YLOGLUCAN FROM T AMARIND S EED

4.3.1 X YLOGLUCAN FROM T AMARIND S EED

4.3.1.1 Source

Xyloglucan is a member of a group of so-called hemicelluloses that are plant cell wall polysaccharides composed of a cellulosic backbone and some branches. An important biological function of xyloglucan is that it binds to cellulose microfibrils and controls the rigidity of the cell wall, thereby controlling cell growth. 59–62 Xylo- glucan polymers incorporated into growing plant cells have been found to increase

the elastic modulus of the cell wall and suppress cell elongation. 62 Similarity in the backbone structure between xyloglucan and cellulose should facilitate noncovalent association of xyloglucan and cellulose. 63–65 In contrast, xyloglucan oligosaccharides activate xyloglucan endotransglycosylase that degrades the polymer in the presence

of the oligosaccharides, loosen the cell wall, and promote cell elongation. 62 Addi- tionally, xyloglucan oligosaccharides induce phytoalexin that protects a plant body from microbial infection, indicating their potential application as an environment- friendly pesticide. 66

Xyloglucan occurs widely in the primary cell wall of higher plants, while the major source of commercially available food-grade xyloglucan is storage xyloglucan

Seed Polysaccharide Gums

in the seed of tamarind tree (Tamarindus indica), indigenous to India and Southeast Asia. 66 Recent studies have revealed potentially therapeutic benefits of xyloglucan extracted from the seed of Detarium senegalense Gmelin, the flour of which has been used as a thickening agent in traditional Nigerian foods. 67,68

4.3.1.2 Methods of Production

Xyloglucan may account for 20 to 30% of the dry weight of the primary cell wall of dicotyledons and nongraminaceous monocotyledons. Tamarind seed xyloglucan

is the only seed xyloglucan currently produced on an industrial scale. 66 Tamarind seeds are washed with water, heated, de-hulled, and ground to provide tamarind kernel powder (TKP). Tamarind seed xyloglucan (TSX) is a water-soluble polysac- charide fraction prepared from TKP. TSX is a permitted food additive in Japan, Korea, and Taiwan. The purity of commercially available TSX can be improved by repeating solubilization in water and precipitation using alcohol.

4.3.1.3 Chemistry and Structural Features

Xyloglucan has a backbone of 1 →4-linked β- D -glucopyranosyl residues, three quar- ters of which is substituted with α- D -xylopyranose at the 6-position. Some of the xylopyranosyl residues are substituted at the 2-position with β- D -galactopyranose. In the living cell wall, a part of the galactosyl residues are further substituted at the 2-position by an α- L -fucosylpyranose. Four types of structural units, or monomers,

have been identified in extracted xyloglucan: Glc 4 Xyl 3 heptasaccharide, two types of Glc 4 Xyl 3 Gal octasaccharides, and Glc 4 Xyl 3 Gal 2 nonasaccharide (Figure 4.2).

Tamarind and detarium seed xyloglucans slightly differ in the content of the galac- tosyl residue (Table 4.1). 69 Synchrotron-radiated small-angle x-ray scattering (SAXS) and molecular dynamics simulation studies on tamarind xyloglucan mono- mers have suggested that they can be regarded as flat ellipsoids in shape. 70 The evaluated length of the shortest semiaxis is constantly 0.22 nm for all types of monomers, while the cross-sectional width increases from 0.62 nm for the heptasac- charide to 0.71 nm for the octasaccharide and 0.75 nm for the nonasaccharide with increasing number of side-chain residues. The longest axis, corresponding to the backbone length, decreases from 1.49 nm for the heptasaccharide to 1.43 nm for the octasaccharide and 1.41 nm for the nonasaccharide, indicating that the β-glucan backbone with a larger number of side-chain residues is more arched or twisted.

