Probing the structure of thermoreversible food polymer gels using gelatin as a model polymer
An investigation of the effect of setting temperature, T, on the scaling properties of the structure of gelatin gels was carried out. The time required for gelatin solutions to gel, tgel, was measured using small amplitude oscillatory rheometry for 4 gelatin solutions with concentrations of 1.3 to 10.0%. As T was increased, tgel increased logarithmically and, when the 4 data sets were extrapolated to infinite tgel, they intersected at 33.6 ± 0.5°C. Static light scattering showed that the theta temperature, T[theta], of the gelatin solution was 34°C, which supports the hypothesis that the maximum temperature at which a gel will form, Tgel, is equivalent to T[theta]. Image analysis of transmission electron microscope images of 2.5 and 5.0% gelatin gels set at 23°C had a fractal dimension, df, = 2.12 ± 0.02, whereas those set at 27°C had df = 2.18 ± 0.02, independent of concentration. For all samples, df decreased after the gel point, although the decrease was larger in gels set at 27°C. The correlation length exponent, [nu], of gels at tgel decreased as the setting temperature increased and approached the Rouse limit, [nu] = 0.5, as T[right arrow]T[theta] indicating that the gels set at higher temperatures were more ordered than those set at lower temperatures. It also suggests that Tgel = T[theta]. The correlation lengths, [xi], of the evolving gels were measured by dynamic light scattering for 2.5 and 5.0% gels set at 23 and 27°C. Two characteristic correlation lengths, [xi] F for a short, fast diffusing component and [xi]S for a longer, slow diffusing component were detected. For 2.5% gels set at 27°C (t gel = 12.7 h), both [xi]F and [xi]S increased before the gel point, reached a maximum value after the gel point, and then decreased. After 48 h, [xi]F ranged from 85-119 nm and [xi] S was 1010-1300 nm depending on gelatin concentration and setting temperature. Rheological measurements showed that [nu] decreased after the gel point indicating that the gel fibrils were becoming more organized as they restructured, and [nu] decreased faster at 27°C than at 23°C reflecting the higher mobility of the polymers in warmer gels.