State of water in starch–water systems in the gelatinization temperature range as investigated using dielectric relaxation spectroscopy

The dielectric response of native wheat starch–water slurries containing 5–60% starch (w/w) was measured in the frequency range of 0.2–20 GHz after heating the slurries to 7 different temperatures between 25 and 90 °C for 30 min. Three relaxations, with relaxation time range of 4–9 ps, 20–25 ps and 230–620 ps at 25 °C, were identified from the dielectric spectra of starch slurries. The fastest relaxation process (4–9 ps) was attributed to bulk water while the two slower relaxations were attributed to the confined water molecules present in the starch–water system. The amount of water exhibiting the slowest relaxation (230–620 ps) was calculated to be 0.08–0.16 g water/g starch, which was close to the monolayer water associated with wheat starch. Mobility of bulk water was significantly reduced (P < 0.001) upon gelatinization at low starch concentration (10% starch), but remained unaffected at higher starch concentrations. The mobility of two slower relaxing water species was not significantly influenced (P > 0.19) by gelatinization at all starch concentrations.