**3.1.2 Hemicelluloses**

Hemicelluloses are a heterogeneous group of polysaccharides constituting the cell walls of higher plants; these polysaccharides are often physically entangled, covalently and/or noncavalently bonded to cellulose and lignins. The structure of hemicelluloses may vary depending on their origin, but they can be divided into four groups based on composition of their main backbone chain:D-xylans with (1→4)-linked β-D-xylose; D-mannans, with (1→4) linked β-D mannose; D-xyloglucans with D-xylopyranose residues attached to the cellulose chain; and D-galactans with (1→3)-linked β-D-galactose. The first three groups are very similar to cellulose in having the main chain backbone linked via (1→4) diequatorial linkages and capable of adopting extended ribbon conformations. Most of the hemicelluloses, however, are substituted with various other carbohydrate and noncarbohydrate residues, and unlike cellulose, they are heteropolysaccharides. This departure from uniformity because of various side branches renders them at least partially soluble in water (Cui, 2005).

#### **3.1.2.1 Mannans and galactomannans**

The cell walls of seeds are especially rich in mannans and galactomannans. D-Mannans, found in tagua palm seeds, have a backbone composed of linear (1→4)-linked β-D-mannose chains. The best known D-galactomannans, locust bean, guar, and tara gums have the same linear mannan backbone but they are substituted with α-D-Gal*p* side units linked to O-6. The degree of substitution in galactomannans, which profoundly affects their solution properties, differs in galactomannans extracted from various plants (Table 2) (Cui, 2005). Widely used galactomannans are from the carob tree (*Ceratonia siliqua*), named as locust bean gum (LBG) or carob bean gum (CBG), and the guar plant (*Cyamopsis tetragonoloba*), namely guar gum (Philips & Williams, 2000). The ratio of D-mannosyl to D-galactosyl units is about 1.8:1 in guar gum and 3.9:1 in LBG. Guar gum containing galactose content of 33- 40% (w/w) is soluble in water of 25°C. The rate of dissolution of guar gum increases with decreasing particle size and with increasing temperature. In the case of LBG, with the larger part containing galactose contents of about 17-21% (w/w), it needs a heat treatment during 10 min at 86-89°C under stirring to dissolve in water. Like most hydrocolloids, both guar gum and LBG shows pseudoplastic, or shear-thinning, behavior in solution. The degree of pseudoplasticity increases with both concentration and molecules weight.

Carboxymethylcellulose (CMC) is an anionic, water-soluble polymer capable of forming very viscous solutions. CMC is prepared by first treating cellulose with alkali (alkali cellulose), and then by reacting with monochloroacetic acid. The degree of substitution (DS) with the carboxyl groups is generally between 0.6 to 0.95 per monomeric unit (maximum DS

Methylcellulose (MC) has thickening, surface activity (due to hydrophobic groups), and film forming properties. MC is prepared by treating alkali cellulose with methyl chloride. Other MC derivatives are also available, of which hydroxypropylmethylcellulose (HPMC) has been widely used. The reagent for HPMC is mixture of methyl chloride and propylene

Hemicelluloses are a heterogeneous group of polysaccharides constituting the cell walls of higher plants; these polysaccharides are often physically entangled, covalently and/or noncavalently bonded to cellulose and lignins. The structure of hemicelluloses may vary depending on their origin, but they can be divided into four groups based on composition of their main backbone chain:D-xylans with (1→4)-linked β-D-xylose; D-mannans, with (1→4) linked β-D mannose; D-xyloglucans with D-xylopyranose residues attached to the cellulose chain; and D-galactans with (1→3)-linked β-D-galactose. The first three groups are very similar to cellulose in having the main chain backbone linked via (1→4) diequatorial linkages and capable of adopting extended ribbon conformations. Most of the hemicelluloses, however, are substituted with various other carbohydrate and noncarbohydrate residues, and unlike cellulose, they are heteropolysaccharides. This departure from uniformity because of various side branches renders them at least partially

