different volatile and

non-volatile constituents between mature and ripe guava

(Psidium guava L) fruits. Journal of Agriculture and

Food Chemistry 40: 846–849.

Itoo S, Yamaguchi T, Oohata J and Ishihata K (1980) Stud-

ies on qualities of subtropical fruits. II. Ascorbic acid and

stone cells of guava fruits (Psidium guava L). Bulletin of

the Faculty of Agriculture, Kogoshima University 30:

47–54.

Guava

Inspection

belt

Wash tank

Elevator

Chute

Cutter

Waste

Pulper

Pump

Finisher

Pump

Filler

Seamer

Labeler

Caser

Cooling belt

or tank

Pump

Flash

pasteurizer

Waste

Sugar

Water

Mixing tank

(for nectar or nectar base)

Water sprays

suspended above

fig0002 Figure 2 Flow sheet for guava-processing line (for frozen products, bypass pasteurization and cooling belt). Adapted from Boyle F,

Seagrave-Smith H, Sakrata S and Sherman G (1957) Hawaii Agric. Exp. Stn., Bull. 111.

GUAVAS 2991

Lim K and Khoo C (1990) Guava in Malaysia. Production,

Pests and Diseases. Kuala Lumpur, Malaysia: Tropical

Press.

Mowlah G and Itoo S (1983) Changes in pectin compon-

ents, ascorbic acid, pectin enzymes and cellulase activity

in ripening and stored guavas. Nippon Shokubin Kogyo

Gakkaishi 30: 454–461.

Nelson PE and Tressler DK (1980) Fruit and Vegetable Juice

Processing Technology, 3rd edn. Westport, Connecticut:

AVI.

Nishimura O, Yamaguch K, Mihara S and Shibamoto T

(1989) Volatile constituents of guava fruits (Psidium

guajava L) and canned puree. Journal of Agriculture

and Food Chemistry 37: 139–142.

Wilson C (1980) Guava-composition, properties and uses.

In: Nagy S and Shaw P (eds) Tropical and Subtropical

Fruits. Westport, Connecticut: AVI.

Yen G, Lin H and Yang P (1992) Changes in volatile flavour

components of guava puree during processing and frozen

storage. Journal of Food Science 57: 679–681, 685.

Yusof S (1989) Characteristics and potential use of guava

(Psidium guajava L.) for processing and storage. PhD

thesis. Universiti Putra Malaysia. Serdang, Selangor,

Malaysia.

Yusof S and Mohamed S (1987) Physico-chemical changes

in guava (Psidium guajava L.) during development and

maturation. Journal of the Science of Food and Agricul-

ture 38: 31–39.

GUMS

Contents

Properties of Individual Gums

Food Uses

Dietary Importance

Nutritional Role of Guar Gum

Properties of Individual Gums

P A Williams and G O Phillips, The North East Wales

Institute, Wrexham, UK

Introduction

0001 The term ‘gums’ is commonly used to describe a

range of polysaccharides that are nowadays widely

used in food products to perform a number of func-

tions including, thickening, gelling, stabilization of

foams, emulsions, and dispersions, inhibition of ice

and sugar crystal formation, and in the controlled

release of flavors. The commercially important gums

and their source of origin are given in Table 1.

0002 The choice of gum for a particular food applica-

tion is dictated by the functional characteristics re-

quired but is inevitably influenced by price and

security of supply. It is for these reasons that starches

are the most commonly used thickening agents. It is

interesting to note, however, that xanthan gum, since

its introduction in the early 1970s, has become the

thickener of choice in many applications, despite its

high price. This is due to the fact that xanthan gum

has unique rheological behavior (see later). Gelatin

(a protein but commonly considered as a gum) is

by far the most widely used gelling agent. However,

with the increasing demand for nonanimal products

and in particular the recent bovine spongiform

encephalopathy (BSE) outbreak in the UK, prices

have increased significantly over the last few years.

There is currently much activity in the devel-

opment of a gelatin replacement. The carrageenan

market has also been unstable over recent years

due to the introduction of cheaper lower-refined

grades (processed euchema seaweed or PES) which

can compete effectively with the traditional puri-

fied grades in applications where gel clarity is not

important. The gum arabic market has been par-

ticularly erratic due to extreme price fluctuations

and security of supply and much effort has been

directed at finding alternatives. A number of starch-

based substitutes are now available. Gellan gum

was approved for food use in Japan in 1988 and

later in the USA. More recently it has been ap-

proved for food use in Europe and is beginning to

establish its own niche market. A summary of the

functional properties of the individual gums and the

value and volume of the gums market is given in

Table 2.

