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Hybrid Solar Dryer for Quality Dried Tomato
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DOI: 10.1080/07373930802467466
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M. A. Hossain a
; B. M. A. Amer b
; K. Gottschalk c
a
Farm Machinery and Postharvest Process Engineering Division, Bangladesh Agricultural Research Institute,
Gazipur, Bangladesh b
Agricultural Engineering Department, Faculty of Agriculture, Cairo University, Cairo,
Egypt c
Postharvest Technology Division, Leibniz-Institut für Agrartechnik Potsdam-Bornim, Potsdam,
Germany
Online Publication Date: 01 December 2008
To cite this Article Hossain, M. A., Amer, B. M. A. and Gottschalk, K.(2008)'Hybrid Solar Dryer for Quality Dried Tomato',Drying
Technology,26:12,1591 — 1601
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M. A. Hossain,1
B. M. A. Amer,2
and K. Gottschalk3
1
Farm Machinery and Postharvest Process Engineering Division, Bangladesh Agricultural Research
Institute, Gazipur, Bangladesh
2
Agricultural Engineering Department, Faculty of Agricultural Engineering, Cairo University,
Cario, Egypt
3
Postharvest Technology Division, Leibniz-Institut für Agrartechnik Potsdam-Bornim,
Potsdam, Germany
A prototype of a hybrid solar dryer was developed for drying of
tomato. It consists of a flat-plate concentrating collector, heat
storage with auxiliary heating unit, and drying unit. It has a loading
capacity of 20 kg of fresh half-cut tomato. The dryer was tested in
different weather and operating conditions. The performance of the
dryer was compared with an open sun-drying method. Drying
performance was evaluated in terms of drying rate, color, ascorbic
acid, lycopene, and total flavonoids. Tomato halves were pretreated
with UV radiation, acetic acid, citric acid, ascorbic acid, sodium
metabisulphite, and sodium chloride. Sodium metabisulphite
(8 g L–1
) was found to be effective to prevent the microbial growth
at lower temperature (45
C).
Keywords Ascorbic acid; Color; Lycopene; Pretreatment;
Solar dryer
INTRODUCTION
Sun drying is a popular and economical method for
drying of food materials in the developing countries. But
drying rate is very low and dependent on weather con-
ditions. Inferior quality of sun-dried products is mainly
due to uneven drying, mixing of dust and dirt, and con-
tamination with insects and microorganisms. Sometimes
the whole amount of product is spoiled in adverse weather
conditions. As an alternative to sun drying, solar drying is
a promising alternative for drying of fruits and vegetables in
developing countries. Mechanical drying, mainly used in
industrialized countries, is not applicable to small farms
in developing countries due to high investment and operat-
ing costs. Solar energy for crop drying is environmental
friendly and economically viable in developing countries.[1]
In natural convection solar dryers, the air flow is due to
buoyancy-induced air pressure, while in forced convection
solar dryers the air flow is provided by using a fan either
operated by electricity=solar module or fossil fuel. But, in
a hybrid solar dryer, drying is continued during off-
sunshine hours by back-up heat energy or storage heat
energy. Therefore, drying is continued and the product
is saved from possible deterioration by microbial
infestation.[2]
Goswami et al.[3]
analyzed the use of a geodesic dome
solar dryer for drying of grapes in India and it was found
to be suitable for drying of fruit in developing countries.
Also, experimental results agreed well with thermal-electrical
simulated results. Yaldyz and Ertekyn[4]
reported a solar
cabinet dryer for drying of vegetables and it was tested for
pumpkin, green pepper, stuffed pepper, green bean, and
onion. Air velocity had an important effect on drying rate.
Vlachos et al.[5]
designed and tested a low-cost solar dryer
for cereal grains. The drying efficiency was found to be
much higher than sun-drying method. Ghazanfari et al.[6]
reported a thin-layer solar air dryer to study the feasibility
of drying pistachio nut. The maximum temperature in the
collector was 56
C, which was 20
C above the ambient tem-
perature. The quality of solar-dried product was better than
the conventional heated air due to slower drying rate.
Bassuoni and Tayeb[7]
studied the effect of temperature
and thickness on tomato slices in solar and sun drying.