The cell walls of seeds are especially rich in mannans and galactomannans. D-Mannans, found in tagua palm seeds, have a backbone composed of linear (1→4)-linked β-D-mannose chains. The best known D-galactomannans, locust bean, guar, and tara gums have the same linear mannan backbone but they are substituted with α-D-Gal*p* side units linked to O-6. The degree of substitution in galactomannans, which profoundly affects their solution properties, differs in galactomannans extracted from various plants (Table 2) (Cui, 2005). Widely used galactomannans are from the carob tree (*Ceratonia siliqua*), named as locust bean gum (LBG) or carob bean gum (CBG), and the guar plant (*Cyamopsis tetragonoloba*), namely guar gum (Philips & Williams, 2000). The ratio of D-mannosyl to D-galactosyl units is about 1.8:1 in guar gum and 3.9:1 in LBG. Guar gum containing galactose content of 33- 40% (w/w) is soluble in water of 25°C. The rate of dissolution of guar gum increases with decreasing particle size and with increasing temperature. In the case of LBG, with the larger part containing galactose contents of about 17-21% (w/w), it needs a heat treatment during 10 min at 86-89°C under stirring to dissolve in water. Like most hydrocolloids, both guar gum and LBG shows pseudoplastic, or shear-thinning, behavior in solution. The degree of

pseudoplasticity increases with both concentration and molecules weight.

is 3), and occurs at *O*-2 and *O*-6, and occasionally at *O*-3 positions.

**3.1.1.2 Carboxymethylcellulose** 

**3.1.2 Hemicelluloses** 

soluble in water (Cui, 2005).

**3.1.2.1 Mannans and galactomannans** 

**3.1.1.3 Methylcellulose and its derivatives** 

oxide. These two cellulose derivatives are non-ionic.


Table 2. Botanical Origin and Main Structure Features of Galactomannans

### **3.1.2.2 Xyloglucans**

Xyloglucans, like cellulose, have linear backbones of (1→4)-linked β-D glucopyranoses. Numerous xylopyranosyl units are attached along the main backbone. In many plant xyloglucans, the repeating unit is a heptasaccharide, consisting of a cellotetraose with three subtending xylose residues (Phillips & Williams, 2000). Some xylose residues may carry additional galactosyl and fucosyl units. A few plants may have arabino- instead of fucogalactosylgroups attached to the xylose residues. One of the best characterized is the xyloglucan from the cotyledons of the tamarind seed (*Tamarindus indica*) (Shirakawa et al., 1998).

#### **3.1.2.3 Glucomannans**

Glucomannans are linear polymers of both (1→4)-linked β-D-mannose and (1→4)-linked β-D-glucose residues. Glucomannans are obtained from dried and pulverized root of the perennial herb *Amorphophallus konjac*. Acetyl groups scattered randomly along the glucomannan backbone promote water solubility. Konjac glucomannan is a high molecular weight polymer (>300 kDa) which can form viscous pseudoplastic solutions. It can form a gel in the presence of alkali.

### **3.1.2.4 Arabinoxylans**

D-Xylans are composed of (1→4)-linked β-D-xylopyranoses with various kinds of side branches, the most common being 4-*O*-methyl-D-glucopyranosyl uronic acid linked mostly to *O*-2 of β-Xyl*p* units and α-L-Araf linked to *O*-3 of β-Xyl*p* units. The amount of arabinose and glucuronic acid in glucuronoarabinoxylans may vary substantially, ranging from substitution at almost all Xyl*p* to polymers having more than 90% of unsubstituted β-Xyl*p* units. Many cereal (wheat, barley, rye, oats) arabinoxylans do not carry glucuronic acid units.

