ISSN 0258-7122

Bangladesh J. Agril. Res. 38(2): 301-319, June 2013

DRYING KINETICS OF GINGER RHIZOME (Zingiber officinale)
1 2 3
M. A. HOQUE , B. K. BALA , M. A. HOSSAIN
AND M. BORHAN UDDIN4

Abstract
This paper presents the drying kinetics of ginger rhizome under blanched and
nonblanched conditions using hybrid solar dryer and mechanical tray dryer at
three temperature levels. The drying rate increases with the increase in drying
air temperature and blanching also increases the drying rate. The drying rate
depends on shape and size of the ginger rhizomes. The highest drying rate was
found for sliced samples of ginger rhizome followed by splitted and whole root
samples. Five thin layer drying models were fitted to the experimental data of
blanched and sliced ginger rhizomes. The Page equation was found to be the
best to predict the moisture content of sliced ginger rhizome in thin layer. The
agreement between the predicted and experimental results was excellent.
Colour of ginger rhizomes was slightly changed after drying. Lightness of
ginger rhizomes decreased with an increase in drying temperature for all
samples except sliced and blanched samples. For drying of ginger rhizome, it
should be sliced and blanched and dried below 70 0C for better quality dried
products.
Keywords: Ginger rhizome, hybrid solar dryer, tray dryer, blanching, thin layer
drying model, colour change.

Introduction
Ginger (Zingiber officinale) is a herb in plant habit. Fresh ginger root is usually
consumed as spice in the tropical countries and dried ginger is used as medicinal
plant internationally. Dried ginger is produced from the mature rhizome. As the
rhizome matures, the flavour and aroma become much stronger. Dried ginger
can be ground and used directly as a spice or in medicinal use and also for the
extraction of ginger oil and ginger oleoresin. Ginger possesses stimulant,
aromatic and carminative properties when taken internally and when chewed it
acts as a sialagogue. It is of much value in tonic dyspepsia, especially if it is
accompanied with much flatulence; and as an adjunct to purgative medicines to
correct griping. Quality specifications for export as medicinal herb, it required
to be properly cut into pieces, well dried and proper storage.
Drying is the most common and fundamental method for post-harvest
preservation of medicinal plants because it is a simple method for the quick

1
Scientific Officer, FMP Engineering Division, Bangladeh Agricultural Research
Institute (BARI), Gazipur, e-mail: arshadulbari@yahoo.com, 2Department of Farm
Power and Machinery, Bangladesh Agricultural University (BAU), Mymemnsingh
2202, 3Senior Scientific Officer, FMP Engineering Division, BARI, Gazipur,
4
Department of Food Technology and Rural Industries, BAU, Mymensingh 2202,
Bangladesh..
302 HOQUE et al.

conservation of the medicinal qualities of the plant material. Quality distinction

