G UAR G UM

4.2.2 G UAR G UM

4.2.2.1 Source

The development of guar gum resulted from the shortage of locust bean gum in the 1940s. The guar plant is an annual summer legume that is cultivated mainly in western India and eastern Pakistan, and to a lesser extent in tropical areas, such as South and Central America, Africa, Brazil, Australia, and the semiarid regions of the U.S. Southwest. The plant grows to about 1 m in approximately 5 months and yields pods slightly smaller than the carob pod. The guar pod contains no more than

10 seeds, in which the endosperm corresponds to 35% by weight.

4.2.2.2 Method of Production

The endosperm of guar is also called splits because of its structure. The manufac- turing process of guar gum is similar to that of locust beam gum: milling of splits, dissolution in hot water, removal of insoluble materials, alcohol precipitation, wash- ing, pressing, drying, grinding, and sieving. Final gum properties depend on the extent of separation. Contamination by rotten black seeds in the raw material causes specks in the gum and a darker color of the powder. Residual germ may cause enzymatic degradation of the gum.

4.2.2.3 Chemistry and Structural Features

The molar ratio of galactose to mannose of guar gum is approximately 1:2. The side group substitution occurs irregularly: side groups are arranged mainly in pairs and

triplets. 6 The average molecular weight varies, typically up to a few million, depend- ing on growth and manufacturing factors. The persistence length of molecularly solubilized guar gum has been reported to be ca. 4 nm, similar to that of molecularly

solubilized locust bean gum. 11 It thus seems that the intrinsic flexibility of the mannan backbone itself is little influenced by galactose substitution. Differences in functional

properties between locust bean and guar gums may rather reflect differences in their solubility and tendency toward intra- and intermolecular aggregation that are relevant

to the extent of galactose substitution.

4.2.2.4 Functional Properties and Applications

Due to the high degree of substitution, guar gum is cold water soluble and serves as an effective thickener or stabilizer in the food and other industries. The thickening ability of commercially available guar gum is often higher than that of locust bean gum if no special care is taken for solubilization. Solution properties are fairly stable over the wide pH range of 4 to 10, but the viscosity significantly decreases at pH over 10. 15,28 The hydration rate reduces in the presence of salts and other water- binding agents, such as sucrose. 29–31

Functional Food Carbohydrates

Rheological properties of guar gum solutions share many common features with those of locust bean gum. Guar gum solutions show pseudoplastic steady-flow behavior

at high shear rates, and Newtonian domains can be observed at sufficiently low shear rates. 32,33 In the Mark–Houwink–Sakurada equation, which describes the relationships between the intrinsic viscosity and molecular weight, the value of the exponent α of

guar gum is reported to be 0.70 to 0.75. 16,32 Additionally, recent ultra-small-angle light- scattering studies have probed the presence of large aggregates in the order of 10 to

100 μm in guar gum aqueous solutions, 34 suggesting that the cold-water solubility does not necessarily guarantee molecular solubilization of guar gum. Dynamic rheo- logical properties of guar gum have features typical of an ordinary random coil polymer. Mechanical spectra, the frequency dependence of the storage (G155) and loss (G'') moduli, show transitions from dilute to semidilute behavior with increasing gum concentration. The relaxation frequency, i.e., G' – G'' crossover frequency, shifts toward lower frequencies with increasing concentration. All mechanical spectra can

be superposed onto a single master curve by dividing the moduli by the concentration and shifting the spectra laterally along the frequency axis (Figure 4.1). 32 Similar to locust bean gum, guar is regarded as a nongelling gum, while syner- gistic interactions are also observed between guar and other gums, such as xanthan, 3 κ-carrageenan, 1,2 and yellow mustard gum. 4 The magnitude of a synergistic increase in gel strength is much smaller than that of locust bean gum. The fact that the synergistic interactions are more pronounced with lowering galactose contents of galactomannans seems to be in line with the hypothesis that unsubstituted smooth regions of the galactomannan chain bind to xanthan or κ-carrageenan helices, and that a coupled gel network is eventually formed. 35,36 Another plausible explanation is that the galactomannans are incompatible with xanthan or κ-carrageenan, and that the effective galactomannan concentration becomes higher than the bulk concentra- tion in the presence of these helix-forming polysaccharides. 2

Borate is known to cause gelation of guar gum. 37 The borate ion reacts with the cis -hydroxyl groups of guar gum, such as the hydroxyl groups at the 2- and 3-positions of the backbone mannose and those at the 3- and 4-positions of the galactose side

10 2 . dl/g) 10 1

FIGURE 4.1 Master curve derived from frequency–concentration superposition of guar galacto- mannans. (Replotted from Robinson, G. et al., Carbohydr. Res., 107, 17, 1982. With permission.)

Seed Polysaccharide Gums

residues. A gel can be formed at room temperature in the presence of borax in alkaline solution, while the gel melts on heating. The pH and temperature dependences of gelling properties can be explained by the pH and Arrhenius type temperature depen- dences of the dissociation and binding equilibrium constants of the borate ion.

Guar gum is also an effective starch modifier. Gelatinized starch paste containing guar gum can be regarded as a suspension of swollen starch granules dispersed in

a guar gum solution. 24 Since the local concentration of guar gum is expected to be higher than the bulk concentration, the viscosity of a starch–guar gum mixture is normally much higher than that of starch or galactomannan alone. Amylose that leaches out of granules during gelatinization should reside in a liquid phase as well. Incompatibility between amylose and guar gum may cause a further increase in the effective concentration of guar gum. 38

The supply of guar gum is stable not only because the plant origin can be harvested every year, but also because sufficient stock is constantly available. Just like locust

bean gum, guar gum has a wide range of applications in food products as a thickener and stabilizer. For example, guar gum is used for obtaining mouth feeling in soup products, improving extensibility of cheese spread and low-sugar jam, and preventing syneresis in meat products. In contrast to locust bean gum, which is susceptible to freeze–thaw processes, guar gum is tolerant to repeated “freeze–thaw cycles” and prevents the formation of large ice crystals during freezing. Guar gum is used to achieve consistency of wheat flour dough elasticity and the water retention property, which vary significantly depending on the gluten content of the flour. A commercial guar gum ingredient may generate yellow color in an alkaline condition, which can be used for adjusting the appearance of products.

4.2.2.5 Physiological Properties and Health Benefits

Physiological activities of guar gum have been relatively well established. By the end of the 1970s, the efficacy of guar gum in reducing plasma cholesterol levels

and postprandial hyperglycemia was well known. 39,40 More recent studies have ensured that guar gum can be an effective tool for the dietary management of elevated plasma total and low-density lipoprotein cholesterol if administered into

normal daily diets. 41 Physiological effects of guar gum are considered due to not only an increased viscosity of digesta, but also to fermentation of the gum by the colonic microflora. 42 Guar gum is, however, a potential deterrent to palatability since organoleptic characteristics of guar-containing food products tend to be poor because of high levels of viscosity. Efforts have been made to suppress excessive thickening and enhance the amount of intake by partially hydrolyzing the gum. 43 Enzymatic hydrolysis has been shown to be effective in controlled depolymeriza- tion of guar gum. 44