Journal of Food Engineering 68 (2005) 439 442
www.elsevier.com/locate/jfoodeng
Influence of drying methods on drying of bell-pepper
(Capsicum annuum)
a,* b c
T.Y. Tunde-Akintunde , T.J. Afolabi , B.O. Akintunde
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 done by traditional sun drying or industrially through
the use of solar dryers or hot air drying (Riva & Peri,
Vegetables and fruits are on important aspect of the 1986). Over the years, solar dying has also been used
human diet in Nigeria because of their nutritional value for drying since the use of solar dryers offer faster, more
(Ihekoronye & Ngoddy, 1985). However they are usu- hygienic, insect and bird free dried products (Ezeike,
ally in short supply during dry seasons because they Echiegu, & Iloje, 1988). However both traditional and
are perishable crops, which deteriorate within a few days industrial sun-drying methods (i.e. drying with forms
after harvest (which occurs mainly in the rainy season). of solar energy) though having low operational installa-
Preserving these crops in their fresh state for months has tion and energy costs, still are labour intensive because
been a problem yet to be solved in the country (Aliyu & of climatic variations. The use of sun drying and solar
Sambo, 1993). drying may also result in quality degradation of the final
A very common method of preservation for these dehydrated products (Tindal, 1983) and increase in cost
agricultural corps is to dry them in order to conserve of sorting food products. The use of hot-air drying is a
the perishable fruits, reduce storage volume and to ex- viable option for drying fruits and vegetables because of
tend their shelf life beyond the few weeks when they the large amounts of fruits produced annually which
are in season (Kordylas, 1991). Drying can either be 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-
*
Corresponding author.
duce the total number of microorganisms in the food
E-mail address: [e-mail] (T.Y. Tunde-
Akintunde). (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
440 T.Y. Tunde-Akintunde et al. / Journal of Food Engineering 68 (2005) 439 442
product more tender and brings about a change in the concentration during the time interval between two sub-
structure of the product in order to make it easier to sequent measurements divided by this time interval, and
dry (Baker, 1997). expressed as kg evaporated water/kg dry solids time.
Bell-pepper (Capsicum annuum) is used for preparing The proximate analysis was determined using the
soaps and stews and it is a source of vitamins, minerals AOAC official methods of analysis (AOAC, 1984).
and energy in the human diet. The aim of this study was The moisture content of the dried materials at the end
to study and compare the drying rate for drying bell- of the drying cycles (sun, solar and artificial) was found
pepper (C. annuum) using sun, solar and hot air drying by vacuum drying at 70 °C for 24 h. Weight reduction
methods. 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
2. Materials and methods a false water concentration will result.
Bell-pepper was obtained from a local market in
Ibadan and was dried either naturally without pre- 3. Results and discussion
treatment or treated by blanching in hot water for 5 min.
The initial moisture content was above 80% (wet basis). The drying curves for bell-pepper (natural and with a
The vegetables were dried using sun, solar and artifi- pre-treatment of blanching) dried using sun, solar en-
cial (i.e. hot-air) drying. The sun drying was achieved by ergy and artificial air dryer are shown in Figs. 1 3 while
placing the pepper under direct sunlight in the dry sea- Figs. 4 6 show the drying rates of the three drying meth-
son with an overall maximum daytime air temperature ods. The amount of water evaporated during the night
of about 37 °C and a minimum night temperature of was lower than that during the day due to the variations
about 20 °C over a 7 day drying cycle with a relatively between the day and night temperatures and this re-
lower air humidity which never exceeded 78% even dur- sulted in the breaks observed in the first drying period
ing nights and with no rain. The drying took place dur- for the sun and solar drying. (Figs. 1 and 2). Apart from
ing the 2003 dry season. The products were placed on these variations, the sun and solar drying of bell-pepper
drying beds placed directly on a stony flat surface. The showed a first drying period of a constant drying rate
pepper was spread evenly and small portions of the followed by a falling rate-drying period. This is similar
products were weighted at various time intervals over to the observation of Karathanos and Belessiotis
the whole drying period. (1997) that during the sun drying of currants, drying
The solar dryer used was a direct cabinet type which took place under a rate approaching the constant drying
are used generally for drying agricultural products espe- period for a significant time. The drying curve of solar
cially fruits and vegetables (Reuss, 1993). It consists drying is much lower than that of sun drying because
essentially of a solar collector and drying chamber con- the higher air temperature in solar drying increases the
structed with wood planks having a cross-sectional area rate of moisture evaporation from the product. The dry-
of 1610 cm2. The dryer base was lined with a reflective ing curves for natural bell-pepper for all the drying proc-
material with the trays fixed in the drying chamber while esses are slightly lower than those of treated bell-pepper,
the collector base was painted black. The top cover was which is expected since blanching increased the moisture
made of glass inclined at an angle of 11° to the horizon- content of the pepper from 4.65% to 6.12% dry basis.
tal, which enhances the flow of air from the collector
unit through openings at the top of the drying chamber.
