Influence of drying methods on drying of bell-pepper

(Capsicum annuum)

T.Y. Tunde-Akintunde

a,*

, T.J. Afolabi

b

, B.O. Akintunde

c

a

Department of Food Science and Engineering, Ladoke Akintola University of Technology, PMB 4000, Ogbomoso, Oyo State, Nigeria

b

Department of Chemical Engineering, Ladoke Akintola University of Technology, Ogbomoso, Oyo State, Nigeria

c

Federal College of Agriculture, PMB 5029, Moor Plantation, Ibadan, Oyo State, Nigeria

Received 4 December 2003; accepted 28 June 2004

Abstract

Drying of bell-pepper was carried out using sun solar and artificial air drying methods. The drying curves and rates obtained

indicated that drying of bell-pepper was done in two drying rate periods, the constant drying rate period (mainly) and the falling
drying rate period. The existence of the constant drying rate period was more pronounced in the artificial air-drying method than in
the other two drying methods and the drying process was also faster. This was attributed to the effect of temperature of drying air on
the diffusion of water from internal regions to the surface of the product.
Ó 2004 Published by Elsevier Ltd.

Keywords: Bell-pepper; Drying method; Drying rate

1. Introduction

Vegetables and fruits are on important aspect of the

human diet in Nigeria because of their nutritional value
(

Ihekoronye & Ngoddy, 1985

). However they are usu-

ally in short supply during dry seasons because they
are perishable crops, which deteriorate within a few days
after harvest (which occurs mainly in the rainy season).
Preserving these crops in their fresh state for months has
been a problem yet to be solved in the country (

Aliyu &

Sambo, 1993

).

A very common method of preservation for these

agricultural corps is to dry them in order to conserve
the perishable fruits, reduce storage volume and to ex-
tend their shelf life beyond the few weeks when they
are in season (

Kordylas, 1991

). Drying can either be

done by traditional sun drying or industrially through
the use of solar dryers or hot air drying (

Riva & Peri,

1986

). Over the years, solar dying has also been used

for drying since the use of solar dryers offer faster, more
hygienic, insect and bird free dried products (

Ezeike,

Echiegu, & Iloje, 1988

). However both traditional and

industrial sun-drying methods (i.e. drying with forms
of solar energy) though having low operational installa-
tion and energy costs, still are labour intensive because
of climatic variations. The use of sun drying and solar
drying may also result in quality degradation of the final
dehydrated products (

Tindal, 1983

) and increase in cost

of sorting food products. The use of hot-air drying is a
viable option for drying fruits and vegetables because of
the large amounts of fruits produced annually which
have to be dried for preservation and the fact that cli-
matic factors do not affect the drying process.

During drying fruits and vegetables may be blanched

as a pre-treatment to lessen changes in colour and re-
duce the total number of microorganisms in the food
(

Roiz, 1997; Roberts & Cox, 1999

). It also makes the

0260-8774/$ - see front matter

Ó 2004 Published by Elsevier Ltd.

doi:10.1016/j.jfoodeng.2004.06.021

*

Corresponding author.

E-mail

address:

[e-mail]

(T.Y.

Tunde-

Akintunde).

www.elsevier.com/locate/jfoodeng

Journal of Food Engineering 68 (2005) 439–442

product more tender and brings about a change in the
structure of the product in order to make it easier to
dry (Baker, 1997).

Bell-pepper (Capsicum annuum) is used for preparing

soaps and stews and it is a source of vitamins, minerals
and energy in the human diet. The aim of this study was
to study and compare the drying rate for drying bell-
pepper (C. annuum) using sun, solar and hot air drying
methods.

2. Materials and methods

Bell-pepper was obtained from a local market in

Ibadan and was dried either naturally without pre-
treatment or treated by blanching in hot water for 5 min.
The initial moisture content was above 80% (wet basis).

The vegetables were dried using sun, solar and artifi-

cial (i.e. hot-air) drying. The sun drying was achieved by
placing the pepper under direct sunlight in the dry sea-
son with an overall maximum daytime air temperature
of about 37

°C and a minimum night temperature of

about 20

°C over a 7 day drying cycle with a relatively

lower air humidity which never exceeded 78% even dur-
ing nights and with no rain. The drying took place dur-
ing the 2003 dry season. The products were placed on
drying beds placed directly on a stony flat surface. The
pepper was spread evenly and small portions of the
products were weighted at various time intervals over
the whole drying period.

The solar dryer used was a direct cabinet type which

are used generally for drying agricultural products espe-
cially fruits and vegetables (

Reuss, 1993

). It consists

essentially of a solar collector and drying chamber con-
structed with wood planks having a cross-sectional area
of 1610 cm

2

. The dryer base was lined with a reflective

material with the trays fixed in the drying chamber while
the collector base was painted black. The top cover was
made of glass inclined at an angle of 11

° to the horizon-

tal, which enhances the flow of air from the collector
unit through openings at the top of the drying chamber.
The mean temperature in the drying chamber was 45

°C.

