result and discussion

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.

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