Drying Kinetics and Colour Change of Mango Slices as Affected by Drying Temperature and Time

Drying Kinetics and Colour Change of Mango Slices as Affected by
Drying Temperature and Time

El-Amin Omda Mohamed Akoy1, Dieter Von Hörsten2, Wolfgang Luecke2
1University of Zalingei, Department of Agricultural Engineering, Sudan, e-mail:
aminoz71@yahoo.com
2Georg-August-Universit¨at Goettingen, Institute of Agricultural Engineering, Germany
Written for presentation at the
2008 Tropentag International Conference on
“Competition for Resources in a Changing World:
New Drive for Rural Development”
Hohenheim, Germany
7-9 October 2008


Abstract. Thin-layer drying behaviour of mango slices (var. Kent) was investigated in a laboratory
scale dryer, using heated ambient air temperatures of 60, 70 and 80°C at a constant air velocity of 1.5m/s
and 3mm slice thickness. Besides the effects of drying air temperature on the drying characteristics,
drying time and quality of dried products were determined. Drying curves obtained from the experimental
data were then fitted to three well-known semi-empirical thin-layer drying models (Lewis model, Page
model and Henderson & Pabis model). Model constants and coefficients were determined by nonlinear
regression method. All the models were compared according to statistical parameters. Among the drying
models investigated, the Page model satisfactory described the drying behaviour of mango slices. The
effective moisture diffusivity varied from 4.97x10-10 to10.83x 10-10 m2/s. Results indicated that drying
took place in the falling rate period. The results have shown that, increasing air temperature causes shorter
drying times. The combined effect of drying temperature and time on colour and re-hydration ratio were
also evaluated. The colour was measured from the surface and expressed in the Hunter L*a*b* system.
Moreover, the total colour change (?E), chroma (colour saturation), hue angle and browning index (BI)
were determined. L* and b* parameters were found to decrease as affected by drying temperature and
drying time, whereas a* parameter increases. Results also indicated that drying time has significant effect
on colour change and rehydration ratio. The lowest total colour change and highest rehydration ratio were
obtained at drying air temperature of 80C then 70 and finally 60C with drying time of 3,5 and 7 hours,
respectively. In contrast to common practice, drying at elevated air temperature (80°C), instead of 60°C
for a longer time, was optimal, since significant colour changes of mango slices were not observed.
Moreover, at increased temperature, drying time was considerably shortened from about 7 h to 3h,
resulting in significant extension of drying capacity.

Keywords: Colour change, drying models, mango slices, re-hydration ratio, thin-layer drying







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Introduction

Mango (Mangifera indica L.) is one of the tropical and subtropical fruit of great importance for
both economical and nutritional point of view. It is considered to be a good source of carbohydrates,
vitamin C and very rich source of pro-vitamin A. In spite of its excellence, the perishable nature of this
fruit and its short harvest season severely limit utilization. Drying may be an interesting method in order
to prevent fresh fruit deterioration.
Drying is one of the most widely used primary methods of food preservation. The objective
drying is the removal of water to the level at which microbial spoilage and deterioration reactions are
greatly minimized (Akpinar and Bicer, 2004). It also provides longer shelf-life, smaller space for storage
and lighter weight for transportation (Ertekin and Yaldiz, 2004). Sun drying is the most common method
used to preserve agricultural products in tropical and subtropical countries. However, being unprotected
from rain, wind-borne dirt and dust, infestation by insects, rodents and other animal, products may be
seriously degraded to the extent that sometimes become inedible and the resulted loss of food quality in
the dried products may have adverse economic effects on domestics and international markets. Therefore,
the drying process of agricultural products should be undertaken in closed equipment (solar or industrial
dryer) to improve the quality of the final product. The re-hydration capacity and colour characteristics
were considered as the most important quality parameters for the dehydrated products. The re-hydration
capacity is used to express ability of the dried material to absorb water.
The first quality judgment made by a consumer on a food at the point of sale is its visual appearance.
Appearance analyses of foods are used in maintenance of food quality throughout and at the end of
processing. Colour is one of the most important appearance attribute of food materials, since it influences
consumer acceptability. Abnormal colours, especially those associated with deterioration in eating quality
or with spoilage, cause the product to be rejected by the consumer (Avila and Silva, 1999). Therefore, the
objectives of this study were:
1-To investigate the drying kinetics of mango fruit
2- To evaluate a suitable drying model for describing the drying process of mango fruit.
3- To determine the optimum drying temperature of mango fruit
4-To investigate the combined effect of drying temperature and time on colour change and re-
hydration ratio

