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LWT - Food Science and Technology 64 (2015) 67e73

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LWT - Food Science and Technology

journal homepage: www.elsevier .com/locate/ lwt

Single layer drying kinetics of papaya amidst vertical and horizontalairflow

Patchimaporn Udomkun a, *, Dimitrios Argyropoulos a, Marcus Nagle a,Busarakorn Mahayothee b, Serm Janjai c, Joachim Müller a

a Universit€at Hohenheim (440e), Institute of Agricultural Engineering, Tropics and Subtropics Group, Stuttgart 70599, Germanyb Silpakorn University, Faculty of Engineering and Industrial Technology, Department of Food Technology, Nakhon Pathom 73000, Thailandc Silpakorn University, Faculty of Science, Department of Physics, Solar Energy Research Laboratory, Nakhon Pathom 73000, Thailand

a r t i c l e i n f o

Article history:Received 28 November 2014Received in revised form4 May 2015Accepted 11 May 2015Available online 27 May 2015

Keywords:Air distributionDrying rateConvective dryingThrough-flowOver-flow

* Corresponding author. Tel.: þ49 711459 22840; faE-mail address: Patchimaporn.Udomkun@uni-hoh

http://dx.doi.org/10.1016/j.lwt.2015.05.0220023-6438/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

The impact of airflow direction, namely through-flow and over-flow modes, on drying kinetics ofosmotically-pretreated papayas was investigated in a convective-type dryer under varied conditions(temperature, humidity and velocity). The Newton model was used to describe thin-layer drying char-acteristics and the dependence of drying air parameters on the drying constant (k) was expressed by anArrhenius-type relationship. It was found that a more uniform airflow distribution in the through-flowchamber resulted in higher product temperature as well as faster drying rate, especially during the initialstage of drying. For both airflowmodes, drying kinetics was most significantly influenced by temperatureand velocity of the air, whereas the specific humidity had less effect on the drying rate. The value of kincreased in parallel with temperature and velocity of the drying air, whereas it was reduced byincreasing humidity. A model incorporating the conditions of drying air was developed for each airflowmode, which can help with optimization of practical drying operations.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Various convective drying approaches are commercially usedfor processing of agricultural products in Thailand. For example,papayas and other tropical fruits are commonly cut, pretreated anddried in fixed-bed- (Nagle et al., 2010), cabinet- (Precoppe, Nagle,Janjai, Mahayothee, & Müller, 2011), solar tunnel- (Janjai, Lamlert,Intawee, Mahayothee, Bala, et al., 2009) or solar greenhouse-dryers (Janjai, Lamlert, Intawee, Mahayothee, Haewsungcharern,et al., 2009) to a target moisture content generally in the range of10e15 g per 100 g w.b. at air temperatures usually between50�80 �C, air velocity around 0.5e2.0 m s�1 and air humidity of10e25% (Babalis & Belessiotis, 2004; Janjai et al., 2011;Tippayawong, Tantakitti, Thavornun, & Peerawanitkul, 2009).Depending on the design of drying equipment, a stream of heatedair is ventilated either through or over the product. For example,fixed-bed and cabinet dryers typically operate in through-flowmode, while most solar dryers impart over-flow conditions. Each

x: þ49 711459 23298.enheim.de (P. Udomkun).

drying application has its own advantages and limitations. The finalproduct obtained may differ in physicochemical and nutritionalproperties as a result of the mode of airflow distribution applied(B€ohner, Barfuss, Heindl, & Müller, 2013; Ghiaus, Margaris, &Papanikas, 1998). Furthermore, airflow direction and distributionin a drying chamber have a significant impact on drying time andenergy efficiency due to the pressure drop and heat losses duringdrying. For instance, Mathioulakis, Karathanos, and Belessiotis(1998) found that the lack of spatial homogeneity of pressure andvelocity above the product in a drying chamber led to variations ofdrying rate. Likewise, Margaris and Ghiaus (2006) indicated thatoptimisation of airflow distribution led to a substantial improve-ment of the product quality together with considerable reductionof energy consumption. Nagle et al. (2010) also mentioned that theuniformity of drying air could produce more homogeneous qualityof longan.