2992 GUMS/Properties of Individual Gums

Functional Characteristics

0003 Gums will dissolve or swell in water, although in

many cases high temperature and vigorous agitation

are required before complete dissolution is achieved.

The solutions formed are usually thick and viscous,

even at very low concentrations (1%). There are,

however, notable exceptions, for example gum arabic

solutions only become viscous at concentrations

> 50%. The viscosity of polymer solutions generally

shows a marked increase at a critical polymer concen-

tration, commonly referred to as C

, which corres-

ponds to the concentration at which the polymer

molecules start to touch and interpenetrate due to

molecular crowding. C

decreases with increasing

hydrodynamic volume of the gum molecules which

in turn is influenced by their molecular mass, shape,

and the presence of charged groups. Hence high-

molecular-mass linear molecules possessing ionizable

groups tend be good thickeners. The main thickeners

used in food products are listed in Table 3.

0004Gum solutions normally exhibit Newtonian behav-

ior at concentrations well below C

, i.e., their viscos-

ity is independent of the rate of shear. However, above

non-Newtonian behavior is usually observed and

the viscosity–shear rate profiles show three distinct

regions, i.e., a high-viscosity low-shear Newtonian

plateau (at shear rates typically < 1 s

1

); a shear thin-

ning region (1  1000 s

1

), and a low-viscosity

high-shear Newtonian plateau (typically > 1000 s

1

0005Some gums, notably xanthan gum, have a tendency

to undergo weak intermolecular chain association

in solution, leading to the formation of a three-

dimensional network structure, thus giving rise to a

very high viscosity. The junction zones formed can

be readily disrupted even at very low shear rates,

resulting in a dramatic drop in viscosity. Other poly-

saccharides self-associate and form stable junction

zones and as a consequence strong gel structures are

produced. The gels are referred to as ‘physical gels’

because the junction zones are formed through phys-

ical interaction, for example, by hydrogen bonding,

hydrophobic association, and cation-mediated cross-

linking and differ from synthetic polymer gels which

normally consist of covalently cross-linked polymer

chains.

0006Some gums form thermoreversible gels and

examples exist where gelation occurs on cooling or

heating. Some form nonthermoreversible gels. In such

cases gelation may be induced by cross-linking poly-

mer chains with divalent cations. Gels may be optic-

ally clear or turbid and a range of textures can be

obtained. Gel formation occurs above a critical min-

imum concentration, which is specific for each gum.

Agarose, for example, will form gels at concentra-

tions as low as 0.2%, while for acid-thinned starch,

a concentration of 15% is required. Typically, how-

ever, concentrations of less than 1% are required. Gel

strength increases with increasing concentration. The

main gelling agents are listed in Table 4.

0007Mixtures of gums are commonly used to impart

novel textural characteristics to food products and

an added incentive is a reduction in costs. Classic

examples include the addition of locust bean gum to

kappa-carrageenan to yield softer, more transparent

gels and also the addition of locust bean gum to

xanthan gum to induce gel formation.

0008Gums are effective at either inducing or preventing

flocculation in particulate dispersions. If insufficient

gum is present to coat all the particles fully, aggrega-

tion occurs owing to polymer adsorbing on to

two or more particles simultaneously, thus inducing

bridging. If the particles are fully coated, particle

aggregation is prevented by steric repulsion, which

arises as a result of interaction between the adsorbed

polymer layers. Although nonadsorbed gum is

tbl0001 Table 1 Source of commercially important gums

Botanical

Trees

Cellulose

Microcrystalline cellulose

Sodium carboxymethyl cellulose

Methyl cellulose

Hydroxypropylmethyl cellulose

Tree gum exudates

Gum arabic (Acacia senegal, A. seyal)

Gum karaya (Stercula urens)

Gum ghatti (Anogeissus latifolia)

Gum tragacanth (Astragalus)

Plants

Starch (corn, maize, waxy maize, wheat, pea, potato, rice,

sago, etc.)

