Radio frequency (RF) heating of starch solutions under continuous flow conditions: Effect of system and product parameters on temperature change across the applicator tube

Studies were conducted to evaluate the effect of system and product parameters on the temperature change (DeltaT) across a 1.5 kW radio frequency heater operating at 27.12 MHz. Starch solutions (1 to 4% w/w) were used at three different flow rates (0.35, 0.5 and 1 L/min) and four power levels (672, 912, 1152 and 1392 W). The average heating rate of starch solutions varied from 6 to 19C/min depending on flow rate, concentration and power level. The corresponding residence time varied from 1.5 to 4.3 min. Central composite designs involving power (830 to 1234 R) and starch concentration (1 to 4% w/w) at 0.5 L/min were used to study the effects of salt, pH and sugar. As expected fluid flow rate, power level and salt concentration had significant impact (P less than or equal to 0.05) on temperature change (DeltaT) across the applicator tube. Although the interaction effect of salt and concentration influenced DeltaT (P < 0.05), observed trends were not clear cut. Sugar and pH had no significant (P > 0.05) influence on DeltaT due probably to their relatively lower conductivities. However, the interaction effect of sugar and starch concentration affected DeltaT. Correlations were developed for estimating DeltaT across the tube as a function of power level, concentration, pH, added salt and sugar. Finally, dimensionless correlations involving the generalized Reynolds, Prandtl, Grashof numbers, dimensionless power and loss-factor ratios were developed for estimating the temperature ratio (U) across the RF applicator.