In general, moisture diffusion is driven by gradients producedthrough numerous mechanisms such as capillary flow (diffusion inliquid phase), migration in the adsorbed layer, vapour-condensation and true diffusion of vapour in air (Karathanos,Villalobos, & Saravacos, 1990). The drying rate, which describesthe mechanisms of internaleexternal and heat-mass transport

Nomenclature

a, b, c, d model coefficientsaw water activityDR drying rate (kg kg�1 d.b. h�1)k drying constant (dimensionless)MAPE mean absolute percentage errorMR moisture ratio (dimensionless)MRexp experimentally observed moisture ratio

(dimensionless)MRpre predicted moisture ratio (dimensionless)N number of observations

x specific humidity of the drying air (g kg�1 dry air)R2 correlation coefficientRH relative humidity (%)Ta temperature of the drying air (�C)Tp temperature of product (�C)t drying time (h)Dt time difference (h)v velocity of drying air (m s�1)X moisture content (kg kg�1 d.b.)X0 initial moisture content (kg kg�1 d.b.)Xe equilibrium moisture content (kg kg�1 d.b.)Xt moisture content at time t (kg kg�1 d.b.)

P. Udomkun et al. / LWT - Food Science and Technology 64 (2015) 67e7368

phenomenon during drying, strongly depends on drying methodand air conditions such as temperature, relative humidity and airvelocity (Lahsasni, Kouhila, Mahrouz, & Jaouhari, 2004). Nonethe-less, Phupaichitkun, Mahayothee, Waldenmaier, and Müller (2008)have found that the air velocity is not as influential of a parameterin thin-layer drying of longan fruits as the temperature, andfurthermore, that the influence of air velocity decreases with dry-ing time. In addition, material properties such as physical andchemical characteristics affect the drying rate (Krokida,Foundoukidis, & Maroulis, 2004; Singh, Jairaj, & Srikant, 2012).Karathanos and Belessiotis (1999) reported that the alteration ofdrying constant of currants, grapes and plums depends on the skinthickness and initial pretreatment process. Barnwal and Tiwari(2008) have observed that the fully mature grapes dried fasterthan the immature grapes. Moreover, the direction of air stream canalso be a crucial factor during drying For example, by significantlyaffecting shrinkage as well as moisture movement in potato slicesduring drying at 60�70 �C (Yadollahinia, Latifi, & Mahdavi, 2009).

Models simulating these drying parameters can be used foroptimization of processing, either for new or for existing dryingsystems, or even for the control of the drying process. From theengineering point of view, the lack of specified information on theeffect of airflow direction inside the drying chamber combinedwith unsuitable drying conditions could result in inefficient dryingas thin-layer models are affected by the airflow mode used. Untilnow, no work has been carried out to compare different modes ofairflow, applied to the product in vertical and horizontal directions,on drying behaviour of fruits. Therefore, the objective of this studywas to investigate the effects of airflow modes on the drying ki-netics of papayas under various conditions. The approach was todevelop simple models based on first order kinetics, in which theinfluence of drying parameters was embodied in the drying con-stant by an Arrhenius type equation.

2. Materials and methods

2.1. Raw materials

Papayas (Carica papaya L. cv. Pluk Mai Lie) harvested from acommercial orchard in Nakhon Nayok province, Thailand werepurchased from a local import company between April�July 2014.Fruits of uniform shape and weight (1.0 ± 0.2 kg fruit�1) wereselected, which exhibited three-quarters ripeness (70 ± 10% ofyellowness skin). The initial material was standardized formoisturecontent (84.5 ± 1.1 g 100 g�1 w.b.), soluble solids content (10.2 ± 0.4�Brix), titratable acidity (0.14 ± 0.02 g citric acid 100 g�1 w.b.) andpH (5.2 ± 0.2). Before preparation for drying, fruits were storedunder refrigeration at temperature of 10 ± 1 �C and relative hu-midity of 20e35%. The storage time was less than 5 days.

