ORIGINAL ARTICLE
https://doi.org/10.1007/s10341-019-00417-5
The Effects of Drying Parameters on Drying Characteristics,
Colorimetric Differences, Antioxidant Capacity and Total Phenols of
Sliced Kiwifruit
Hakan O. Mengeş1· Ahmet Ünver2· Mehmet Musa Özcan2· Can Ertekin3· Mehmet Hakan Sonmete1 Received: 21 September 2017 / Accepted: 21 December 2018 / Published online: 7 February 2019
© Springer-Verlag GmbH Deutschland, ein Teil von Springer Nature 2019, corrected publication 2019
Abstract
The influence of drying parameters on drying characteristics, colorimetric differences, antioxidant capacity and total phenols of sliced kiwifruit were researched. The kiwi fruits dried between 2.6 h and 12.1 h for different drying conditions. Total phenol content of dried fruits were ranged from 2.03–2.71 mg GAE/L. Free radical scavenging activity were varied from 26.04% to 40.91%. The effect of different drying temperatures were not very effective on the total phenol content of kiwi fruits. But, the free radical scavenging activity were variable. While L* value were in the range of 51.41 and 72.90, the a* value were ranged between –8.22 and 3.47 and the b* value between 22.24 and 40.37. The most suitable model is Midilli et al. model with low RMSE, reduced chi-square and high modeling efficiency values. While the effective diffusivity ranged between 2.63 × 10–10and 1.29 × 10–9m2.s–1, the activation energy was between 28.51 and 34.16 kJ mol–1.
Keywords Kiwifruit · Total phenol · Radical scavenging activity · Colour · Drying
Auswirkungen verschiedener Trocknungsparameter auf Trocknungsverhalten, kolorimetrische Unterschiede, antioxidative Kapazität und Gesamtphenolgehalt von in Scheiben geschnittenen Kiwifrüchten
Schlüsselwörter Kiwi · Gesamtphenolgehalt · Radikalfänger-Eigenschaft · Farbe · Trocknung
Introduction
Recently, the necessity of high quality fast-dried foods is leading to a renewed interest in drying operations (Maskan
2001a). Hot air drying has been used as a simple and com-mon drying method for the drying of vegetables and fruits (Ogura 1993; Leonid et al. 2006; Orikasa et al. 2008). Maskan (2001b) reported the drying characteristics of sliced kiwifruit, such as drying rate, shrinkage and rehydration ca-pacities. Drying studies are usually performed at constant
Can Ertekin
ertekin@akdeniz.edu.tr
1 Department of Agricultural Machinery, Faculty of
Agriculture, University of Selçuk, 42031 Konya, Turkey
2 Department of Food Engineering, Faculty of Agriculture,
University of Selcuk, 42031 Konya, Turkey
3 Department of Agricultural Machinery, Faculty of
Agriculture, University of Akdeniz, 07070 Antalya, Turkey
drying conditions of temperature, velocity and humidity us-ing a thin layer of samples. Kiwi fruit (Actinidia deliciosa) is well known as a nutritious fruit due to its high contents of ascorbic acid, folic acid and antioxidants (Martin2003). Several vegetable and fruits contain significant levels of biologically active components with physiological and bio-chemical functions which benefit human health. Fruits and vegetables are also the source of these substances. Fruit and vegetables represent a major source of dietary antiox-idants (Tavarini et al.2008). The fruit of Actinidia species is known worldwide as kiwi fruit, appreciated for its sweet, slightly acidic flesh and high nutritional value, especially due its high content in vitamin C (Ferguson and McRae
1991; Salinero et al. 2009). Kiwi fruits are rich in many vitamins, flavonoids and minerals. In particular, they con-tain a high amount of Vitamin C (more than oranges), as much potassium as bananas and a good amount of beta-carotene. Vitamin C is a water-soluble antioxidant that has been proven to protect our body from free radicals, dramat-ically improving the health of individuals who consumed it
regularly against all kinds of disease, from cardiovascular problems to cancer and obesity (Anonymous2010). The aim of this work was to evaluate the influence of drying pa-rameters on drying characteristics, several kiwi fruit quality attributes, such as colorimetric differences, antioxidant ca-pacity and total phenols and thin layer drying model.
Material and Method
Ripened kiwi fruits (about 10 kg) were purchased from lo-cal market in Konya, Turkey. The fruit samples were put into plastic bags for transport to the laboratory. The fruits were dried under the air circulation, and cleaned in an air screen cleaner to remove all foreign matters such as dust, dirt and chaff as well as immature. The initial moisture con-tent of fruits was determined by using a standard method (Brusewitz1975). The remaining material was kept in cold storage until use.
Laboratory Dryer
Drying experiments were performed in a modified labora-tory dryer manufactured in the Department of Farm Ma-chinery, Faculty of Agriculture, Selcuk University, Konya, Turkey. A schematic view of the experimental arrangement is shown in Fig.1.
The dryer consists of three basic units: a fan providing desired drying air velocity, electrical heaters controlling the temperature of drying air and drying chamber. The airflow rate for drying is kept at the desired level by arranging the cycle number of the electrical motor. The air is heated to the desired dry bulb temperature by a heater inside the air channel. The heater comprises four independently operated circuit elements, each of 1000 W power. A resistance con-nected in series to one of these circuit elements turns it on or off according to changes in temperature via the tempera-ture control unit, thus adjusting the temperatempera-ture to maintain the same level during the experiments. The changes in the
Fig. 1 Schematic view of the experimantel arrengament: 1, fan; 2, damper mechanism; 3, diffuser; 4, heater (4 × 1000 w); 5, variator; 6, relative humidity measurement and control unit; 7, temperature measurement and control unit; 8, measurement point of temperature, velocity and relative humidity; 9, drying region; 10, bucket; 11, isola-tion; 12, air velocity adjustment handle
weight of drying material were determined using an electric balance having an accuracy of 0.01 g.
