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doi:10.1016/j.biosystemseng.2006.05.001PH—Postharvest Technology

Biosystems Engineering (2006) 95 (1), 43–49

Thin-layer Drying Kinetics of Raw Mango Slices

R.K. Goyal1; A.R.P. Kingsly2; M.R. Manikantan1; S.M. Ilyas1

1Central Institute of Post Harvest Engineering and Technology, P.O. : PAU campus, Ludhiana 141004, India;e-mail of corresponding author: rkgoyal63@yahoo.co.in

2Central Institute of Post Harvest Engineering and Technology, Malout-Hanumangarh Byepass, Abohar 152116, Punjab, India;e-mail: arpkingsly@rediffmail.com

(Received 12 September 2005; accepted in revised form 2 May 2006; published online 12 July 2006)

Thin-layer drying behaviour of raw mango slices was studied in a laboratory model tunnel dryer. The sampleswere dried at 55, 60 and 65 1C air temperature with control, blanching and blanching in 1% potassiummetabisulphide (KMS) solution as pre-treatments. Six thin-layer drying models (Newton, Page, ModifiedPage, Henderson and Pabis, logarithmic and Wang & Singh) were fitted to the moisture ratio data. Among thedrying models investigated, the Page model satisfactorily described the drying behaviour of raw mango slices.The effective moisture diffusivity varied from 2�62� 10�10 to 4�39� 10�10m2s�1.r 2006 IAgrE. All rights reserved

Published by Elsevier Ltd

1. Introduction

Drying of fruits and vegetables is one of the oldestmethods of preservation. Drying prevents occurrence ofundesirable changes due to microbial activity. Althoughpreservation is the primary reason for drying, it alsolowers the product mass and volume. The reduction inmass and volume improves the efficiency of packaging,storing and transportation.Mango (Mangifera indica L.) is a good source of

carbohydrates, vitamin C and a very rich source of pro-vitamin A (Bhatnagar & Subramanyam, 1973). In India,raw mango has been dried historically to produce‘amchur’, a dried powder for use in culinary prepara-tion. Commercially, amchur is used for the manufactureof pickles, chutneys and sauces (Kesarwani et al., 2000).It is used to give a slightly sour and tangy taste to dishes.Usually the dropped immature mangoes are dried indirect sunlight for preparation of amchur. Sun dryingtakes 16–18 h for complete drying (Teaotia & Pruthi,1987). During drying, exposure to the open environmentfor a long time may cause contamination and spoilageof the product (Vagenas & Marinos-Kouris, 1991).Industrial dryers enable a uniform, hygienic driedproduct to be produced in within a shorter timescale(Abdelhaq & Labuza, 1987; Karathanos & Belessiotis,1997; Doymaz & Pala, 2002). The drying rate can also

1537-5110/$32.00 43

be enhanced by pre-treatments like blanching andsulphitation (Dabhade & Khedkar, 1980; Doymaz,2004). Also, pre-treatments prevent fruits from darken-ing. Dehydration of raw mango slices after sulphitationin potassium metabisulphide solution (KMS) (1–4%, w/w) has helped in better retention of sulphur dioxideduring storage (Mehta & Tomar, 1982).

The drying kinetics of food is a complex phenomenonand requires simple representations to predict the dryingbehaviour, and for optimising the drying parameters.Recently, studies have been done on drying kinetics offruits and vegetables (Sabarez & Price, 1999; Togrul &Pehlivan, 2002; Soysal, 2004; Doymaz, 2004; Cao et al.,2004; Jain & Pathare, 2004). However, no detailedstudies were found in literature on drying kinetics of rawmango. The objectives of this study were: (a) to studythe effect of temperature and pre-treatments on dryingbehaviour of raw mango; and (b) to evaluate a suitablethin-layer drying model.

2. Materials and methods

2.1. Experimental material

Raw mango (cv: Dasehari) used for the dryingexperiments were purchased from local market, Ludhi-

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Notation

a,b,c drying coefficientsDeff effective moisture diffusivity, m2s�1

EMB mean bias errorERMS root mean square errork drying constantL thickness of slice, mM moisture content at time t, kg [moisture] kg�1

[dry matter]Me equilibrium moisture content, kg [moisture]

kg�1 [dry matter]M0 initial moisture content, kg [moisture] kg�1 [dry

matter]MR dimensionless moisture ratio

N number of observationsn drying exponentr2 coefficient of determinationt drying time, minz number of drying constantsw2 reduced mean square of the deviation

Subscripts

exp experimentalpre predicted

500

500

1020

1

23

R.K. GOYAL ET AL.44

ana, India. The mango was sliced uniformly (averagethickness 2�870�3mm) and was dried on the same day.The initial moisture content of mango was 800�90%(d.b.) and was estimated by the AOAC method no.934�06 (AOAC, 2000).