Materials and Methods

Raw Material
Fresh mangoes, var. “Kent”, from Mali, were purchased at the wholesale market in Goettingen, Germany.
The mangoes were left for 5days for post harvest ripening at 25±2ºC and 50% relative humidity (Pott et al,
2005). Fresh fruits were washed, manually peeled using stainless steel knife and sliced using an electric
food-slicer (Krups variotronic, Germany) at 3 mm thickness.
Drying experiments
Drying experiments were performed in a laboratory cross flow dryer, Heraeus (Schütt, UT 6120 type,
Germany). This is consisted of heating unit, temperature control unit, drying chamber and centrifugal fan
of constant air velocity of 1.5 m/s. The average initial moisture content of the mango fruit was 82.5±0.4 %
( w.b.), as determined by convective air drying at 135°C for 2h (AOAC, 2000)
Prior to starting the experiments, the dryer was run idle for about half an hour to reach thermal
stabilization. Then the samples were uniformly spread within the tray as single layer of 3mm thickness. A
representative sample of sliced mango for moisture loss assessment was placed in circular wire mesh of
10cm diameter and placed on to the centre of the tray. For measuring the mass of sample at any time
during experimentation, sample on circular wire mesh was taken out of drying chamber and weighed on
the digital balance and placed back into the drying chamber every 30 min during drying. The digital top
pan balance 600±0.001g, (Sartorius, Goettingen, Germany) was kept near to the drying unit and weight
measurement process took less than 10 seconds time. The drying process was stopped when the moisture
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content decreases to about 9±0.2 % (w.b). All the experiments were replicated two times at each air
temperature and the average values were used for calculation.

Table.1.Slected drying models for describing mango slices drying data
Model name
Equation
References
Newton
MR= exp (-kt)
Lewis (1921
Henderson and Pabis
MR= a exp (-kt)
Henderson
and Pabis
(1961
Page
MR= exp (-ktn) Page
(1949)

Non-linear regression analysis was used to evaluate the parameters of the selected models. The goodness
of fit was determined using the four statistical parameters, i.e. coefficient of determination (R2), the sum
square error (SSE), The reduced chi-square ( ?2) and root mean square error (RMES). These parameters
can be described in the following equations:
(SSE) = 1/N ?Ni=1 (MR exp,i - MRpred,i)² (1)
(?2 ) = ?Ni=1 (MR exp,i - MRpred,i)² (2)
N-n
(RMSE) = (1/N ?Ni=1 (MR exp,i - MRpred,i)²)½ (3)
Where:
exp. = Experimental data
pred. = Predicted data
N = Number of observations
n = Number of constants

Colour measurements

Colour parameters were measured with a Minolta CR-310 Chroma-meter (Minolta, Japan). The chroma
meter consists of the respective measuring head and the data processor. The measuring head of the chroma
meter CR-310 uses wide-area illumination and a 0° viewing angle and has 50 mm-diameter measuring
area to average the reading. Hunter scale (L*, a*, b*) system was used. The Chroma meter was calibrated
with a white standard tile with illuminant D65(Y=94.3, x= .3156 and y= .3324) equivalent to HL system:
HL = 97.10, a = -0.17 and b= 1.80. Calibration was made at each experiment. The parameter L* represents
the brightness of the colour, a* the hue range of the colours red (+) and green (-) and b*hue range of
colours yellow (+) and blue (-). Three measurements were made on the surface of sliced mango fresh and
after drying and average values were made for calculation and each experiment was duplicated. From the
colour values, total colour change (?E), chroma (C), hue angle (h) and browning index (BI) were
calculated using equations described by Maskan (2001):
?E = ((L0-L)2 + (a0 - a)2 + (b0 – b)2)½ (4)
Chroma = (a2 + b2)½ (5)
Hue angle = tan-1(b/a) (6)
Where; subscript “0” refers to the colour reading of fresh mango slices. Fresh mango was used as a
reference and a larger ?E denotes greater colour change from the reference material (Maskan, 2001).
BI = [100(x-0.31)] / 0.17 (7)
Where:
x = (a + 1.75L)/ (5.645L + a –3.012b)
Re-hydration ratio (Reconstitution ratio)
Re-hydration ratio was determined according to the official method of AOAC (1984), 5g of dried sample
was soaked for 30 and 60 minutes in 50 ml distilled water, filtered through filter paper and then the
filtrates were weighed.
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Re-hydration ratio (R/R) = W2/W1 (8)
Where: W2 = weight of drained material, g
W1 = weight of dried material, g