Experimental Procedure
Thin layers of kiwi were dried at drying air temperatures of 50, 60, 70 and 80 °C, drying air velocities of 2.0 and 3.5 m s–1 and slice thicknesses of 10 and 15 mm. Moisture
content was determined by drying the samples at 70 °C in a vacuum oven until the weight became constant. The air velocity and temperature was measured by a hot wire anemometer with an accuracy of 0.1% and thermocouples, respectively.
Extraction
The samples were grounded and extracted in ethanol (1 g sample/10 ml ethanol). The extraction duration were 24 h. After filtration, the filtrate were evaporated under vacuum less than 45°C to obtain the extract.
Total Phenolic Content
Folin-Ciocalteu colorimetric method were applied and the results were expressed as mg GAE/L extract (Slinkard and Singelton1977).
Phenolic content was determined by the Folin-Ciocalteu method by using Gallic acid as a standard for the calibration curve (Slinkard and Singelton1977). Results were calcu-lated and expressed as milligrams of Gallic acid equivalent (GAE) per liter of extract. Samples diluted 1:10 were added to Folin-Ciocalteu reagent (diluted 1:10) to obtain the ratio 1:5 (v/v) after 1 min and before adding sodium carbonate solution (700 mmol/L) and incubating for 8 min (2:3 v/v). Results were read at 760 nm after 2 h.
Free Radical Scavenging Activity
Free radical scavenging activity of the diluted extracts (0.4 g/ml) were determined by DPPH method and the re-sults were expressed as percent inhibition of 1.1-diphenyl-2-picrylhydrazyl (Gyamfi et al.1999).
Colour Measurement
Colour was measured using a Minolta C.M. 2002 spectro-colourimeter with specular component included, C illumi-nant and an observer with an angle of 2°, using CIE L*a*b* coordinates.
Evaluation of colour changes of the product were done by using the total chromatic aberration (E*), chrome (C*) and hue angle (H*) (Soysal et al. 2005). The changes in each colour parameters were calculated as follows;
Table 1 Mathematical models applied to the drying curves
Model Name
MR = exp(–kt) Newton Doymaz2012
MR = exp(–ktn) Page Kumar et al.2012
MR = exp[(–kt)n] Modified Page I Demiray and Tulek2012
MR = exp[–(kt)n] Modified Page II Yi et al.2012
MR = a exp(–kt) Henderson ve Pabis Evin2012
MR = a exp(–kt) + c Logarithmic Kurozawa et al.2012
MR = a exp(–kt) + b exp(–k1t) Two term Chowdhury et al.2011
MR = a exp(–kt) + (1 – a)exp(–kat) Two term exponential Bahloul et al.2011
MR = 1 + at + bt2 Wang ve Singh Therdthai et al.2011
t = a ln(MR) + b(ln(MR))2 Thompson Akpinar2011
MR = a exp(–kt) + (1 – a)exp(–kbt) Diffision approximation Dissa et al.2011
MR = a exp(–kt) + (1 – a)exp(–gt) Verma et al. Fan et al.2011
MR = a exp(–kt) + b exp(–gt) + c exp(–ht) Modified Henderson ve Pabis Balasubramanian et al.2011
MR = a exp(–ktn) + bt Midilli et al. Akhondi et al.2011
E=q.L/2+.a/2+.b/2 (1) L=L sample−Lstandard (2) a=a sample−astandard (3) b=b sample−bstandard (4) H = tan−1b a (5) C=pa2+b2 (6) where as;
a*: Red colour deviation b*: Yellow colour deviation L*:Colour brightness deviation E*:Total chromatic aberration H*: Metric hue
H*:Metric hue angle deviation C*: Metric chrome
C*:Chrome aberration
Fresh kiwifruits were used as the reference and a larger E denotes greater colour change from the reference ma-terial. Browning index (BI) represents the purity of brown colour and is considered as an important parameter related with browning and could be calculated as follows (Maskan
2001a);
x = .a+ 1.75L/
.5.645L+a− 3.012b/ (7)
BI = Œ100 .x − 0.31/
0.17 (8)
Mathematical Modelling of Drying Curves
Mathematical modeling of convection drying has been widely and effectively used for analysis of drying of differ-ent agricultural products. In order to establish the equations of mass transfer during convection drying, the following considerations were taken into account; the process was isothermal, the main mass transfer mechanism is of dif-fusional nature and sample deformations and shrinkage during drying were negligible. The convection drying of vegetables and fruits occurs in the falling rate drying period, thus well-known semi emprical and empirical models could be applied to the drying data. The widely used models are shown in Table1. The drying curves obtained were fitted with fourteen different thin layer drying moisture ratio models. However, the moisture ratio (MR) was simplified to M/Moinstead of the (M – Me) / (Mo– Me); where M is the
moisture content in decimal dry basis at any time t, Mois
the initial moisture content in decimal dry basis, Meis the
equilibrium moisture content in decimal dry basis (Mengec and Ertekin2006; Akpinar2010; Unal and Sacilik2011).
The reduced χ-square, root mean square error (RMSE) and modeling efficiency (EF) were used as criteria for ad-equacy of the fit. The lower the values of the reduced χ-square, the better the goodness of the fit. The RMSE gives the deviation between the predicted and experimental values and it is required to reach zero. The EF also gives the ability of the model and its highest value is 1. These statis-tical analysis values can be calculated as follows (Menges and Ertekin2006; Corzo et al.2010; Kayisoglu and Ertekin
2011; Hii et al.2009; Ojediran and Raji2011);
X2= PN i=n MRexp;i− MRpre;i2 N − n (9)
RMSE = sPn i=1 MRpre;i− MRexp;i2 N (10) EF = Pn i=1
MRi;exp− MRi;expmean2−Pni=1MRi;pre− MRi;exp2 Pn
i=1MRi;exp− MRi;expmean
2
(11)
where MRexp,iis the ith experimental moisture ratio, MRpre,i
is the ithpredicted moisture ratio, N is the number of
obser-vations, n is the number of constants in the drying model and MRexp meanis the mean value of experimental moisture
ratio.
