Earticle

The effect of molar substitution (MS, 0.030-0.118) on the dynamic rheological properties of hydroxypropylated rice starch pastes (5%, w/w) was investigated by small-deformation oscillatory measurements during aging. The magnitudes of storage (G#) and loss (G") moduli measured at 4℃ before aging increased with an increase in MS in the range of 0.030-0.118, while those of tan δ (the ratio of G"/G#) decreased. The G# values of hydroxypropylated rice starches, as a function of aging time (10 hr) at 4℃, increased rapidly at the initial stage, and then reached a plateau region at shorter aging times. However, for the native starch, the plateau values were not observed for G# after a long aging time. Increasing the MS resulted in a decrease in plateau values. The rate constant (K) for structure development during aging was described by first-order kinetics. The K values of hydroxypropylated rice starches at 0.086 and 0.118 MS were much lower than the K value at 0.030 MS.

The effect of five commercial gums (carboxylmethylcellulose, CMC; guar gum, GG; hydroxypropylmethyl-cellulose, HPMC; locust bean gum, LBG; and xanthan gum) at a concentration of 0.25% on the rheological properties of rice flour (RF) dispersions was investigated in steady and dynamic shear. The steady shear rheological properties showed that RF gum mixture dispersions (5%, w/w) at 25℃ had high shear-thinning flow behavior (n=0.20-0.31) exhibiting a yield stress. Magnitudes of consistency index (K), apparent viscosity (??,100), and Casson yield stress (?oc) of RF-gum mixtures were much higher than those of RF dispersion with no added gum (control). Activation energy values (6.67-10.8 kJ/mole) of RF-gum mixtures within the temperature range of 25-70℃ were lower than that (11.9 kJ/mole) of the control. Dynamic rheological data of log (G', G") versus log frequency (w) of RF-gum mixtures had positive slopes (0.15-0.37) with G' greater than G" over most of the frequency range (0.63-63 rad/sec), demonstrating a frequency dependency. Tan ? (G"/G') values of RF-gum mixtures, except for xanthan gum, were much higher than that of the control.

Dynamic rheological properties of commercial 0.8, 1.0, and 1.2% gums [carboxylmethylcellulose (CMC), guar gum, hydroxypropylmethylcellulose (HPMC), tara gum, and xanthan gum], which can be dissolved in cold water, were investigated by small-deformation oscillatory measurements. Magnitudes of storage (G') and loss (G") moduli increased with increasing concentration of gum solutions except for xanthan gum. Guar gum exhibited greatest G' and G" values among all gums except for G' value at 0.8% concentration. Slopes of G' and G" decreased with increasing concentration of gum solutions except for xanthan gum. Tan (G"/G') values decreased with increasing concentration of gum solutions except for xanthan gum. Tan values of xanthan gum solutions were much lower than those of other gum solutions, indicating that xanthan gum solutions were predominantly more elastic than viscous.

The effect of rice starch at different concentrations (0, 4, 6, and 8%) on dynamic rheological properties of surimi sols was investigated by small-deformation oscillatory measurements at 10℃ and during heating from 10 to 95℃. Dynamic frequency sweeps at 10℃ showed that the magnitudes of storage modulus (G') decreased with increasing starch concentration while those of tan increased. G' values of surimi-rice starch sols during heating decreased with increasing starch concentration, indicating that the pattern of G' changes during heating was influenced by the concentration of the added rice starch. In general, the characteristic G' thermograms of all samples showed a similar sol-gel transition pattern.

Surimi samples were prepared with the addition of three different sugars (sucrose, glucose, and fructose) at 8% and the thermal gelation of surimi sols was investigated by small-deformation oscillatory measurements of storage (G') and loss (G") moduli. The magnitudes of G' at 10℃ were much greater than G" over the entire range of frequency (w), with little dependence on . In general, G' values of surimi sol containing sucrose during heating (from 10 to 95℃) was pronounced than those of glucose and fructose, showing the following order: sucrose>glucose>fructose. The transition peaks of surimi sols containing sugars were in the temperature range of 34.8-37.4℃.

Rheological properties of acorn flour-guar gum mixtures (4% w/w) at different guar gum concentrations (0, 0.2, 0.4, 0.6, and 0.8% w/w) were evaluated in steady and dynamic shear. The acorn flour-guar gum mixtures at 25℃ showed high shear-thinning flow behavior (n= 0.20-0.27). Consistency index (K), apparent viscosity (,100), and Casson yield stress (oc) increased with the increase in guar gum concentration. Within the temperature range of 25-70℃, the ,100$ of mixtures obeyed the Arrhenius relationship with high determination coefficient (R2 = 0.974-0.994). Activation energy values (5.37-6.77 kJ/mole) of acorn flour dispersions in the mixtures with guar gum (0.2-0.8%) were much lower than that (12.5 kJ/mole) of acorn flour dispersion (0% guar gum). Storage modulus (G'), loss modulus (G"), and complex viscosity () increased with the increase in guar gum concentration. Dynamic rheological data of 1n (G', G") versus ln frequency (w) of guar gum-acorn flour mixtures had positive slopes with G' greater than G" over most of the frequency range, indicating that they exhibited weak gel-like behavior.