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Influence of airflow velocity on kinetics of convection apple drying

D. Velic *, M. Planinic, S. Tomas, M. Bilic

Department of Process Engineering, Faculty of Food Technology, University J.J. Strossmayer of Osijek, F. Kuhaca 18,

P.O. Box 709, 31000 Osijek, Croatia

Received 30 May 2003; accepted 13 September 2003

Abstract

The aim of this experiment was to investigate airflow velocity influence (0.64, 1.00, 1.50, 2.00, 2.50 and 2.75 m s1) on the kinetics

of convection drying of Jonagold apple, heat transfer and average effective diffusion coefficients. Drying was conducted in a con-

vection tray drier at drying temperature of 60 C using rectangle-shaped (20 20 5 mm) apple samples. Temperature changes ofdried samples, as well as relative humidity and temperature of drying air were measured during the drying process. Rehydratation

ratio was used as a parameter for the dried sample quality. Kinetic equations were estimated by using an exponential mathematical

model.

The results of calculations corresponded well with experimental data. Two well-defined falling rate periods and a very short

constant rate period at lower air velocities were observed. With an increase of the airflow velocity an increase of heat transfer

coefficient and effective diffusion coefficient was found. During rehydratation, about 72% of water removed by the drying process

was returned.

2003 Elsevier Ltd. All rights reserved.

Keywords: Airflow velocity; Convection apple drying; Exponential drying model; Effective diffusion coefficient; Heat transfer coefficient

Apple is an important raw material for many foodproducts and apple plantations are cultivated all over

the world in many countries. Thus, it is very important

to define the conditions under which the characteristics

of fresh apples can be preserved and to define optimal

parameters for their storage and reuse.

Drying is a frequently and used procedure for food

preservation. Convection drying as well as other tech-

niques for drying are used in order to preserve the

original characteristics of apples. Dried apples can be

consumed directly or treated as a secondary raw mate-

rial.

High temperatures and long drying times required to

remove the water from the fruit material in convectionair drying may cause serious damage in flavour, colour,

nutrients and can reduce the bulk density and rehydra-

tation capacity of the dried product (Lin, Durance, &

Scaman, 1998).

There is a growing interest in the food industry in

the development of economical methods for food

production with high organoleptic and nutritionalvalue. The purpose of this study was to study con-

vection drying of apple in laboratory conditions and

to investigate the influence of airflow velocities on

drying kinetics, heat transfer coefficient and effective

diffusion coefficient.

1. Materials and methods

1.1. Drying equipment

Drying was performed in a pilot plant tray dryer(UOP 8 Tray Dryer, Armfield, UK). The dryer operates

on the thermogravimetric principle. The dryer (Fig. 1) is

equipped with controllers for controlling the tempera-

ture and airflow velocity. Air was drawn into the duct

through a diffuser by a motor driven axial flow fan

impeller. In the tunnel of the dryer there were carriers

for trays with samples, which were connected to a bal-

ance. The balance was placed outside the dryer and

continuously determined and displayed the sample

weight. A digital anemometer at the end of the tunnel

measured airflow velocity.

Journal of Food Engineering 64 (2004) 97102

www.elsevier.com/locate/jfoodeng

* Corresponding author. Tel.: +385-31-224-352; fax: +385-31-207-

115.

E-mail address: [email protected] (D. Velic).

0260-8774/$ - see front matter 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2003.09.016
http://mail%20to:%[email protected]/http://mail%20to:%[email protected]/http://mail%20to:%[email protected]/


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1.2. Material

Apples (Jonagold) were obtained from a local super-

market and stored at +4 C. After 2-h stabilization at

the ambient temperature, apples were hand peeled and

cut to the rectangle-shaped slices, dimensions: 20 20 5

mm.

1.3. Drying procedure

The dryer was operated at air velocities 0.64, 1.0, 1.5,

2.0, 2.5 and 2.75 m s1, with 60 C dry bulb temperature

and average relative humidity of 9%. Air flowed parallel

to the horizontal drying surfaces of the samples. Drying

process started when drying conditions were achieved

(60 C and constant air velocity). The apple samples on

trays were placed into the tunnel of the dryer and the

measurement started from this point. During the drying

process temperature changes of dried samples were

continuously recorded by thermocouples connected to a

PC. Testo 350 probes placed into the drying chamber

measured relative humidity and drying air temperature.