A batch tray drier was used for the hot-air drying

method. The perforated trays had an area of 1990 cm

2

and they were filled with one layer of wet product. There
was a gap of 10 cm between the trays to allow for ade-
quate air movement. The dryer had four trays and the
air was heated using an electrical burner and the temper-
ature in the dryer was controlled at 60

°C.

The tray containing the samples was weighed at var-

ious time intervals ranging from 30 min at the beginning
of the drying cycle to 60 min at the latter stages of the
drying process.

The weight of the material in the tray was expressed

as moisture content, dry basis (kg water/kg dry solids).
The drying rate was found from the decrease in water

concentration during the time interval between two sub-
sequent measurements divided by this time interval, and
expressed as kg evaporated water/kg dry solids time.
The proximate analysis was determined using the
AOAC official methods of analysis (

AOAC, 1984

).

The moisture content of the dried materials at the end

of the drying cycles (sun, solar and artificial) was found
by vacuum drying at 70

°C for 24 h. Weight reduction

due to longer temperatures and times of drying is not
only due to water evaporation but partial sugar decom-
position to water vapour and carbon dioxide, therefore
a false water concentration will result.

3. Results and discussion

The drying curves for bell-pepper (natural and with a

pre-treatment of blanching) dried using sun, solar en-
ergy and artificial air dryer are shown in

Figs. 1–3

while

Figs. 4–6

show the drying rates of the three drying meth-

ods. The amount of water evaporated during the night
was lower than that during the day due to the variations
between the day and night temperatures and this re-
sulted in the breaks observed in the first drying period
for the sun and solar drying. (

Figs. 1 and 2

). Apart from

these variations, the sun and solar drying of bell-pepper
showed a first drying period of a constant drying rate
followed by a falling rate-drying period. This is similar
to the observation of

Karathanos and Belessiotis

(1997)

that during the sun drying of currants, drying

took place under a rate approaching the constant drying
period for a significant time. The drying curve of solar
drying is much lower than that of sun drying because
the higher air temperature in solar drying increases the
rate of moisture evaporation from the product. The dry-
ing curves for natural bell-pepper for all the drying proc-
esses are slightly lower than those of treated bell-pepper,
which is expected since blanching increased the moisture
content of the pepper from 4.65% to 6.12% dry basis.

0

1

2

3

4

5

6

7

0

50

100

150

Drying time (hours)

Moisture

content

(dry

basis)

(kg

water

/

kg

dry

solids)

Normal

Treated

Fig. 1. Sun drying curves of bell-pepper; natural (no treatment) and
pre-treated by blanching.

440

T.Y. Tunde-Akintunde et al. / Journal of Food Engineering 68 (2005) 439–442

The solar and sun drying of bell-pepper was largely

under the constant drying rate period (

Figs. 4 and 5

).

This period has been associated with the existence and
evaporation of free water at a constant rate (

Baker,

1997

) The second stage, the falling drying rate period

takes place as a result of the predominance of internal
diffusion mechanism because of the presence of bound
water. During the initial part of the falling drying rate
period, water in larger capillaries is removed followed

by that in smaller capillaries, resulting in a reduction
in the rate of evaporation. The final stage is the removal
of water highly bound to sites of water-holding compo-
nents (e.g. protein and starch) and thus water extraction
becomes more difficult and drying rate decreases as the
drying time progresses (

Baker, 1997

). The drying curves

for bell-pepper (normal and treated in the artificial air
dryer) are shown in

Fig. 3

. An examination of these

curves indicate that the drying of normal bell-pepper is
faster than treated bell-pepper which is the same experi-
ence in both solar and sun drying, with the only differ-
ence in the drying technique being the time for drying
in the three cases. In the case of artificial air drying, dry-
ing takes place mostly in the constant drying rate period
(

Fig. 6

) and a lower moisture content was obtained than

in the other two drying methods. This is similar to the
observations of

Saravacos and Charm (1962)

but

Karathanos and Belessiotis (1997)

observed that the

artificial drying of grapes was mostly in the falling rate
period and that of currants was in two drying rate peri-
ods (almost constant and a falling rate drying period).
This constant drying rate period for artificial drying of
bell-pepper may be due to the fact that initially during
the drying, evaporation of water takes place entirely at

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2

4

6

Moisture Content (kg water / kg dry solids)

Drying Rate (kg evap. water

/ kg dry solids*h)

Normal

Treated

Fig. 4. Curves of drying rate vs moisture content of solar dried bell-
pepper.

0

0.1

0.2

0.3

0.4

0.5

0 5

10

Moisture Content,% d.b. (kg water/kg

dry solids)

Drying Rate (kg evap.

water/kg dry solids*h)

Normal

Treated

Fig. 6. Drying curves of drying rate vs moisture content of bell-pepper
dried in an artificial dryer at 60

°C.