Hawlader et al.[8]
also studied the drying characteristics
of tomato under various drying conditions and developed
a diffusion model considering tomato slice as a flat plate
and taking shrinkage into account. Queiroz et al.[9]
reported the drying kinetics of tomato using a heat pump
dryer and an electric resistance dryer. The heat pump dryer
was found to be 40% more energy economic compared to
the electric resistance dryer. The Page model was found
suitable for prediction of tomato moisture content and
the model parameters were mainly affected by drying air
temperature. Andritsos et al.[10]
commercially dried tomato
Correspondence: M. A. Hossain, Farm Machinery and
Postharvest Process Engineering Division, Bangladesh Agricul-
tural Research Institute, Gazipur-1701, Bangladesh; E-mail:
mahossain64@yahoo.com
Drying Technology, 26: 1591–1601, 2008
Copyright # 2008 Taylor Francis Group, LLC
ISSN: 0737-3937 print/1532-2300 online
DOI: 10.1080/07373930802467466
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using a large (dimensions: 14 m 1 m 2 m) geothermal
energy dryer. Sacilik et al.[11]
investigated the thin-layer
drying characteristics of half fruit organic tomato in a
polyethylene-covered solar tunnel dryer (dimensions:
8 m 2.5 m 1.8 m) and fitted the experimental data to
10 thin-layer drying models. Mastekbayeva et al.[12]
designed and fabricated a solar-biomass hybrid dryer at
the Asian Institute of Technology (AIT), Thailand. A
biomass stove-heat exchanger chimney using briquetted
rice husk as fuel has been incorporated in the dryer. Separ-
ate experiments were carried out for drying of chilli and
mushroom in the hybrid dryer and the results were
compared with solar only and open sun drying. Time
required to dehydrate chilli from moisture content of 75
to 4% (w.b.) was 12 h in a hybrid dryer, 2 days in a solar
photovoltaic (PV) dryer, and 5–18 days in open sun drying.
The capacity of the dryer is 20 kg chilli and 40 kg mush-
room per batch.
Due to changing lifestyles especially in developed coun-
tries, there is a great demand for a wide variety of dried
food products with emphasis on high quality.[13]
Demand
for ready-to-use products, which have similar health bene-
fits to the original raw products, has also increased in
recent years.[14]
There is an increasing interest in quality
dried tomato in the international market. On the other
hand, tomato is nutritionally recognized for ascorbic acid
(vitamin C) and lycopene. Drying conditions, including
high temperature, light, and oxygen exposure, may cause
lycopene degradation and thereby affect the attractive
color and nutritive value of the final products.[15]
During
sun drying, quality losses may result from color degradation
(browning caused by enzymatic and nonenzymatic reac-
tion), microbial growth (mostly caused by molds and
yeasts), and poor rehydration (caused by injuries and case-
hardening during processing), along with losses of color,
ascorbic acid, and lycopene.[16]
Some investigators[17,18]
examined the effects of several pretreatments like blanching,
ascorbic acid, citric acid, sulphur, sodium metabisulphite,
and salt at different doses on drying and storage of tomato.
Little information is available on full-scale solar drying
of tomato that considers the quality. An efficient solar dryer
need to be designed to dry tomato that can produce quality
dried product. This study attempts to design an efficient
solar dryer and to test the performance of the dryer for
drying of tomato for production of quality dried product.
MATERIALS AND METHODS
A hybrid solar dryer was designed, fabricated, and
installed at Institut für Agrartechnik Potsdam-Bornim,
Germany. The dryer basically consisted of a solar collector,
a heat exchanger with auxiliary heat storage unit, and a
drying unit. A schematic view and a photograph of the
solar dryer are shown in Figs. 1 and 2, respectively.
Solar Collector
The dimensions of the horizontal solar collector were
2.8 m long, 1.8 m wide, and 0.18 m high. The transparent
cover of the collector was 4-mm-thick clear glass. About
200 mm below the glass cover, 2-mm black painted corru-
gated iron sheet was used as an absorber plate. To increase
the efficiency of the solar collector, a flat-type reflector
made of bright aluminum was added at top of the solar col-
lector. The dimensions of the reflector were the same as
those of the solar collector so that it can be used as a reflec-
tor in the daytime and a cover at nighttime. This could be
changed according to the change of the sun’s angle to col-
lect higher amount of sun rays that fall down on the solar
collector. In addition, the collector was placed on six legs
with a 150-mm wheel to turn it horizontally and change
its direction according to the change of the sun’s angle.
The solar collector was insulated with 50-mm-thick poly-
styrene. A 0.75-kW centrifugal blower was used to draw
the atmospheric air inside the collector and provide the
heated air to the dryer at a desired air velocity. There were
FIG. 1. Schematic diagram of solar collector and dryer.
FIG. 2. Photograph of solar collector and dryer.
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three air controllers at the inlet, outlet, and just before the
suction opening of the air blower to control the air flow at
the inlet, outlet, and mixing point, and these air controllers
could be controlled manually as shown in Fig. 3.
Heat Exchanger with Heat Storage Unit
The heat exchanger consisted of 15-mm-diameter copper
tubes placed inside the solar collector, 100 mm below the
glass cover and 100 mm above the absorber plate. The heat
exchanger consisted of 70 tubes covering the whole area of
the drying collector. These tubes were fixed and placed
over a metal holder of width 20 mm at each side of the solar
collector. Two ends of the copper tubes were connected to
the water storage tank with 15-mm-diameter plastic tubes.
The capacity of the water tank was 500 L. Water circulated
through the plastic and copper tubes by a small water
pump of capacity 20 L=h from water tank. The heat
exchanger provided a part of the heat collected during
the sunshine, which was carried by air inside the solar col-
lector to the water inside the copper tubes. The water
passed very slowly inside the copper tubes to be able to
take heat from the hot air contacted with the external sur-
face of the tubes. This water was stored inside a plastic
tank of 500-L volume and insulated by 50-mm fiberglass.