was made some 4000 years ago in ancient Egypt between medicinal plants dried
in the sun and those dried in the shade (Heeger, 1989). However, factors such as
scale of production, availability of new technologies and pharmaceutical quality
standards must be considered for medicinal plant drying.
Natural drying, i.e., drying without auxiliary energy either in the field or in
sheds, should only be considered for drying of small quantities. In cases of mass
production, the use of mechanical drying is indispensable. For the preservation
of active ingredients of medicinal plant materials, comparatively low drying
temperatures are recommended and, as a result, the drying duration is
comparably long.
Drying represents 30 to 50% of the total costs in medicinal plant production
(Qass and Schiele, 2001) and therefore, it is crucial that factors determining the
high costs are identified. Currently, energy demand of drying represents a
significant cost factor, especially with the increased price of fossil fuels. This is
largely due to the high moisture content of the flowers, leaves or roots to be
dried. Moreover, drying performance takes authoritative influence on the quality
of the product and, therefore, on its value.
Drying temperature should be chosen as high as possible without reducing
the quality of the medicinal plant to achieve increased dryer capacity. Maximum
allowable temperatures depend mainly on the chemical composition of the
active ingredients of the medicinal plant species under consideration. For
glycoside species, a maximum temperature of 1000C is recommended, for
mucilage species 65 0C and for essential-oil species 35 to 45 0C (Maltry et al.,
1975). For some products drying either in the shade is superior to other methods
or solar drying is superior to sun drying in terms of essential oil contents
(Ozguven et al., 2007). Numerous types of belt and batch dryers have been
frequently used in practice for drying herbal and medicinal plants. Soysal and
Öztekin (2001) reported the performance and economic analysis of a heated air
tray dryer for drying of Mentha piperita and Hypericum perforatum.
Several studies have been reported for solar drying of herbal and medicinal
plants, such as mint, sage, and hops (Muller et al., 1989), aonla (Haque and
Bala, 1996) and rosella flower (Janjaia, 2008). Solar drying results in
considerable reduction in drying time and production of quality dried products.
Arabhosseini (2005) fitted a number of thin layer drying equations to the
experimental data of tarragon leaves as well as chopped plants and the Page
equation was selected. Müller (2007) compiled experimental studies in terms of
models for drying kinetics and losses of active ingredients during drying at
different drying conditions.
DRYING KINETICS OF GINGER RHIZOME 303

The effects of solar drying on the appearance, aroma/flavour, pungency and

the ginger oil/oleoresin yield of dried ginger and analysis of the ginger
oil/oleoresin contents are essential for processing of ginger. Therefore, a
suitable method of drying and storage of powder of ginger rhizome tubers is
needed (Yiljep et al., 2005). Although several studies have been reported on
drying of herbs and medicinal plants (Soysal and Öztekin, 2001; Janjaia et al.,
2008; Arabhosseini, 2005; Yahya et al., 2004; Müller, 2007; Böhm et al., 2002;
Heindl and Müller, 2002; Heindl, 2005; Arabhosseini et al., 2007) no study has
been reported on drying of ginger rhizome. The purpose of this study is to
determine the drying kinetics of ginger rhizome using solar hybrid and tray
dryer; to determine the optimum drying air temperature, develop models of
drying kinetics and also assess the quality attributes of dried ginger rhizome in
terms of colour.

Materials and Method

The hybrid dryer installed at Bangladesh Agricultural Research Institute (BARI)
was used for drying of ginger rhizome at 50 0C (Hossain and Hoque, 2008) and
the tray dryer at BARI laboratory was used for drying of ginger rhizome at 60
0
C and 70 0C and three drying experiments were conducted. Fresh ginger
rhizomes were collected from local market of Gazipur district in Bangladesh. The
ginger rhizomes were processed in three different sizes e.g., whole (60 mm
length @ 20 mm dia. finger), splitted (60 mm length @ 20 mm dia finger splitted
longitudinally to make two) and sliced (20 mm dia. finger @ 4 mm thickness) .
Three samples of 100 g each of the different shape and size of the product were
water blanched and another three samples were not blanched.
Fresh ginger rhizomes were dried in three forms- whole, longitudinally splitted,
and sliced. Whole fresh rhizomes were cut longitudinally to make equal two
parts by knives. Slicing was done with a mechanical slicer in 4 mm in thickness.
Three samples of all three forms were blanched at 80 0C for 5 minutes (Murad et
al., 2004). The initial moisture content of the ginger rhizome was determined by
oven method drying at 105 0 C for 24 hours (Park et al., 2002).
Comparison of temperature at 50, 60, and 70 0C were made in this study.
Here temperature is factor and there is no effect of method of temperature
(whether it is raised by solar, cabinet dryer or other means). It is convenient to
raise temperature 50 0C in solar dryer and 60 and 70 0C temperature in cabinet
dryer. So, solar and cabinet dryer was used to dry ginger at different
temperature.