Sample weight loss was recorded every 5 min during the

drying process using a digital balance (with precision of

0.01 g). Dehydration lasted until a moisture content of

about 20% (wet base) was achieved. Airflow velocity was

measured every 5 min with a digital anemometer that

was placed at the end of the tunnel. Dried samples were

kept in airtight glass jars until the beginning of rehy-dratation experiments.

1.4. Determination of the total solid/moisture content

The moisture content of the dried samples was de-

termined by using a standard laboratory. Small quanti-

ties of eachsample were dried in a vacuum oven (6 h at of

70 C and 30 mbar pressure). Time dependent moisture

content of the samples was calculated from the sample

weight and dry basis weight. Weight loss data allowed

the moisture content to be calculated such as follows:

Nomenclature

a, K parameters in model (8)

c specific heat (J kg1 K1)

CP critical point

h heat transfer coefficient (W m2 K1)

k, n parameters in model (2)L length (m)

sample thickness (m)m weight (kg)

Nu Nusselt number

Pr Prandtl number

Re Reynolds number

t drying time (min)

T temperature (K)

X moisture (kgw kg1db )

X0 dimensionless moisture

dX0=dt drying rate (min1)v air velocity (m s1)

k heat conductivity (W m1 K1)

l dynamic viscosity of air (Pa s)

# temperature (C)q density of air (kg m3)

Subscripts

db dry basis

w water

0 initial

a air

e equilibrium

f film

s surface

Fig. 1. Schematic diagram of the convection drying equipment (UOP 8 Tray Dryer, Armfield, UK).

98 D. Velic et al. / Journal of Food Engineering 64 (2004) 97102


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Xt mw=mdb

1.5. Rehydratation

The rehydratation characteristics were used as a

quality index of a dried product. Approximately 3 g

(0.01 g) of dried samples were placed in a 250 ml

laboratory glass (2 parallels for each sample), 150 ml

distilled water was added, the glass was covered and

heated up to the boiling point within 3 min. The content

of the laboratory glass was then cooked for 10 min by

mild boiling and cooled. Cooled content was filtered for

5 min under vacuum, and weighed.

The rehydratation ratio (R) was used to express

ability of the dried material to absorb water (Lewicki,

1998). It was determined by the following equation:

Rehydratation ratio%

mass of water absorbed during rehydratationmass of water removed during drying

100

1

1.6. Drying rate curve determination

The exponential model successfully describes the

drying kinetics of some porous materials, such as clay

(Skansi & Tomas, 1995; Tomas, Skansi, & Sokele,

1994), AlNi catalyst (Sander, Tomas, & Skansi, 1998)

and food materials (Tomas & Skansi, 1996). The au-

thors also used this model to describe the changes ofmoisture content and drying rates. The time dependent

weight of samples was converted for the given time de-

pendent to moisture content.

To avoid some ambiguity in results because of the

differences in initial sample moisture, the sample mois-

ture was expressed as dimensionless moisture ratio

(X0 Xt=X0). The drying curve for each experimentwas obtained by plotting the dimensionless moisture of

the sample vs. the drying time. For the approximation

of experimental data and calculating drying curves

(Eq. (2)) and drying rate curves (Eq. (3)), the simplified

model was used, as follows:

X0t expkt

n 2

dX0

dt k n tn1 X0t 3

The parameters k and n were calculated by non-linear

regression method (Quasi-Newton) using Statistica 6.0

computer program. The correlation coefficient (r2) was

used as a measure of model adequation. The first and

second critical points were determined as a maximum

and point of inflexion of the function (dX=dt) (Tomas& Skansi, 1996).

1.7. Calculation of the heat transfer coefficient

Convective heat transfer occurs between a moving

fluid and a solid surface. This work investigated con-

vective heat transfer for forced convection flow over a

flat plate. The viscosity of the fluid requires that the fluid

has zero velocity at the plates surface. Because a

boundary layer exists, the flow is initially laminar but

can proceed to turbulence once the Reynolds number of

the flow is sufficiently high (Pitts & Sissom, 1977).

