An experimental study was performed to determine the drying characteristics of hull-less seed pumpkin using hot air, solar tunnel and open sun drying methods. For the hot air drying, the test samples were dried in a laboratory scale hot air dryer at a constant air velocity of 0.8 m/s and air temperature in the range of 40–60 C. For solar drying experiments, a solar tunnel dryer was constructed at a low cost with locally obtainable materials. The moisture transfer from the test samples was described by applying the Fick’s diffusion model and the effective diffusivity was calculated. Temperature dependence of the effective diffusivity was described by the Arrhenius-type relationship. The experimental drying data of hull-less seed pumpkin were used to fit the Page, Henderson and Pabis, logarithmic and two-term models, and drying rate constants and coefficients of models tested were determined by non-linear regression analysis. Among the various models tested to interpret the drying behaviour of hull-less seed pumpkin, one was selected which presented best statistical indicators.
Trang 1Effect of drying methods on thin-layer drying characteristics
of hull-less seed pumpkin (Cucurbita pepo L.)
Kamil Sacilik * Department of Agricultural Machinery, Faculty of Agriculture, Ankara University, 06130 Ankara, Turkey
Received 22 August 2005; accepted 6 January 2006 Available online 28 February 2006
Abstract
An experimental study was performed to determine the drying characteristics of hull-less seed pumpkin using hot air, solar tunnel and open sun drying methods For the hot air drying, the test samples were dried in a laboratory scale hot air dryer at a constant air velocity
of 0.8 m/s and air temperature in the range of 40–60C For solar drying experiments, a solar tunnel dryer was constructed at a low cost with locally obtainable materials The moisture transfer from the test samples was described by applying the Fick’s diffusion model and the effective diffusivity was calculated Temperature dependence of the effective diffusivity was described by the Arrhenius-type relation-ship The experimental drying data of hull-less seed pumpkin were used to fit the Page, Henderson and Pabis, logarithmic and two-term models, and drying rate constants and coefficients of models tested were determined by non-linear regression analysis Among the var-ious models tested to interpret the drying behaviour of hull-less seed pumpkin, one was selected which presented best statistical indicators
2006 Elsevier Ltd All rights reserved
Keywords: Hull-less seed pumpkin; Solar tunnel drying; Hot air drying; Moisture ratio; Effective diffusivity; Activation energy
1 Introduction
The drying technique is probably the oldest and the
most important method of food preservation practiced by
humans The removal of moisture prevents the growth
and reproduction of microorganisms which cause decay,
and minimises many of the moisture-mediated
deteriora-tive reactions It brings about substantial reduction in
weight and volume, minimizing packing, storage and
trans-portation costs and enables storability of the product under
ambient temperatures (Mujumbar, 1995) During drying
many changes take place; structural and physic-chemical
modifications affect the final product quality, and the
qual-ity aspects involved in dry conversation in relation to the
quality of fresh products and applied drying techniques
(Baysal, Icier, Ersus, & Yıldız, 2003) Currently hot air
dry-ing is the most widely used method in post-harvest technol-ogy of agricultural products Using this method, a more uniform, hygienic and attractively coloured dried product can be produced rapidly (Doymaz, 2004) However, it is
an energy consuming operation and low-energy efficiency,
so more emphasis is given on using solar energy sources due to the high prices and shortage of fossil fuels Solar dryers are now being increasingly used since they are a bet-ter and more energy efficient option The solar dryers could
be an alternative to the hot air and open sun drying meth-ods, especially in locations with good sunshine during the harvest season (Pangavhane, Sawhney, & Sarsavadia,
2002) However, large-scale production limits the use of the open sun drying Among these are lack of ability to control the drying process properly, weather uncertainties, high labour costs, large area requirement, insect infesta-tion, mixing with dust and other foreign materials and so
on (Basunia & Abe, 2001)
Solar drying is essential for preserving agricultural prod-ucts Using a solar dryer, the drying time can be shortened
0260-8774/$ - see front matter 2006 Elsevier Ltd All rights reserved.
doi:10.1016/j.jfoodeng.2006.01.023
*
Tel.: +90 312 596 15 92; fax: +90 312 318 38 88.
