Sanehisa Nakamura

Gelation mechanism of agarose

Abstract The non-Newtonian behavior and dynamic viscoelasticity of a series of aqueous solutions of agarose were measured with a rheogoniometer. The flow curve, at 25°, of agarose solution approximated to plastic behavior at 0.1, 0.13, and 0.15% concentrations. Gelation occurred at concentration of 0.13% at low temperature (0°). The dynamic modulus of agarose showed a very high value at low temperature, and increased with an increase in temperature, showing a maximum value at 30°, then it decreased. In the presence of NaCl, KCl, CaCl 2 , and MgCl 2 for a solution of agarose at 0.08% concentration, the transition temperature, at which dynamic modulus decreased rapidly, was observed at 60°. Gelation was also observed at low temperature (0°) in acid and alkaline range after reaching pH values of 2.3 and 9.5, respectively, by addition of 100m m HCl, H 2 SO 4 , NaOH, and Ca(OH) 2 to a 0.08% agarose solution. A possible mode of intra- and inter-molecular hydrogen bonding within and between the agarose molecules in aqueous solution is proposed.

Synergistic interaction between xanthan and guar gum

Abstract The non-Newtonian behavior and dynamic viscoelasticity of a series of aqueous mixtures of xanthan and guar gum were measured with a rheogoniometer. At a concentration of 0.2% of total gums, gelation did not occur at room temperature but occurred at a low temperature (0°). A much stronger interaction was observed with a mixture of deacetylated xanthan than that with native xanthan. The maximum dynamic modulus was obtained when the ratio of xanthan to guar gum was 2:1. The transition temperatures of dynamic viscoelasticity for mixtures with native and deacetylated xanthan were observed at 25 and 30°, respectively. It was concluded that the side chains of the guar gum molecular prevent an intermolecular interaction with the side chains of the xanthan molecule. An intermolecular interaction between xanthan and guar gum at low temperature might be promoted between the periphery of the side chains of the xanthan molecule and the backbone of the guar gum molecule and dissociation takes place at the transition temperature.

Indicative evidence for a conformational transition in ι-carrageenan

Abstract The non-Newtonian behavior and dynamic viscoelasticity of ι-carrageenan solutions were measured with a rheogoniometer. The K salt of ι-carrageenan showed Newtonian behavior 1.5%, whereas the Ca salt of ι-carrageenan showed plastic behavior even at 0.3%. A gelation occurred in polysaccharide concentrations >1.5 and 0.5% for K and Ca salt of ι-carrageenan upon cooling, respectively. The transition temperature, at which dynamic viscoelasticity decreased rapidly, was observed at 45 or 50° at various concentrations of the Ca salt of ι-carrageenan. The dynamic modulus of the Na and K salts forms of ι-carrageenan showed a very high value at low temperature, and increased with an increase in temperature, showing a maximum value at 5°, then it decreased at 0.2% in the presence of CaCl 2 . However, in the presence of KCl, the K salt form of ι-carrageenan showed a very low dynamic modulus upon cooling. The rheological characteristics of the Ca salt of the ι-carrageenan molecule might be essentially attributed to an intramolecular association, contributed by sulfate groups of adjacent 3,6-anhydro- d -galactose and d -galactose residues through Ca 2+ ions with ionic forces.

D-Mannose-specific interaction between xanthan and D-galacto-D-mannan

A gelation occurred in a mixed solution of xanthan and locust‐bean gum at room temperature; in contrast, gelation did not occur in a solution of xanthan and guar gum. The maximum dynamic modulus was obtained when the mixing ratio of xanthan and locust‐bean gum was 1:2 at 0.2% total gums. A mixture of deacetylated xanthan and locust‐bean gum showed the highest dynamic modulus, about twice that of the mixture of native xanthan. We concluded that the intermolecular interaction between xanthan and locustbean gum might occur between the side chains of the former and back‐bone of the latter molecules in a lock‐and‐key arrangement.