Mineo Watase

Effects of sugars and polyols on the gel-sol transition of agarose by differential scanning calorimetry

Abstract Effects of various sugars and polyols such as ethylene glycol, glycerin, ribose, glucose, fructose, mannose, galactose, sucrose, maltose, raffinose on the gel-sol transition of agarose gels are studied by differential scanning calorimetry. Melting temperature T m , and setting temperature T s shift to lower temperatures by the addition of polyols and ribose. All the other sugars examined in this study shift T m and T s to higher temperatures. The shift of T m and t s to higher temperatures is explained by the increase of the number of zippers created by hydrogen bonds between hydroxyl groups in sugars and agarose. The shift of T m and T s is linearly related to the dynamic hydration number or the number of equatorially attached hydroxyl groups in sugars or polyols.

Effect of alkali pretreatment on the rheological properties of concentrated agar-agar gels

Abstract Differential scanning calorimetry and thermogravimetry in the solid state and dynamic mechanical measurements of gels have been carried out for agar-agars of Chilean and Argentinian origin in order to elucidate the rheological changes in the gel as a result of alkali pretreatment. The elastic modulus of the gel prepared from Chilean agar-agar increased with increasing sodium hydroxide concentration up to 10%, while that of Argentinian agar-agar increased with increasing sodium hydroxide concentration up to 7%, and then began to decrease at higher concentrations. The increase in elastic modulus has been attributed to the structural stabilisation induced by the formation of 3,6-anhydro- l -galactose, while the decrease in elastic modulus in Argentinian agar-agar has been ascribed to chain breakage.

Effect of potassium ions on the rheological and thermal properties of gellan gum gels

Abstract To clarify the effect of cations on the gelling ability of gellan gum, the cation-free gellan gum was prepared by treating with ethylenediaminetetraacetic acid and ethanol. Storage Youngs modulus E′ and mechanical loss tangent tan δ were observed for cation-free gellan gum (F1) gels, non-treated gellan gum (F2) gels and potassium added F1 gels. E′ increased and tan δ decreased in the presence of potassium ions indicating the increase of the number of junction zones, and the critical polymer concentration below which no gelation occurs shifted to lower concentrations by adding cations. An endothermic peak accompanying gel-to-sol transition in heating differential scanning calorimetry curves shifted to higher temperatures by adding potassium ions. It was confirmed that potassium ions increase the number of junction zones and make the junction zones more heat resistant. F2 gels containing sodium, calcium and magnesium ions in addition to potassium ions showed multiple broad endotherms in addition to a main endothermic peak indicating that junction zones are made from various aggregations of double helices, with different bonding energies and/or different degrees of rotational freedom of parallel links consisting a single zipper.

Characterization and properties of gellan-κ-carrageenan mixed gels

Abstract This paper concerns the properties of gellan and κ-carrageenan mixtures in the dilute and semi-dilute regimes. For the purified K-forms of the two polymers at low polymer concentrations, a cooperative interaction between their ordered conformations is demonstrated from viscosity measurements. At higher polymer concentrations a two-phase system is formed as shown from differential scanning calorimetry and rheological measurements. From these data, it is concluded that the gellan and κ-carrageenan mixed gel is a phase separated gel. The role of ionic impurities (such as Ca 2+ ions) on the position of the melting temperatures of the two phases is also discussed.

Rheological and thermal properties of agarose and kappa-carrageenan gels containing urea, guanidine hydrochloride or formamide

Abstract Measurements of dynamic Youngs modulus E′ and differential scanning calorimetric studies of agarose and kappa-carrageenan gels were carried out in the presence of urea, guanidine hydrochloride or formamide at various concentrations. The dependence of the values of E′ and the endothermic peak temperatures on the concentration of these organic solvents suggests that the mechanism by which urea weakens gel is the same for agarose and kappa-carrageenan gels, but the influence of guanidine hydrochloride on gel properties is different for agarose and kappa-carrageenan. Formamide was more effective at weakening the gels than the other two organic solvents.

DSC study on properties of water in concentrated agarose gels

Abstract The state of water in concentrated agarose gels was studied by differential scanning calorimetry. An endothermic peak was observed at temperatures 20% w/w. It was found also that there must be some non-freezing water, because the sum of the frozen disordered water and the frozen free water, which was estimated qualitatively from the area enclosed by a DSC peak and a baseline, was less than the total content of water in gels. It was suggested that the disordered frozen water and the non-freezing water exist mainly in junction zones of agarose gels.