The heat stored during the day in the water tank could
be used again at night. The temperature of this water could
be raised by 6-kW water heaters located inside the tank to
provide the desired temperature for drying during the night
and for maintaining the temperature throughout the drying
process.
Drying Unit
The prototype of the dryer was designed for drying of
about 20 kg fresh tomatoes (half-cut) per batch and it is
suitable for small farmholders in developing countries.
The dryer was designed on the basis of heat and mass bal-
ance equations. The overall dimensions of the dryer were
1.0 m 1.0 m 1.0 m. Air could be blown from the collector
to the drying chamber either from the bottom or top end of
the dryer. The air inlet connection was a 120-mm-diameter
flexible pipe with insulation. There were five trays placed
on after another in the drying chamber with an air gap
of 120 mm between two trays. Each tray was
600 mm 520 mm and made of a wooden frame and plastic
net. The trays were placed in such a way that hot air can
flow through, over, and under the products. There was a
door in front of the dryer to open and close the drying
chamber. The sidewalls, roof, and floor of the dryer were
made with 1-mm-thick metal sheet insulated with 5-mm
polystyrene.
Experimental Procedure
Several experimental runs under different drying con-
ditions for solar and sun drying of tomato were carried
out at Leibniz Institut für Agrartechnik Potsdam-Bornim
(ATB), Germany, during the period June to September
2006 (mid-European summer conditions). Fresh and uni-
form size of ripe tomatoes (variety Roma) were purchased
from Potsdam supermarket. Average diameters varied
from 49.3 to 61.7 mm and average weights varied from
73.1 to 139.3 g. Color of fresh and dried tomato surface
was measured by a Minolta CR-300 Chromameter
(Minolta Co., Tokyo, Japan) in L
(from black to white),
a
(from green to red), and b
(from blue to yellow) chroma-
ticity coordinates using a CIElab color difference meter. The
instrument was standardized each time with a white ceramic
plate. Firmness of fresh tomato was determined by a pen-
etrometer (Zwick Universal Testing Machine, Germany)
with a 60-mm-wide flat plate probe at a constant speed
of 2 mm=min. Before starting an experimental run, the
whole apparatus was operated for at least one hour to
stabilize the air temperature and air velocity inside the dryer.
Tomatoes were cut into halves with a sharp knife and
then placed in a single layer on the drying trays placing
cut side up in the dryer. To compare the performance of
the dryer with that of sun drying, control samples of
half-cut tomatoes were also placed on trays in a single layer
beside the dryer in the open sun. Drying was started after
completion of the loading, usually at 9 a.m., and discontin-
ued up to reach the final moisture content of tomato.
Weight loss of both the samples in the solar dryer and
the control samples (open sun) were measured during the
drying period at one-hour intervals with an electronic bal-
ance (BP 310S, Sartorius AG, Göttingen, Germany). The
positions of the collector and its reflector were adjusted
according to solar angle so that maximum solar radiation
could be captured by the solar collector as well as by the
reflector. In the afternoon, after 5 p.m., the samples
remained in the dryer and the collector was covered by
reflector and the control samples. The next morning, at
9 a.m. the cover of the dryer and also the cover from the
control samples were removed and subjected to drying by
solar radiation. A data logger (Almeno 5590, Ahlborn
Mess-und Regelungstechnik GmbH, Germany) was used
FIG. 3. A schematic diagram of the air damper configuration.
HYBRID SOLAR DRYER FOR QUALITY DRIED TOMATO 1593
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to record the air temperatures and relative humidities at 10-
min intervals. A solar meter (Solarwatt, GmbH, Dresden,
Germany) was used to measure the global solar radiation
and total radiation (global þ reflected from reflector) dur-
ing the daytime drying period. Velocity of drying air was
measured with an anemometer (TA-5, Airflow Develop-
ment Limited, Buckinghamshire, England). The moisture
content of the tomato sample was measured by drying
the samples in an air ventilated oven at 105
C for 24 h.[8]
Pretreatment
Treatments were used before drying of tomato halves to
find out the effect on quality and efficacy to control
microbial growth during low-temperature (45
C) drying.
Different treatments used before drying and their appli-
cation methods are given in Table 1. The treated and con-
trol samples (with no treatments) were dried up to the final
moisture content.
Quality Analysis
Ascorbic Acid
Ascorbic acid was analyzed according to the method
described by Denniston and Wimmers[19]
and Goula and
Adamopoulos.[20]
Ten grams of tomato sample were placed
in a mortar and 20 mL oxalic acid (0.25 M) solution and a
pinch of sand was added into it. Again, 20 mL oxalic acid
was added and macerated thoroughly for 3 min. The
macerated mixture was thoroughly filtered with Whatman
#1 filter paper. Filtrated extract was collected in a 100-mL
volumetric flask. Then 1.0 mL DCIP (2,6-dichloroindophe-
nol) solution and 0.1 mL metaphosphoric acid was added
and diluted with 3.0 mL of tomato extract. The absorbance
of the solution was measured at 520 nm on a UV-VIS spec-
trophotometer (Model: CADAS 200, Bruno Lange GmbH
Co. Kg, Berlin, Germany) with oxalic acid as blank. The
ascorbic acid of tomato extract was determined from the
standard curve.