Description of the hybrid solar dryer

The dryer basically consists of a solar collector and a drying unit. A schematic
view of the solar dryer is shown in Fig. 1. The dimensions of the flat plate
concentrating solar collector were 2.3 m long, 1.6 m wide, and 0.5 m high. The
304 HOQUE et al.

transparent cover of the collector was 4 mm thick clear glass. Black painted
corrugated iron sheet about 200 mm below the glass cover was used as an
absorber plate. To increase the efficiency of the solar collector, flat type
reflector made of glass mirror was added at top of the solar collector. The
dimensions of the reflector were the same as those of the solar collector so that
it could be used as a reflector in day time and as a cover in night time or in
adverse weather. This reflector had adjustable angles that could be changed
according to the change of the sun’s angle during the day to collect higher
amount of sun rays that fall down on the solar collector. In addition, the
collector was placed on 4 legs with 140 mm wheel to turn the solar collector
horizontally and change its direction according to the change of the sun’s angle.
The solar collector was insulated by 50 mm thick polystyrene. A centrifugal
blower operated by a 0.75 kW, 220 V electric motor was connected at one side
of the collector to draw the atmospheric air in the collector and push out the
heated air into the dryer at a desired air velocity. Air flow was controlled by a
variac connected with the electric motor. For auxiliary heating, two electric
heaters (2 kW x 2= 4 kW) were installed at the entry of the collector. A
temperature controller was set to maintain constant temperature inside the dryer.

Fig. 1. Schematic view of a solar hybrid dryer: (1) Leg , (2) base plate, (3)
wheel, (4) tray, (5) floor, (6) insulation, (7) absorber plate, (8) glass
cover, (9) reflector, (11and 12) reflector frame, (13) reflector adjusting
support and (14) hinge.
The length and width of the solar dryer were same as the collector (2.30 m
×1.60 m). It was located directly under the solar collector and 200 mm under the
absorber plate. It was divided into 4 parts with equal dimensions. In each of the
parts, there were 2 trays for drying. This allows the usage of 8 drying trays in
the
DRYING KINETICS OF GINGER RHIZOME 305

drying unit. The drying air was passed across the asparagus placed in thin layers
on 8 horizontally stacked trays and arranged in two vertical columns. Each tray
was made of wooden frame and plastic net with dimensions of 1040 mm × 780
mm. The drying air was heated up in the solar collector and passed to the drying
chamber. The drying air from the solar collector was passed through a curved
passage downward, then again turned into the drying unit to flow over and
under all the drying trays and then exhausted through an outlet.

Description of the mechanical tray dryer

The mechanical tray dryer consists of a drying chamber, heater, electric blower
etc. The overall dimensions of the drying chamber of the mechanical tray dryer
are 1.42 m  0.64 m  0.86 m. Inner dimensions of the chamber are 0.80 m 
0.50 m  0.60 m. There were arrangements for fixing five trays. Heated air was
passed in over the trays through fourteen holes and passed out with same
numbers of holes. Air was circulated over the electric heater installed on the
bottom of the dryer with a fan sucking fresh air from the right side of the dryer.
There was an arrangement to control the velocity of the air by reducing or
increasing the opening of the holes. Sensors were used to detect the temperature
level. When the temperature is higher than the desired temperature, the heating
system is off and the reverse is true when the temperature is below the desired
temperature.

Experimental procedure
The ginger rhizome as a whole, splitted, and sliced (4 mm) were dried under
blanched and non-blanched conditions at 50 0C using solar hybrid dryer and at
60 0C and 70 0C using tray dryer. Before starting an experimental run, the whole
apparatus was operated for at least one hour to stabilize the air temperature and
air velocity in the dryer. Drying was started usually at 09:00 am and continued
until it reached the final moisture content (about 8 to 9%, wb). Ambient
temperature and temperature inside the dryer temperature was measured with a
digital thermometer (K202, Voltcraft digital thermometer, Germany) connected
with k type thermocouples. Solar radiation was measured with a Lux meter
(LX- 9626, China) during the day time. Velocity of drying air was measured
with a thermo-anemometer (AM-3848, China). Weight losses of the samples in
the solar dryer were recorded during the drying period at two hours of interval
with an electronic balance (EK-200g, Max 200  0.01g). After completion of
drying, the dried samples were collected, cooled in a desiccator to the ambient
temperature and then sealed it in the plastic bags.

