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13 April 2012 1

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

Temperature and Time1El-Amin Omda Mohamed Akoy

2Dieter von Hörsten and 2W. Luecke1

Assistant ProfessorUniversity of Zalingei, West Darfur,

Zalingei, Sudan.2Dept of Crop Sciences, section Agric.

Eng., University of Goettingen, Germany.



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Introduction

• Mango ( Mangifera indica L.) is one of thetropical and subtropical fruit of great importancefor 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

its utilization.• Drying may be an interesting method in order toprevent fresh fruit deterioration.



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• 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 colour measurements can be used in an

indirect way to estimate colour change of foods,

since it is simpler and faster than chemical

analysis



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Study objectives

1-To investigate the drying kinetics of mangofruit

2-To determine the optimum drying

temperature for mango fruit

3-To investigate the combined effect of drying

temperature and time on colour change and re-

hydration ratio4-To evaluate a suitable drying model for

describing the drying process of mango fruit.



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Material and Methods

Raw Material

•Fresh mangoes, var. “Kent”, from Mali, werepurchased at the wholesale market in Goettingen,

Germany.•The mangoes were left for 5days for post harvestripening at 25±2ºC and 50% relative humidity (Pott etal, 2005).

• Fresh fruits were washed, manually peeled usingstainless steel knife and sliced using an electric food-slicer at 3 mm thickness.



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2.2. Drying experiments

•Drying experiments were performed in a laboratory air-

oven.

•The average initial moisture content of the mango fruit was

82.5±0.4 %( w.b.), as determined by convective air dryingat 135°C for 2h (AOAC, 2000)



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• 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 formoisture loss assessment was placed in circular

wire mesh of 10 cm diameter and placed on to

the centre of the tray.



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•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 drying process was stopped when the moisturecontent 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 forcalculation.



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Colour measurements

•Colour parameters were measured with a Minolta

CR-310 Chroma-meter (Minolta, Japan).

• Hunter scale (L*, a*, b*) system was used.



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From the colour values, total colour change (∆E),

chroma (C), hue angle (h) and browning index (BI)

were calculated using equations described byMaskan (2001):

ΔE = ((L0-L)2 + (a

0- a)2 + (b

0 – b)2)½ (1)

Chroma = (a2 + b2)½ (2)Hue angle = tan-1(b/a) (3)

BI = [100(x-0.31)] / 0.17 (4)

Where:

x = (a + 1.75L)/ (5.645L + a – 3.012b)



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2.2.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 thefiltrates were weighed.

Re-hydration ratio (R/R) = W2 /W1

Where: W2 = weight of drained material, g

W1

= weight of dried material, g



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Mathematical modelling of

drying curvesModel

name

Equation Reference:

Newton MR= exp (-kt Lewis (1921)

Henderson

and Pabis

MR= a exp (-kt) Henderson and

Pabis (1961)

Page MR= exp (-ktn) Page (1949)



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•The moisture ratio (MR) and drying rate of mango

slices during drying experiments were calculated

using the following equations:

• MR = Mt/M0

•Drying rate (Mt-Mt+dt)/dt



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•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 squareerror (SSE), The reduced chi-square ( χ 2) and root

mean square error(RMES).



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Results

Drying Kinetics of Mango Slices:

• Effect of drying temperatures on drying

kinetics of mango slices



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0

0.5

1

1.5

2

2.5

3

3.5

0 0.2 0.4 0.6 0.8 1 1.2

Moisture ratio

D r y i n g

r a t e ( k g / k g h

r )

T=60C

T=70C

T=80C

Effect of drying air temperature and moisture ratio on the drying rate

of mango slices.



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0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8

Drying time (h)

M o i s t u r e r a t i o ( d i m e n s i o n l e s s )

T=60C

T=70C

T=80C

Effect of drying air temperature and drying time on the moisture

ratio of mango slices.



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0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

60 70 80

drying temperature C

R e - h y d r a t i o n r a t i o

combined effect of drying temperature and time on re-

hydration ratio of mango slices.



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colour parameters versus drying time

-20

0

20

40

60

80

100

120

0 60 70 80

drying time

c o l o u r p a r a m e t e r s

L*value

a* value

b* value

E differenceChroma value

Hue angle

browning index



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Colour parameters change versus

drying temp.

0

1

2

3

4

5

6

60 70 80

drying temperature

c o l o u r p a r a m

e t e r c h a n g e

L0-L

a0-a

b0-b



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0

0.10.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3 3.5

Drying time

M o i s t u r e r a t i o ( d i m e n

s i o n l e s s )

Measured MR Pred. MR for Lewis model Pred. MR for Page Pred. MR for Hederson & Pabis

Measured and Predicted MR versus drying time



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Conclusions

•The following conclusions can be drawn

from this study: 1- All the drying process of mango slices occurs in fallingrate drying period.

2- The optimum drying temperature of mango slices is

found to be 80ºC. 3- Drying temperature and drying time have significanteffect on colour change of mango fruit

4- Page model was found to predict thin-layer dryingcharacteristics of mango fruit well.



13 April 2012 24

• Thank you for

listening

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