Lycopene
Lycopene was analyzed according to the method
described by Opiyo and Ying.[21]
For extracting lycopene,
1 g of homogenized fresh or dried tomato sample was
weighed and covered with aluminum foil to exclude light
and the lycopene from the sample. Approximately
12–15 mL of distilled water was added to 1 g of dried tomato
powder. The whole tomato was ground in an enamel mor-
tar for uniform consistency. Then a 39-mL mixture of
hexane-acetone-ethanol (1:1:1, v : v : v) was added to the
sample, which was placed on the rotary mixer for 30 min.
Agitation was continued for another 2 min after adding
10 mL of distilled water. The solution was then left to sep-
arate into distinct polar and non-polar layers and then the
hexane layer was collected in a 50-mL flask. The absor-
bance of the hexane layers was measured at 503 nm on a
spectrophotometer using hexane as a blank. The amount
of lycopene in the tomato samples was determined using
the formula as: lycopene (mg=100 g) ¼ 312 Absorbance
at 503 nm.[21]
Total Flavonoids
Total flavonoids was analyzed according to the method
reported by Zhishen et al.[22]
and Toor and Savage.[23]
Four
grams of finely homogenized sample were extracted twice
with 10 mL of hexane in the dark. The final extract was fil-
tered through a 0.5-mm filter paper. A known volume
(1 mL) of the extract or standard solution of rutin solutions
was added to a 10-mL volumetric flask. Distilled water was
added to make a volume of 5 mL. At zero time, 0.3 mL of
TABLE 1
Different treatments and their application method
Treatments Application procedure
Control No pretreatment was applied
UV radiation (750 Wm2
) Put tomato halves under radiation with cut side up
for 5 min
) Put tomato halves under radiation with cut side up
for 10 min
Acetic acid (4 mg L1
) Placed the tomato halves in gas enclosure for 30 min
Acetic acid (6 mg L1
) Placed the tomato halves in gas enclosure for 30 min
Citric acid (3 g L1
) Sprayed on the tomato halves with cut side up
Citric acid (6 g L1
Ascorbic acid (3 g L1
Sodium metabisulphite (2 g L1
) Dipped the tomato halves for 3 min
Sodium chloride (10 g L1
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5% w=v sodium nitrite was added to the flask. After 5 min,
0.6 mL of 10% w=v AlCl3 was added and, after 6 min, 2 mL
of 1 M NaOH was added to the mixture, followed by the
addition of 2.1 mL distilled water. Absorbance was mea-
sured at 510 nm on a spectrophotometer against the blank
(water) and total flavonoids and determined from the stan-
dard curve.
Rehydration Ratio
About 20g of dried tomato halves was submerged into
distilled water with a product-to-water ratio of 1:8 at room
temperature (20 2
C) for 24h.[24]
Then the samples were
drained of excess water for about 3 min and adhering water
was soaked into tissue paper and the sample was weighed
again. The rehydration ratio or index is the ratio of the weight
of sample after rehydration to the original dried weight.[16]
Microorganisms Detection
During the drying period, microbial infestation in the
sample was determined by manual enumeration method.
When a microorganism-infected sample was observed,
the sample was collected in a cleaned plastic Petri dish
and put in a refrigerator at 5
C. This procedure was
continued until the completion of the drying experiments.
After completion of the drying experiments, the infected
samples were observed on a microscope to diagnose the
severity of infestation and the type of microorganisms
were identified.
Efficiency Calculation
The thermal efficiency of the solar collector and drying
efficiency of the solar dryer were calculated using following
formula:
(a) Collector efficiency during daytime
(i) Considering global solar radiation
gcdg ¼
_
m
maCpaðTi ToÞ
AcIg
ð1Þ
(ii) Considering total solar radiation (global þ reflected
from reflector)
gcdt ¼
_
m
maCpaðTi ToÞ
AcIt
ð2Þ
(b) Collector efficiency during nighttime:
gcn ¼
_
m
maCpaðTi ToÞ
_
m
mwCwðTw TiÞ
ð3Þ
(c) Drying efficiency of the solar dryer:
(i) Solar dryer efficiency in daytime
gdd ¼
mwhL
AcItt þ Qf þ Qp
ð4Þ
(ii) Solar dryer efficiency at nighttime
Drying with hot water flow without using water heater
gnd ¼
mwhL
Qf þ Qp
ð5Þ
Drying with hot water flow using water heater
gnd ¼
mwhL
Qh þ Qf þ Qp
ð6Þ
Statistical Analysis
The analysis of variance (ANOVA) of color values,
ascorbic acid, lycopene, and total flavonoids of solar,
sun-dried, and different pretreated samples were statisti-
cally analyzed using the software SPSS 9.0. The mean
obtained from each set of variable was compared
by Duncan’s multiple range test (DMRT) based on the
complete randomized design (CRD).
