Changein moisture content during drying Fig. 1 represents drying curves offresh, grated beetroot during CMWC (i.e. pulse ratio 1.
0) at 60%, 70%, 80%microwave power levels and 40°C, 50°C, 60°C inlet air temperature while Fig. 2,3 and 4 represent drying curves of those dried in IMWC mode with microwave intermittence/pulseratio (PR) 2.0, 3.0 and 4.0 respectively and atvarious power levels and inlet air temperatures.
It is clearly evidentfrom the figures that moisture content of samples decreased with the increasein drying time until they reached a constant value. Average initial moisturecontent of samples during CMWC (PR-1.0) and IMWC at (PR-2.0, 3.
0 and 4.0 respectively)drying was 6.109 g of water/ g of dry matter, which reduced faster duringinitial stages of drying as evident by steeper slope of drying curves, however,as the drying proceeded the slope of curves become flatter, indicating slowerdrying.Dependingon the drying treatment, the final moisture content ranged between 0.0483and 0.0753 g of water/ g of dry matter in about 25–40 min during CMWC drying(Fig.
1). Drying time decreased with anincrease in convective drying air temperature. Also,as the microwave power input increased from 60% to80% (output power 321.003W to 446.613 W), drying time decreasedconsiderably. The drying curve became steeper withthe increase in microwave power input indicating faster drying of the product.
AtPR-2.0 (Fig. 2), the final moisture content wasreduced to the range of 0.0494-0.
0753 g of water/ g of dry matter was observed in 40–55 min, depending on the level of microwavepower and the temperature of drying air. Similarly, the final moisture contentdecreased to the range of 0.0564-0.
0789 g of water/ g of dry matter in 45–60 min depending on the level of microwave power andthe temperature of drying air at PR-3.0 (Fig. 3) and to the range of 0.
0561- 0.0806 g of water/ g ofdry matter in 45–75 min depending on the level of microwavepower and the temperature of drying air at PR-4.0 (Fig. 4).Whenmicrowave input power increased from 60% to 80% ( output power 321.
003W to446.613 W), drying time progressively decreased at given drying air temperatureand PR. Likewise, as the air temperature increased, the drying time considerablydecreased at given microwave input powerand PR. However, the drying time increased with an increase in PR at the givenconvective drying air temperature and microwave power. This suggests that theapplied microwave power, PR and drying air temperature had a crucial effect onthe drying rate during intermittent microwave–convective air drying.
3.2Validity of Drying ModelsThemodels (Table 1) adopted from several literatures (Tuncay et al., 2005; Saeedet al., 2008) have reported that a higher value of coefficient of determination(R2) and lower values of ?2 as well as RMSE for a modelindicate that the model fits better to the given set of experimental data.
Itwas found that all the models had highly satisfactory fitting tests for the experimentaldata. ForCMWC drying, the Midilli-Kucuk model was found to be the best fit with thehighest R2 value of 0.9998 and lowest ?2 and RMSE values(0.00004 and 0.00495, respectively) for the drying condition: 40°C inlet airtemperature, 60% microwave power.Table 1: Various drying models fitted to drying data Name of model Model equation# Reference Lewis Model M.R = exp(-kt) Aghbashlo et al. (2009), Asiru et al.
(2013) Henderson & Pabis Model M.R = a exp(-kt) Yaldiz et al. (2001), Asiru et al. (2013) Page’s Model M.
R = exp(-ktn) Arumuganathan et al.(2009) Two term Exponential Model M.R = a exp(-kt) + (1-a) exp(-kat) Sharaf-Elden et al. (1980) Wang & Singh Model M.R = 1 + at + bt2 Wang, Singh (1978) Logarithmic Model M.R = c + a exp(-kt) Yaldiz et al. (2001) Midilli –Kucuk Model M.R = a exp(-ktn) + bt Midilli et al.
(2002) #Where,a, b, c, n and k are constants in the drying models. M.R. – moisture ratio,t-time (sec)ForIMWC drying with PR 2.0, the highest R2 (0.9992) and the lowest ?2(0.00014) both were obtained for Wang & Singh model fitted to dryingcondition: 50°C inlet air temperature, 60% microwave power but lowest RMSE(0.01037) was obtained for Midilli-Kucuk model fitted to drying condition: 50°Cinlet air temperature, 80% microwave power while for PR 3.0, Midilli-Kucukmodel was the best fit with highest R2 (0.9986) and lowest ?2 andRMSE values (0.00032 and 0.01336, respectively) for the drying condition: 50°Cinlet air temperature, 70% microwave power and for PR 4.0 also Midilli-Kucukmodel was the best fit with highest R2 (0.9987) and lowest ?2 andRMSE values (0.00022 and 0.01116, respectively) for the drying condition: 50°Cinlet air temperature, 80% microwave power.