It was assumed that the plate (sample) was main-

tained at constant temperature (Ts) and the plate length

(L) was sufficiently short so that turbulent flow was

never triggered (Fig. 2).

Average heat transfer coefficient was calculated using

Pohllhausen equation (4) for laminar flow and other

correlations (5) and (6) that are given below:

Nulam 0:664 Re1=2lam Pr

1=3 valid for Re < 2 105

4

Nu h L

k; Re

L v q

l; Pr

l c

k5

All calculations were performed at the average film

temperature (Tf):

Tf Ta Ts

2K 6

where are: Taair temperature [K], Tsaverage tem-

perature of sample surface [K].

1.8. Determination of the effective diffusion coefficient

The simplified method (Zogozsa, Maroulis, & Mari-

nos-Kouris, 1994) was used for determination of the

effective diffusion coefficient. For a thin plate the solu-

tion of Ficks law of diffusion, with assumptions of

moisture migrating only by diffusion, negligible shrink-

ing, constant temperature and diffusion coefficients and

long drying times, are given below (Baroni & Hubinger,

1998):

X XeX0 Xe

Xn1n0

8

2n 12p2

expDeff2n 1

2p2t

42

!

7where Xe and X0 represent equilibrium and initial

moisture contents, and is the slab thickness. The value

Fig. 2. Convection heat transfer for forced flow over a flat plate.

D. Velic et al. / Journal of Food Engineering 64 (2004) 97102 99


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of the equilibrium moisture content is relatively small

(low air relative humidity) compared to X or X0. Thus

X Xe=X0 Xe is simplified to X0 X=X0 (dimen-

sionless moisture ratio) (Doymaz & Pala, 2002).

Where sample thickness is small (0.005 m) and drying

time is relatively large, only the first term of Fickans

solution series is need, and Eq. (7) becomes:

X0 a expK t 8

where K Deffp2=42 is represent the slope ofX0 vs. t

plotting on the semi-logarithmic diagram.

2. Results and discussion

The results of numerical adoptions of experimental

data are summarized in Table 1. The moisture contents

(experimental and modelled data) vs. drying time at dif-

ferent airflow velocities are shown in Fig. 3. It can be seen

that a good agreement between experimental data and

chosen mathematical model exists, which is confirmed by

high values of correlation coefficient (0.99910.9995).

Results show that the airflow rate had a significant effect

on drying rates of apple. With the increase of the air flows

velocity, the time required to achieve certain moisture

content decreased.

Fig. 4 shows typical drying curves, which are char-

acterised by two falling rate periods with no undoubt-

edly apparent constant rate period. However, it might

be possible to have a very short constant rate period at

lower airflow velocities (0.64, 1.0 and 1.5 m s1) followed

after the initial period of increasing drying rate. In thisperiod samples retained almost constant temperature

(Fig. 5) and then kept growing. After the first critical

point (in interval from 0.8176 to 0.9186 kgw kg1db ), the

internal resistance of product increase, resulted in the

first falling rate period. The second falling rate period

started after the second critical point (around 0.3

kgw kg1db according to air velocity). If the slope of tan-

gent to the drying curve (dX=dt) was considered as the

drying rate of the sample, the results suggested that in

the second period drying was faster than in the first

falling rate period. Similar results were obtained during

the drying of sweet potato slices (Diamante, 1994).

Rehydratation did not show a clear dependence of

rehydratation ability of dried apple on airflow velocity.

During the rehydratation, dried sample absorbed be-

Table 1

Results of numerical analyses [model (2)]. Time, dimensionless moisture and drying rate in the first and second critical points at different airflow rate

v (m s1) 0.64 1.0 1.5 2.0 2.5 2.75

k 0.002917 0.004230 0.005829 0.009298 0.009846 0.012683

n 1.252203 1.225387 1.201714 1.116508 1.110101 1.092722

r2 0.999140 0.999309 0.999186 0.999486 0.999516 0.999185

t (min) CP1 29.4 21.7 20.2 8.7 8.0 5.7

CP2 122.5 99.3 91.9 72.1 69.9 58.6

X0 CP1 0.81758 0.83199 0.83170 0.90091 0.90558 0.91865

CP2 0.30077 0.30607 0.30596 0.33143 0.33314 0.33795

dX0=dt(min1)

CP1 0.00701 0.00863 0.00932 0.01204 0.01245 0.01496

CP2 0.00369 0.00447 0.00483 0.00566 0.00581 0.00683

CP1first critical point; CP2second critical point.