RESULTS AND DISCUSSION
The solar collector as well as solar dryer were tested at
four different operational modes. Mode 1: daytime solar
drying using solar radiation but nighttime drying with
ambient air (no auxiliary heating). Mode 2: daytime solar
drying and storing heat in the heat storage tank and recir-
culating hot water from the storage tank during the night.
Mode 3: daytime solar drying and storing heat in the heat
storage tank recirculating hot water from the storage tank
and also using external water heating during the night.
Mode 4: in adverse weather, day and nighttime drying by
circulating hot water in the collector using water heater.
Collector Performance
The variations of ambient air temperature, air tempera-
ture at the outlet of the collector, and absorber plate tem-
perature with solar radiation of drying mode 1 are shown
in Fig. 4. Absorber plate temperature was about 80
C at
the mid noon (12:00–13:00 hr) of the days. But the highest
collector outlet air temperature was about 60
C. This col-
lector outlet air temperature was maintained during high
insolation by increasing air flow because above 60
C drying
air temperature, quality of the dried product deteriorates.
Average ambient air temperature in the day and nighttime
were found to be 24 and 18
C, respectively. In the daytime
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(9:00 a.m. to 5:00 p.m.), collector outlet air temperature
was about 30
C higher than the ambient air temperature.
But at nighttime (without hot water flow), absorber plate
temperature and collector outlet air temperature decreased
rapidly and approached the ambient air temperature.
Variations of ambient air temperature, outlet air
temperature, absorber plate temperature, and water
temperature with solar radiation of mode 2 are shown in
Fig. 5. The daytime temperature was about 30
C higher
than the ambient air temperature. During the daytime
(solar radiation period), solar heat energy was stored in
the water tank by circulating water in the collector. This
storage heat energy was released at night in the collector
air (from water to air through copper tube) by recirculating
hot water in the collector. As a result, air temperature in
the collector did not drop drastically (as happened for
mode 1) but was reduced slowly down to about 30
C the
next morning. During the nighttime, ambient air tempera-
ture dropped to about 15
C. Air temperature developed in
the daytime at the outlet of the collector was sufficient for
tomato drying. But nighttime temperature was not suitable
for tomato drying, because nighttime temperature
(30
C) susceptible for microbial growth. Hossain and
Gottschalk[25]
reported that tomato should be dried in the
air temperature range of 45 to 55
C to prevent microbial
growth and to prevent case-hardening and quality loss.
Variations of ambient air temperature, temperature in
the collector, and water temperature with solar radiation
of mode 3 are shown in Fig. 6. During the daytime, collec-
tor outlet air temperature was about 60
C and it was about
30
C higher than the ambient air temperature. During the
daytime (solar radiation period) a part of solar heat energy
was stored in the water tank by circulating water in the col-
lector and at nighttime additional heat was supplied to
water using a 6-kW thermostat-controlled water heater.
The water heater was adjusted such that water temperature
did not go below 65
C. During the nighttime, air tempera-
ture at the outlet of the collector was about 50
C when the
ambient air temperature dropped to 14
C. This collector
outlet air temperature is quite suitable to prevent microbial
growth for nighttime drying.
For mode 4, i.e., in adverse weather conditions (cloudy
and rainy), the suitability of the solar collector as well as
the dryer was tested. The weather in this period was mostly
cloudy with scattered rains. Day and nighttime ambient air
temperature was low (20
C). During this period solar
radiation was very uncertain and irregular and not suitable
for solar and sun drying (Fig. 7). In this period (04.09.06–
07.09.06), hot water flow was continued with the water
heater until the end of the drying experiment. Day and
nighttime air temperature at the outlet of the collector
FIG. 4. Variations of ambient air temperature and temperatures in the
collector with solar radiation of drying mode 1 (01.07.06–06.07.06).
FIG. 5. Variations of ambient air temperature and temperatures in the
collector with solar radiation of drying mode 2 (14.07.06–18.07.06).
FIG. 6. Variations of ambient air temperature and temperature in the
collector with solar radiation of mode 3 (11.09.06–14.09.06).
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was about 50
C against the ambient temperature of
15–20
C in the same period.
Collector and Dryer Efficiency
The relationship between total radiation (global þ reflected
from reflector) and global radiation on the glass cover is
shown in Fig. 8. The following regression equation was
developed for total radiation in terms of global radiation.
It ¼ 117:72 þ 1:1699Ig ðR2
¼ 0:89Þ ð7Þ
The thermal efficiency of the collector was calculated con-
sidering both global solar radiation and total solar radiation
in the collector. Variations of collector efficiency with global
solar radiation at different times of a typical day are shown
in Fig. 9. Collector efficiency followed a similar pattern of
the solar radiations. Collector efficiency found to be higher
for total (global þ reflected) solar radiation than that of global
solar radiation.
