Solution Structure of Molecular Associations Investigated Using NMR for Polysaccharides: Xanthan/Galactomannan Mixtures.

Although the intermolecular nuclear Overhauser effect (NOE) signal was valuable to elucidate molecular association structure, it could not always be observed for associated molecules due to the short spin-spin relaxation time T2 in NMR measurements, especially for high molar mass systems. While almost no study has been reported for high molar mass polymers (>1 × 10(6)), especially for polysaccharide-polysaccharide interactions, NOE signals were observed for the first time between two different types of polysaccharides, xanthan and galactomannan (locust bean gum), forming a synergistic gel, as a direct evidence of intermolecular binding of polysaccharides. The NOE peak was found between pyruvic acid in xanthan and anomeric proton of mannose of galactomannan. This NOE signal was observed only when mixing time >0.5 s, indicating indirect NOEs caused by spin diffusion. Therefore, this NOE could not be used to construct the molecular models. However, it is a direct evidence for the binding between two different types of polysaccharide to elucidate the synergistic gelation. This NOE signal was observed only for low molar mass galactomannans (1.4 × 10(4)). T2 of pyruvate methyl drastically decreased at low temperatures in the presence of synergistic interaction, suggesting that pyruvate group at terminal end of side chain in xanthan plays an essential role in synergistic interaction.

Effects of Time and Temperature of Annealing on Rheological and Thermal Properties of Rice Starch Suspensions during Gelatinization.

Effects of annealing time, t a , and annealing temperature on the rheological and thermal properties of rice starch suspensions during the gelatinization were studied by dynamic viscoelasticity, steady shear viscosity, η, and differential scanning calorimetry measurements. When t a increased, for the sample which was annealed at almost the same temperature as the gelatinization temperature (= T gel ; 62C), 58 or 62C, the onset temperatures at which the storage and loss moduli, G' and G″, started to increase shifted to higher temperatures, and G' and G″ decreased in the temperature range from 65 to about 75C. Further, G' and G″ of the sample annealed at higher T than T gel , 65C, increased in the temperature range from 50 to 75C. The amorphous and thermally unstable portion in the starch decreased with increasing t a . Effects of annealing on the temperature dependence of η were found to be almost the same as those of G' and G″. The annealing tended to prevent starch sample from shear thinning. PRACTICAL APPLICATIONS: This article shows the effects of annealing time, t a , and annealing temperature on the rheological and thermal properties of rice starch suspensions. When t a increased, the storage and loss moduli, G' and G″, of rice starch decreased in the temperature range from 65 to about 75C and the gelatinization temperature increased due to the increase of the amount of leached-out amylose. For the sample annealed at 62C for 15 min, it was observed that the annealing tended to prevent starch sample from shear thinning. These findings are helpful to control rheological and thermal properties during cooking of starch-based foods.

Single molecular pair interactions between hydrophobically modified hydroxyethyl cellulose and amylose determined by dynamic force spectroscopy.

Interactions among HMHEC (hydrophobically modified hydroxyethyl cellulose) and between HMHEC and amylose were investigated by means of dynamic force spectroscopy of single molecular pairs. The technique was realized using a scanning probe based platform, and the molecular pair interactions were investigated in aqueous solutions over a range of force loading rates. Both hydrophobic interactions among hydrophobe C(16) alkyl side chains in HMHEC and association between these hydrophobes in HMHEC and amylose showed a stretching type peak. The distribution analysis of rupture force based on Bell-Evans's model revealed that the peaks had a most probable rupture force ranging from 27 pN at a force loading rate r(f) = 0.43 nN/s to 125 pN at r(f) = 170 nN/s for HMHEC-HMHEC, and from 13 pN at r(f) = 0.20 nN/s to 34 pN at r(f) = 33.7 nN/s for HMHEC-amylose interactions. The distance of the energy barrier relative to the minimum, x(beta), and the apparent lifetime in the absence of external force, tau, were found to depend on the force loading rate, and the average values are estimated to be 0.99 nm and 0.89s for HMHEC-HMHEC and 0.31 nm and 0.075s for HMHEC-amylose interactions. The obtained data for these pairwise molecular interactions are underpinning the associative behavior of the macroscopic properties of aqueous solutions of these polysaccharides.

