**3.1 Plant hydrocolloids**

#### **3.1.1 Cellulose and derivatives**

Cellulose is the most abundant naturally occurring polysaccharide on earth. It is the major structural polysaccharide in the cell walls of higher plants. It is also the major component of cotton boll (100%), flax (80%), jute (60 to 70%), and wood (40 to 50%). Cellulose can be found in the cell walls of green algae and membranes of fungi. Acetobacter xylinum and related species can synthesize cellulose. Cellulose can also be obtained from many agricultural byproducts such as rye, barley, wheat, oat straw, corn stalks, and sugarcane. Cellulose is a high molecular weight polymer of (14)-linked -D-glucopyranose residues. The -(14) linkages give this polymer an extended ribbon-like conformation. The tertiary structure of cellulose, stabilized by numerous intermolecular H-bonds and van der Waals forces, produces three-dimensional fibrous crystalline bundles. Cellulose is highly insoluble and impermeable to water. Only physically and chemically modified cellulose finds applications in various foodstuffs (Cui, 2005).

#### **3.1.1.1 Microcrystalline cellulose**

Microcrystalline cellulose (MCC) is purified cellulose, produced by converting fibrous cellulose to a redispersible gel or aggregate of crystalline cellulose using acid hydrolysis. Microcrystalline cellulose is prepared by treating natural cellulose with hydrochloric acid to partially dissolve and remove the less organized amorphous regions of this polysaccharide. The end product consists primarily of crystallite aggregates. MCC is available in powder form after drying the acid hydrolysates. Dispersible MCC is produced by mixing a hydrophilic carrier (e.g., guar or xanthan gum) with microcrystals obtained through wet mechanical disintegration of the crystallite aggregates (Cui, 2005). These colloidal dispersions are unique when compared to other soluble food hydrocolloids. They exhibit a variety of desirable characteristics including suspension of solids, heat stability, ice crystal control, emulsion stabilization, foam stability, texture modification and fat replacement (Imeson, 2010).

Hydrocolloids in Food Industry 23

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.,

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

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

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

Galactomannan Species of Origin Man:Gal Ratio

Locust bean gum Ceratonia siliqua 3.5 Senna gum Senna occidentalis 3.5 Guar gum Cyamopsis tetragonolobus 1.6 Tara gum Caesalpinia spinosa 1.3 Fenugreek gum Trigonella foenum graecum 1

Table 2. Botanical Origin and Main Structure Features of Galactomannans

**3.1.2.2 Xyloglucans** 

**3.1.2.3 Glucomannans** 

gel in the presence of alkali.

**3.1.2.4 Arabinoxylans** 

**3.1.2.5 β-D-Glucans** 

1998).

units.

#### **3.1.1.2 Carboxymethylcellulose**

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