Initial selection of the drying models were done using above mentioned criteria. The drying constants and coef-ficients of the best suited model were then related to the drying air temperature, velocity and slice thickness by mul-tiple combinations of the different equations as the simple linear, logaritmic, power, exponential and arhenius type;
Linear: Y = a + bX (12) Logarithmic: Y = a + b ln.X/ (13) Power: Y = aXb (14) Exponential: Y = a exp.bX/ (15) Arhenius: Y = a exp.b=X/ (16)
The drying model with lowest reduced chi-square and RMSE and highest EF was chosen as the best model de-scribing the thin layer drying behaviour of carrots.
Moisture Diffusion and Activation Energy
The most widely studied theoretical model in thin layer drying of various foods is given by the solution of Fick’s second law. The solution of Fick’s second law for diffusion
out of spheres may be used to fit the experimental drying data (Crank1975): MR = M − Me Mo−Me = 6 2 1 X n=1 1 n2 exp −n 22D efft r2 (17)
Eq. 17 assumes that the effective diffusivity (Deff) is
constant and shrinkage of the sample is negligible. For long drying times (setting n = 1), several researchers demon-strated that Eq.17could be further simplified to a straight-line equation as (Nuh and Brinkworth 1997; Pala et al.
1996; Riva and Peri1986):
ln.MR/ = ln 6 2 − 2D eff r2 t (18)
The effective diffusivities were determined using the method of slopes. Effective diffusivities are typically de-termined by plotting experimental drying data in terms of ln (MR) versus time. From Eq.18, a plot of ln (MR) versus time gives a straight line with a slope (k1) of:
k1= 2D
eff
r2 (19)
In order to obtain the influence of temperature on the effective diffusivity, ln (Deff) are plotted versus 1/T. The
plot was found to be essentially a straight line in the range of temperatures investigated, indicating Arrhenius depen-dence: Deff=Doexp − Ea R .T + 273.15/ (20)
where Do is the pre-exponential factor of Arrhenius
equa-tion (m2s–1), E
a is the activation energy (kJ mol–1), T is
the temperature of air (°C) and R is the gas constant (kJ mol–1K–1).
Results and Discussion
The mean initial moisture content of the kiwi fruit was 83.57% (w.b.) and dried to a final moisture content of 20% (w.b.). The effects of drying parameters can be seen in the results (Fig. 2). The drying time decreased by increasing drying air temperature at different drying air velocities and slice thicknesses. It was ranged between 2.6 h and 12.1 h for different conditions. For example, at constant drying air velocity of 2 m s–1and slice thickness of 10 mm, drying
time was between 2.8 and 7.9 h at drying air temperature of 80 °C and 50 °C, respectively. This drying time was also decreased by increasing drying air velocity. While it was
Fig. 2 Moisture content changes during drying process
0,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00 90,00 0,00 1,00 2,00 3,00 4,00 5,00 6,00 7,00 8,00 9,00 10,00 ) %. b.. w( tn et no c er ut si o M Drying me (h) 50 C 60 C 70 C 80 C
Drying air velocity: 2.0 m.s-1
Slice thickness: 10 mm 0,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00 90,00 0,00 1,00 2,00 3,00 4,00 5,00 6,00 7,00 8,00 9,00 10,00 ) %. b. w( tn et no c er ut si o M Drying me (h) 50 C 60 C 70 C 80 C
Drying air velocity: 3.5 m.s-1
Slice thickness: 10 mm
7.9 h at drying air velocity of 2.0 m s–1, it was decreased to
7.3 h at 3.5 m s–1at constant drying air temperature of 50 °C
and slice thickness of 10 mm. These values were 2.8 and 2.6 h for drying air temperature of 80 °C at the same condi-tions, respectively. When the slice thickness of the product were increased from 10 to 15 mm, for example, drying time increased from 7.3 h to 11.0 h at 50 °C and from 2.6 to 4.6 h at 80 °C at constant drying air velocity of 3.5 m s–1.
Kaya et al. (2008) investigated the effects of drying air
temperature, velocity and relative humidity on drying time in convective drying process of kiwi. The drying time was diminished by both increasing drying air temperature and velocity and also decreasing drying air relative humidity. Kaya et al. (2010) determined the effect of drying air tem-perature, relative humidity and velocity in convective kiwi drying process at constant slice thickness of 7 mm. The dry-ing time influenced by drydry-ing air temperature, increasdry-ing temperature from 35 to 65 °C, decreased drying time about
62.5%. In addition, decreasing relative humidity from 85 to 40%, decreased drying time about 56.4%. Drying air veloc-ity changes does not have a considerable effect on drying time. Increasing velocity from 0.3 to 0.9 m s–1 decreased
drying time from 240 to 224 min. Maskan (2001b) deter-mined the drying time as 24 min, 225 min and 136.5 min for microwave, hot air and hot air and microwave combination drying methods for kiwi, respectively. Mohammadi et al. (2009), dried kiwi fruits under convection at constant dry-ing air velocity of 1.5 m s–1and slice thickness of 4 mm by
changing drying air temperature. The drying process took 450, 340, 270, 210 and 180 min for drying air temperatures of 40, 50, 60, 70 and 80 °C, respectively. Doymaz (2009), found drying time as 390 min at drying air temperature of 60 °C and as 570 min at 50 °C for convective kiwi drying.