Fig. 3. Experimental and calculated moisture contents vs. drying time.

Fig. 4. Drying rate vs. drying time for different airflow velocities with

first and second critical points.



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tween 63.80% and 79.25% of water, which was removed,

by drying (Table 2).

As the airflow velocity increased, the heat transfer

coefficient for drying apples also increased almost pro-

portionally (Table 2; Fig. 7).

The semi-logarithmic dimensionless moisture vs.

drying time plot for falling rate period at different air

velocities is shown in Fig. 6.

Two well-defined falling rate periods are observed,

each corresponding to an approximately constant slope

from which the effective diffusion coefficients are calcu-

lated. With increasing the airflow rate, Deff increases too

in the both periods. For the examined airflow rate, the

value of the average effective diffusion coefficient in the

first falling rate period ranged from 1.7 109 to

3.0 109 m2 s1 (Table 2). That accords with the liter-ature data for food products such as: vegetable wastes

(Lopez, Iguaz, Esnoz, & Virseda, 2000), carrot and po-

tatoes (Mulet, 1994), apple cubes (Simal, Deya, Frau, &

Rosselo, 1997) and apple tissues (Feng, Tang, & Dixon-

Werren, 2000). Whereas, in the second falling rate pe-

riod average Deff was around 1.6 times greater than in

the first period, and it ranged from 2.9 109 to

4.4 109 m2 s1. This is corresponds with conclusions

that the rate of diffusion is proportional to the sample

temperature (Diamante, 1994), which in this case de-

pends on the airflow velocities and heat transfer coeffi-

cient (Fig. 7), and that the value ofDeff increases in time

(Simal, Rossello, Berna, & Mulet, 1994).

3. Conclusion

The drying kinetics of Jonagold apple, average heat

transfer coefficient and average effective diffusion coef-

ficient at airflow rate: 0.64, 1.0, 1.5, 2.0, 2.5 and 2.75

m s1 were obtained by a thermogravimetric method.

As can be observed, by using the exponential model it

is possible to accurately simulate the drying kinetics of

apple at different air velocities. An increase of airflow

velocity resulted in increase of moisture removal rate.

Two well-defined falling rate periods with different

drying rates and effective diffusion coefficients were ob-

served at all examined air rate.

Table 2

Rehydratation ratio, heat transfer coefficient and effective diffusion coefficient for drying apple at different airflow ratio

v (m s1) 0.64 1.0 1.5 2.0 2.5 2.75

R (%) 79.15 63.80 72.20 79.25 72.00 75.95

h (W m2 K1) 21.43 26.73 32.72 37.87 42.28 44.30

Deff;1 (m2 s1) 1.70109 2.06 109 2.26 109 2.56 109 2.64 109 3.02 109

Deff;2 (m2 s1) 2.91109 3.37 109 3.61 109 3.72 109 3.62 109 4.45 109

Deff;1effective diffusion coefficient in the first falling rate period; Deff;2effective diffusion coefficient in the second falling rate period.

Fig. 5. Temperature of sample vs. drying time for different airflow

velocities.Fig. 6. Semi-logarithmic dimensionless moisture ratio vs. drying time

in the falling rate period for different airflow velocities.

Fig. 7. The effect of airflow velocity on the heat transfer coefficient and

effective diffusion coefficients.

D. Velic et al. / Journal of Food Engineering 64 (2004) 97102 101


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The average Deff increased with airflow rate, and

ranged form 1.7 109 to 3.0 109 m2 s1 for the first

and from 2.9 109 to 4.4 109 m2 s1 for the second

falling rate period.

With the increase of the airflow velocity, heat transfer

coefficient increased too, and it ranged between 21.4 and

44.3 W m2 K1.

During rehydratation of dried apples, 63.8079.25%

of water removed by the drying process was returned.

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