E-mail address: sacilik@agri.ankara.edu.tr
www.elsevier.com/locate/jfoodeng
Trang 2by about 65% compared to sun drying because, inside the
dryer, it is warmer than outside; the quality of the dried
products can be improved in terms of hygiene, cleanliness,
safe moisture content, colour and taste; the product is also
completely protected from rain, dust, insects; and its
pay-back period ranges from 2 to 4 years depending on the rate
of utilization The most important feature of solar dryers is
that the product does not include any kind of preservatives
or other added chemical stuffs, which allows its use for
peo-ple suffering from various allergic reactions from chemical
preservatives and other added stuffs Furthermore, the
product is not exposed to any kind of harmful
electromag-netic radiation or electromagelectromag-netic poles (Tiris, Tiris, &
Dincer, 1996) Although for agricultural products, solar
dryers with solar air heater offer better control of required
drying air conditions, solar tunnel dryers based on plastic
tunnel greenhouses have a great potential and does not
require any other energy during operation Therefore, solar
tunnel dryer may become a more convenient alternative for
rural sector and other areas in which electricity is scarce
and in regular supply Also, it can reduce crop losses,
improve the quality of dried product significantly and is
economically beneficial compared to traditional drying
methods
Sun shines in Ankara, situated in Middle Anatolian
Region in Turkey, over an average 2466 h/year, delivering
about 1525 kWh/m2year of solar radiation on the
horizon-tal surface Hours of sunshine and solar radiation between
June and September, namely drying period in Ankara,
make up about 46.59% and 49.64% of these values,
respec-tively (Sacilik, Keskin, & Elicin, 2006) Though other
sources of energy may be used for drying of agricultural
products, solar energy is preferred more and more since
it is abundant in Ankara, inexhaustible and non-polluting
It can be tapped at relatively low cost and has no
associ-ated environmental dangers (Basunia & Abe, 2001;
Mohamed et al., 2005)
Pumpkin seed is of considerable nutritional value for
human consumption due to its 37.8–45.4% oil and 25.2–
37.0% protein It enjoys valuable dietetic and medicinal
advantages besides being a source of edible oils, proteins and minerals of good quality (Yoshida, Shougaki, Hirak-awa, Tomiyama, & Mizushina, 2004) Hull-less seed pump-kin (var styriaca) or naked seed pumppump-kin is grown widely
in the southern regions of Austria (Styria province) and the adjacent regions in Slovenia and Hungary The pumpkin cultivated in Styria has a high content of green seeds with-out husks The seeds itself can be eaten as a snack and show good results in curing prostate Pumpkin seed oil extracted from the seed is used widespread as salad oil (Murkovic & Pfannhauser, 2000) Recently, the hull-less seed pumpkin have been grown in the some parts of Tur-key, notably Nallıhan province, Ankara The efficient pro-cessing and long-term storage of pumpkin seed requires that the moisture content be reduced to suitable levels by various drying methods To the knowledge of the author, there is no literature specific to the drying behaviour of hull-less seed pumpkin found Therefore, the present study was conducted with the following objectives:
(1) to study and compare the thin-layer drying character-istics of hull-less seed pumpkin using the open sun, solar tunnel and hot air drying methods; and (2) to fit the experimental data obtained to semi-theoret-ical models widely used to describe thin-layer drying behaviour of agricultural products
2 Material and methods 2.1 Material
The hull-less seed pumpkins used in this study were obtained from a local grower of Nallıhan, Turkey during the summer season of 2003 In order to preserve its original quality, they were stored in a refrigerator at 4C until dry-ing experiments The initial moisture content of seed was determined using the vacuum oven method at 70C for
24 h (AOAC, 1990) These experiments were replicated thrice to obtain a reasonable average After drying, the
Nomenclature
a, b, c coefficients in models
Deff effective diffusivity, m2/s
D0 pre-exponential factor, m2/s
Ea activation energy, kJ/mol
EMD mean relative percent deviation, %
ERMS root mean square error
H half-thickness of the slab in sample, m
k, k0 drying rate constants in models, l/h
m exponent in drying model
M moisture content at any time, kg [H2O]/kg [DM]
Me equilibrium moisture content, kg [H2O]/kg [DM]
M0 initial moisture content, kg [H2O]/kg [DM]
MR dimensionless moisture ratio
n positive integer
N number of observations
R universal gas constant, kJ/mol K
R2 coefficient of determination
Ta absolute air temperature, K
z number of constants
v2 reduced chi-square
Trang 3sample was found to have a moisture content of about 67%
dry basis (d.b.)
