Study On Combined Heat Pump And Microwave Drying Technology Of Yacon

China is a super power in production and export of dehydrated fruits and vegetables with total export quantity accounted for 40% of the total world trade. However, traditional drying is a high energy consumption process and results in poor quality, which decline in the international competition ability. Combined drying is refers to a composite drying technology in stages, utilizing the merits of two or more than two kinds of drying according to the properties of the materials. The heat pump and microwave combined drying of yacon were carried out to abtain product with hing quality, low energy consumption and high drying efficiency. The main research contents and conclusions are as follows:1. Effects of sample thickness, surface load, drying temperature, air velocity and evaporator bypass air ratio (BAR) on the drying and quality characteristics of yacon during heat pump drying (HPD) have been investigated. Experimental results showed that higher thickness and surface load resulted in lower drying rate (DR). DR increased with increasing drying temperature and air velocity. Most drying rate curves displayed both long time falling rate period and relatively short time constant rate period. While some curves existed only falling rate period. Thickness of 2-4 mm, surface load of 1-2 kg/m, drying air temperature of 25-35℃ and air velocity of 1.5-2.0m/s were preferred by considering specific moisture evaporation rate (SMER), total color difference (△E), shrinkage rate (SR) and rehydration rate (RR). However, BAR has little influence on the heat pump drying process of yacon. Eight mathematical models were selected to describe and compare the drying kinetics of yacon. Comparisons were based on the coefficient of determination (R2), reduced chi-square (X2) and root mean square error (RMSE). Among the models tested, the Midilli et al. model achieved the best fit. Moisture transfer from yacon slice was described by applying the Fick’s diffusion model. Effective diffusivity coefficients (Deff) increased with increasing drying temperature and were found to range from 1.092 to 7.388×10-7 m2/s. The temperature dependence of the effective diffusivity was described by the Arrhenius-type relationship and the activation energy for the diffusion of the moisture associated with the yacon was found to be 30.00 kJ/mol.2. Response surface methodology (RSM) was employed to investigate the effects of drying air temperature (15-45℃), drying air velocity (1.0-2.0m/s), moisture content at conversion point (40-60%) and microwave power (1.0-3.0W/g) on the drying rate (DR), specific moisture evaporation rate (SMER), total colour difference (AE), rehydration rate (RR) and shrinkage (SR) of heat pump and microwave combined dried yacon and to determine the optimised conditions for heat pump and microwave combination drying of yacon. A central composite rotatable design was used to develop models for responses. The order of factors that affected DR of heat pump and microwave combination drying of yacon were temperature (very significant), velocity (very significant), moisture content at conversion point (very significant), microwave power. The order of factors that affected SMER were temperature (very significant), velocity (very significant), moisture content at conversion point, microwave power. The order of factors that affected AE were temperature (very significant), velocity (significant), moisture content at conversion point, microwave power. The optimum conditions were determined to obtain the criteria of maximum DR and SMER and minimum AE. Based on response surface and desirability functions, the optimum conditions for yacon drying by HP and MW combination were found to be drying air temperature of 42.7℃, drying air velocity of 1.69m/s, moisture content at conversion poin of 50% and microwave power of 2W/g.3. The moisture sorption isotherm (MSI) of heat pump-microwave dried yacon at 25℃ was studied by static weight method. The GAB model fitted the experimental data to again the moisture absorption characteristics and the monolayer moisture content. Differential scanning calorimetry (DSC) was used to determine the Tg and freezing point (TF) of yacon. Gordon-Taylor equation and Clausius-Clapeyron equation were used to fit Tg and TF data, by which the state diagram of yacon can be aquired. According to the understanding of state diagram, the shelf life prediction model of the HP-MV dried yacon was established by Williams-Landel-Ferry (WLF) equation.The results showed that the equilibrium moisture content (EMC) of yacon increased with increasing aw. The GAB monolayer moisture content was calculated to be 7.95g/100g (d.b.). Tg decreased from 3.2 to-73.3℃ as moisture content of yacon increased from 0.030 to 0.324 (g/g, w.b.).TF decreased from -4.8 to-21.5℃ as the solid content of yacon increased from 0.288 to 0.629g/g. The state diagram provided an estimate of maximally-freeze-concentrated solutes at 0.706 g/g with the characteristic temperatures of end point of freezing (Tm’) being -28.6℃ and glass transition (Tg’) being -66.6℃. According to the state diagram, the best storing conditions and shelf life model can be proposed by WLF equation as follows: Where τg and τ are time constants for crystallization at To and T, respectively. C/and C2 are general constants, C1=20, C2=155.The relationship between aw preservation theory and the glass transition theory and their effects on storgae stability of HP-MD dried yacon were discussed. The results showed that based on aw preservation theory, the predicted To will be lower than safe storage temperature. Likewise, based on glass transition theory, the forecast will be also lower than its safety aw values. Therefore, there is a considerable discrepancy in the temperature related stability criteria predicted by the concepts of water activity and glass transition.