Change

in moisture content during drying

Fig. 1 represents drying curves of

fresh, 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/pulse

ratio (PR) 2.0, 3.0 and 4.0 respectively and at

various power levels and inlet air temperatures. It is clearly evident

from the figures that moisture content of samples decreased with the increase

in drying time until they reached a constant value. Average initial moisture

content 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 during

initial stages of drying as evident by steeper slope of drying curves, however,

as the drying proceeded the slope of curves become flatter, indicating slower

drying.

Depending

on the drying treatment, the final moisture content ranged between 0.0483

and 0.0753 g of water/ g of dry matter in about 25–40 min during CMWC drying

(Fig. 1). Drying time decreased with an

increase in convective drying air temperature. Also,

as the microwave power input increased from 60% to

80% (output power 321.003W to 446.613 W), drying time decreased

considerably. The drying curve became steeper with

the increase in microwave power input indicating faster drying of the product. At

PR-2.0 (Fig. 2), the final moisture content was

reduced 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 microwave

power and the temperature of drying air. Similarly, the final moisture content

decreased 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 and

the temperature of drying air at PR-3.0 (Fig. 3) and to the range of 0.0561- 0.0806 g of water/ g of

dry matter in 45–75 min depending on the level of microwave

power and the temperature of drying air at PR-4.0 (Fig. 4).

When

microwave input power increased from 60% to 80% ( output power 321.003W to

446.613 W), drying time progressively decreased at given drying air temperature

and PR. Likewise, as the air temperature increased, the drying time considerably

decreased at given microwave input power

and PR. However, the drying time increased with an increase in PR at the given

convective drying air temperature and microwave power. This suggests that the

applied microwave power, PR and drying air temperature had a crucial effect on

the drying rate during intermittent microwave–convective air drying.3.2

Validity of Drying ModelsThe

models (Table 1) adopted from several literatures (Tuncay et al., 2005; Saeed

et al., 2008) have reported that a higher value of coefficient of determination

(R2) and lower values of ?2 as well as RMSE for a model

indicate that the model fits better to the given set of experimental data. It

was found that all the models had highly satisfactory fitting tests for the experimental

data. For

CMWC drying, the Midilli-Kucuk model was found to be the best fit with the

highest R2 value of 0.9998 and lowest ?2 and RMSE values

(0.00004 and 0.00495, respectively) for the drying condition: 40°C inlet air

temperature, 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)

For

IMWC 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 drying

condition: 50°C inlet air temperature, 60% microwave power but lowest RMSE

(0.01037) was obtained for Midilli-Kucuk model fitted to drying condition: 50°C

inlet air temperature, 80% microwave power while for PR 3.0, Midilli-Kucuk

model was the best fit with highest R2 (0.9986) and lowest ?2 and

RMSE values (0.00032 and 0.01336, respectively) for the drying condition: 50°C

inlet air temperature, 70% microwave power and for PR 4.0 also Midilli-Kucuk

model was the best fit with highest R2 (0.9987) and lowest ?2 and

RMSE values (0.00022 and 0.01116, respectively) for the drying condition: 50°C

inlet air temperature, 80% microwave power.