Colour measurement
The colour of fresh and dried ginger rhizome samples were measured by a
chromameter (CR-400, Minolta Co. Ltd., Japan) in CIE (Commission
306 HOQUE et al.

Internationale l’Eclairage) Lab chromaticity coordinates. L *, a* and b* represent

black to white (0 to100), green to red (-ve* to +ve) and from blue to yellow (-ve
to +ve) colours, respectively. Out of five available colour systems, the L *a*b*
(Krokida et al., 1998; Lozano and Ibarz, 1997 and Maskan, 2001) and L *C*h*
(Zhang et al., 2003) systems were selected because these are the most used
systems for evaluation of the colour of dried food materials. The instrument was
standardized each time with a white ceramic plate. Three readings were taken at
each place on the surface of root samples and then the mean values of L *, a* and
b* were averaged. The different colour parameters were calculated using the
following equations (Camelo and Gomez, 2004).
Hue angle indicating colour combination is defined as:
Hue angle =tan-1(b*/a*) (when a*>0) (1)
Hue angle =1800+ tan-1(b*/a*) (when a*<0) (2)
and Chroma indicating colour saturation is defined as:
Chroma = (a*2+b*2)1/2 (3)

Thin layer drying equations

Thin layer drying equations and expressions for the drying parameters as a
function of drying conditions are required for simulation of the drying systems
(Bala, 1997). Three general approaches for thin layer drying are the
development of (1) empirical equations, (2) theoretical equations, and (3) semi-
theoretical equations. Theoretical approach concerns either the diffusion
equation or simultaneous heat and mass transfer equations. Semi-theoretical
approach concerns approximated theoretical equations. The main justification of
the empirical approach is a satisfactory fit to all experimental data. The details
of the development of thin layer drying models and expressions of the drying
parameters are given in Bala (1997). This study considers empirical and semi-
theoretical thin layer equations.
Mathematical models for thin-layer drying of sliced asparagus root under
blanched conditions were developed by using direct least square method
between moisture ratio (MR) and drying time (t). Moisture ratio was defined as
follows:

M M
MR  M t  Me (4)
0 e

Me values were obtained from drying curves and were set equal to moisture
content at which sample weight became constant with drying time. Five
commonly used thin layer equations were selected to fit the experimental data
of drying of sliced ginger rhizome by the direct least square method using
SPSS
DRYING KINETICS OF GINGER RHIZOME 307

11.5 and these are shown in Table 1. The constant final moisture contents were
considered as the equilibrium moisture contents of the samples.
The equations were evaluated in terms of coefficients of determination (R2)
and root mean square errors (RMSE) and these are defined as:

R2  M exp pred


2
(5)
M

 M M exp 2 pred
2

N M M 2
RMSE   pred exp 
 (6)
1  df 
Residuals of each model were plotted with experimental moisture contents.
If residual plots indicate a systematic pattern, there is a systematic error in
model prediction (Chen and Morey, 1989; Kaleemullah and Kailappan, 2004).
A model was considered to be the best when the residual plots indicated
uniformly scattered points i.e. random; RMSE is a minimum value and R 2 is a
maximum value (close to 1.0).
Table 1. Thin layer drying models.
Serial
Name of model Model expression
No.
1 Newton Equation Mt  Me
 exp(kt)
M 0  Me
2 Henderson and Pabis Mt  Me
 a exp(kt)
M 0  Me
3 Page Equation Mt  Me
 exp(ktn )
M 0  Me
4 Approximation of Mt  Me
 a exp(kt)  (1  a) exp(kbt)
Diffusion Equation
M 0  Me

5 Midilli et al. (2002) Mt  Me

 a exp(ktn )  bt
Equation M 0  Me

Results and Discussions

Effect of drying
The effect of temperature for different shapes and sizes of ginger rhizomes on
drying characteristics under blanched and non-blanched conditions are shown in
Fig. 2 and Fig. 3, respectively.
308 HOQUE et al.