2.2 Experimental set up
The experimental set ups used for determining the
influ-ence of various drying methods on the thin-layer drying
behaviour of hull-less seed pumpkin are presented inFigs
1 and 2 The description of the laboratory scale hot air and
solar tunnel dryer used in present study was described in
detail elsewhere in Sacilik et al (2006) and Sacilik and
Elicin (2006), respectively
2.3 Experimental procedure
The hot air drying experiments were conducted at 40, 50
and 60C air temperatures and a constant air velocity of
0.8 m/s In each experiment, about 100 g of pumpkin seed
samples were used After the system was run for at least
half an hour to reach steady conditions for the operation
temperatures, the samples were evenly distributed within
the sample tray as a single layer and dried there Moisture
losses of samples were recorded at 10 min intervals for first
one hour and 20 min subsequently thereafter for
determi-nation of drying curves Drying was continued until no
fur-ther changes in their mass were observed The dried
samples were allowed to cool down at an ambient
temper-ature for 15 min and then packed in low-density polyethyl-ene bags
Three sets of the solar tunnel drying experiments were carried out during the periods of August–September 2003 under the climatic conditions of Ankara Each experiment started at 08:00 am and continued till 06:00 pm The test samples were uniformly spread on wire mesh tray the load-ing density of which was about 1.5 kg/m2 To determine the moisture loss of drying samples during experiments, pump-kin samples were taken from three points, namely inlet, middle and outlet of the solar tunnel dryer and weighed
at various time intervals, ranging from 30 min at the begin-ning of the drying to 1 h during the last stage of the pro-cess The moisture loss of samples was determined with the help of a digital electronic balance having an accuracy
of 0.01 g After 06:00 pm, the pumpkin seeds in the solar tunnel dryer were collected and placed in plastic boxes in order to induce fermentation and diffusion of moisture within the drying samples These were again spread in the dryer in the next morning and the drying process was con-tinued until no further changes in their mass were observed Also, to compare the performance of the solar tunnel dryer with that of open sun drying, control samples
of pumpkin seeds were distributed on a tray at the same loading density near the solar tunnel dryer Both experi-mental and control samples were dried simultaneously under the same weather conditions
2.4 Analysis of drying data The experimental drying data obtained were fitted to the four well-known drying models given inTable 1 The mois-ture ratio is given as follows:
MR¼ M Me
M0 Me
ð1Þ where MRis the dimensionless moisture ratio, M, Meand
M0are the moisture content at any time, the equilibrium moisture content and the initial moisture content in kg [H2O]/kg [DM], respectively
However, MR is the simplified to M/M0 instead of
Eq (1) due to the continuous fluctuation of the relative humidity of the drying air during their drying processes (Diamente & Munro, 1993) The drying rate constants and coefficients of models were estimated using a non-linear regression procedure The estimation method was
Fig 1 Experimental set up of laboratory dryer: (1) centrifugal blower, (2)
air heating chamber, (3) drying chamber, (4) perforated floor, (5)
electronic balance, (6) holding wire, (7) sample tray, (8) sensors, (9) Pc,
(10) door.
Fig 2 Schematic representation of the solar tunnel dryer.
Table 1 Thin-layer drying models given by various workers for drying curves
Page M R = exp(kt m
Munro (1993) Henderson
and Pabis
M R = a exp(kt) Westerman et al.
(1973) Logarithmic M R = a exp(kt) + c Tog˘rul and
Pehlivan (2003) Two term M R = a exp(kt) + b exp(k 0 t) Henderson (1974)
Trang 4Levenberg–Marguardt and the statistical validity of models
was evaluated and compared by means of the coefficient of
determination R2, mean relative percent deviation EMD,
root mean square error ERMS and reduced chi-square v2
These comparison criteria methods can be calculated as
follows:
EMD¼100
N
XN
i¼1
ERMS¼ 1
N
XN
i¼1
ð3Þ
v2¼
PN
where: MR,ex,i is the ith experimental dimensionless
mois-ture ratio; MR,pre,iis the ith predicted dimensionless
mois-ture ratio; N is the number of observations; and z is the
number of constants
R2 was used as the primary comparison criteria for
selecting the best model to fit the four models to the
exper-imental data Also, a model is considered better than
another if it has a lower value of the EMD, ERMS, v2
3 Results and discussion
3.1 Hot air drying of hull-less seed pumpkin
The moisture content versus drying time curves for hot
air drying of hull-less seed pumpkin as affected by various
air temperatures are shown inFig 3 The pumpkin samples
of average initial moisture content of around 0.67 kg
[H2O]/kg [DM] were dried to the final moisture content
of about 0.04 kg [H2O]/kg [DM] until no further changes
in their mass were observed It is evident from these curves
that the moisture content decreases continuously with the
drying time As expected, the air temperature had a
signif-icant effect on the moisture content of samples During
the hot air drying experiments, the time to reach the final
moisture content for samples were found to be 9.0, 7.5 and 6.0 h at the air temperatures of 40, 50 and 60C, respectively The increase in the air temperature resulted