930 2130

5

4

740

Fig. 1. Schematic diagram of tunnel dryer: (1) table; (2)drying chamber; (3) motor; (4) conveyor; (5) control panel:

All dimensions in mm.

2.2. Drying equipment

The thin-layer drying experiments were performed ina pilot plant cross-flow tunnel dryer (NSW-600, NarangScientific Works, New Delhi). The schematic diagram ofthe dryer is shown in Fig. 1. The dryer consisted of atunnel, electrical heater, fan and a temperature con-troller (30–110 1C, dry bulb temperature). The speed ofthe tunnel was fixed at 0�004m s�1. The samples weredried in multiple passes in the dryer. It took 8min forthe trays to complete a single passage in the tunnel.

2.3. Drying procedure

Mango samples were dried with pre-treatmentsnamely control (untreated sample), blanching (50 1Cfor 2min) and blanching with 1% KMS. Experimentswere conducted at 55, 60 and 65 1C. After the dryerreached the set conditions, sliced raw mango samples(150 g) were uniformly spread in rectangular aluminiumtrays (size: 310mm by 210mm by 30mm) and kept inthe tunnel for drying. Moisture loss was recorded in30min intervals by a digital balance of 0�01 g accuracy(Scaltec instruments, Germany). The drying was con-tinued until there was no large variation in the moistureloss. Experiments were replicated three times.

2.4. Mathematical modelling

Moisture ratio of samples during drying was ex-pressed by the following equation:

MR ¼ ðM �MeÞ=ðM0 �MeÞ (1)

where: MR is the dimensionless moisture ratio; and M,M0 and Me are the moisture content at any given time,the initial moisture content and equilibrium moisturecontent, respectively.

Since the drying was conducted in the tunnel dryer,during drying, the samples were not exposed to uniformrelative humidity and temperature continuously. So, themoisture ratio was simplified according to Pala et al.

(1996) and Doymaz (2004), to

MR ¼M=M0 (2)

To select a suitable model for describing the dryingprocess of mango slices, drying curves were fitted withsix thin-layer drying equations. The moisture ratiomodels are presented in Table 1. Non-linear regression

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Table 1

Thin-layer drying models

Equation Name References

MR ¼ exp (�kt) Newton Liu and Bakker-Arkema (1997), O’Callaghan et al. (1971)MR ¼ exp (�ktn) Page Zhang and Litchfield (1991)MR ¼ exp (�(kt)n) Modified Page Overhults et al. (1973)MR ¼ a exp (�kt) Henderson and Pabis Henderson and Pabis (1961), Chhinnman (1984)MR ¼ a exp (�kt)+c Logarithmic Yaldiz et al. (2001)MR ¼ 1+at+bt2 Wang and Singh Wang and Singh (1978)

MR, moisture ratio; k, drying constant; t, drying time; n, drying exponent; a, b, c, drying coefficients specific to individual equations.

Table 2

Drying time of raw mango slices

Drying temperature, 1C Pre-treatment Drying time, min

55 Control 210Blanched 210

Blanched with 1% potassium metabisulphide solution 180

60 Control 210Blanched 180

Blanched with 1% potassium metabisulphide solution 150

65 Control 180Blanched 180

Blanched with 1% potassium metabisulphide solution 150

THIN-LAYERDRYING KINETICS 45

analysis was performed using SPSS (Statistical Packagefor Social Scientists) 11�5 program. The coefficient ofdetermination r2 was one of the main criteria forselecting the best equation. In addition to the coefficientof determination, the goodness of fit was determined byvarious statistical parameters such as reduced meansquare of the deviation w2, mean bias error EMB androot mean square error ERMS. For quality fit, r2 valueshould be higher and w2, EMB and ERMS values shouldbe lower (Pangavhane et al., 1999; Sarsavadia et al.,1999; Togrul & Pehlivan, 2002; Erenturk et al., 2004;Demir et al., 2004). The above parameters can becalculated as follows:

w2 ¼PN

i¼1ðMR;exp;i �MR;pre;iÞ2

N � z(3)

EMB ¼1

N

XN

i¼1

ðMR;pre;i �MR;exp;iÞ (4)

ERMS ¼1

N

XN

i¼1

ðMR;pre;i �MR;exp;iÞ2

" #1=2(5)

where: MR,exp,i and MR,pre,i are the experimental andpredicted dimensionless moisture ratios, respectively; N

is the number of observations; and z is the number ofdrying constants.