Total phenol content of the dried kiwi fruits were ranged from 2.03–2.75 mg GAE/L. Free radical scavenging activity were varied from 26.04 to 40.92%. According to the results, the effect of different drying air temperatures were not very effective on the total phenol content of kiwi fruits. But, the free radical scavenging activity were variable. So, no meaningfull correlation were found between the free rad-ical scavenging activity and the total phenol content. The highest free radical scavenging activity were obtained for the samples dried at 50 °C, 3.5 m s–1air velocity and 15 mm
slice thickness.
Results on antioxidant activities of dried kiwi fruits were in agreement with the reported literature data (Fisk et al.
2006; Jung et al. 2005; Leong and Shui2002; Park et al.
2006a,2006b). Lee and Kader (2000) stated that the con-tent of phytochemical substances is influenced by numerous factors such as ripening time, genotype, cultivation tech-niques, climatic conditions that occur during the pre-har-vest period but also the operations carried out during the post-harvest storage are very important. Imeh and Khokhar (2002) also reported various factors as agronomic, genomic, pre- and post-harvest conditions and processing, may affect the chemical composition of plant foods and they may have a significant role in determining the phenolic composition and the bioactivity of these compounds.
Kiwi fruits are recommended by most investigators, due to high antioxidant activity (Jayaprakasam et al.2005; Jung et al. 2005; Leontowicz et al. 2007; Scalzo et al.
2005). Some researchers reported high correlation coef-ficient between the Folin-Ciocalteu assay and the DPPH radical scavenging assay, also others (Katsube et al.2004; Park et al.2008,.2011; Krupa et al. 2011). According to the prediction of some researchers, it may be due to the similarity of the mechanism of the both methods (Lim et al.
2007; Ozgen et al. 2006; Park et al. 2008). Scalzo et al. (2005) determined the phenolic content and antioxidant ca-pacity of some fruits. They reported the hydrophilic Trolox equivallent antioxidant activity for the kiwi fruit as 2.47 µM
TE/g FW. Tavarini et al. (2008) studied on influences of harvest time and storage on the quality indices and nutri-tional content of kiwifruit. They reported that the harvest time had not induce any change in antioxidant capacity and phenol content. Toledo et al. (2008) studied on some bioac-tivity indices of four kiwifruit cultivars. The range of total polyphenols (mg gallic acid equivalent (GAE)/100 g fresh weight (FW)) for the studies samples were ranged from 260.8 to 384.2. Park et al. (2011) studied on four different cultivars of kiwifruit (‘Hayward’, ‘Daeheung’, ‘Haenam’ and ‘Bidan’) to find the best for human consumption. They reported the polyphenols and ascorbic acids were highest in ‘Bidan’ (25.9 ± 1.3 mgGAE/g and 152 ± 10.4 mg/g DW, re-spectively). The ‘Bidan’ cultivar was also found to have the highest antioxidant activity (mM TE/gDW) among the oth-ers, in terms of CUPRAC (121 ± 5.8), ABTS (109 ± 11.2), DPPH (102 ± 6.6) and FRAP (94 ± 4.7) radical scavenging assays. Park et al. (2008) evaluated ethylene treatment of kiwi fruits on antioxidant activity and protein content. They reported the contents of phenolic compounds (mg GAE/g DW) in the ET kiwi fruit extracts during 10 days treatment were in the range from 14.92 ± 1.51 to 26.70 ± 2.87 and for flavonoids (mg catechin equivalents/g) from 2.09 ± 0.31 to 3.25 ± 0.38 and in the AT kiwifruit extracts for phenolics (mg GAE/g DW) were in the range from 25.17 ± 2.49 to 24.37 ± 1.77 and for flavonoids (mg catechin equivalents/g) from 2.27 ± 0.27 to 1.99 ± 0.19. Femenia et al. (2009) eval-uated the effects of air-drying temperature on the cell wall components of fresh kiwifruits at different degrees of ripening, unripe, half-ripe and ripe samples. They reported that half-ripe kiwifruit samples exhibited the higher quality values when samples corresponding to the three ripening stages were dehydrated at the same air-drying temperature. According to their results, they suggested the importance of taking into consideration the stage of ripening of the fresh kiwifruits before being processed, in order to preserve the physicochemical and nutritional properties of the cell wall polymers and, therefore of the dietary fiber, present in the kiwifruit.
The initial L* value of kiwi samples was 53.98, which is the indicator of the brightness. The L* values affected from drying air temperature and increased by increasing temper-ature at all examined drying air velocities and slice thick-nesses. In addition, the L* value was higher for the sam-ples dried at drying air velocity of 2.0 m s–1in comparison
to 3.5 m s–1at slice thickness of 10 mm, while it was higher