Normal
The mean temperature in the drying chamber was 45 °C.
A batch tray drier was used for the hot-air drying
7 Treated
method. The perforated trays had an area of 1990 cm2
6
and they were filled with one layer of wet product. There
5
was a gap of 10 cm between the trays to allow for ade-
4
quate air movement. The dryer had four trays and the
3
air was heated using an electrical burner and the temper-
ature in the dryer was controlled at 60 °C.
2
The tray containing the samples was weighed at var-
1
ious time intervals ranging from 30 min at the beginning
0
of the drying cycle to 60 min at the latter stages of the
0 50 100 150
drying process.
Drying time (hours)
The weight of the material in the tray was expressed
as moisture content, dry basis (kg water/kg dry solids).
Fig. 1. Sun drying curves of bell-pepper; natural (no treatment) and
The drying rate was found from the decrease in water pre-treated by blanching.
water / kg dry solids)
Moisture content (dry basis) (kg
T.Y. Tunde-Akintunde et al. / Journal of Food Engineering 68 (2005) 439 442 441
7
Normal
Normal
6
0.5 Treated
Treated
5
0.4
4
0.3
3
0.2
0.1
2
0
1
0 2 4 6
0
Moisture Content, % d.b.(kg water / kg dry
0 50 100 150
solids)
Drying time (hours)
Fig. 5. Curves of drying rate vs moisture content of sun dried bell-
Fig. 2. Solar drying curves of bell-pepper; natural and pre-treated by
pepper.
blanching.
8
0.5
Treated
Normal
6
0.4
Normal
Treated
0.3
4
0.2
2
0.1
0
0
020 40 60
0 5 10
Drying time (hours)
Moisture Content,% d.b. (kg water/kg
Fig. 3. Drying curves (moisture content vs drying time) of bell-pepper
dry solids)
dried 60 °C in an artificial air dryer.
Fig. 6. Drying curves of drying rate vs moisture content of bell-pepper
dried in an artificial dryer at 60 °C.
Normal
0.7
Treated
by that in smaller capillaries, resulting in a reduction
0.6
in the rate of evaporation. The final stage is the removal
0.5
of water highly bound to sites of water-holding compo-
0.4
nents (e.g. protein and starch) and thus water extraction
becomes more difficult and drying rate decreases as the
0.3
drying time progresses (Baker, 1997). The drying curves
0.2
for bell-pepper (normal and treated in the artificial air
0.1
dryer) are shown in Fig. 3. An examination of these
curves indicate that the drying of normal bell-pepper is
0
0 2 4 6 faster than treated bell-pepper which is the same experi-
ence in both solar and sun drying, with the only differ-
Moisture Content (kg water / kg dry solids)
ence in the drying technique being the time for drying
Fig. 4. Curves of drying rate vs moisture content of solar dried bell-
in the three cases. In the case of artificial air drying, dry-
pepper.
ing takes place mostly in the constant drying rate period
(Fig. 6) and a lower moisture content was obtained than
The solar and sun drying of bell-pepper was largely in the other two drying methods. This is similar to the
under the constant drying rate period (Figs. 4 and 5). observations of Saravacos and Charm (1962) but
This period has been associated with the existence and Karathanos and Belessiotis (1997) observed that the
evaporation of free water at a constant rate (Baker, artificial drying of grapes was mostly in the falling rate
1997) The second stage, the falling drying rate period period and that of currants was in two drying rate peri-
takes place as a result of the predominance of internal ods (almost constant and a falling rate drying period).
diffusion mechanism because of the presence of bound This constant drying rate period for artificial drying of
water. During the initial part of the falling drying rate bell-pepper may be due to the fact that initially during
period, water in larger capillaries is removed followed the drying, evaporation of water takes place entirely at
solids)
Drying Rate (kg water
Moisture content (dry
evaporated/kg dry solid*h)
basis) (kg water / kg dry
2
/ kg dry solids)
Moisture content (kg H O
Drying Rate (kg evap.
water/kg dry solids*h)
/ kg dry solids*h)
Drying Rate (kg evap. water
442 T.Y. Tunde-Akintunde et al. / Journal of Food Engineering 68 (2005) 439 442
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