Pectin (citrus fruits, apple pomace)

Cellulose (cotton)

Seeds

Guar gum (Cyamopsis tetragonoloba)

Locust bean gum (Ceratonia siliqua)

Tara gum (Cesalpinia spinosa)

Tubers

Konjac mannan (Amorphophallus konjac)

Algal

Red seaweeds (Rhodophyceae)

Agar (Gelidium and Gracilaria spp.)

Carrageenan (Euchema cottonii, E. spinosum, Chondus crispus,

Gigartina sp.)

Brown seaweeds (Phyophyceae)

Alginate (Laminaria hyperborea, Macrocystis pyrifera,

Ascophyllum nodosum) Propylene glycol alginate

Microbial

Xanthan gum (Xanthomonas campestris)

Curdlan (Alcaligenes faecalis var. myxogenes 10C3)

Gellan gum (Auromonas elodea)

Animal

Gelatin (collagen from animal skins and bones)

Chitosan (shells of shrimps, crabs)

GUMS/Properties of Individual Gums 2993

generally considered to enhance the stability of dis-

persions (and emulsions) by increasing the viscosity of

the aqueous phase, it can have the opposite effect and

give rise to depletion flocculation. This phenomenon

occurs as a consequence of polymer molecules being

excluded from the space between approaching par-

ticles. The difference in osmotic pressure between

the depleted region and the bulk solution results in

weak interparticle attractive forces, which induce

aggregation.

0009 Some gums, such as gum arabic, methyl cellulose

and propylene glycol alginate, show amphiphilic

character which has led to their use in the stabiliza-

tion of emulsions and foams owing to their affinity

to adsorb at the oil–water or air–water interface.

0010 In systems containing sugar or ice crystals, gums

can retard crystal growth either by adsorbing on to

the crystals or by competing for available water

molecules.

0011Although gums have primarily been used in foods

to control texture, organoleptic properties, and sta-

bility, consumers are now being made increasingly

aware of their nutritional and health benefits. Many

gums (e.g., locust bean gum, guar gum, konjac man-

nan, gum arabic, xanthan gum, pectin) have been

shown to reduce blood cholesterol levels. Others

(e.g., inulin, gum arabic) have been shown to have

prebiotic effects. They are resistant to our digestive

enzymes and pass through the stomach and small

intestine without being metabolized. They are fer-

mented in the large intestine to yield short-chain

fatty acids and stimulate the specific growth of bene-

ficial intestinal bacteria, notably bifidobacteria, and

reduce the growth of harmful microorganisms such as

clostridia.

Properties of Individual Gums

Tree Gum Exudates

0012The four main tree gum exudates are gum arabic,

gum karaya, gum tragacanth, and gum ghatti, with

gum arabic being by far the most important commer-

cially. Gum arabic occurs as a sticky liquid that oozes

from the stems and branches of acacia trees (Acacia

senegal and A. seyal) which grow across the Sahelian

belt of Africa, principally Sudan. The gummosis

process occurs when the tree is subjected to stress

conditions such as heat, drought, or wounding. The

liquid dries in the sun to form glassy nodules, which

tbl0002 Table 2 Value and volume of the gums market

Hydrocolloid Principal function Thousand tonnes (1998) $ million (1998)

Agar Gelling agent

Alginate Gelling agent 30 90

Propylene glycol alginate Emulsifier and foam stabilizer

Arabic Emulsifier 30 150

Carrageenan Gelling agent 35 270

Processed euchema seaweed Gelling agent

Carboxymethyl cellulose Thickener 34

Hydroxypropylmethyl cellulose Thickener and emulsifier

Methyl cellulose Thickener, emulsifier, and gelling agent

Cellulose (microcrystalline) Thickener and gelling agent

Gelatin Gelling agent 162 350

Guar gum Thickener 30 80

Karaya Thickener

Konjac mannan Thickener and gelling agent

Locust bean gum Thickener 10 100

Pectin Gelling agent 25 250

Starch (modified) Thickener and gelling agent 1000 650

Tragacanth Thickener

Xanthan gum Thickener 16 160

Source: Lillford PJ (2000) Gums and Stabilisers for the Food Industry. In: Williams PA and Phillips GO (eds) Royal Society of Chemistry Publication 251.