for the samples dried at drying air velocity of 3.5 m s–1at
slice thickness of 15 mm. While the initial a* value was –8.11, it was ranged between –8.22 and 3.47 after drying process. There was an increase by the increase of the ap-plied drying air temperature. And also, by the increase of the drying air velocity and the slice thickness of the sam-ples, the a* value were increased. The initial b* value was
Ta b le 2 M at h em at ical m odel s appl ie d to d ryi n g curv es (for sl ic e thi ckness of 10 m m ) M odel equat io n V el oci ty ms –1 50 °C 60 °C 70 °C 80 °C RMSE χ 2 EF RM SE χ 2 EF RM SE χ 2 EF RM SE χ 2 EF Ne wton 2. 0 0 .034096 0. 001195 0. 979663 0. 028822 0. 000860 0. 986381 0. 020747 0. 000452 0. 993732 0. 011072 0. 000130 0. 998385 3. 5 0 .036646 0. 001380 0. 975773 0. 036521 0. 001389 0. 977791 0. 022951 0. 000553 0. 992125 0. 021440 0. 000488 0. 993512 Pa g e 2. 0 0 .005880 0. 000037 0. 999395 0. 005711 0. 000035 0. 999465 0. 007348 0. 000060 0. 999214 0. 006064 0. 000042 0. 999515 3. 5 0 .007858 0. 000066 0. 998886 0. 005237 0. 000030 0. 999543 0. 006860 0. 000052 0. 999296 0. 004655 0. 000025 0. 999694 M odi fied P age I 2. 0 0 .034096 0. 001229 0. 979663 0. 028822 0. 000892 0. 986381 0. 020747 0. 000476 0. 993732 0. 011072 0. 000139 0. 998385 3. 5 0 .036646 0. 001420 0. 975773 0. 036521 0. 001450 0. 977791 0. 022951 0. 000582 0. 992125 0. 021440 0. 000521 0. 993512 M odi fied P age II 2. 0 0 .005881 0. 000037 0. 999395 0. 005713 0. 000035 0. 999465 0. 007348 0. 000060 0. 999214 0. 006065 0. 000042 0. 999515 3. 5 0 .007858 0. 000065 0. 998886 0. 005237 0. 000030 0. 999543 0. 006860 0. 000052 0. 999296 0. 004655 0. 000025 0. 999694 Henderson v e Pa b is 2. 0 0 .022432 0. 000532 0. 991197 0. 020597 0. 000456 0. 993045 0. 015983 0. 000282 0. 996280 0. 009425 0. 000101 0. 998830 3. 5 0 .024006 0. 000609 0. 989604 0. 026991 0. 000792 0. 987869 0. 018678 0. 000386 0. 994785 0. 018396 0. 000384 0. 995223 Logari th m ic 2. 0 0 .017699 0. 000341 0. 994520 0. 016733 0. 000312 0. 995410 0. 013847 0. 000224 0. 997208 0. 008970 0. 000098 0. 998940 3. 5 0 .019679 0. 000421 0. 993014 0. 020988 0. 000501 0. 992665 0. 015360 0. 000275 0. 996473 0. 014362 0. 000251 0. 997089 Tw o -t er m 2. 0 0 .022429 0. 000564 0. 991199 0. 007046 0. 000058 0. 999186 0. 012061 0. 000180 0. 997882 0. 008091 0. 000086 0. 999138 3. 5 0 .008996 0. 000091 0. 998540 0. 010563 0. 000133 0. 998142 0. 009037 0. 000101 0. 998779 0. 008277 0. 000090 0. 999033 Tw o -T er m exponent ia l 2. 0 0 .010681 0. 000121 0. 998004 0. 006384 0. 000044 0. 999332 0. 003637 0. 000015 0. 999807 0. 008660 0. 000085 0. 999012 3. 5 0 .014357 0. 000218 0. 996281 0. 013117 0. 000187 0. 997135 0. 005216 0. 000030 0. 999593 0. 003561 0. 000014 0. 999821 W ang v e Si ngh 2. 0 0 .081111 0. 006955 0. 884909 0. 082583 0. 007325 0. 888192 0. 075006 0. 006218 0. 918076 0. 076298 0. 006598 0. 923302 3. 5 0 .088539 0. 008287 0. 858576 0. 087241 0. 008273 0. 873267 0. 091753 0. 009305 0. 874140 0. 091373 0. 009462 0. 882153 Thom pson 2. 0 0 .135101 0. 019295 0. 997438 0. 099494 0. 010632 0. 997737 0. 084148 0. 007826 0. 996910 0. 087981 0. 008773 0. 994840 3. 5 0 .115554 0. 014116 0. 998126 0. 083064 0. 007500 0. 997877 0. 151438 0. 025348 0. 989993 0. 082144 0. 007647 0. 995502 Dif fus ion approxi m at io n 2. 0 0 .003928 0. 000017 0. 999730 0. 002179 0. 000005 0. 999922 0. 002963 0. 000010 0. 999872 0. 003372 0. 000014 0. 999850 3. 5 0 .004305 0. 000020 0. 999666 0. 002322 0. 000006 0. 999910 0. 004296 0. 000022 0. 999724 0. 003392 0. 000014 0. 999838 Ve rm a et al . 2. 0 0 .004791 0. 000026 0. 999598 0. 010329 0. 000119 0. 998251 0. 009936 0. 000115 0. 998563 0. 011073 0. 000149 0. 998385 3. 5 0 .036646 0. 001461 0. 975773 0. 036547 0. 001518 0. 977759 0. 008470 0. 000084 0. 998928 0. 008448 0. 000087 0. 998993 M odi fied H en-derson and P api s 2. 0 0 .004023 0. 000019 0. 999717 0. 002230 0. 000006 0. 999918 0. 002969 0. 000012 0. 999872 0. 007032 0. 000076 0. 999349 3. 5 0 .006414 0. 000049 0. 999258 0. 009911 0. 000129 0. 998364 0. 008107 0. 000092 0. 999017 0. 010748 0. 000179 0. 998370 Mid illi et al . 2. 0 0 .003438 0. 000013 0. 999793 0. 004171 0. 000020 0. 999715 0. 006451 0. 000051 0. 999394 0. 005465 0. 000039 0. 999606 3. 5 0 .005711 0. 000037 0. 999412 0. 002264 0. 000006 0. 999915 0. 006666 0. 000055 0. 999336 0. 004584 0. 000028 0. 999703