#### **3.1.2.5 β-D-Glucans**

β-D- Glucans are high molecular weight, viscous polysaccharides. Mixed linkage (1→3), (1→4) β-D-glucans are present in the grass species, cereals, and in some lichens (e.g., *Cetraria islandica*). Cereal β-D-glucans contain predominantly (1→4) linked β-D-Glc*p* units (~70%) interrupted by single (1→3)-linked β-D-Glc*p* units (~30%). The distribution of β-(1→4) and β-(1→3) linkages is not random; this leads to a structure of predominantly β-(1→3)-linked cellotriosyl and cellotetraosyl units. There are also longer fragments of contiguously β- (1→4)-linked glucose units (cellulose fragments) in the polymer chain. The main source of

Hydrocolloids in Food Industry 25

**Origin General structure Concentration** 

arabinogalactan 5.01 717

1.0 3.0

1.0

glucuronomannoglycan 5.01 2,882,440

Substituted acidic

arabinogalactan and

rhamnogalacturonan

rhamnogalacturonan

Acacia gum, also known as gum arabic, is a natural, vegetable exudate from acacia trees (primarily in Africa) known since antiquity and used for thousands of years in foods as an additive and ingredient, in the pharmaceutical industry and for technical purposes (Imenson, 2010). The thorny trees grow to a height of 7 to 8 meters, and the gum is obtained by cutting sections of the bark from the tree. The structure of gum arabic is relatively complex. The main chain of this polysaccharide is built from (1→3) and (1→6)-linked β-Dgalactopyranosyl units along with (1→6)-linked β-D-glucopyranosyl uronic acid units. Side branches may contain α-Lrhamnopyranose, β-D-glucuronic acid, β-D-galactopyranose, and α-L-arabinofuranosyl units with (1→3), (1→4), and (1→6) glycosidic linkages. Gum arabic has a high water solubility (up to 50% w/v) and relatively low viscosity compared to other exudate gums. The highly branched molecular structure and relatively low molecular weight of this polymer are responsible for these properties. Another unique feature of gum arabic is its covalent association with a protein moiety. It is thought that the protein moiety rich in hydroxyproline (Hyp), serine (Ser), and proline (Pro) constitutes a core to which polysaccharide subunits are attached via Ara-Hyp linkages (the wattle blossom model). The protein moiety of gum arabic is responsible for the surface activity, foaming, and

In the *European Pharmacopoeia* (6th edition, 2007), *gum tragacanth* is defined as 'the airhardened gummy exudates, flowing naturally or obtained by incision from the trunk and branches of *Astragalus gummifer* Labillardiere and certain other species of *Astragalus* from western Asia (mostly in Iran, some in Turkey)'. Tragacanth gum contains a water-soluble fraction and a water-insoluble fraction and the water-soluble fraction is accounted for 30 to 40% of the total gum. The water soluble fraction (tragacanthin) is a highly branched neutral polysaccharide composed of 1→6-linked D-galactosyl backbones with L-arabinose side chains joined by 1→2-, 1→3- and/or 1→5-linkages. The water-insoluble fraction (~60 to 70%), is tragacanthic acid (bassorin) which is a water-swellable polymer and is consisted of D-galacturonic acid, D-galactose, L-fucose, D-xylose, L-arabinose and L-rhamnose. It has a

**% (Pas×10-3)a**

5.0 300,045,000

1,000 >10,000

 **Viscosity** 

Mixture of

glycano-

glycano-

glycano-

Table 3. Main botanical and structural characteristics exudates gums

emulsifying properties of this polymer (Phillips & Williams, 2000).

**Gum Species of** 

Senegal

urens

latifolia

a Viscosity obtained at shear rates 10-S

Astragalus gummifer

Gum arbic Acacia

Gum karaya Sterculia

**3.1.4.1 Gum arabic** 

**3.1.4.2 Tragacanth gum** 

Gum ghatti Anogeissus

Gum tragacanth

food β-D-glucans are the kernels of oats, barley, wheat, and rye. β-D- glucans have been ascribed cholesterol and blood glucose lowering properties.

#### **3.1.2.6 Arabinogalactan**

Arabinogalactan is a major D-galactan obtained from soft-woods such as pine, larch, cedar, and spruce. This polymer has a main backbone of (1→3)-linked β-D-galactopyranosyl residues with β-(1→6)-linked disaccharides of β-D-Gal*p*-(1→6)-β-D-Gal*p* and α-(1→6) linked disaccharides of β-L-Ara*f*-(1→3)-α-L-Ara*f*. Arabinogalactan is generally a highly branched polymer with arabinose and galactose ratio of 1:6. Commercially available arabinogalactan, obtained from the butt wood of Western larch, has a relatively low molecular weight of 15,000 to 25,000, little impact on viscosity, color, and taste. It is used as a low-calorie additive in beverages to increase the fiber content.