Results and discussion
Drying characteristics
The effect of three temperatures on the drying curves of mango slices are shown in Fig. 1. it can be
observed that there is no constant rate drying period in the drying of mango slices, and all the drying
process occurred in the falling rate period. This indicates that diffusion is the dominant physical
mechanisms governing moisture movement in the samples. These results are in agreement with Goyal et
al (2006) for mango fruit.
3.5
3
)
r
h 2.5
g
/
k
2
T=60C
(kg
T=70C
te
a 1.5
r
T=80C
g
i
n
1
y
Dr 0.5
0
0
0.2
0.4
0.6
0.8
1
1.2
Moisture ratio

Fig. 1. Effect of drying air temperature and moisture ratio on the drying rate of mango slices.

1.2
1
l
ess)
n
i
o 0.8
s
n
e
T=60C
i
m 0.6
T=70C
(
d
t
i
o
T=80C
a 0.4
r
r
e
t
u 0.2
i
s
Mo
0
0
2
4
6
8
Drying tim e (h)

Fig. 2. Effect of drying air temperature and drying time on the moisture ratio of mango slices.



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Combined effect of drying temperature and time on product colour

colour parameters versus drying time
120
100
L*value
rs
80
a* value
t
e
e
m
b* value
60
ra
a
E difference
40
r
p
Chroma value
u
l
o
20
o
Hue angle
c
0
browning index
0
60
70
80
-20
drying time

Fig.3. Colour parameters versus drying temperature
Colour parameters change versus
drying temp.
6
5
4
e
t
e
r

c
h
a
n
g
e
L0-L
a0-a
3
b0-b
r
a
m
2
pa 1
0
c
o
l
our
60
70
80
drying temperature

Fig.4. Colour parameters changes versus drying temperature














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Re-hydration ratio of dried mango slices
5
4.5
4
o
ti 3.5
a
r
3
n
o
ti 2.5
r
a
d
2
y
1.5
Re-h
1
0.5
0
60
70
80
drying temperature C

Fig.5. Combined effect of drying temperature and time on rehydration ratio of mango slices


Conclusions
The following conclusions can be drawn from this study: -
1- All the drying process of mango slices occurs in falling rate drying period.
2- The optimum drying temperature of mango slices is found to be 80ºC.
3- Drying time has significant effect on colour change of mango fruit
4- Page model was found to predict thin-layer drying characteristics of mango fruit well.
References
Akpinar, E. K., and Bicer, Y. (2004). Modelling of the drying of eggplants in thin-layers. International
journal of food science and technology, 39:1-9.
AOAC (1984). Association of official analytical chemist. Official methods of analysis 14th ed.
Washington. D.C.
AOAC. (2000). Official method of analysis. No.920.149149(c).17th ed. Horwitz, USA.
Avila, I.M.L.B. and Silva, C.L.M. (1999). Modeling kinetics of thermal degradation of colour in
peach puree. Journal of food engineering, 39(2):161-166.
Ertekin, C. and Yaldiz, O. (2004). Drying of eggplant and selection of a suitable thin-layer drying
model. Journal of food engineering, 63:349-359.
Goyal, R . K .; Kingsly, A. R. P.; Manikantan, M. R. and Ilyas, S. M. (2006). Thin-layer drying
kinetics of raw mango slices. Biosystems enginerring, 95(1):43-49.
Henderson, S. M. and Pabis, S. (1961). Grain drying theory I: temperature effect on drying
coefficient. J. Agric. Eng. Res. 6(3): 169-174.
Lewis, W.K.(1921). The rate of drying solid materials. J. of Industrial Eng., 13 (5): 427-432.
Maskan, M. (2001). Kinetics of colour change of kiwifruits during hot air and microwave drying.
Journal of food engineering, 48:169-175.
Page, G. (1949). Factors influencing the maximum rates of air drying shelled corn in thin layers.
Unpublished M.Sc. Thesis, Purdue University, Lafayette, IN
Pott, I.; Neidhart, S.; Muhlbauer, W. and Carle, R. (2005). Quality improvement of non-sulphited
mango slices by drying at high temperatures. Innovative food science and emerging
technologies, 6: 412-419.

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