Cambridge, UK.

tbl0003 Table 3 Thickeners

Xanthan gum

Very high viscosity at low shear rates (yield stress), highly

shear thinning, maintains viscosity in the presence of

electrolyte, over a broad pH range and at high temperatures

Carboxymethyl cellulose

High viscosity but reduced by the addition of electrolyte and

at low pH

Methyl cellulose and hydroxypropylmethyl cellulose

Viscosity increases with temperature (gelation may occur)

not influenced by the addition of electrolytes or pH

Galactomannans (guar and locust bean gum)

Very high viscosity at low shear rates and strongly shear

thinning. Not influenced by the presence of electrolyte but

can degrade and lose viscosity at high and low pH and when

subjected to high temperatures

2994 GUMS/Properties of Individual Gums

are collected by hand. The gum from A. senegal

(traditionally the main source of gum arabic) is a

complex polysaccharide consisting of galactopyra-

nose (44%), arabinopyranose, arabinofuranose

(25%), rhamnopyranose (14%), glucuropyrano-

syl uronic acid (15.5%) and 4-O methyl glucuro-

pyranosyl uronic acid (1.5%). It also contains a

small amount (2%) of protein as an integral part

of the structure. Analysis of the carbohydrate struc-

ture has shown that it consists of a core of (1,3)-b-d-

galactose units with extensive branching at the C6

position. The branches consist of galactose and ara-

binose and terminate with rhamnose and glucuronic

acid (Figure 1a). As a consequence of the highly

branched structure, viscous solutions are only formed

at high gum concentrations (50% w/w). It has been

shown by gel permeation chromatography that the

gum consists of three molecular mass fractions which

have also been shown to differ principally in their

protein contents. Most of the gum (90%) contains

very little protein and has a molecular mass of

2.5 10

. A second fraction (10% of the total)

contains 10% protein and has a molecular mass of

1–2 10

and has been shown to have a ‘wattle-

blossom’-type structure where blocks of carbohy-

drate of molecular mass 2.5 10

are connected

to a common polypeptide chain. The third fraction

(1% of the total) contains up to 50% protein and

has a molecular mass of 2 10

. Fraction 2 has

been shown to be responsible for the gum’s excellent

ability to stabilize oil-in-water emulsions. It has been

proposed that the hydrophobic polypeptide chains

adsorb on to the oil droplets and anchor the

molecules to the surface, while the hydrophilic

carbohydrate blocks protrude out into solution and

prevent droplet aggregation and coalescence due to

electrosteric repulsions. Its ability to act as an emulsi-

fier has led to its use in stabilizing flavor oil emulsion

concentrates for the soft drinks industry and also in

the production of spray-dried encapsulated flavors

for use in dry packaged products such as soup and

cake mixes. In the latter case the gum forms a film

around the flavor particle, preventing oxidation and

evaporation, and the gum’s high solubility facilitates

rapid flavor release. The ability of gum arabic to form

concentrated solutions of low viscosity has led to its

widespread use in confectionery products, particu-

larly those with a high sugar content such as pastilles

where it functions by reducing sugar crystallization.

0013Gum tragacanth consists of a water-swellable frac-

tion called tragacanthic acid (or bassorin) (60–70%)

and a water-soluble fraction called tragacanthin. The

former consists of a main chain of (1,4)-a-d-galacto-

pyranosyluronic acid residues with branches linked

through (1,3)-b-d-xylose units terminating in l-fuco-

pyranose, as illustrated in Figure 1b. The latter is

a highly branched arabinogalactan with a main

chain of d-galactopyranosyl units either (1,6)- or

(1,3)-linked with side chains consisting mainly

of l-arabinofuranose but with a small proportion of

d-galacturonic acid and l-rhamnose. Gum traga-

canth gives rise to high-viscosity solutions even at

1% concentration. The viscosity decreases irrevers-

ibly on heating. The gum solution is stable under acid

conditions and shows good emulsification character-

istics, which has led to its use in salad dressings and

sauces. Its cost and availability, however, have meant

that it has been largely replaced in these products by

other gums, notably xanthan gum.