Collector and dryer efficiencies for different drying
modes are given in Table 2. On sunny days, the daytime
collector efficiencies at different drying modes were almost
similar but in adverse weather (mode 4) it reduced to
24.36%. On sunny days, the contribution of the reflector
on the thermal efficiency of the collector was about 10%.
In adverse weather, sometimes the reflector was used as a
cover on top of the collector. Therefore, the reflected
energy from the reflector was small. At nighttime, with
hot water flow, collector efficiency increased from 0 to
10.53% and further increased 24.31% using the water hea-
ter. In the daytime, dryer efficiency varied from 17.73 to
29.35% depending on weather conditions. At nighttime,
dryer efficiency increased from 0 to 6.88% for hot water
flow and again increased by 18.75% using the water heater.
FIG. 8. Relationship between global solar radiation and total solar
(global þ reflected) radiation on the glass cover.
FIG. 9. Variations of collector efficiency considering global radiation
and both global and reflected radiation at different times of a day
(11.09.06).
FIG. 7. Variations of ambient air temperature and temperature in the
collector with solar radiation of mode 4 (04.09.06–07.09.06).
TABLE 2
Collector and drier efficiency at different drying modes
Collector efficiency (%)
Day Dryer efficiency (%)
Drying
modes
Using
reflector
Without
reflector Night Day Night
Mode 1 43.78 34.02 0.00 17.92 0.00
Mode 2 45.80 35.29 10.53 22.48 6.88
Mode 3 44.19 33.89 24.31 29.35 18.75
Mode 4 24.36 22.36 20.83 17.73 16.90
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Dryer Performance
Effects of fruit and slice sizes on solar and sun drying of
tomato are shown in Fig. 10. These were dried in the same
weather conditions in the dryer as well as in the open sun.
The moisture reduction of small tomato was higher than
the large size for both solar and sun-drying methods. The
reason might be that the distance from the center to the
evaporating surface of slices of small size tomato was lower
than that of large size and the initial moisture content of
large tomatoes was higher than smaller ones. Again, the
moisture reduction of quarter-size fruit was higher than
half fruit. But shrinkage of quarter-cut fruit was higher
(40%) than the half-cut fruit. Also, the shrinkage of the
half-cut small tomato was higher (20%) than the half-cut
large tomato. The greater the shrinkage, the less the consu-
mer’s acceptance as well as market price.
Drying performance of solar and sun drying of tomato
at four operating modes is given in Table 3. For mode 1,
moisture contents of identical samples were reduced from
27.74 to 0.17 kg kg1
(d.b.) in 126 and 288 h (days and
nights) in solar and sun-drying methods, respectively.
Hence, the time saved in solar drying over sun drying is
56.25%. Average drying rate by solar and sun drying were
0.218 and 0.096 kg kg1
(d.b.) h1
, respectively. There was
no hot water flow at nighttime, so dryer air temperature
dropped below the ambient temperature and both solar
and sun-drying samples were infected by molds and fungus.
Moisture contents were reduced to 0.18 from initial moist-
ure content of 18.86 kg kg1
(d.b.) in 96 and 240 h by solar
and sun drying, respectively, in mode 2. Average drying
rates by solar and sun drying were 0.195 and 0.077 kg
kg1
(d.b.) h1
, respectively. Time reduced in solar drying
is 60%. Molds and fungus were detected during drying
since nighttime temperature reduced below 45
C.[25]
For
mode 3, moisture contents of tomato were reduced from
22.15 to 0.19 kg kg1
(d.b.) in 72 and 220 h by solar and
sun drying, respectively. Hence, the time saving in solar
drying is 70%. Drying rates by solar and sun drying were
0.305 and 0.099 kg kg1
(d.b.) h1
, respectively. No molds
or fungus growth was observed in solar drying samples as
the drying air temperature in the dryer was always above
45
C. But sun-drying samples were infected by molds and
fungus. For drying mode 4, sample moisture content was
reduced from 21.14 to 0.16 kg kg1
(d.b.) in 88 h by solar
drying method and the average drying rate was 0.235 kg
kg1
(d.b.) h1
. The samples were not infected by micro-
organisms. But the sun-drying samples were severely
infected by molds and fungus and completely damaged
due to bad weather.