Ta b le 3 M at h em at ical m odel s appl ie d to d ryi n g curv es (for sl ic e thi ckness of 15 m m ) M odel V el oci ty (m /s ) 50 0C6 0 0C7 0 0C8 0 0C RMSE χ 2 EF RM SE χ 2 EF RM SE χ 2 EF RM SE χ 2 EF Ne wton 2. 0 0 .032291 0. 001063 0. 976143 0. 040040 0. 001648 0. 970988 0. 029882 0. 000925 0. 985195 0. 015874 0. 000265 0. 996386 3. 5 0 .043393 0. 001922 0. 963196 0. 036285 0. 001358 0. 976866 0. 029059 0. 000880 0. 986457 0. 026222 0. 000716 0. 988824 Pa g e 2. 0 0 .006487 0. 000044 0. 999037 0. 008771 0. 000081 0. 9986078 0. 004482 0. 000022 0. 999667 0. 002385 0. 000006 0. 999918 3. 5 0 .006926 0. 000050 0. 999062 0. 003756 0. 0000150 0. 9997520 0. 003444 0. 000013 0. 999810 0. 003928 0. 000017 0. 999749 M odi fied P age I 2. 0 0 .032291 0. 001084 0. 976143 0. 040040 0. 001695 0. 9709881 0. 029882 0. 000959 0. 985195 0. 015874 0. 000279 0. 996386 3. 5 0 .043393 0. 001963 0. 963196 0. 036285 0. 0014015 0. 9768660 0. 029059 0. 000918 0. 986457 0. 026222 0. 000747 0. 988824 M odi fied P age II 2. 0 0 .006487 0. 000044 0. 999037 0. 008772 0. 0000813 0. 9986076 0. 004483 0. 000022 0. 999667 0. 002385 0. 000006 0. 999918 3. 5 0 .006930 0. 000050 0. 999061 0. 003757 0. 0000150 0. 9997520 0. 003445 0. 000013 0. 999810 0. 003928 0. 000017 0. 999749 Henderson v e Pa p is 2. 0 0 .019329 0. 000388 0. 991452 0. 025657 0. 0006959 0. 9880871 0. 021440 0. 000494 0. 992379 0. 012711 0. 000179 0. 997683 3. 5 0 .026796 0. 000749 0. 985966 0. 025799 0. 000709 0. 988305 0. 021878 0. 000520 0. 992323 0. 021870 0. 000520 0. 992225 Logari th m ic 2. 0 0 .016227 0. 000279 0. 993975 0. 021119 0. 000485 0. 991929 0. 015747 0. 000277 0. 995889 0. 008928 0. 000093 0. 998857 3. 5 0 .021665 0. 000500 0. 990826 0. 019042 0. 000399 0. 9936288 0. 015142 0. 000261 0. 996323 0. 013827 0. 000217 0. 996892 Tw o -t er m 2. 0 0 .002368 0. 000006 0. 999872 0. 003761 0. 000016 0. 9997440 0. 004192 0. 000020 0. 999709 0. 004650 0. 000027 0. 999690 3. 5 0 .003812 0. 000016 0. 999716 0. 003737 0. 000016 0. 9997545 0. 004994 0. 000030 0. 999600 0. 006155 0. 000045 0. 999384 Tw o -T er m exponent ia l 2. 0 0 .008905 0. 000082 0. 998186 0. 016219 0. 000278 0. 995240 0. 007770 0. 000065 0. 998999 0. 005777 0. 000037 0. 999521 3. 5 0 .019881 0. 000412 0. 992275 0. 012678 0. 0001711 0. 9971756 0. 007274 0. 000058 0. 999151 0. 007776 0. 000072 0. 999017 W ang v e Si ngh 2. 0 0 .080361 0. 006711 0. 852240 0. 081388 0. 0070025 0. 8801269 0. 082322 0. 007279 0. 887642 0. 077182 0. 006584 0. 914569 3. 5 0 .093112 0. 009039 0. 830545 0. 088207 0. 0082825 0. 8632869 0. 082106 0. 007328 0. 891878 0. 105575 0. 013269 0. 818825 Thom pson 2. 0 0 .192835 0. 038644 0. 995212 0. 170918 0. 030882 0. 995900 0. 073601 0. 005818 0. 998762 0. 039335 0. 001710 0. 999325 3. 5 0 .237173 0. 058645 0. 995500 0. 121616 0. 015745 0. 997390 0. 038978 0. 001651 0. 999566 0. 127289 0. 017611 0. 995371 Dif fus ion approxi m at io n 2. 0 0 .005791 0. 000036 0. 999233 0. 006000 0. 000039 0. 999349 0. 006452 0. 000046 0. 999310 0. 003033 0. 0000107 0. 999868 3. 5 0 .004906 0. 000026 0. 999530 0. 005073 0. 000028 0. 999548 0. 004911 0. 000027 0. 999613 0. 005707 0. 0000370 0. 999471 Ve rm a et al . 2. 0 0 .002344 0. 000006 0. 999874 0. 003772 0. 000016 0. 999743 0. 004152 0. 000019 0. 999714 0. 003049 0. 0000108 0. 999867 3. 5 0 .003873 0. 000016 0. 999707 0. 003653 0. 000015 0. 999766 0. 004893 0. 000027 0. 999616 0. 005733 0. 0000373 0. 999466 M odi fied H ender-son and P api s 2. 0 0 .003422 0. 000013 0. 999732 0. 009533 0. 000109 0. 998355 0. 005779 0. 000042 0. 999446 0. 005470 0. 0000419 0. 999571 3. 5 0 .005447 0. 000034 0. 999420 0. 004752 0. 000028 0. 999603 0. 008172 0. 000088 0. 998929 0. 007044 0. 000065 0. 999194 Mid illi et al . 2. 0 0 .002197 0. 000005 0. 999890 0. 005496 0. 000034 0. 999453 0. 004080 0. 000019 0. 999724 0. 002322 0. 000007 0. 999923 3. 5 0 .001756 0. 000003 0. 999940 0. 002178 0. 000005 0. 999917 0. 003326 0. 000013 0. 999823 0. 001980 0. 000005 0. 999936
Table 4 Statistical results of Midilli et al. model and its constants and coefficients for kiwi under different drying conditions (for slice thicknesses of 10 and 15 mm) Drying air temperature (°C) Drying air velocity (m s–1) a k n b RMSE χ2 EF MR = a exp(–k tn) + b t Slice thickness: 10 mm 50 2.0 0.997261 0.561468 0.771128 –0.002060 0.003438 0.000013 0.999793 3.5 0.992737 0.608215 0.756907 –0.002050 0.005711 0.000037 0.999412 60 2.0 0.996597 0.715519 0.80289 –0.001860 0.004171 0.000020 0.999715 3.5 0.998404 0.811888 0.750891 –0.003020 0.002264 0.000006 0.999915 70 2.0 0.994533 0.914878 0.858799 –0.002020 0.006451 0.000051 0.999394 3.5 0.995371 1.072627 0.845465 –0.000630 0.006666 0.000055 0.999336 80 2.0 0.996725 1.197736 0.923375 –0.001650 0.005465 0.000039 0.999606 3.5 0.996904 1.291324 0.853392 –0.000130 0.004584 0.000028 0.999703 Slice thickness: 15 mm 50 2.0 0.996844 0.420822 0.775229 –0.001802 0.002197 0.000005 0.999890 3.5 0.998940 0.506539 0.707283 –0.002275 0.001756 0.000003 0.999940 60 2.0 0.996840 0.559801 0.730169 –0.003245 0.005496 0.000034 0.999453 3.5 1.001012 0.658451 0.761258 –0.001483 0.002178 0.000005 0.999917 70 2.0 0.995268 0.713522 0.810402 –0.000709 0.004080 0.000019 0.999724 3.5 0.996314 0.809190 0.818919 –0.000189 0.003326 0.000013 0.999823 80 2.0 0.997782 0.951431 0.897939 0.000196 0.002322 0.000007 0.999923 3.5 0.998939 1.017905 0.834960 0.001543 0.001980 0.000005 0.999936