Drying rate of whole ginger rhizomes under blanched and non-blanched

conditions was extremely low and the desired moisture contents were reduced
from initial moisture content 87.98% and 84.97% (wb) to the moisture content
75.73% and 68.70% (wb) under blanched condition and to the moisture content
81.98% (wb) and 77.46% (wb) under non-blanched condition after 20 hours of
drying at 50 and 60 0C, respectively. Though drying rate was, relatively, high at
70 0C, still the moisture content reduction were about 40.18% and 59.85% (wb)
after 20 h of drying under blanched and non-blanched conditions.
The drying rate of splitted ginger rhizomes increased with the drying air
temperature, but the rate of increase of drying rate was relatively low at 50 0C.
Blanched splitted ginger rhizomes were dried at 70 0C in a mechanical dryer
from 85.76% (wb) to a moisture content of 9.11% (wb) in 20 hours, while at 60
0
C, it took about 32 h to obtain a moisture content of 11.32% from the initial
moisture content of 84.97% (wb). Non-blanched splitted ginger rhizomes were
dried at 70 0C in a mechanical dryer from 85.76% (wb) to a moisture content of
13.44% (wb) in 20 hours while at 60 0C it took about 32 hours to obtain a
moisture content of 13.65% from the initial moisture content of 84.97% (wb).
Splitted ginger rhizomes were dried from the initial moisture content of 87.98%
(wb) to 22.54% and 32.96% (wb) under blanched and non-blanched conditions
in 32 hours of drying at 50 0C. This implies that drying rate of splitted ginger
rhizome increases with the increase of drying temperature.
Sliced ginger rhizomes were dried from an initial moisture content of
85.76% (wb) to moisture content of 7.41% and 8.73% (wb) under blanched and
non- blanched conditions in 16 and 18 hours, respectively, at 70 0C in
mechanical tray dryer, while at 60 0C, sliced ginger rhizomes were dried from
an initial moisture content of 84.97% (wb) to moisture content of 8.21% (wb)
and 8.59% (wb) under blanched and non-blanched conditions in 20 and 24
hours. But sliced ginger rhizomes were dried from an initial moisture content of
87.98% (wb) to moisture content of 10.27% and 14.62% (wb) under blanched
and non-blanched conditions in 26 and 28 hours at 50 0C. This implies that the
drying rate increases with the increase of temperature from 50 0C to 60 0C, but
the drying rate is almost same for drying either at 60 0C or 70 0C and it is much
more prominent under blanched conditions. Ginger rhizomes should be dried to
about 8% (wb) of moisture content so that samples can be powdered and for this
reason ginger rhizomes should be dried in the sliced form either at 60 0C or 70
0
C. Logically mechanical drying or solar assisted drying may be recommended
for drying of ginger rhizomes.
The highest drying rate was found for sliced samples followed by splitted
and whole rhizome samples. This highest drying rate of the sliced samples
might be due to higher diffusion for sliced samples because of its two cut
surfaces with small diffusion length to travel towards the cut surfaces. The
splitted samples had one cut surface to the drying environment and the other
surface was the skin
DRYING KINETICS OF GINGER RHIZOME 309

resulting high diffusion length and less area exposed to the environment.
Diffusion rate of whole rhizome was very small and drying rate was also
extremely low.
Whole at 50°C Whole at 60°C Whole at 70°C
Splitted at 50°C Splitted at 60°C Splitted at 70°C
slices at 50°C slices at 60°C slices at 70°C
100

90

80
Moisture Content, %(wb)

70

60

50

40

30

20

10

0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

Drying time, h

Fig. 2. Effect of temperature for different shape and size of blanched ginger
rhizomes
Whole at 50°C Whole at 60°C Whole at 70°C
Splitted at 50°C Splitted at 60°C Splitted at 70°C
slices at 50°C slices at 60°C slices at 70°C
100
90
Moisture Content, %(wb)

80
70
60
50
40
30
20
10
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
Drying time, h

Fig. 3. Effect of temperature for different shape and size of non-blanched ginger
rhizome.
310 HOQUE et al.

