SOLUTION PROPERTIES OF KONJAC GLUCOMANNAN

3.3 SOLUTION PROPERTIES OF KONJAC GLUCOMANNAN

3.3.1 I NTRINSIC V ISCOSITY

The intrinsic viscosity was determined in water and in 4 M urea for the four fractions, F1 to F4. 10 The intrinsic viscosity observed in 4 M urea was higher than that observed in water for all the fractions. The conformation of konjac glucomannan molecules may be more expanded in urea because hydrogen bonds between hydroxyl groups in konjac glucomannan molecules are broken.

A fraction with a high molecular weight began to deviate from the straight line in the plot of the reduced viscosity vs. konjac glucomannan concentration at a lower concentration than a fraction with a lower molecular weight. This has also been

observed for pullulan 14 and many other polymers.

3.3.2 Z ERO -S HEAR S PECIFIC V ISCOSITY

The relationship between the logarithm of the zero-shear specific viscosity (η sp0 ) and the logarithm of the KGM concentration (c) is shown in Figure 3.2. Log η sp0

increased linearly with increasing log c when the value of log η sp0 was lower than about 1. The double logarithmic plots of the zero-shear specific viscosity η sp0

against the coil overlap parameter (c[ η]) are shown in Figure 3.3. For dilute solutions, slopes of the plots were close to 1.4 for all the fractions, as observed for many polysaccharide solutions. It was not possible to obtain a clear inflection

Konjac Glucomannan

FIGURE 3.2 Concentration dependence of the zero-shear specific viscosity (η sp0 ) of konjac glucomannan solution. UnF (unfractionated material, ●), F2 (䡩), F3 (Δ), F4 (▫). (From Kohyama, K. and Nishinari, K., Jpn. Agric. Res. Q., 31, 301, 1997. With permission.)

FIGURE 3.3 Dependence of the zero-shear specific viscosity (η sp0 ) of konjac glucomannan solution on the coil overlap parameter, c[ η]. Symbols are the same as in Figure 3.2. (From Kohyama, K. and Nishinari, K., Jpn. Agric. Res. Q., 31, 301, 1997. With permission.)

Functional Food Carbohydrates

10 3 C.(%)

0.35% G ʹ 10 2 0.35% G ʺ

0.70% G ʹ 0.70% G ʺ 10 1 1.05% G ʹ

ʺ/Pa 1.05% G ʺ ʹ, G

1.40% G ʹ G 10 0 1.40% G ʺ

ω/rad ⋅ s −1

FIGURE 3.4 Frequency dependence of G' and G" of KGM aqueous dispersions in various concentrations. C: concentration of KGM (wt %) (From Yoshimura, M. et al., Carbohydr. Polym ., 35, 71, 1998. With permission.)

point of the curves in Figure 3.3, because the low solubility of konjac glucomannan made it difficult to prepare solutions that exhibited large c[η] values. However,

the inclination of the slope of the curves increased gradually with increasing log c[ η], suggesting that significant coil overlap and entanglement had already started when c[ η] > 1. The onset of coil overlap occurs at lower concentrations for KGM molecules than for other polysaccharides.

3.3.3 D YNAMIC V ISCOELASTICITY OF KGM D ISPERSIONS

The frequency dependence of the storage shear modulus, G', and the loss shear modulus, G'', for KGM dispersions of different concentrations is shown in Figure

3.4. The behavior of KGM plus water is typical of a concentrated polymer solution. In concentrated polymer solutions the response is liquid-like, i.e., G'' is larger than G' and both moduli increase with increasing frequency at lower frequencies, while the behavior approaches that of solid-like materials, i.e., G' is larger than G'', and both moduli become frequency independent at higher fre- quencies. The molecular chains can disentangle and rearrange during the long period of oscillation, so G'' is larger than G' at lower frequencies in concentrated polymer solutions. At higher frequencies, molecular chains cannot disentangle during the short period of oscillation, so G' is larger than G'', because the entanglement points play the role of a temporary cross-linking junction zone. Moreover, the crossover frequency of G' and G'' shifted as expected to lower frequencies with increasing KGM concentration.

Konjac Glucomannan

3.3.4 O THER S OLUTION P ROPERTIES

The partial specific volume of konjac glucomannan was determined as a function of pH by density measurements 16 and showed a steep rise at around pH = 11.5

and 3, as shown in Figure 3.5. This suggests that the conformational change is necessary for gel formation of konjac glucomannan. The typical results of light- scattering measurements, carried out for acetylated konjac glucomannan Ac21 prepared by using acetic anhydride in the presence of zinc chloride as a catalyst,

are shown in Figure 3.6. 17 From the intercept with the ordinate and the slopes

FIGURE 3.5 Apparent partial specific volume of konjac glucomannan as a function of pH at 25.0˚C. The pH was adjusted by addition of HCl or NaOH. Concentration of konjac gluco- mannan: 0.22 w/w%. (From Kohyama, K. and Nishinari, K., in Gums and Stabilisers for the Food Industry 5 , Phillips, G.O. et al., Eds., IRL Press, Oxford, 1990, p. 459. With permission.)

c →0 /mol

M w = 3.17 × 10 5

10 Ac21 in cadoxen at 25 °C

R G,z = 42.8 nm

A 2 = 0.67 × 10 –4 mol cm 3 g − 2 0 −

sin 2 (θ/2) - 792 (c/g cm − 3 )

FIGURE 3.6 Zimm plot of the acetylated KGM sample Ac21 in cadoxen at 25˚C. 17

Functional Food Carbohydrates

against the concentration and scattering angle, weight-average molecular weight, M w, second virial coefficient, A 2 , and z-average radius of gyration, R G,z , were evaluated, respectively. KGM forms a biphasic liquid crystal (LC) phase in water at 7 wt% concentration and becomes completely anisotropic above 10 wt% concentrations, as observed by polarized optical microscopy. 18 No order–disorder transition was observed when the LC solutions were heated on the hot stage. Circular dichroism spectra show positive bands at 210 and 290 nm for the LC phase and the shear-induced birefringence, respectively. Increases in the intensity of wide-angle x-ray diffraction patterns of the films cast from LC solutions provided further evidence of the existence of mesophase in the KGM solutions.