#### **3.1.3 Pectins**

Pectins are polysaccharide and are the major components of most higher plant cell walls; they are particularly prevalent in fruits and vegetables. Commercial pectins are prepared mostly from some by-products of the food industry, such as apple pulp, citrus peels, and sugarbeet pulp. Pectins are the most complex class of plant cell wall polysaccharides. They comprise of two families of covalently linked polymers, galacturonans and rhamnogalacturonans.

Galacturonans are segments of pectins with (1→4)-linked α-D-galactosyluronic acid residues in the backbone, such as those in the linear homogalacturonans, in the substituted xylogalacturonans and in rhamnogalacturonans type II (RG II) (Cui, 2005). The carboxylic acid groups in galacturonans may be methyl esterified; the degree of esterification has an important effect on the conformation and solution properties of these polymers. Based on the degree of esterification, pectins are divided into two categories: low methyl (LM) pectin that contains less than 50% methyl esters, and high methyl (HM) pectin with more that 50% methyl esters. Xylogalacturonans are relatively recently discovered subunits of pectic polysaccharides, present in storage tissue of reproductive organs of peas, soybeans, apple fruit, pear fruit, onions, cotton seeds, and watermelon. The rhamnogalacturonans type II have been found in the cell walls of many tissues of edible plants, such as apple (juice), kiwi, grape (wine), carrot, tomato, onion, pea, and radish. Rhamnogalacturonans type I (RG I) have a backbone composed of alternating (1→2)-linked α-L rhamnosyl and (1→4)-linked α-D-galacturonic acid residues. Depending on the source of pectins, 20 to 80% of rhamnose residues may be branched at *O*-4 with side chains which vary in length and composition. The side branches may be composed of arabinans, galactans and type I arabinogalactans. Pectins with type I arabinogalactans have been found in potato, soybean, onion, kiwi, tomato, and cabbage.

#### **3.1.4 Exudate gums**

Exudate gums are polysaccharides produced by plants as a result of stress, including physical injury and/or fungal attack. Gum arabic, gum tragacanth, gum karaya, and gum ghatti have been used by humans for many thousands of years in various food and pharmaceutical applications (Table 3). Generally, these gums are structurally related to arabinogalactans, galacturonans, or glucuronomannans. They all contain a high proportion of glucuronic or galacturonic acid residues (up to 40%).

food β-D-glucans are the kernels of oats, barley, wheat, and rye. β-D- glucans have been

Arabinogalactan is a major D-galactan obtained from soft-woods such as pine, larch, cedar, and spruce. This polymer has a main backbone of (1→3)-linked β-D-galactopyranosyl residues with β-(1→6)-linked disaccharides of β-D-Gal*p*-(1→6)-β-D-Gal*p* and α-(1→6) linked disaccharides of β-L-Ara*f*-(1→3)-α-L-Ara*f*. Arabinogalactan is generally a highly branched polymer with arabinose and galactose ratio of 1:6. Commercially available arabinogalactan, obtained from the butt wood of Western larch, has a relatively low molecular weight of 15,000 to 25,000, little impact on viscosity, color, and taste. It is used as a

Pectins are polysaccharide and are the major components of most higher plant cell walls; they are particularly prevalent in fruits and vegetables. Commercial pectins are prepared mostly from some by-products of the food industry, such as apple pulp, citrus peels, and sugarbeet pulp. Pectins are the most complex class of plant cell wall polysaccharides. They comprise of two families of covalently linked polymers, galacturonans and