The effect of blanching on drying characteristics of ginger samples of

different shapes and sizes for temperature levels of 50, 60, and 70 0C are shown
in Fig. 4, Fig. 5, and Fig. 6, respectively. Blanching increases the drying rate
(Bala, 1997). There is a significant difference between the drying curves for
blanched and non-blanched samples for splitted and sliced ginger rhizomes and
this difference becomes a minimum at 70 0C. This might be due to the fact that
during blanching, the samples were partially cooked and some cells or tissues of
splitted and sliced ginger rhizome might be disrupted or loosened. As a result,
moisture diffusion was higher and hence the drying rate was higher. The effect
becomes more prominent with the increase of the temperature. Similar results
have been reported by Hossain et al. (2007) for red chilli.
However, the moisture content of the whole ginger rhizome remains almost
constant during the drying period and this is true for either blanched whole
ginger rhizomes samples or non-blanched whole ginger rhizomes samples. This
implies that the thick skin of the whole ginger rhizomes prevents the moisture
diffusion through the skin.

Whole non-blanched Whole blanched Splitted non-blanched

Splitted blanched Slices non-blanched Slices blanched

100

90
80
Moisture Content, %(wb)

70

60

50
40

30

20
10

0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
Drying time, h

Fig. 4. Effect of blanching on drying for different shape and size of ginger
rhizome dried at 50 0C.
DRYING KINETICS OF GINGER RHIZOME 311

Whole non-blanched Whole blanched Splitted non-blanched

Splitted blanched Slices non-blanched Slices blanched

90

80

70
Moisture Contrnt (%,

60

50

40

30
wb)

20

10

0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
Drying time, h

Fig. 5. Effect of blanching on drying for different shape and size of ginger
rhizome dried at 60 0C.

Whole non-blanched Whole blanched Splitted non-blanched

Splitted blanched Slices non-blanched Slices blanched

100
90
80
Moisture Contrnt (%, wb)

70
60
50
40
30
20
10
0
0 2 4 6 8 10 12 14 16 18 20 22
Drying time, h

Fig. 6. Effect of blanching on drying for different shape and size of ginger
rhizome dried at 70 0C.
 312
Table 3. Model parameters, coefficient of determination (R2), root mean standard error (RMSE), grade and ranking of thin
layer drying models at different temperatures.
Grade
Models Temp. (°C) a b k n R2 RMSE Point Rank

Newton 50 0.159942 0.9874 0.0039

60 0.277501 0.9883 0.0018 3.16 4
70 0.399923 0.9900 0.0036
Henderson and Pabis 50 1.05370 0.167445 0.9900 0.0596
60 1.037016 0.285821 0.9895 0.0430 1.83 5
70 1.019779 0.405722 0.9904 0.0652
Page 50 0.087498 1.296297 0.9991 0.0170
60 0.160821 1.369787 0.9987 0.0148 4.00 1
70 0.236738 1.45232 0.9999 0.0041
Approxim-ation of 50 20.3279 1.039328 0.285134 0.9992 0.0179
Diffusion
60 37.05262 0.989062 0.183674 0.9920 0.0401 3.33 3
70 17.96628 1.060838 0.793326 0.9999 0.0046

Midilli et al.(2002) 50 0.99819 5.28E-06 0.086745 1.299746 0.9992 0.0196

HOQUE et al.
60 1.004872 3.79E-05 0.163046 1.364374 0.9987 0.0169 3.83 2
70 1.000421 5.72E-05 0.674964 1.214447 0.9999 0.0035
DRYING KINETICS OF GINGER RHIZOME 313