Reported values of the molecular weight evaluated based on light scattering vary from 880,000 to 1,160,000 for tamarind xyloglucan to 2,690,000 for detarium xyloglucan. 69,71 X-ray fiber diffraction analyses have confirmed that xyloglucan adopts an extended twofold helix conformation similar to cellulose in solid state. 70 The conformation in an aqueous solution of xyloglucan polymer has been investi-

gated using light scattering and synchrotron-radiated SAXS. 71 The evaluated cross- sectional radius of gyration is 0.58 nm for tamarind xyloglucan and 0.49 nm for detarium xyloglucan. Earlier SAXS studies have reported a cross-sectional radius

of gyration value of tamarind xyloglucan to be 0.29 nm. 72 The light-scattering profiles of tamarind xyloglucan suggest an extended and stiff molecular chain with the Kuhn

Functional Food Carbohydrates

Heptasaccharide (XXXG)

α D Xylp1

α D Xylp1

α D Xylp1

4β D Glcp1 4β D Glcp1 4β D Glcp1 4β D Glcp1 Octasaccharide (XLXG)

β D Galp1 2

α D Xylp1

α D Xylp1

α D Xylp1

4β D Glcp1 4β D Glcp1 4β D Glcp1 4β D Glcp1 Octasaccharide (XXLG)

β D Galp1 2

α D Xylp1

α D Xylp1

α D Xylp1

4β D Glcp1 4β D Glcp1 4β D Glcp1 4β D Glcp1 Nonasaccharide (XLLG)

β D Galp1

β D Galp1

α D Xylp1

α D Xylp1

α D Xylp1

4β D Glcp1 4β D Glcp1 4β D Glcp1 4β D Glcp1 FIGURE 4.2 Structural features of tamarind and detarium seed xyloglucans. (Adapted from

Wang, Q. et al., Carbohydr. Res., 283, 229, 1996.)

TABLE 4.1 Comparison of Composition of Monosaccharide and Oligosaccharide

Tamarind and Detarium Xyloglucans 75

Source Oligosaccharides Monosaccharides XXXG

Galactose Glucose

Tamarind 1 0.42 2.07 6.2 1 0.51 1.34 Detarium

Note : X = xylose-substituted glucose residue; L = galactosylxylose-substituted glucose resi- due; G = unsubstituted glucose residue.

Seed Polysaccharide Gums

segment length, a measure of chain rigidity, ranging from 108 to 184 nm. 70 The backbone is supposed to be twisted like cellobiose in a solution. 63 Atomic force microscopy has been utilized to directly visualize tamarind xyloglucan polymers. 73 Xyloglucan molecules spread onto the molecularly flat surface of mica appear as largely linear chains with some branches. The widely ranging contour length of the main chain, approximately 0.1 to 1.5 μm, suggests a highly polydisperse nature of the molecular weight. The heights of chains, measures of the cross-sectional chain diameter, are fairly uniformly about 0.6 nm. On the other hand, reported light- scattering profiles of detarium xyloglucan are inconsistent with those of a linear polymer, but more similar to those of a branched polymer with long side-arm

chains. 74 SAXS profiles of enzymatically carboxylated detarium xyloglucan have also supported the hypothesis that the main chain of detarium xyloglucan is highly branched, and thus the overall conformation is more compact than an extended tamarind xyloglucan molecule.

4.3.1.4 Functional Properties and Applications

The intrinsic viscosity has been determined to be 6.0 dl/g for tamarind xyloglucan 72 and 8.9 dl/g for detarium xyloglucan, 69 consistent with the higher molecular weight of detarium xyloglucan. Steady-flow characteristics of dilute solutions of xyloglucan are described essentially as Newtonian. Solutions at higher xyloglucan concentra- tions (ca. >0.5% w/w) exhibit a constant viscosity at relatively low shear rates, and

at higher shear rates, shear thinning is observed. 74 The onset of shear thinning shifts to a lower shear rate with increasing xyloglucan concentration. Dynamic rheological properties of xyloglucan solutions are similar to those of ordinary polymer solutions. Mechanical spectra exhibit a transition from a spectrum typical of dilute polymer solutions (G' < G'' at all frequencies) to that of semidilute polymer solutions (G' <

G '' at low frequencies and G' > G'' at higher frequencies) with increasing concen- tration. 74,75 Rheological properties are stable against heat (e.g., 100°C for 2 h) and pH (e.g., for 45 days in the presence of 2.25% acetic acid and 1.0% salt), making this polysaccharide a promising candidate for a physically functional food ingredi-

ent. 66 The absence of ionic groups in the molecule may indicate insensitivity of solution properties to the presence of salts. Tamarind xyloglucan can be used as a starch replacer in food products since its solution properties are similar to gelatinized starch but more stable against heat, pH, and mechanical distortion.