Table 5 Effect of drying air temperature, velocity and thickness on midilli et al. Model constants and coefficients
MR = [1.0322025 exp(–0.827279/L)] exp[[–0.551205 exp(2.791808/L)] t[–4.824614 + 1.372992 ln(T)]] + [0.006679 + (–0.008447 ln(V))] t
Drying air temperature (°C) Drying air velocity (m/s) RMSE χ2 EF
50 2.0 0.065378 0.004473 0.923936 3.5 0.044745 0.002100 0.962226 60 2.0 0.041608 0.001843 0.970104 3.5 0.021841 0.000512 0.991813 70 2.0 0.032958 0.001181 0.982981 3.5 0.050365 0.002773 0.959758 80 2.0 0.079890 0.007133 0.911972 3.5 0.088103 0.008579 0.900926
17.78 and generally it was increased after drying applica-tion. It was ranged between 22.24 and 40.37. While the chroma value were ranged between 22.55 and 40.38 after drying process, the hue angle between –89.93 and 89.39. The browning index also changed between 47.35 and 89.03. Increasing drying air velocity decreased the chroma values and browning index. While the lowest total colour change value was 6.42 at drying air temperature of 50 °C, and veloc-ity of 3.5 m s–1, the highest value was 29.10 at temperature
of 70 °C and 2.0 m s–1at the same slice thickness of 15 mm.
Mohammadi (2008a) examined the effect of drying air tem-perature in convective drying process of kiwi on L*, a*, b*, total colour change (E*), chroma and hue. They were all affected from the drying temperature. When temperature increased, a*, E* and BI increased but L*, b*, chroma and
hue angle decreased. According to the browning index, dry-ing process caused more brown compounds in kiwi fruit. Maskan (2001a) dried kiwi fruit with hot air, microwave and hot air with combination of microwave finishing dry-ing methods. Accorddry-ing to the results, all colour parameters were affected from the drying method and also drying con-ditions. While the L* and b* value decreased with drying time, a* value increased. The fresh kiwi fruits had L* value of 47.0, a* value of –2.2 and b* value of 17.8. The L* value decreased to 30.0, 39.8 and 33.4 in microwave, hot air and hot air with microwave drying, respectively. The b* value decreased to 15.3, 16.0 and 14.1 and a* value increased to 4.3, 2.7 and 6.5 for the same drying methods. Increasement of L* value means that, its colour turning darker. Usage of microwave increased the rate of colour deterioration.
Drying air velocity:2.0 m/s, 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00 0 2 4 6 8 10 12 14
Drying time (hours)
oit ar er ut si o M 50 C-Experimental 50 C-Predicted 60 C-Experimental 60 C-Predicted 70 C-Experimental 70 C-Predicted 80 C-Experimental 80 C-Predicted
Fig. 3 Experimental and predicted moisture ratio during drying process for slice thickness of 15 mm and drying air velocity of 2 m/s
Drying air velocity: 3,5 m/s
0,00
0,10
0,20
0,30
0,40
0,50
0,60
0,70
0,80
0,90
1,00
0
2
4
6
8
10
12
Drying time (hours)
oi t ar er ut si o M 50 C-Experimental 50 C-Predicted 60 C-Experimental 60 C-Predicted 70 C-Experimental 70 C-Predicted 80 C-Experimental 80 C-Predicted
Fig. 4 Experimental and predicted moisture ratio during drying process for slice thickness of 15 mm and drying air velocity of 3.5 m/s cording the the BI results, microwave drying produced more
brown products.
For modeling purpose, the experimental moisture content data on a dry weight basis were used and converted to the MR value. These moisture ratio data at any time of drying process obtained at different drying air temperature, veloc-ity and slice thickness conditions were fitted against the drying time. Fourteen thin layer drying models were com-pared according to their statistical results such as RMSE, chi-square and EF (Table 2and 3). The results indicated that, the lowest values of RMSE and chi-square and
high-est values of EF were obtained at Midilli et al. model for all different drying conditions. This model could be shown as;
MR =a exp.−ktn/ + bt (21)
where MR is the moisture ratio, k drying rate constant (h–1),
t time (h), a, n and b are experimental constants. RMSE var-ied between 0.002264 and 0.006666, the reduced chi-square between 0.000006 and 0.000055, EF between 0.999336 and 0.999915 for the slice thickness of 10 mm,
respec-Fig. 5 Effective diffusivity at different drying conditions
tively. These statistical values were between 0.001756 and 0.005496, 0.000003 and 0.000034, 0.999453 and 0.999940 for slice thickness of 15 mm, respectively. Thus, the Midilli et al. model may be accepted to represent the drying be-haviour of kiwi fruits. This model can be used satisfactorily to predict the experimental moisture ratio values.
