MIXTURE OF KONJAC WITH OTHER POLYSACCHARIDES

3.5 MIXTURE OF KONJAC WITH OTHER POLYSACCHARIDES

3.5.1 K ONJAC –X ANTHAN M IXTURES

Since the discovery of the synergistic effect of xanthan and locust bean gum, there have been many investigations on the interaction between two different polysac- charides.

Individually, neither konjac nor xanthan dilute aqueous solution forms a gel at

a neutral pH and room temperature; however, on mixing, a gel can be produced. Annable and her coworkers 26 studied the interaction of these polymers and found that the mixture on heating above 55 °C formed a gel. This temperature is lower than the conformational transition temperature of xanthan molecules (Figure 3.13). It was concluded, therefore, that the junction zones in mixed gels are formed by the

association of konjac and the helical form of xanthan. 26 The effects of salts were also examined, and it was found that their addition shifted the gelation temperature of the mixture to lower temperatures. This was attributed to the self-association of

Konjac Glucomannan

Frequency/Hz

FIGURE 3.13 G' as a function of frequency for xanthan in 0.04 mol/dm 3 NaCl in the presence of glucomannan measured at 25˚C after heating to 25˚C (o), 35˚C (+), 45˚C (䉭), 55˚C (●), and 65˚C (▫). Cooling rate, approximately 1˚C/min. (From Annable, P. et al., Macromolecules,

xanthan molecules rather than the association of konjac and xanthan (Figure 3.14). 26 The reason why gelation temperature shifted to lower temperature by the addition of salt is that electrolyte promotes xanthan self-association at the expense of xan- than–KGM interaction.

3.5.2 K ONJAC – Κ -C ARRAGEENAN M IXTURES

Although κ-carrageenan can form a gel, in Japan the mixture of konjac and κ- carrageenan has been used to produce dessert jellies containing fruit juices. The texture of the mixed gel is more rubber-like than that of κ-carrageenan gels. Williams et al. observed two exothermic peaks in cooling differential scanning calorimetry (DSC) curves for mixtures of konjac and κ-carrageenan, with the mixing ratio KGM/CAR from 0.1/0.5 to 0.2/0.4 (total polysaccharide concentration, 0.6 wt%),

as shown in Figure 3.15. 27 Only one exothermic peak was observed for mixtures with CAR content below 0.3/0.3, and this was attributed to the formation of an ordered structure by the interaction between konjac glucomannan and κ-carrag- eenan. When the CAR content became higher than 0.45/0.15, the lower temperature peak began to appear in addition to the exothermic peak originating from the formation of the ordered structure by the interaction between konjac glucomannan and κ-carrageenan. Exothermic enthalpy per unit mass of κ-carrageenan accompa- nying gelation of the mixtures was significantly smaller than that for κ-carrageenan alone, indicating that the gel formation of κ-carrageenan is strongly affected by the presence of konjac glucomannan. Electron spin resonance (ESR) spectra for mixtures

Functional Food Carbohydrates

Temperature/°C

FIGURE 3.14 G' as a function of temperature for xanthan–KGM mixtures (1:1, 0.6 wt%) in the presence and absence of monovalent cations (0.04 mol/dm 3 ), measured at 3 Hz. H 2 O (●), NaCl (o), KCl (▫), CsCl (䉱), NH 4 Cl (䉭). (From Annable, P. et al., Macromolecules, 27, 4204, 1994.)

0.02 mW

Exothermic E F

CB 30 40 50 60 70 80 A

FIGURE 3.15 DSC cooling curves for various ratios of κ- carrageenan –konjac mannan mixtures in the presence of 50 mM KCl (0.6% total polymer concentration). Scanning rate was 0.1°C/min. % κ-carrageenan/% konjac mannan: (A) 0.1/0.5, (B) 0.2/0.4, (C) 0.3/0.3, (D) 0.4/0.2, (E) 0.45/0.15, (F) 0.5/0.1, and (G) 0.6/0. (From Williams, P.A. et al., Macromolecules,

Konjac Glucomannan

and konjac glucomannan alone showed that the segmental motion of konjac gluco- mannan is reduced by the interaction with κ-carrageenan molecules.

Kohyama et al. 8 carried out a large deformation extension measurement on mixed gels of konjac glucomannan with different molecular weights and κ-carrageenan molded into a ring shape, and found that the breaking stress increased with increasing molecular weight of KGM, indicating that KGM chains contribute to the network structure. Since the gel-to-sol transition temperature of mixed gels did not depend so much on the molecular weight of KGM, it was suggested that the KGM chains interact weakly with CAR; this can contribute to strengthening the network mechan- ically, but does not affect its thermal stability to a great extent.

