Influence of drying methods on drying of bell-pepper (Tunde-Akintunde, Afolabi, Akintunde).pdf

Journal of Food Engineering 68 (2005) 439–442

www.elsevier.com/locate/jfoodeng

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 DepartmentofFoodScienceandEngineering,LadokeAkintolaUniversityofTechnology,PMB4000,Ogbomoso,OyoState,Nigeria

b Department of Chemical Engineering,LadokeAkintolaUniversity of Technology,Ogbomoso,OyoState,Nigeria

FederalCollege ofAgriculture,PMB5029,MoorPlantation, Ibadan, OyoState,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.

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,

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

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

* Corresponding author.

E-mail address: toyositunde@yahoo.co.uk (T.Y. Tunde-

Akintunde).

0260-8774/$ - see front matter 2004 Published by Elsevier Ltd.

doi:10.1016/j.jfoodeng.2004.06.021

440

T.Y. Tunde-Akintunde et al./ JournalofFoodEngineering68 (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 (Capsicumannuum) 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.

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 oIcial 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.

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 (kgwater/kgdry solids).

The drying rate was found from the decrease in water

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.

Normal

Treated

100

150

Drying time (hours)

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

pre-treated by blanching.

T.Y. Tunde-Akintunde etal. / Journal ofFoodEngineering68 (2005)439–442

441

Normal

Treated

Normal

Treated

0.5

0.4

0.3

0.2

0.1

0 2 4 6

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

solids)

100

150

Drying time (hours)

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

blanching.

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

pepper.

0.5

Treated

Normal

0.4

Treated

0.3

0.2

0.1

Drying time (hours)

0 5 10

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

dry solids)

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

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.

0.7

Normal

Treated

0.6

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 di I cult 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

0.5

0.4

0.3

0.2

0.1

0 2 4 6

Moisture Content (kg water / kg dry solids)

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

pepper.

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

442

T.Y. Tunde-Akintunde et al./ JournalofFoodEngineering68 (2005)439–442

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.

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