OPTIMIZATION OF COMBINED HEAT PUMP AND MICROWAVE DRYING OF YACON (SMALLANTHUS SONCHIFOLIUS) USING RESPONSE SURFACE METHODOLOGY

Response surface methodology was employed to investigate the effects of drying temperature (15.0-45.0C), air velocity (1.00-2.00 m/s), moisture content at conversion point (40.0-60.0%) and microwave (MW) power (1.0-3.0 W/g) on the average drying rate (DR), specific moisture evaporation rate (SMER), total color difference (E), rehydration ratio (R-R) and shrinkage ratio (S-R) of yacon dried by combined heat pump (HP) and MW methods. A central composite rotatable design was used to develop models for responses. The coefficients of determination R-2 of DR, SMER and E were higher than 0.7. However, R-2 of R-R and S-R were 0.540 and 0.343, respectively. Based on response surface and desirability functions, the optimum conditions for combined HP and MW drying of yacon were drying temperature of 42.7C, air velocity of 1.69m/s, moisture content at conversion point of 50% and MW power of 2.0W/g. At this optimum point, DR, SMER and Delta E were 0.262 kg(H2O)/kg((d.b.)).h, 0.222kg/kW.h and 23.59, respectively.