3.5.3 K ONJAC –G ELLAN M IXTURES

The rheological properties of mixtures of konjac glucomannan with different molec- ular weights and gellan (GELL) were also examined. 28 The elastic modulus of mixtures of gellan with a lower and a medium molecular weight KGM as a function of mixing ratio exhibited a maximum at a certain KGM content, while that with higher molecular weight KGM increased with increasing KGM content. The relax- ational strength of mixtures, defined as the difference between the loss shear moduli as a function of temperature at a lower temperature and at a higher temperature, was smaller than that of gellan gum alone, indicating that the gel formation of gellan is strongly affected by the presence of KGM. The decrease in the relaxational strength was more significant in mixtures with higher molecular weight KGM, indicating that the higher molecular weight KGM inhibits the gel formation of gellan. The storage modulus of mixtures of gellan and KGM with a medium molecular weight showed a maximum at the mixing ratio gellan/KGM = 0.3/0.5. The effect of sodium chloride and calcium chloride on this mixture was examined by rheology and DSC. The midpoint transition temperature shifted to higher temperatures with increasing concentration of sodium chloride or calcium chloride, while the relaxational strength showed a maximum at a certain concentration of calcium chloride, as shown in

Figure 3.16. 29 The sodium or calcium ions shield the electrostatic charge of the carboxyl groups of gellan and promote the self-aggregation of gellan and network formation by the attachment of KGM molecules on the surface of gellan aggregates. Excessive salt addition promotes the self-aggregation of gellan and hinders the attachment of KGM, thus leading to phase separation. 29

The values of T s ,T m , or ΔH m determined from DSC as a function of KGM concentration indicated that KGM little influenced the thermal stability of gellan

solutions in comparison with sugars; however, KGM with relatively lower molecular weight significantly increased the G' observed from the rheological measurement.

Thus, in the presence of KGM with relatively lower molecular weight, the effective concentration of gellan may increase by immobilizing water molecules in gellan solutions, so that KGM with relatively lower molecular weight could indirectly promote the helix–coil transition of gellan molecules. Therefore, the effect of the KGM with relatively lower molecular weight on the rheological and thermal prop- erties of gellan solution seems to be essentially different from that of sugar, which stabilizes junction zones of gellan molecules by forming hydrogen bonds. KGM

Functional Food Carbohydrates

0 2 4 6 8 0 2 4 6 8 CaCl 2 Conc./mmol · l –1

CaCl 2 Conc./mmol · l –1 (a)

(b)

FIGURE 3.16 Dependence of the midpoint temperature of transition T M (a) and the relaxation strength ΔG(b) for mixtures with GELL/KGM = 0.3/0.5 (total polysaccharide concentration,

0.8%), or 0.3 or 0.8% GELL solutions on the CaCl 2 concentration: (●) mixture, (䉭) 0.3% GELL, and (▫) 0.8% GELL. (From Miyoshi, E. et al., J. Agric. Food Chem., 44, 2486, 1996.)

with relatively higher molecular weight may inhibit helix formation in gellan mol- ecules; moreover, it may hinder further aggregation of gellan helices, because KGM with relatively higher molecular weight decreased both ΔG observed by rheological measurement and ΔH m observed by DSC. 30

3.5.4 K ONJAC –A CETAN M IXTURES

Diffraction patterns of deacetylated acetan and KGM blend fibers were recorded on

a microfocus x-ray generator. 31 One of the best patterns is shown in Figure 3.17. Background scattering extends to the outer edge, yet few sharp Bragg reflections

and some diffuse spots stand out in the interior. The distribution of intensity is consistent with good axial orientation and short-range lateral organization of the helices in the fiber. The meridional reflection on the sixth layer line suggests that the binary complex is a sixfold helix of pitch 55.4 Å. A molecular modeling study incorporating this information reveals that a double helix in which one strand is acetan and the other glucomannan is stereochemically feasible. While the backbone and side groups are sufficiently flexible to allow the chains to associate with the same or opposite polarity, the parallel model is superior in terms of unit cell packing. The results are compatible with the observed synergy, namely, the weak gelation behavior of the complex. The molecular model can be generalized for the binary system when acetan is replaced by xanthan, or glucomannan by galactomannan.

3.5.5 K ONJAC –S TARCH M IXTURES

When the konjac was added to a starch dispersion, both the elastic modulus and the breaking stress of the resulting gel increased. However, following storage for a long

Konjac Glucomannan

FIGURE 3.17 (a) Diffraction pattern from a fiber of the 1:1 acetan:glucomannan complex characterized by a sharp meridional reflection on the sixth layer line and a few diffuse spots. The ring is from calcite (d-spacing, 3.035 Å) for internal calibration. (b) A schematic of the pattern is shown superposed on the Bernal chart for the hexagonal unit cell a = 17.3 and c =

55.4 Å. The (hk) indices of the row lines, and layer lines 0, 6, and –6 are marked. (From Chandrasekaran, R. et al., Carbohydr. Res., 338, 2889, 2003. With permission.)

time (2 weeks), the starch gel containing konjac showed smaller values for these parameters. The retrogradation ratio, ΔH 2 / ΔH 1 , the regelatinization enthalpy observed in the second-run DSC heating curve divided by the gelatinization enthalpy observed in the first-run DSC heating curve, showed the same tendency to rheological observation. Therefore, the addition of a small amount of konjac to starch promotes the retrogradation for short storage. However, it prevents the retrogradation after

long storage. 32 The addition of a small amount of konjac to the gelatinized dispersion of starch was found to be effective in preventing the syneresis and the consequent phase separation, as shown in Figure 3.18. 33