Synergistic interaction of xyloglucan and xanthan investigated by rheology, differential scanning calorimetry, and NMR.

A new synergistic interaction between tamarind seed xyloglucan and xanthan was found and investigated by rheology, differential scanning calorimetry (DSC), and NMR. The effect of the acetyl and pyruvate groups in the side chain in xanthan on the synergistic interaction was also examined. The shear moduli G' and G' ' of the mixture solution of xyloglucan and native (or acetate-free) xanthan increased steeply at around 22 degrees C upon cooling. An exothermic DSC peak appeared at the same temperature. A drastic decrease in the of the acetyl and pyruvate groups of the xanthan side chain was observed from 1H NMR spectra only in the mixture at low temperatures (<25 degrees C). It was found that the pyruvate group is more restricted in the mixture solution compared with the acetyl group. The mixture of xyloglucan and pyruvate-free xanthan showed no synergistic interaction. We concluded that this synergistic interaction is caused by the intermolecular binding between xyloglucan and xanthan, and, in the heterotypic junction zones, the xanthan side chain becomes a new state that is different from both the coil and helix states.

Effect of annealing temperature on gelatinization of rice starch suspension as studied by rheological and thermal measurements.

The effect of annealing temperature (Ta) on the rheological behavior of 10 wt % rice starch suspension was investigated by the dynamic viscoelasticity, the differential scanning calorimetry (DSC), and the amount of leached out amylose and the swelling ratio of starch suspension. The rheological behaviors of the annealed samples are classified into three types in terms of Ta: Ta1, 48 and 55 degrees C, which are much lower than the gelatinization temperature, Tgel (=62 degrees C); Ta2, 58, 60, and 62 degrees C, which are almost the same as Tgel; and Ta3, 65, 68, 70, and 73 degrees C, which are much higher than Tgel. For the samples annealed at Ta2, the onset temperature of the storage and the loss moduli, G' and G'', increased with increasing T(a), and G' and G" in the temperature range from 65 to 90 degrees C gradually increased though smaller than those for the nonannealed sample, the control. This can be understood by the partial gelatinization; i.e., the leached out amylose prevents further amylose from leaching out. The rheological property of the samples annealed at Ta1 is not so different from that of the control, and the samples annealed at Ta3 are almost gelatinized. The rheological behavior of starch suspension can be controlled by Ta.

The effect of the linear charge density of carrageenan on the ion binding investigated by differential scanning calorimetry, dc conductivity, and kHz dielectric relaxation.

The effect of the linear charge density of natural polyelectrolyte, carrageenan, on the ion binding to carrageenan molecules in relation to the gelation was investigated by using the dielectric relaxation spectroscopy, dc conductivity, optical rotation, and differential scanning calorimetry (DSC). Although carrageenan is an anionic polysaccharide, carrageenan molecules in the helix state at low temperatures can bind not only cation, such as potassium and cesium, but also anion, such as iodide. The dc conductivity steeply decreases just below the coil-helix transition temperature, which indicates the binding of ion to the carrageenan molecules in the helix state due to the increase of the linear charge density compared with that in the coil state. The addition of NaI promotes the helix formation, and prevents from aggregation of helices, which was suggested by the results of the dynamic shear modulus and the DSC, and resulted in an increase of the relaxation amplitude of the lowest frequency relaxation ( approximately kHz) attributed to the fluctuation of the tightly bound counter ions along the high charge density region (helix). It is concluded that binding of iodide induces (1) the increase in the amount of tightly bound counterions to carrageenan molecules and (2) the formation of non-aggregated helix.