3. Results and discussion

3.1. Drying behaviour of raw mango slices

The effect of treatment and drying temperature on thetime taken to reach the final moisture content ispresented in Table 2. The final moisture content ofsamples dried under different conditions ranged from8% to 12% (d.b.). It is evident that pre-treatment hadeffect on moisture movement from the samples. In allthe drying temperature selected, the samples blanchedwith 1% KMS had shorter drying time than the controland blanched samples. Similar findings were reported indrying of apricots (Pala et al., 1996; Doymaz, 2004) andgrapes (Doymaz & Pala, 2002). The drying airtemperature has also an important effect on drying ofraw mango slices. At the higher temperature of 65 1C,the drying time was less for control and blanchedsamples. Similar observations have been reported fordrying of garlic slices (Madamba et al., 1996) and onionslices (Sarsavadia et al., 1999).

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0

0.2

0.4

0.6

0.8

1

0 50 100 150 200

Drying time, min

Moi

stur

e ra

tio

R.K. GOYAL ET AL.46

Effect of pre-treatment on drying curves of rawmango dried at 55, 60 and 65 1C are presented in Figs

2–4. The drying curves show that moisture ratiodecreases continuously with drying time. During theinitial period of drying, the drying rate was similar in allthe treatments. After the removal of surface moisture,the drying rate of pre-treated samples was higher.Drying of raw mango slices occurred in falling rateperiod and no constant rate period was observed. Thedrying in falling rate period indicates that, internal masstransfer has occurred by diffusion. Similar results havebeen reported for the drying studies on onion slices(Rapusas & Driscoll, 1995) and apricots (Doymaz,2004).

00.10.20.30.40.50.60.70.80.9

1

0 40 80 120 160 200 240Drying time, min

Moi

stur

e ra

tio

Fig. 2. Effect of pre-treatments on drying time at drying airtemperature of 55 1C:&, control; W, blanched; � , blanched

with 1% potassium metabisulphide solution

0

0.2

0.4

0.6

0.8

1

0 40 80 120 160 200 240Drying time, min

Moi

stur

e ra

tio

Fig. 3. Effect of pre-treatments on drying time at drying airtemperature of 60 1C:&, control; W, blanched; � , blanched

with 1% potassium metabisulphide solution

Fig. 4. Effect of pre-treatments on drying time at drying airtemperature of 65 1C:&, control; W, blanched; � , blanched

with 1% potassium metabisulphide solution

3.2. Mathematical modelling of drying curves

The moisture ratio data of raw mango slices dried atdifferent temperature with different pre-treatments werefitted into the thin-layer drying models listed in Table 1.The values of r2, w2, EMB and ERMS are summarised inTable 3.

In all cases, the value of r2 was greater than 0�90,indicating a good fit (Madamba et al., 1996; Erenturk et

al., 2004). The values of r2 for the Page and logarithmicmodel were above 0�99. But the Page model gavecomparatively higher r2 values in all the dryingtreatments (0�9991–0�9999), and also the w2(0�008� 10�3

–0�504� 10�3), EMB (0�006 � 10�3–0�360� 10�3) andERMS (0�0024–0�0190) values were lower. Hence, thePage model may be assumed to represent the thin-layerdrying behaviour of raw mango slices. Demir et al.

(2004) reported a similar result for air-drying of bayleaves.

3.3. Calculation of effective moisture diffusivity

The continuous decrease in moisture ratio withincrease in drying time shows that the results can beinterpreted by using Fick’s diffusion model. Fick’sdiffusion equation for particles with slab geometry isgiven as (Crank, 1975):

MR ¼8

p2exp

�p2Deff t

L2

� �(6)

where: Deff is the effective moisture diffusivity in m2s�1;L is the thickness of the samples in m; and t is the dryingtime in s. Since the mango samples were sliced inuniform thickness, the samples were considered ashaving the geometry of slab.

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Table 3

Values of model constants and statistical parameters

Model Drying temp., 1C Pre-treatments r2 w2� 10�3 EMB� 10�3 ERMS

Newton 55 Control 0�9787 0�310 0�270 0�0164Blanched 0�9798 0�040 0�004 0�0060

B+ 1% KMS� 0�9761 0�987 0�846 0�029160 Control 0�9761 0�995 0�871 0�0295

Blanched 0�9769 0�848 0�727 0�0270B+ 1% KMS� 0�9784 0�464 0�386 0�1965

65 Control 0�9893 0�179 0�154 0�0124Blanched 0�9785 0�399 0�342 0�0185

B+ 1% KMS� 0�9871 0�858 0�715 0�0267

Page 55 Control 0�9991 0�243 0�182 0�0135Blanched 0�9998 0�045 0�034 0�0058

B+ 1% KMS� 0�9984 0�504 0�360 0�019060 Control 0�9996 0�125 0�094 0�0097

Blanched 0�9998 0�064 0�046 0�0068B+ 1% KMS� 0�9992 0�296 0�197 0�0141

65 Control 0�9999 0�013 0�009 0�0031Blanched 0�9990 0�315 0�225 0�0150

B+ 1% KMS� 0�9999 0�008 0�006 0�0024

Modified Page 55 Control 0�9887 3�449 2�587 0�0509Blanched 0�9898 0�048 0�036 0�0060