0

2

4

6

8

0

20

40

60

Drying time (hours)

Moisture content (kg H

2

O

/

kg

dry

solids)

Treated

Normal

Fig. 3. Drying curves (moisture content vs drying time) of bell-pepper
dried 60

°C in an artificial air dryer.

0

1

2

3

4

5

6

7

0

50

100

150

Drying time (hours)

Moisture content (dry

basis) (kg water / kg dry

solids)

Normal
Treated

Fig. 2. Solar drying curves of bell-pepper; natural and pre-treated by
blanching.

0

0.1

0.2

0.3

0.4

0.5

0 2

4 6

Moisture Content, % d.b.(kg water / kg dry

solids)

Drying

Rate

(kg

water

evaporated/kg

dry

solid*h)

Normal

Treated

Fig. 5. Curves of drying rate vs moisture content of sun dried bell-
pepper.

T.Y. Tunde-Akintunde et al. / Journal of Food Engineering 68 (2005) 439–442

441

the surface and the drying rate was controlled by con-
vective heat and mass transfer (

Van Brakel, 1980

).

The temperature gradient in the product is negligible

and the temperature is close to but above the wet bulb
temperature of the flowing air. However it is assumed
that for the falling rate period to start all the free water
is evaporated and the evaporation front moves into the
interior of the product. When this occurs, (moisture)
mass transfer is then by molecular (liquid) diffusion or
vapor diffusion or by capillary forces in the wet (i.e. inte-
rior) regions to the surface of the product where evapo-
ration occurs. This diffusion process of water through
the dry outer layers to the surface is influenced by high
temperatures (

Karathanos & Belessiotis, 1997

). As a re-

sult, the diffusion process was faster during the drying of
bell-pepper in an artificial dryer than in sun and solar
drying because of the higher drying air temperature.
Thus the existence of the constant rate drying period
was more pronounced in the artificial air drying than
in sun and solar drying.

Another reason for the larger existence of the falling

dry rate period in sun and solar drying as compared with
artificial air drying is that the thick skin of the bell-
pepper may have also resulted in an impedance to water
motion during drying and this decreased the rate of dif-
fusion of water from the inner regions to the product
surface. However in the case of the artificial air drying,
the diffusion process was increased due to the high tem-
perature of the drying air.

Thus the drying of bell-pepper was mostly during the

constant drying rate period while the falling rate drying
rate period occurred towards the end of drying. The
high temperature of the drying air using the artificial
air drying method resulted in faster diffusion of water
and thus the drying proceeded faster than in the other
two methods.

References

Aliyu, M., & Sambo. (1993). Comparative performance studies of glass,

PVC, and Perspex covered solar dryers. Paper presented at National
solar Energy Forum (NASEF Õ93). Abubakar Tatari Ali Polytech-
nic, Bauchi, Nigeria, 1–4 December 1993.

AOAC (1984). Official methods of analysis (14th ed.). Washington, DC:

Association of Official Analytical Chemists.

Baker, C. G. J. (Ed.). (1997). Industrial drying of food (pp. 7–30).

London: Blackie Academic and Professional/Chapman and Hall.

Ezeike, G. O. I., Echiegu, E. A., & Iloje, O. C. (1988). The design,

construction and performance testing of a passive solar manure dryer.
Technical Report of National Centre for Energy Research and
Development, University of Nigeria, Nsukka.

Ihekoronye, A. I., & Ngoddy, P. O. (1985). Integrated food science and

technology for the tropics (pp. 189–190). London/Basingstoke:
Macmillan Publishers Ltd.

Karathanos, V. T., & Belessiotis, V. G. (1997). Sun and artificial air

drying kinetics of some agricultural products. Journal of Food
Engineering, 31(1), 35–46.

Kordylas, J. M. (1991). Processing and preservation of tropical and

subtropical foods. London: Macmillan Education Ltd.

Reuss, M. (1993). Solar drying in Europe. In Proceedings of an expert

workshop on drying and conservation with solar energy, Budapest,
Hungary.

Riva, M., & Peri, C. (1986). Kinetics of sun and air drying of different

varieties of seedless grapes. Journal of Food Technology, 16(1),
78–81.

Roberts, T., & Cox, R. (1999). Drying fruits and vegetables. Virginia

Cooperative Extension Publication No. 348–597. Available from

http://www.ext.vt.edu/pubs/foods/348-597/348-597.html

.

Roiz, J. F. (1997). Drying foodstuffs (p. 33). GERES. Leiden: Backhuys

Publishers.

Saravacos, G. D., & Charm, S. E. (1962). A study of the mechanism of

fruit and vegetable dehydration. Journal of Food Technology, 16(1),
78–81.

Tindal, H. D. (1983). Vegetables in the tropics pp. 325–328. London:

The Macmillian Press Ltd.

Van Brakel, J. (1980). Mass transfer in convective drying. In A. S.

Mujumdar (Ed.). Advances in drying (Vol. 1, pp. 217–267).
Washington: Hemisphere Publishing.

442

T.Y. Tunde-Akintunde et al. / Journal of Food Engineering 68 (2005) 439–442