Thin layer drying equations

Model parameters, coefficient of determination (R 2), root mean square error
(RMSE), grade points and ranking of thin layer drying models at different
temperatures are presented in Table 3. From the table, it is observed that based
on the highest average coefficient of determination and lowest root mean square
error, Page model posses highest grade point and ranked first. Here, the highest
R2 and the lowest RMSE values indicated the highest grade point and lowest
rank. The lowest ranked model was considered to be the best fitted model.
Hence, Page model ranked one and the Midilli et al. (2002) model ranked two
followed by the Approximation of diffusion model, Newton model, and
Henderson and Pabis model.
Residual plots of different models for single layer drying of ginger rhizome
for drying temperature of 50, 60, and 70 0C are shown in Fig.7. For Page model
the residual plots indicated a scattered pattern and the residuals are very close to
X-axis leading to suitability for predicting single layer drying of ginger. For
other models, the residual plots indicated a systematic pattern and/or the
residuals are not close to X-axis.

Newton Model Page model

Henderson and Pabis Approximation of diffusion model
model Midilli et al.(2002)
model
1
0.8

0.6

0.4

0.2
Residual

0
0 2 4 6 8
-0.2

-0.4

-0.6

-0.8

Moisture Content kg/kg (db)

Fig. 7. Residual plots of different models for single layer drying of sliced ginger.

Estimation of different drying parameters

The parameters of Page model at variable temperatures (50 0 to 70 0C) are found
to be a linear function of air temperature. Following regression equations were
developed for the parameters of Page model as a function of temperature.
314 HOQUE et al.

k  0.0075Ta  0.286
(R2= 0.999) (7)
n  0.0078Ta 
(R2= 0.998) (8)
0.9047
Substituting the values of k and n from the equations (7) and (8) into the
Page equation in Table no 1, we get the following equation in terms of
temperature
M  Me  (Mo  Me) exp((0.0075Ta  0.286)t 0.0078Ta0.9047 ) (9)
Fig. 8 shows the comparison between the experimental moisture content and
moisture content predicted from the Page model for the drying temperature of
500, 600 and 70 0C respectively. The predicted data mainly banded around the
straight line which showed the suitability of the model in describing single layer
drying behaviour. Furthermore, the predictions are within the acceptable limit
(1.70%) (O’Callaghan et al., 1971).

8
R2 = 0.9992
7
Predicted Moisture Content (kg/kg,

1
db)

0
0 2 4 6 8
Experimental Moisture Content (kg/kg, db)

Fig. 8. Experimental and predicted moisture content for single layer drying of
ginger rhizome at 500, 600 and 70 0C.
Comparison of the experimental data and predicted results from the Page
equation for drying of ginger rhizome at 50 0C, 60 0C and 70 0C are shown in
Fig. 9(a), Fig 9(b) and Fig 9(c), respectively. The agreement between the
predicted and experimental results is excellent.
DRYING KINETICS OF GINGER RHIZOME 315

6
8
Observed Observed
7 5

M o is t u re C ont en t (k g/ k g , d b
M ois ture C ontent (k g/k g, db)

Predicted Predicted
6
4
5

4 3

3
2
2

1 1

)
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 0
0 2 4 6 8 10 12 14 16 18 20 22
Drying time, h
Drying time, h

(a) Dried at 50 0C (b) Dried at 60 0C

7
Observed
6
Predicted
M o is ture C o nte nt (k g / k g,

2
db )

0
0 2 4 6 8 10 12 14 16 18
Drying time, h

(c) Dried at 70 0C
Fig. 9. Comparison of predicted results of the Page model with the experimental
data for dried at (a) 50 0C, (b) 60 0C and (c) 70 0C.

