Guar

The solid state of guar gum is basically the same as that of locust bean gum. In contrast to locust bean gum, guar gum dissolves at temperatures in the region of 25 to 40°C with stirring, and this solubility at lower temperatures occurs as a result of the lower M:G ratio in guar gum (Morris, 1990b). The lower M:G ratio in guar gum and the more even distribution of galactose residues along the mannan backbone mean that there are fewer galactose-deficient segments in the polymer chain, hence fewer intermolecular interactions between them and consequently, less heat required for solubilization of the polymer.

The viscosity behavior of guar gum solutions is similar to that of locust bean gum solutions. Typical random-coil-like behavior is observed below the onset of coil overlap but the transition from dilute solution behavior to concentrated solution behavior (c*) occurs at a lower degree of space occupancy (c[n] ~ 2.5), while the concentration dependency of viscosity above c* is greater than expected for typical random coil polymers. Unlike locust bean gum which has a significantly higher M:G ratio, there is little evidence for any renaturation of intermolecular structure on aging guar gum solutions. Guar gum solutions have zero yield values and their behavior under shear is typically random-coil-like, being pseudoplastic and showing the same general shear-rate dependency of viscosity as locust bean gum and other random coil hydrocolloids.

As with locust bean gum, the main functional properties of guar gum arise as a result of its highly efficient thickening properties and water-binding capacity. In most applications, guar gum is utilized at concentrations below 1%, and at concentrations above this, solutions of guar gum, although still possessing zero yield value, have an almost gellike appearance due to their high viscosity and viscoelastic properties (Seaman, 1980b). Figure 9.6 shows the mechanical spectra of guar gum solutions at different concentrations. At low frequencies of oscillation and low concentrations, the response of the guar gum solution is predominantly liquid-like (i.e., G" » G' ) but, with increasing frequency of oscillation and concentration, the mechanical spectrum indicates a more solid-like response (i.e., G' > G"). This crossover from predominantly liquid-like behavior to predominantly solid-like behavior is typical of concentrated solutions of random coil hydrocolloids, and as discussed earlier, has its origins in the inability of an entangled network of polymer chains to disentangle within the time scale of an oscillation.

The effects of salts and pH on the viscosities of guar gum solutions are primarily the same as for locust bean gum solutions, with typical salts used in the food industry having little effect. The viscosities of guar gum solutions are reasonably stable between pH 3.5 and pH 9.0 (Herald, 1986b). The pH, however, does affect the rate of hydration of guar gum, with the maximum hydration rate occurring at a pH of about 8. As with locust bean gum, the effect of high shear rates and prolonged exposure to high temperatures can cause irreversible degradation of guar gum with the associated reduction in solution

Figure 9.6 Frequency dependence of G' (filled symbols) and G" (open symbols) for guar gum solutions at different concentrations (1%, 2% and 3%). (Reprinted from, Robinson, G., Morris, E.R. and Ross- Murphy, S.B., Carbohydr. Res., 107, Viscosity-molecular weight relationships, intrinsic chain flexibility and dynamic solution properties of guar galactomannan, 17, 1982. With permission from Elsevier Science BV, Amsterdam Publishing Division, Sara Burgerhartstraat 25, 1055 KV, Amsterdam, The Netherlands.)

viscosity as a consequence of polymer chain cleavage. Guar gum also shows synergistic effects with other food hydrocolloids such as carrageenan and xanthan but, unlike locust bean gum, guar gum does not interact with xanthan gum to give thermoreversible gels, but gives only a synergistic viscosity increase (Morris, 1990b).

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