in a decrease in the drying time Other researchers have reported similar trend (Akpinar, Bicer, & Yildiz, 2003a; Ertekin & Yaldiz, 2004)
3.2 Solar drying of hull-less seed pumpkin Fig 4shows the variations of the ambient air tempera-ture, relative humidity and solar radiation during the solar tunnel and open sun drying of hull-less seed pumpkin for a typical day of September 2003 in Ankara During the dry-ing experiments, the weather was generally sunny and no rain appeared The daily mean values of the ambient air temperature, relative humidity and solar radiation changed from 21.6 to 39.7C, 12.1% to 51.5% and 205.1 to 796.2 W/m2, respectively The drying air temperature and relative humidity in solar tunnel dryer varied continuously from morning to evening The ambient air temperature and solar radiation were reached the highest figures between 12:00 and 15:00, whereas the relative humidity was reached the lowest figures during this time The difference between the drying air temperature and ambient temperature was observed to be the highest during this time In other words, inside the solar dryer, it is warmer than outside This clearly indicates that the drying rate in the solar tunnel dry-ing would be higher than open sun drydry-ing
Fig 5suggests drying curves for hull-less seed pumpkin dried by solar tunnel and open sun drying methods The interruptions of the lines in this figure represent the night periods of the drying process The pumpkin samples of average initial moisture content of around 0.67 kg [H2O]/
kg [DM] were reduced to the final moisture content which changed between 0.05 and 0.07 kg [H2O]/kg [DM] It is clear fromFig 5that the moisture content decreases con-tinuously with the drying time During the experiments, the time to reach the final moisture content of samples
0.0
0.2
0.3
0.5
0.6
0.8
Drying time (h)
T=60 ˚C T=50 ˚C T=40 ˚C
Fig 3 Drying curves for hull-less seed pumpkin under hot air drying
condition at indicated air temperatures.
0 10 20 30 40 50 60
08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00
Drying hours (h)
0 100 200 300 400 500 600 700 800 900
2 )
Fig 4 Variations of ambient air temperature (j), relative humidity (d) and solar radiation (m) with the drying hours for a typical day of December 2003.
Trang 5for solar tunnel were found to be between 27 and 30 h,
while the drying time for the open sun drying changed
between 32 and 34 h Solar tunnel dryer had a shorter
dry-ing time than the open sun drydry-ing In other words, drydry-ing
time was reduced to about 17.9% by the solar tunnel dryer
according to the open sun drying Depending on weather
conditions, the solar tunnel dryer developed shortened half
day the drying time of hull-less seed pumpkin samples The
decrease in the drying time could be attributed to the values
of higher temperature and lower relative humidity obtained
in dryer Similar results have been reported by Schirmer,
Janjai, Esper, Smitabhindu, and Mu¨hlbauer (1996); Bala
and Mondol (2001); Bala, Mondol, Biswas, Das
Chowd-ury, and Janjai (2003)
3.3 Determination of effective diffusivity and activation
energy
The effective diffusivity of the samples is estimated by
using the simplified mathematical Fick’s second diffusion
model The solution of Fick’s second law in slab geometry,
with the assumptions of moisture migration being by
diffu-sion, negligible shrinkage, constant diffusion coefficients
and temperature was as follows (Crank, 1975):
MR¼ M Me
M0 Me
¼ 8
p2
X1
n¼1
1 ð2n þ 1Þ2 exp
ð2n þ 1Þ2p2Defft 4H2
!
ð5Þ For long drying periods, Eq.(5)can be further simplified to
only the first term of the series and the moisture ratio MR
was reduced to M/M0 because Me was relatively small
compared to M and M0 Then, Eq (5) can be written in
logarithmic form:
lnM
M0
¼ ln 8
p2 p
4H2
ð6Þ where H is the half-thickness of the slab in sample in m, n is
a positive integer and D is the effective diffusivity in m2/s
The effective diffusivity is typically calculated by plotting experimental drying data in terms of ln(MR) versus drying time From Eq.(6), a plot of ln(MR) versus the drying time gives a straight line with a slope of
Slope¼p
The values of effective diffusivity for various drying methods are presented inTable 2 Among the three drying methods, the hot air drying offered the highest values of
Deff for all drying air temperatures, followed by the solar tunnel drying and open sun drying In hot air drying, the values of Deffincreased with an increase in the air temper-ature The value of Deff for the solar tunnel drying was slightly higher than that for the open sun drying These results were compatible with the previous literature studies
on the drying of grape leather (Maskan, Kaya, & Maskan,
2002) and potato slice (Akpinar, Midilli, & Bicer, 2003b) The effect of the temperature on the effective diffusivity
is often expressed using the Arrhenius-type relationship:
Deff ¼ D0exp Ea
RT
ð8Þ where D0 is the pre-exponential factor of the Arrhenius equation in m2/s, Eais the activation energy in kJ/mol, R
is the universal gas constant in kJ/mol K and Ta is the absolute air temperature in K
The activation energy was calculated by plotting the nat-ural logarithm of Deffversus reciprocal of the absolute tem-perature as presented inFig 6 The plot was found to be a
0.0
0.2
0.3
0.5
0.6
0.8
Drying time (h)
Solar tunnel drying Open sun drying
Fig 5 Drying curves for hull-less seed pumpkin dried by solar tunnel and
open sun drying methods.