Colour degradation
The colour of ginger rhizomes were measured before and after drying.
Variations of colour of fresh and dried ginger rhizomes of all forms dried at
different temperatures are shown in Table. 4. Colour of ginger rhizomes in
combination of L* (from black to white), a* (from green to red) and b* (from
blue to yellow) were slightly changed after drying. Lightness decreased with an
increase in drying temperature for all samples except sliced and blanched
samples. Lightness of blanched sliced samples and fresh rhizomes were not
significantly changed with the drying temperature. Values of hue angle of all
dried samples were significantly changed from the fresh samples but values of
hue angle of blanched sliced samples were not significantly changed with the
drying temperature. The sliced and blanched asparagus was dried at 50,
60 and 70 0C of drying
316 HOQUE et al.

temperature and there were some differences in colour indexes as shown in Fig.
10. From Fig. 10, we find that the lightness decreased at 60 0C but again
increased at 70 0C, hue angle increased but there was no significant difference
between hues for 60 and 70 0C. The chroma of the dried product increased for
drying at 60 0C than chroma of ginger dried at 50 0C and then again decreased in
70 0C. This implies a small change in the colour intensity with increase in
temperature. Thus, colour of the dried ginger rhizome for different drying air
temperatures is almost same with a small increase in colour saturation.
Table 4. Colour variations of fresh and dried ginger rhizomes dried at different
temperatures.
Colour Value
Treatments
L* a* b* h* (0)
Fresh 71.43d -6.73a 36.30e 100.61d
0
Non-blanched whole 50 C 49.26b 4.6f 24.64bcd 79.39a
0
Non-blanched splitted 50 C 49.76b 4.33f 26.18cd 80.59a
Non-blanched sliced 50 0C 63.36cd 0.54cd 25.14bcd 88.74b
Blanched whole 50 0C 49.28b 3.82ef 27.62d 82.06a
0
Blanched splitted 50 C 62.01cd 0.45cd 24.33bcd 89.02b
0
Blanched sliced 50 C 65.43cd -1.06bc 24.15bcd 92.64bc
0
Non-blanched whole 60 C 42.80ab 4.21f 24.31bcd 80.18a
0
Non-blanched splitted 60 C 46.76b 5.15f 27.42cd 79.61a
0
Non-blanched sliced 60 C 61.60cd 0.24bcd 25.14bcd 89.47b
Blanched whole 60 0C 49.28b 3.82ef 27.62d 82.06a
0
Blanched splitted 60 C 60.28c 0.33cd 23.23bc 89.22b
0
Blanched sliced 60 C 61.65cd -1.06bc 26.85cd 92.12bc
0
Non-blanched whole 70 C 47.80b 4.21f 23.64bcd 79.89a
0
Non-blanched splitted 70 C 47.33b -0.21bcd 16.67a 90.67bc
0
Non-blanched sliced 70 C 35.81a 1.8de 17.13a 84.05a
0
Blanched whole 70 C 40.62a 3.82ef 21.62b 80.04a
0
Blanched splitted 70 C 36.67a 1.88de 17.21a 83.78a
Blanched sliced 70 0C 63.09cd -1.83b 21.82b 94.85c

Common letter in the same column does not significantly differ at 5% level by Duncan’s
Multiple Range Test (DMRT).
DRYING KINETICS OF GINGER RHIZOME 317

100
90
80
70

Colour index
60
50 L*
Hue angle
40 Chroma
30
20
10
0
50 60 70
Drying temperature, °C

Fig. 10. Influence of drying on lightness (L*), hue angle (h*) and chroma (C) of
sliced and blanched ginger.

Conclusions
Drying characteristics of ginger rhizomes of different shapes and sizes at three
temperature levels of 50, 60, and 70 0C under blanched and non-blanched
conditions were investigated. Blanching increases the drying rate and there is a
significant difference between the drying curves for blanched and non blanched
samples. The drying rate depends on shape and size of the ginger rhizomes. The
highest drying rate was found for sliced samples of ginger rhizome followed by
splitted and whole root samples. The moisture content of the whole ginger
rhizome remain almost constant during the whole drying period and this is true
for either blanched the whole samples or non-blanched the whole samples. The
drying rate increased with the drying air temperature. The rate of increase of
drying rate was relative low at low temperature. The drying time decreases with
increase in drying temperature. Lightness of ginger rhizomes decreased with an
increase in drying temperature for all samples except sliced and blanched
samples.

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