B+ 1% KMS� 0�9884 1�185 0�846 0�290960 Control 0�9861 1�161 0�871 0�0295

Blanched 0�9969 1�017 0�727 0�0270B+ 1% KMS� 0�9984 0�580 0�387 0�0197

65 Control 0�9893 0�216 0�154 0�0124Blanched 0�9885 0�479 0�342 0�0185

B+ 1% KMS� 0�9871 1�073 0�715 0�0267

Henderson and Pabis 55 Control 0�9887 0�338 0�254 0�0159Blanched 0�9898 0�047 0�035 0�0059

B+ 1% KMS� 0�9863 1�128 0�806 0�028460 Control 0�9865 1�046 0�784 0�0280

Blanched 0�9871 0�934 0�667 0�0258B+ 1% KMS� 0�9885 0�570 0�380 0�0195

65 Control 0�9894 0�202 0�144 0�0120Blanched 0�9785 0�464 0�332 0�0182

B+ 1% KMS� 0�9873 1�002 0�668 0�0258

Logarithmic 55 Control 0�9987 0�406 0�254 0�0159Blanched 0�9988 0�510 0�032 0�0056

B+ 1% KMS� 0�9986 0�139 0�079 0�008960 Control 0�9989 0�403 15�87 0�0003

Blanched 0�9985 0�595 0�340 0�0184B+ 1% KMS� 0�9982 0�378 0�190 0�0137

65 Control 0�9988 0�081 0�046 0�0068Blanched 0�9986 0�057 0�330 0�00181

B+ 1% KMS� 0�9982 0�404 0�202 0�0142

Wang and Singh 55 Control 0�9862 4�404 2�752 0�0525Blanched 0�9896 1�023 6�392 0�0799

B+ 1% KMS� 0�9884 0�629 0�359 0�018960 Control 0�9884 0�583 0�364 0�0191

Blanched 0�9797 1�210 0�690 0�0263B+ 1% KMS� 0�9845 2�700 1�350 0�0367

65 Control 0�9738 2�450 1�400 0�0374Blanched 0�987 5�100 2�920 0�0540

B+ 1% KMS� 0�9783 0�851 0�426 0�0206

r2, coefficient of determination; w2, reduced mean square of the deviation; EMB, mean bias error; ERMS, root mean square error.�Blanching with 1% potassium metabisulphide.

THIN-LAYERDRYING KINETICS 47

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Table 4

Effective moisture diffusivity for drying of raw mango slices

Drying temperature,1C

Pre-treatment Moisture diffusivity (Deff),m2s�1

Coefficient of determination(r2)

55 Control 2�62� 10�10 0�9903Blanched 2�73� 10�10 0�9956

Blanched with 1% potassium metabisulphidesolution

3�23� 10�10 0�9419

60 Control 2�95� 10�10 0�9931Blanched 3�17� 10�10 0�9975

Blanched with 1% potassium metabisulphidesolution

3�78� 10�10 0�9938

65 Control 3�19� 10�10 0�9979Blanched 3�33� 10�10 0�9952

Blanched with 1% potassium metabisulphidesolution

4�39� 10�10 0�9858

R.K. GOYAL ET AL.48

The effective moisture diffusivity was calculated usingmethod of slopes (Maskan et al., 2002; Doymaz, 2004).Values of Deff for different drying treatments with r2 aregiven in Table 4.

Drying at higher temperature gave highest Deff values.The values ranged from 2�62� 10�10 to 4�39� 10�10m2 s�1. It can also be seen that pre-treatmenthas affected the moisture diffusivity. Similar results ofthe influence of pre-treatments on the moisture diffu-sivity during air drying has been found in apricots (Palaet al., 1996; Doymaz, 2004).

4. Conclusions

The effect of temperature and pre-treatments on thin-layer drying of raw mango slices in a tunnel dryer wasinvestigated in this study. Increase in drying airtemperature from 55 to 65 1C decreased the drying timefrom 210 to 180min for both control and blanchedsamples. Mango slices blanched with 1% potassiummetabisulphide solution have shorter drying timescompared to control and blanched samples. The entiredrying process occurred in falling rate period andconstant rate period was not observed. Six thin-layerdrying equations were investigated for their suitability todescribe the drying behaviour of mango slices. The Pagemodel showed the best fit with high values for thecoefficient of determination (0�9991–0�9999) and low w2,EMB and ERMS values. The effective moisture diffusivityvaried from 2�62� 10�10 to 4�39� 10�10m2 s�1.

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