Table 2 Values of effective diffusivity obtained from various drying methods Drying method Temperature (C) Effective diffusivity
D eff · 10 11
(m2/s)
R 2 = 0.9999
-23.5 -23.2 -22.9 -22.6 -22.3
1/Ta (K -1 )
Fig 6 Arrhenius-type relationship between the effective diffusivity and absolute temperature.
Trang 6straight line in the range of air temperatures studied,
indi-cating Arrhenius dependence Then, the dependence of the
effective diffusivity of hull-less seed pumpkin on the
tem-perature can be represented by the following equation:
Deff¼ 1:95 105exp 3987:51
Ta
ð9Þ The activation energy for hull-less seed pumpkin was
found to be 33.15 kJ/mol, which is within the range of
15–40 kJ/mol for various foods reported byRizvi (1986)
3.4 Fitting of the drying curves
Tables 3 and 4list the estimated parameter and
compar-ison statistics of four drying models for the hot air and
solar tunnel drying of hull-less seed pumpkin, respectively
All the four models for hot air drying gave an excellent fit
to the experimental data with a value for R2of greater than
0.9931 Of all the models tested, the two-term model
offered the highest value for R2, followed by the
logarith-mic model However, the values of EMDfor the logarithmic
model were less than 10% in all cases, which is in the
acceptable range Also, the values for ERMS and v2
obtained from this model were less than those attained from other models Therefore, the logarithmic model was considered the best model in present study to represent the hot air drying behaviour of hull-less seed pumpkin within the experimental range of study
For solar tunnel drying, all models other than the Henderson and Pabis model provided an adequate fit to the experimental data with a value for R2of greater than 0.9888, indicating a good fit The values of EMDfor the log-arithmic and two-term model were less than 10%, which is
in the acceptable range However, the two-term model was rejected due to its higher value for ERMSand v2despite its high value for R2 Hence, the logarithmic model may be assumed to represent the thin-layer solar tunnel drying behaviour of hull-less seed pumpkin
Figs 7 and 8 suggest comparisons of the experimental and predicted moisture ratio obtained using the logarith-mic model for the hot air and solar tunnel drying, respec-tively It can be seen from these there was a good conformity between experimental and predicted moisture ratios This indicates the suitability of the logarithmic model in describing the drying behaviour of hull-less seed pumpkin in the hot air and solar tunnel drying process
Table 3
Parameter estimation, R 2 , E MD , E RMS and v 2 of the four drying models for the hot air drying at an air temperature of 40 C
Table 4
Estimated parameters and comparison criteria of the four drying models for solar tunnel drying
Trang 74 Conclusions
Based on the results of the investigations, the following
conclusions were drawn
(1) The three drying methods used greatly affected the
drying characteristics of hull-less seed pumpkin
(2) The solar tunnel dryer was found to be more efficient
than the open sun drying and resulted in saving to
extent of about 17.9% of drying time In addition,
the samples of solar tunnel dryer were completely
protected from insects, birds, rain and dusts
(3) The effective diffusivity varied from 8.53 to
17.52· 1011m2/s in the air temperature range of
40–60C The activation energy was found to be
33.15 kJ/mol
(4) The value of effective diffusivity for the solar tunnel
and open sun drying process were found to be
1.94· 1011and 1.66· 1011m2/s, respectively
(5) Of all the four models tested, the logarithmic model gave an excellent fit to the experimental data obtained with a value for R2 of greater than 0.99 for the hot air and solar tunnel drying process
Acknowledgement
I am grateful to Mr Rıfat Orkan from Orkan Agricul-tural Products Company in Nallıhan, Ankara, for his assis-tance related to providing hull-less seed pumpkin
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