0014Gum karaya is a heavily acetylated polysaccharide

composed of chains of a-d-galacturonic acid and

a-l-rhamnose. The acid groups are glycosylated with

tbl0004 Table 4 Gelling agents

Thermoreversible gelling agents

Gelatin Gel formed on cooling. Molecules undergo a coil-to-helix transition

followed by aggregation of helices

Agar Gel formed on cooling. Molecules undergo a coil-to-helix transition

followed by aggregation of helices

Kappa-carrageenan Gel formed on cooling in the presence of salts, notably potassium salts

Iota-carrageenan Gel formed on cooling in the presence of salts

Low-methoxyl (LM) pectin Gels formed in the presence of divalent cations, notably calcium at low pH

(3–4.5)

Gellan gum Gels formed on cooling in the presence of salts

Methyl- and hydroxypropylmethyl cellulose Gels formed on heating

Xanthan gum with locust bean gum or konjac mannan Gels formed on cooling mixtures

Thermally irreversible gelling agents

Alginate Gels formed on the addition of polyvalent cations, notably calcium or at low

pH (< 4)

High-methoxyl (HM) pectin Gels formed at high soluble solid (e.g., 50% sugar) content at low pH < 3.5

Konjac mannan Gels formed on addition of alkali

Locust bean gum Gels formed after freezing due to association of galactose-deficient regions

GUMS/Properties of Individual Gums 2995

b-d-galactose or b-d-glucuronic acidresidues, whereas

about half of the rhamnose groups carry b-d-galac-

tose units as side chains (Figure 1c). Gum karaya

swells in water to yield highly viscous solutions, how-

ever, solutions show a permanent loss in viscosity on

heating. The gum finds some application in salad

dressings.

0015Gum ghatti has a main chain of alternating (1,4)-b-

d-glucopyranosyluronic acid and (1,2)-a-d-manno-

pyranose units and contains numerous side chains

of l-arabinose, d-galactose, and d-glucuronic acid

(Figure 1d). It contains a water-soluble (> 80%) and

water-swellable fraction and was originally de-

veloped in the early 20th century as replacement for

U-β-D-Galp

6)-β-

D-Galp-(1,3)-β-D-Galp -(1

β-

D-Galp -∇

U-β-

D-Galp -(1,3)-β-D-Galp

3)-β-

D-Galp -(1,3)-β-D-Galp -(1,3)-β-D-Galp -(1,3)-β-D-Galp -(1,3)-β-D-Galp -1)

β-

D-Galp β-D-Galp

β-

D-Galp -(1,3)-β-D-Galp

U-β-

D-Galp -(1,3)-β-D-Galp

β-

D-Galp

U(1,3)-β-

D-Galp -(1,3)-β-D-Galp

β-

D-Galp β-D-Galp

4)-α-D-Galp -A(1→4)-α-D-GalpA(1,4)-α-D-GalpA(1,4)-α-1)-GalpA

β-

D-Xylp

β-

D-Xylp

β-

D-Galp.

β-

D-Xylp

α-

L-Fucp.

2)-α-

L-Rhap -(1→4)-α-D-GalpA-2)-α-L-Rhap -(1,4)-α-D-GalpA

β-

D-Galp β-D-Galpβ-D-GlcpA

(d)

4)-β-

D-GlcpA-2)-α-D-Manp -4)-β-D-GlcpA-2)-α-D-Manp

LArap L-Arap

-6)β-

D-Galp -6-β-D-Galp -6)-β-D-Galp. β-D-Galp -6)-β-D-Galp

β-

D-Galp

β-

D-GlcpA

(a)

(b) (c)

fig0001 Figure 1 (a)Acacia senegal representative segment. U ¼2)-a-L-Rhap-(1,4)-b-D-GlcA- or 4)-b-D-methyl-GlcpA. r¼L-Araf or L-Arap

terminated (1,3)-linked short chains or a-

D-Galp-(1,3)-L-Araf. (b) Tragacanthic acid. (c) Some structural features of gum karaya. (d)

Some structural features of gum ghatti: r¼

L-Araf or L-Arap terminated side chains. Reproduced from Gums: Properties of Individual

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

Press.

2996 GUMS/Properties of Individual Gums

gum arabic since it forms solutions of low viscosity at

high concentrations and is also a good emulsifier.

Cellulosics

0016 Cellulose is the most abundant of all the polysacchar-

ides, being the structural component of land plants. It

is composed of linear chains of (1,4)-b-d-glucopyra-

nose units which are associated through hydrogen

bonding, giving rise to a number of crystalline

forms. Although it is insoluble in water, it can be

chemically modified to form a number of water-sol-

uble derivatives with valuable functional properties.