Quality of Fresh and Dried Tomato
Quality (color, ascorbic acid, lycopene, total flavonoids,
and rehydration ratio) of fresh and solar-dried samples was
measured. To compare the quality of solar-dried tomato
with the dried tomato available in the supermarket, the
quality of commercially available dried tomato was
determined. Quality of fresh, solar-dried, sun-dried, and
commercially available dried tomato is presented in
Table 4. Significantly highest hue angle was found for fresh
tomato followed by sun-dried samples. Significantly lowest
hue angles were found for solar-dried sample with 15 and
30% final moisture contents, since these were statistically
similar but significantly lower than solar-dried samples
with 25% moisture content and that of commercial sam-
ples. Hue angle is the actual color and it is a combination
of green, red, blue, and yellow colors and widely used to
express tomato color changes.[26]
The lower the hue angle,
the redder the color.[27]
The hue angle of all dried samples
was significantly lower than that of fresh tomato. Hence,
the red color of tomato concentrated to deep red after dry-
ing. But the hue angle of sun-dried samples was signifi-
cantly higher than the solar-dried samples. The reason
might be that sun-dried samples were contaminated with
dust, dirt, and microorganisms and also exposed to direct
sunlight. Significantly lower ascorbic acid, lycopene, and
total flavonoids were found in all dried samples in com-
parison to fresh samples. These are antioxidant compo-
nents and these components reduced significantly during
drying. It is reported that ascorbic acid, lycopene, and total
flavonoids of tomato decrease during drying in different
amounts by oxidative heat damage depending upon the
drying air temperature and other drying con-
ditions.[14,20,23,28,29]
The lowest ascorbic acid was obtained
at the lowest moisture content (15%). There was no signifi-
cant difference of lycopene among the dried samples. The
lowest amount of total flavonoids was found in commer-
cially available dried sample in comparison to solar and
FIG. 10. Effect of size and slices on solar and sun drying of tomato
(18.09.06–22.09.06).
1598 HOSSAIN ET AL.
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sun-dried samples. Total flavonoids decreased with the
reduction of moisture content of dried samples but there
was no significant difference in total flavonoids for 15
and 25% moisture contents. The lowest amount of total
flavonoids was obtained for commercially available sam-
ples. The highest and lowest rehydration ratio was found
for solar-dried samples, with 15% moisture content, and
commercial samples, with 37% moisture content. There
was no significant difference of rehydration ratio for other
samples. It is clear from the results that rehydration ratio
mainly depends on the moisture content and surface hard-
ness of the dried product. Similar results have been
reported by Latapi and Barrett.[30]
It is evident from the
results that retention of surface color, ascorbic acid, lyco-
pene, and total flavonoids of solar-dried samples is higher
than in commercially available samples. Hence, the color
and nutritional quality of solar-dried samples are better
than those of the commercially available samples in the
European market.
Effect of Pretreatments
Effects of different pretreatments on quality and
microbial infestation of solar-dried tomato are given in
Table 5. The lower hue angles were found for all pretreated
samples over control samples. Again, the lowest hue angles
were obtained from citric acid–, ascorbic acid–, sodium
metabisulphite–, and sodium chloride–treated samples than
acetic acid, UV radiation, and control samples, but there
were no significant differences among them. Latapi and
Barrett[30]
found better color retention from sodium metabi-
sulphite–, sulphur dioxide–, and sodium chloride–treated
samples than non-treated (control) samples. The highest
amount of ascorbic acid was obtained for ascorbic acid–
pretreated samples and the lowest amount was found for
sodium chloride–treated samples. Statistically, the same
amount of ascorbic acid was found for acetic acid–, citric
acid–, and sodium metabisulphite–pretreated samples.
There was no significant difference of ascorbic acid between
UV radiation and control samples. The highest amounts of
lycopene were obtained for 6 mg L1
acetic acid and 2 and
8 g L1
sodium metabisulphite–pretreated samples. The
lycopene content of other pretreated samples was statisti-
cally the same but significantly higher than control samples.
The highest amount of total flavonoids was found for
sodium metabisulphite–, ascorbic acid–, and citric acid–
pretreated samples than for other samples and these were
statistically alike. Sodium chloride pretreatment reduced
significantly the highest amount of total flavonoids followed
by control, UV radiation, and acetic acid–pretreated sam-
ples but there were no significant differences among them.
TABLE 4
Quality of fresh, solar-dried, sun-dried, and commercially available samples of tomato
Samples
Color
(hue angle,
)
Ascorbic acid
(mg=100 g)
Lycopene
(mg=100 g)
Total flavonoids
(mg=100 g)
Rehydration
ratio
Fresh (95% MC) 39.09a
238.38a
62.59a
217.54a
—
Solar dried (15% MC) 18.26b
172.54e
44.52b
122.51d
3.26a
Solar dried (25% MC) 15.53c
181.58c
45.98b
128.54d
2.83b
Solar dried (30% MC) 14.29c
184.24b
49.04b
195.61b
2.80b
Sun dried (30% MC) 25.02b
176.69d
44.13b
145.71c
2.78b
Commercial (37%MC) 19.50c
176.71d
44.06b
96.76e
2.16c
Significance level (p) 0.05 0.05 0.05 0.05 0.05
MC ¼ moisture content of sample in wet basis. Different letters in the same column are significantly different from each other by
DMRT.