Tamarind xyloglucan can form a gel in the presence of alcohol 76 or a large amount (40 to 70% w/w) of sugar. 77 Gels made with alcohol are hard and melt at a lower temperature than gels with sugar. SAXS studies have confirmed the absence of ordered structures in alcohol-induced gels at the nanometer scale, suggesting that cross-linking domains are composed of randomly aggregated polysaccharide chains,

and the side groups prevent substantial aggregation or precipitation. 76 Sugar-induced gels are elastic and have good water-holding properties. Freeze–thaw processes can make the gels harder and more elastic.

Xyloglucan is categorized as an amyloid that exhibits a characteristic blue color when an iodine–potassium iodide solution is added. In the case of iodine–amylose reaction, amylose molecules transform into single helices and form inclusion

Functional Food Carbohydrates

compounds with iodine. The architecture of an iodine–xyloglucan complex may be different: an iodine molecule is held between two laterally associated xyloglucan chains. Supporting evidence of this structural model is the fact that a thermoreversible gel is formed in the presence of iodine at a sufficiently high

concentration. 66 It is likely that an iodine–xyloglucan complex plays a role as a cross-link in such a gel network. Tamarind xyloglucan is also known to form a gel with polyphenols such as catechin. 66

Not much information is available regarding interactions of xyloglucan with polysaccharides other than cellulose. Effects of xyloglucan on gelatinization and retrogradation of corn starch have been investigated, but no significant interaction

has been recognized. 75 Synergistic effects on dynamic viscoelasticity have been found between xyloglucan and gellan, a microbial-produced gelling polysaccha- ride, and the synergistic effect is attributed to the exclusion effects of highly hydrophilic xyloglucan molecules that effectively increase the local concentration of the gellan polysaccharide. 73

The galactoxylose branch of xyloglucan is considered to generate steric hin- drance and prevent intermolecular association. Thus, ordinary xyloglucan is soluble in cold water and forms a gel only in the presence of alcohol or a substantial amount of sugar. It has been revealed that enzymatic elimination of more than 35% of original galactose residues with β-galactosidase imparts gelling ability to the modified xylo-

glucan. 78 Additionally, sol–gel transition behavior of this galactose-cleaved xyloglu- can (up to ca. 60% of the total galactose residues) presents a quite unique feature:

a sol of modified xyloglucan turns into a gel at a certain temperature on heating, indicating involvements of intermolecular association driven by hydrophobic inter-

actions, and the gel melts at a higher temperature on further heating. 78 Cross-linking domains in such a gel are considered to be composed of randomly aggregated polysaccharide chains, similar to the case of alcohol- or sugar-induced gelation of unmodified xyloglucan. The unique temperature sensitivity of enzyme-modified xyloglucan may indicate its potential use as a controlled drug delivery material.

4.3.1.5 Physiological Properties and Health Benefits

Xyloglucan is regarded as a dietary fiber that can increase the viscosity of digesta in the stomach and small intestine to reduce the rate and extent of absorption of

nutrients. A dietary fiber would also have an impact on fermentable intestinal bacteria as prebiotics. Tamarind xyloglucan has been shown to reduce plasma and liver

cholesterol levels in rats on high-cholesterol diets. 79 However, a high viscosity can

be regarded as a drawback since texture or mouth feel of food products is also influenced. A strategy to reduce the viscosity is to reduce the molecular weight of

the polymer. Partially hydrolyzed xyloglucans have been reported to be less viscous but maintain hypocholesterolemic effects, albeit to a lesser extent than the intact

polymer. 79 Other studies have reported that tamarind xyloglucan prevents suppres- sion of immune responses in mice exposed to ultraviolet irradiation. 80 Detarium xyloglucan has been found to have promise in the treatment of diabetes and hyper- lipidemia. 67,68,81 Postprandial plasma glucose and insulin levels of healthy human subjects significantly decreased after consumption of meals supplemented with

Seed Polysaccharide Gums

detarium seed flour. 68 Detarium seed flour also significantly reduced plasma choles- terol levels in rats. 81