Effects of pH and DMSO content on the thermal and rheological properties of high methoxyl pectin-water gels

It was shown that high methoxyl pectin forms a gel with a high elastic modulus in the presence of dimethyl sulfoxide (DMSO). Differential scanning calorimetry (DSC) and dynamic viscoelastic measurements were carried out for high methoxyl pectin-DMSO gels prepared at pH 2·4 and 6·86. Elastic moduli of pectin gels as a function of DMSO content showed a maximum around 0·2 mole fraction (mf) DMSO; however, acidic gels (pH = 2·4) needed less DMSO to form a gel and the elastic moduli for the acidic gels were larger than those for neutral gels (pH = 6·86). Endothermic peaks accompanying the gel-to-sol transition observed in heating DSC curves were sharpest for the acidic pectin gels with 0·168 mf DMSO and for neutral pectin gels with 0·277 mf DMSO. A small amount of DMSO, less than 0·3 mf, promotes gel formation, whilst excessive amounts of DMSO lower the gelling ability. The mean end-to-end distance rm of chains which connect junction zones decreases, and the bonding energy e increases with increasing DMSO content up to 0·277 mf; then rm increases and e decreases beyond this DMSO content. Judging from the heat absorbed on forming 1 mole of junction zones, junction zones in acidic gels are more heat-resistant than those in neutral gels. The electrostatic repulsion between carboxylate ions is reduced at lower pHs because of the suppression of the dissociation of carboxylic groups, hence the elastic modulus, the endothermic enthalpy accompanying gel-to-sol transition, and gel-to-sol transition temperature increase with decreasing pH.

Thermal measurements of curdlan in aqueous suspension during gelation

Abstract Differential scanning calorimetry (DSC) measurements of curdlan in aqueous suspension were carried out in a temperature range of 40–200°C by using silver pans with and without heat treatment, respectively. Silver pans with heat treatment were heated at 200°C and then cooled. Heating DSC curves of curdlan in aqueous suspension showed an endothermic peak around 60°C and were identical below 120°C, but quite different above 120°C. A large exothermic peak was observed at 140–190°C in a heating DSC curve when a non-heated pan was used, whereas a small endothermic peak was observed at 140–160°C and no exothermic peak was observed at higher temperatures when a heated pan was used. It was suggested that the large exothermic peak was attributed to an interaction between the non-heated silver pan and water above 120°C. Only the endothermic peak at 140–160°C reflected the reality of the thermal property of curdlan in aqueous suspension, which was caused by the melting of the curdlan gel formed.

Polysaccharide-protein interaction: a rheological study of the gel-sol transition of a gelatin-methylcellulose-water system.

Viscoelastic parameters for mixtures of gelatin and methylcellulose were measured as a function of temperature, in order to study the gel-sol transition of the system in which biopolymers forming thermo-setting and thermo-melting gels coexist. At higher temperatures than 45 degrees C, the gel network is mainly formed by methylcellulose while at lower temperatures around 5 degrees C, it is mainly formed by gelatin. At higher temperatures than 45 degrees C, gelatin inhibits the gelation of methylcellulose. A small amount of methylcellulose helps gelatin to form a network at lower temperatures. However, excessive amounts of methylcellulose inhibit the growth of network structure. Therefore, this mixture forms a phase separated gel at higher temperatures while it is not completely phase separated at lower temperatures, probably by some interaction between non-substituted hydroxyl groups in methylcellulose with carboxylic groups in gelatin.

Effect of Sugars and Polyols on Water in Agarose Gels

Effects of ribose, glucose, sucrose, ethylene glycol, glycerin, propylene glycol, and sorbitol on water in concentrated agarose gels were studied by differential scanning calorimetry at low temperatures. Changes in the phase transition temperatures of 40% agarose gels, induced by the addition of these chemical reagents, are discussed, together with rheological and thermal data for the same systems at ambient and higher temperatures. Both sugars and polyols are believed to reduce the amount of freezable water and to promote plasticization and molecular mobility of agarose chains in gels, thus shifting the glass transition temperatures to lower temperatures. However, the effects of decreasing freezable water, and the direct effect on the junction zones, produced by sugars seem to be different from the effects produced by polyols.