Derivatization usually involves etherification of the

reactive hydroxyl groups on the glucose residues. The

most common water-soluble cellulose derivatives

used in food applications are carboxymethyl (CMC),

methyl (MC), and hydroxypropylmethyl (HPMC)

cellulose. Derivatization is carried out following con-

version of the cellulose to the sodium form by treat-

ment with concentrated alkali. This process destroys

the crystalline structure and yields alkali cellulose.

The alkali cellulose is then reacted with the appropri-

ate reagent, namely sodium monochloroacetate for

CMC, methyl chloride for MC, and methyl chloride/

propylene oxide for HPMC. Since the reactions are

heterogeneous, substitution can be very irregular.

0017 CMC is the most widely used of the cellulose ethers

and can be obtained in various grades, which vary in

molecular mass and degree of substitution (DS). The

minimum DS is 0.4 (i.e., 4 hydroxyls substituted

per 10 glucose residues), since below this the CMC is

insoluble. Above this CMC dissolves readily in water

to form viscous solutions but, since it is anionic, the

viscosity of dilute solutions (< 1%) is reduced on

addition of electrolyte and at low pH due to compac-

tion of the polymer chains. A synergistic increase in

viscosity occurs in blends with guar gum and locust

bean gum. CMC finds widespread use in icecream

and other frozen desserts as well as salad dressings,

sauces, and gravies.

0018 The nonionic cellulose ethers, MC and HPMC, are

also supplied in a range of molecular sizes and DS.

For HPMC, since the substituted moieties contain

hydroxyl groups they may also participate in the

reaction. Hence a molar substitution (MS) is also

quoted to characterize the polymers. MC and

HPMC dissolve readily in water to produce viscous

solutions which on heating form thermoreversible

gels. The gelation temperature is dependent on the

degree of substitution. For MC containing 30%

methoxyl groups, gelation occurs at 50–55



whereas for HPMC containing 20% methoxyl and

8% hydroxypropyl, gelation occurs at 85



C. Gel-

ation is believed to be a consequence of molecular

association through the hydrophobic methyl and

hydroxypropylmethyl substituents. This functional

characteristic has led to their use in combination

with starch in soups and gravies to prevent undesir-

able loss of viscosity on heating. A further application

is their use in fried foods, where gelation on heating

helps retain the structural integrity of the product and

additionally serves to prevent moisture loss and oil

absorption. Since MC and HPMC are also surface-

active they are used in baked goods to aid uniform gas

cell formation and in salad dressings to stabilize emul-

sion droplets.

Plant Extracts

0019Starch is also a very abundant material and is derived

commercially principally from corn and potato and,

to a lesser extent, from waxy corn, wheat, tapioca,

rice, cassava, sorghum, pea, and sago. It is obtained

in the form of granules, which have varying degrees

of structural order and consist of two polysacchar-

ides, namely amylose and amylopectin. The propor-

tions of each depend on the source, for example,

corn and potato starch contain 27% and 21%

amylose respectively, while waxy corn contains

< 2% amylose.

0020Amylose consists of linear (1,4)-a-d-glucopyranose

chains with very little branching (10 branch points

per molecule) and has a molecular mass of typically

2 10

–2 10

. Amylopectin also contains se-

quences of (1,4)-a-glucopyranose units; however, it

has extensive branching via (1,6) linkages and has a

molecular mass of 5 10

– 4 10

. The starch

granules are insoluble in water but on heating they

swell and burst, releasing amylose, forming a viscous

paste. The temperature at which the granules burst is

very characteristic of the source of the starch and is

(perhaps inappropriately) referred to as the gelatin-

ization temperature. Typical values for corn and

potato starch are 67



C and 60



C respectively. On

cooling, the amylose molecules readily self-associate,

a process known as retrogradation, and it is then that

a gel is formed. Natural starches form turbid gels

which are prone to syneresis. Hence most of the com-

mercial starches used are derivatives which have a

lesser tendency for retrogradation. Derivatives in-

clude hydroxypropyl starches, starch phosphates,

oxidized starch, acid, or enzyme-degraded starches.

0021Pectin is the general term for a group of polyuro-

nans that occur as the structural components of

plants. Commercial pectins are obtained from the

pomace of apple or the peel of citrus fruits following

hydrolysis, which renders the pectin water-soluble.

Pectin molecules consist of linear chains of (1,4)-a-

d-galacturonic acid residues, up to 80% of which

occur as the methyl ester. The hydroxyls in the C2

and C3 positions may be acetylated. The chains also

GUMS/Properties of Individual Gums 2997