TABLE 3
Summary of drying performance of solar and sun drying at different drying conditions
Average day
temperature (
C)
Average night
temperature (
C)
Average drying
rate (kg kg1
(d.b.) h1
)
Drying time (h)
(day þ night)
Drying modes Drier Ambient Drier Ambient Solar Sun Solar Sun Time saving (%)
Mode 1 55 25 13 15 0.218 0.096 126 288 56.25
Mode 2 58 30 28 16 0.195 0.077 96 240 60.00
Mode 3 57 28 48 15 0.305 0.099 72 220 70.90
Mode 4 53 20 48 15 0.235 — 88 — —
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All pretreatments partially prevented the microbial
growth but only 8 g L1
sodium metabisulphite prevented
completely the microbial infestation when night tempera-
ture reduced to below 45
C. Control samples were com-
pletely infested by microorganisms. Microorganisms were
identified as Zygomycetes, Aspergillus sp., Aspergillus
niger, and Fusarium sp., which are tropical molds and
fungus. At the drying air temperature of 45
C, control
samples (untreated) were not infected by any micro-
organisms. Temperature has a great impact on growth
of microorganisms. These types of molds and fungus
(Aspergillus sp., Fusarium sp.) grow well at temperatures
over 40
C and can grow at a temperature range of 20–
50
C.[31,32]
Therefore, if drying temperature is maintained
at 45
C, there is no need to apply any treatments before
drying of tomato. It will reduce the cost of additives and be
more hygienic.
CONCLUSIONS
Average air temperature at the outlet of the collector
was found to be about 30
C higher than the average ambi-
ent temperature during the normal sunny days. Collector
efficiency was increased by 10% using the solar reflector.
The capacity of the solar dryer was 20 kg of half-cut fresh
tomato to produce 2 kg of dried product per batch. The
average drying system efficiency of the solar dryer varied
from 17 to 29% depending on different operating con-
ditions. The drying process significantly reduced the color,
ascorbic acid, lycopene, and total flavonoids of tomato but
the losses of color and nutritional components were higher
than the commercially available samples in the European
market. All pretreatments significantly improved the color
of dried tomato compared to non-treated sample. No pre-
treatment could completely control the microbial infes-
tation except sodium metabisulphite (8 g L1
) at lower
temperature (45
C). Therefore, it is recommended that
if tomato is dried with continuous air temperature of
45
C or above then no pretreatment is required. If drying
air temperature is reduced below 45
C, then tomato should
be pretreated with 8 g L1
sodium metabisulphite to
prevent microbial growth.
NOMENCLATURE
Ac Area of collector (m2
)
Cpa Specific heat of air (kJ=kgK)
Cpw Specific heat of water (kJ=kgK)
hL Latent heat of water in the tomato (kJ=kg)
Ig Global solar radiation (W=m2
)
It Total solar radiation (global þ reflected) (W=m2
)
_
m
ma Mass flow rate of air (kg=s)
_
m
mw Mass flow rate of water (kg=s)
mw Mass of evaporated moisture (kg)
Qf Heat used by fan for air flowing (kW)
Qh Heat produced by water heater (kW)
Qp Heat used by water pump (kW)
Ti Inside air temperature (
C)
To Outside air temperature (
C)
Tw Water temperature (
C)
t Time (min)
TABLE 5
Effect of different pretreatment on quality of solar-dried tomato (drying air temperature during day was 50–55
C and
night was 25–40
C)
Pretreatment
Color
(hue angle,
)
Ascorbic acid
(mg=100 g)
Lycopene
(mg=100 g)
Total flavonoids
(mg=100 g)
Microbial
infestation (%)
Control sample 24.19a
118.29c
34.52c
124.18b
100
) 22.75b
137.36c
43.98b
123.36b
50
) 19.87b
149.42c
46.04b
124.51b
50
Acetic acid (4 m g L1
) 20.49b
176.69b
47.13b
124.09b
50
Acetic acid (6 m g L1
) 17.64c
176.71b
49.06a
125.39b
40
Citric acid (3 g L1
) 20.56b
181.58b
46.64b
129.14a
20
Citric acid (6 g L1
) 18.19c
184.24b
45.91b
128.57a
20
) 17.35c
230.56a
44.75b
133.91a
60
) 18.09c
249.71a
47.15b
134.87a
40
) 18.79c
180.36b
48.50a
132.79a
10
) 16.65c
183.49b
49.57a
135.92a
0
Sodium chloride (10 g L1
) 19.13c
90.41d
41.76b
119.52c
40

Untreated sample (temp. 45
C) 22.83b
134.39c
42.08b
120.32b
0
Different letters in the same column are significantly different from each other by DMRT at p 0.05.

Sample was dried at the drying air tmperature of 45
C. Other samples were died at th air temperature of 45
C.
1600 HOSSAIN ET AL.
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Greek Letters
gcdg Collector efficiency in the daytime considering
global solar radiation (ratio)
gcdt Collector efficiency in the daytime considering
total solar radiation (ratio)
gcn Collector efficiency at nighttime (ratio)
gdd Dryer efficiency in the daytime (ratio)
gnd Dryer efficiency at nighttime (ratio)
ACKNOWLEDGEMENTS
This research was carried out under the postdoctoral
research fellowship of Alexander von Humboldt Foun-
dation, Germany. The authors are thankful to the Foun-
dation for granting the fellowship to carry out this research.
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