Galacturonans are segments of pectins with (1→4)-linked α-D-galactosyluronic acid residues in the backbone, such as those in the linear homogalacturonans, in the substituted xylogalacturonans and in rhamnogalacturonans type II (RG II) (Cui, 2005). The carboxylic acid groups in galacturonans may be methyl esterified; the degree of esterification has an important effect on the conformation and solution properties of these polymers. Based on the degree of esterification, pectins are divided into two categories: low methyl (LM) pectin that contains less than 50% methyl esters, and high methyl (HM) pectin with more that 50% methyl esters. Xylogalacturonans are relatively recently discovered subunits of pectic polysaccharides, present in storage tissue of reproductive organs of peas, soybeans, apple fruit, pear fruit, onions, cotton seeds, and watermelon. The rhamnogalacturonans type II have been found in the cell walls of many tissues of edible plants, such as apple (juice), kiwi, grape (wine), carrot, tomato, onion, pea, and radish. Rhamnogalacturonans type I (RG I) have a backbone composed of alternating (1→2)-linked α-L rhamnosyl and (1→4)-linked α-D-galacturonic acid residues. Depending on the source of pectins, 20 to 80% of rhamnose residues may be branched at *O*-4 with side chains which vary in length and composition. The side branches may be composed of arabinans, galactans and type I arabinogalactans. Pectins with type I arabinogalactans have been found in potato, soybean, onion, kiwi,

Exudate gums are polysaccharides produced by plants as a result of stress, including physical injury and/or fungal attack. Gum arabic, gum tragacanth, gum karaya, and gum ghatti have been used by humans for many thousands of years in various food and pharmaceutical applications (Table 3). Generally, these gums are structurally related to arabinogalactans, galacturonans, or glucuronomannans. They all contain a high proportion

ascribed cholesterol and blood glucose lowering properties.

low-calorie additive in beverages to increase the fiber content.

**3.1.2.6 Arabinogalactan** 

**3.1.3 Pectins** 

rhamnogalacturonans.

tomato, and cabbage.

**3.1.4 Exudate gums** 

of glucuronic or galacturonic acid residues (up to 40%).


a Viscosity obtained at shear rates 10-S

Table 3. Main botanical and structural characteristics exudates gums

#### **3.1.4.1 Gum arabic**

Acacia gum, also known as gum arabic, is a natural, vegetable exudate from acacia trees (primarily in Africa) known since antiquity and used for thousands of years in foods as an additive and ingredient, in the pharmaceutical industry and for technical purposes (Imenson, 2010). The thorny trees grow to a height of 7 to 8 meters, and the gum is obtained by cutting sections of the bark from the tree. The structure of gum arabic is relatively complex. The main chain of this polysaccharide is built from (1→3) and (1→6)-linked β-Dgalactopyranosyl units along with (1→6)-linked β-D-glucopyranosyl uronic acid units. Side branches may contain α-Lrhamnopyranose, β-D-glucuronic acid, β-D-galactopyranose, and α-L-arabinofuranosyl units with (1→3), (1→4), and (1→6) glycosidic linkages. Gum arabic has a high water solubility (up to 50% w/v) and relatively low viscosity compared to other exudate gums. The highly branched molecular structure and relatively low molecular weight of this polymer are responsible for these properties. Another unique feature of gum arabic is its covalent association with a protein moiety. It is thought that the protein moiety rich in hydroxyproline (Hyp), serine (Ser), and proline (Pro) constitutes a core to which polysaccharide subunits are attached via Ara-Hyp linkages (the wattle blossom model). The protein moiety of gum arabic is responsible for the surface activity, foaming, and emulsifying properties of this polymer (Phillips & Williams, 2000).

#### **3.1.4.2 Tragacanth gum**

In the *European Pharmacopoeia* (6th edition, 2007), *gum tragacanth* is defined as 'the airhardened gummy exudates, flowing naturally or obtained by incision from the trunk and branches of *Astragalus gummifer* Labillardiere and certain other species of *Astragalus* from western Asia (mostly in Iran, some in Turkey)'. Tragacanth gum contains a water-soluble fraction and a water-insoluble fraction and the water-soluble fraction is accounted for 30 to 40% of the total gum. The water soluble fraction (tragacanthin) is a highly branched neutral polysaccharide composed of 1→6-linked D-galactosyl backbones with L-arabinose side chains joined by 1→2-, 1→3- and/or 1→5-linkages. The water-insoluble fraction (~60 to 70%), is tragacanthic acid (bassorin) which is a water-swellable polymer and is consisted of D-galacturonic acid, D-galactose, L-fucose, D-xylose, L-arabinose and L-rhamnose. It has a

Hydrocolloids in Food Industry 27

be increased substantially. It has been shown that the neutral (1→4)--D-glucan fraction of yellow mustard mucilage synergistically interacts with galactomannans. Yellow mustard is used in processed meat formulations and salad dressing as a stabilizer and bulking agent.