When the Midilli et al. model was analysed according to the different drying air temperature (T), velocity (V) and slice thickness (L) conditions, individual constants could be obtained for different drying conditions (Table4). While the RMSE ranged between 0.002264 and 0.006666 for 10 mm, it was between 0.001756 and 0.005496 for 15 mm. The reduced chi-square values were between 0.000006 and 0.000055, 0.000003 and 0.000034 for the same slice thicknesses, respectively. The EF values were higher then 0.999 for all experimental conditions.
Further regressions were made to take into account the effect of drying air temperature, velocity and thickness on the Midilli et al. models constants and coefficients by mul-tiple regression analysis. When we examined the effect of all these drying conditions on Midilli et al. model, the RMSE values changed between 0.021841 and 0.088103 (Table 5). The reduced chi-square values were between 0.000512 and 0.008579 and 0.9942 and EF values between 0.900926 and 0.991813. Although RMSE and reduced chi square increased and EF values were decresed, these criteria are still in acceptable ranges for moisture ratio predictions. It can be seen from Figs.3and4that, this model was in good agreement with the experimental results under all dry-ing conditions for kiwi. Mohammadi (2008b) determined the Page model to explane convective drying behaviour of kiwi fruits. According to the results, chi-square ranged
be-tween 0.000065 and 0.000404, RMSE bebe-tween 0.000210 and 0.019261 and modeling efficiency between 0.995218 and 0.999269 in convective. They also showed the temper-ature effect on the Page model. Mohammadi (2009) exam-ined some different thin layer drying models such as New-ton, Page, Modified Page, Henderson and Pabis, Modified Henderson and Pabis, logarithmic, two term, two term expo-nential, Wang and Singh, diffusion approximation, Verma et al. and Midilli et al. models and compared them ac-cording to the reduced chi-square, RMSE and modeling efficiency. The best results were 0.99932 for modeling ef-ficiency, 0.03254 for RMSE and 0.00112 for reduced chi-square for Midilli et al. model. Maskan (2001b) described the drying behavior of kiwi with Newton model for mi-crowave and hot air drying process. Simal et al. (2005 ), dried kiwi fruit samples under convection at different drying air temperatures. Thin layer drying models such as New-ton, Page and Fick’s diffusional models were compared. According to the results Page model was superior to the others. Diamante et al. (2010 ) evaluated Page, logarith-mic, Henderson and Pabis and newly porposed thin layer drying models in convective drying of kiwifruits. The best model was the proposed new model and chi square were between 0.000067 and 0.000681, RMSE between 0.001256 and 0.004424 and mean relative percentage error (P) be-tween 5.4 and 37.2%. Ceylan et al. (2007) used heat pump dryer for kiwi fruits and evaluated Newton, Page Modi-fied Page, Henderson and Pabis, logarithmic and Wang and Singh models according to the determination coefficient and standard error of estimate. According to the results, Newton model could be used to describe drying behavior of kiwi fruits. Doymaz (2009) compared some different models such as Newton, Page, Modified Page, Henderson and Pabis, Modified Henderson and Pabis, logarithmic, two term, exponential two term, approximation of diffusion, Verma et al., Wang and Singh and Midilli et al. models according to determination coefficient, mean relative per-centage error, reduced chi-square and RMSE. While Mod-ified Henderson and Pabis model was the best one for 50 and 55 °C, Verma et al. model was the best for 60 °C. Kaya et al. (2010) evaluated Newton, Henderson and Pabis and two term models according to the determination coefficient, and two term model showed superiority for convective dry-ing of kiwi.
The drying process took place at the falling rate dry-ing stage. So, the governdry-ing factor of drydry-ing is diffusion for convective kiwi drying process. Effective diffusivity is changed between 2.63 × 10–10 and 1.23 × 10–9m2.s–1 for
different drying conditions. It was increased by increasing drying air temperature, velocity and also slice thickness (Fig. 5). These values are in the general range of 10–15
and 10–6m2.s–1 for food materials (Mujumdar 1995). The
for slice thickness of 10 mm, 28.51 and 32.64 kJ mol–1for
slice thickness of 15 mm at drying air velocities of 2.0 and 3.5 m s–1, respectively. It is clear that, while increasing
drying air velocity increased activation energy, increasing slice thickness decreased activation energy. Kaya et al. (2010) found that, the effective diffusion coefficient was increased by increasing drying air temperature and velocity and decreasing relative humidity. The effective diffusivity were between 0.59–6.6 × 10–10m2.s–1and activation energy
27.7–29.1 kJ/mol for convective drying of kiwi. Orikasa et al. (2008) used convective drying at different drying air temperatures and found diffusion coefficient between 3.79 × 10–12 and 7.53 × 10–12m2s–1and activation energy as
38.6 kJ mol–1 during the drying process. Doymaz (2009)
found the effective diffusivity between 1.7 × 10–10 and
2.2 × 10–10 m2/s and activation energy as 22.48 kJ mol–1for
convective drying of kiwi. Simal et al. (2005) found the effective diffusion coefficient between 3 × 10–10 m2.s–1at
30 °C and 17.7 × 10–10m2.s–1at 90 °C. The activation energy
was 27.0 kJ mol–1for convective drying of kiwi fruit. Acknowledgements The authors wish to thank Selcuk and Akdeniz Universities Scientific Research Project Units.
Conflict of interest H.O. Menge¸s, A. Ünver, M.M. Özcan, C. Ertekin and M.H. Sonmete declare that they have no competing interests.
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