Flaxseed mucilage can be easily extracted from the seeds by soaking them in warm water. The mucilage constitutes the secondary wall material in the outermost layer of the seed. Upon hydration of the seeds, it expands, breaks the mucilage cells, and exudes on the surface of the seeds. Flaxseed mucilage contains 50 to 80% carbohydrates and 4 to 20% proteins and ash. Flaxseed mucilage contains a mixture of neutral polysaccharides, composed mainly of xylose, arabinose and galactose residues, and acidic polysaccharides, containing galactose, rhamnose, and galacturonic acid residues. The neutral fraction of flaxseed mucilage has a backbone of (1→4)-linked β-D-xylopyranosyl residues, to which

The acidic fraction of flaxseed mucilage has a rhamnogalacturonan backbone with (1→4) linked α-D-galacturonopyranosyl and (1→2)-linked α-L-rhamnopyranosyl residues. The ratio of neutral to acidic polysaccharides in flaxseed may vary substantially with their origin. Unfractionated flaxseed mucilage forms a viscous solution, but it is the neutral fraction that mainly contributes to the high viscosity and weak gel-like properties of this gum. Flaxseed mucilage has not yet been widely utilized mostly because of limited information about the structure and functional properties of this gum. Similar to other gums, flaxseed mucilage can be used as a thickener, stabilizer, and water-holding agent.

Fructans are reserve polysaccharides in certain plants, either complementing or replacing starch. They can also be produced by certain species of bacteria. A main kind of fructans is inulin. Inulins are found in roots or tubers of the family of plants known as Compositae, including dandelions, chicory, lettuce, and Jerusalem artichoke. They can also be extracted from the Liliacae family, including lily bulbs, onion, tulips, and hyacinth. Inulin is a low

Alginates constitute the primary structural polysaccharides of brown seaweeds (*Phaeophyceae*). The alginate molecules provide both flexibility and strength to the plants and these properties are adapted as necessary for growth conditions in the sea. The major species of seaweeds that produce alginates are *Macrocystis pyrifera,* grown primarily along the California coast of the USA*,* south- and north-western coasts of South America, and coasts of Australia and New Zealand. Other good sources of alginates are *Laminaria hyperborea*, *Laminaria digitata*, and *Laminaria japonica*, grown along the north Atlantic coast of the USA, Canada, France, and Norway. Alginates can also be synthesized by bacteria, *Pseudomonas aeruginosa* and *Azobacter vinelandii*. Alginates are unbranched copolymers of (1→4)-linked β-D-mannuronic acid (M) and α-L-guluronic acid (G) residues. If the uronic acid groups are in the acid form (–COOH), the polysaccharide, called alginic acid, is water insoluble. The sodium salts of alginic acid (–COONa), sodium alginates, are water soluble. The sequence of mannuronic and guluronic residues significantly affects the physicochemical properties of alginates. The ratio of β-D-mannuronic acid to α-L-guluronic

arabinose and galactosecontaining side chains are linked at *O*-2 and/or *O*-3.

molecular weight polysaccharide containing (2→1) linked β-D-Fru*p* residues.

**3.1.5.3 Flaxseed mucilage** 

**3.1.6 Fructans** 

**3.2.1 Alginates** 

**3.2 Seaweed hydrocolloids** 

(1→4)-linked α-D-galacturonopyranosyl backbone chain with randomly substituted xylosyl branches linked at the 3 position of the galacturonic acid residues. In spite of the availability of alternative materials, the continued use of the gum is the result of its unique functional properties combined with a high degree of stability in a range of conditions.