2.2. Sample preparation

2.2.1. Osmotic pretreatmentPapayas were osmotically-pretreated according to the proce-

dure described by Udomkun, Mahayothee, Nagle, and Müller(2014). Fruits were hand-peeled and cut into slabs of20 � 30 � 20 mm using a specially-designed stainless steel cutter.The samples were rinsed with fresh water and then soaked in25 g L�1 calcium lactate solution for 1 h under controlled temper-ature (20 ± 2 �C), then blanched at 60 ± 2 �C for 1 min. Subse-quently, samples were immersed in 30 �Brix osmotic solution at astarting temperature of 60 ± 2 �C and then allowed to stand at roomtemperature for 6 h. The osmotic solution was prepared by dis-solving 429 g of 99.9% refined sucrose in 1 L of water to obtain therequired concentration and then pH was adjusted to 4.0 using citricacid. The weight ratio of osmotic solution to fruit samples was 1:1.After removal from the solution, the samples were rinsed, drainedand blotted with absorbent paper to remove the surface waterbefore drying. The average water activity (aw) of samples wasreduced from 0.990 to 0.980 by osmotic pretreatment.

2.2.2. Hot air dryingConvective drying was carried out using a high precision hot air

laboratory dryer (Institute of Agricultural Engineering, Tropics andSubtropics Group, University of Hohenheim, Germany). A descrip-tion of the experimental dryer has been given by Argyropoulos,Heindl, and Müller (2011) and Argyropoulos, Khan, and Müller(2011). The temperature, air velocity and humidity inside the dry-ing chamber were monitored using PLC software. The flow of airwas measured by differential pressure monitoring sensors (orificemeter) at an accuracy of ±0.05 m s�1. The dry and wet bulb tem-peratures of the air are measured by Pt-100 sensors at differentlocations (accuracy ±0.1 �C). The humidity was controlled byadjusting the dew point temperature using a psychometric chart.

Two airflow modes were applied in this study: through-flowmode, where the airflow was applied vertically to the productand over-flowmode, where the air was applied horizontally. Dryingexperiments were carried out under various conditions by settingdrying temperature (T) at 50, 60, 70 and 80 �C, specific humidity (x)at 10 and 25 g kg�1 dry air and air velocity (v) at 0.2, 0.5 and0.7 m s�1 in through-flow mode and 0.5, 0.7 and 1.0 m s�1 in over-flow mode. The experimental conditions for drying conditions aredenoted by codes such as TF/50/10/02, which are ordered by airflowdirection, T, x and v, respectively.

For each experiment, 450e500 g of sample was placed in thedrying tray. Round (diameter of 240 mm) and rectangular (diam-eter of 325 mm) stainless steel trays were used for through-flowand over-flow mode drying, respectively. To study temperatureprofile within the samples, K-type thermocouples were

P. Udomkun et al. / LWT - Food Science and Technology 64 (2015) 67e73 69

indiscriminately inserted at the geometric centre of three papayaslabs. The mass reduction of the samples was automaticallyrecorded for the determination of the drying curves and estimationof total drying time (t). Drying was executed until mass of thesamples was achieved corresponding to X of 0.16 ± 0.02 kg kg�1 d.b.with aw ranging between 0.5 ± 0.05. Experiments were performedin triplicate.

2.3. Airflow simulation

In order to assess the effect of different airflow modes on aircirculation in the drying chamber, fluid dynamic simulations werecarried out using FLUENT (Ansys Workbench 15.0 with the solverFluent, Ver. 15.0, ANSYS, Inc., USA). The simulations were per-formedwith an input v¼ 1.0 m s�1 at the dryer inlet. The resistanceto airflow due to the changes in thickness of papaya during dryingwas assumed to be negligible.

2.4. Mathematical modelling

Themoisture ratio (MR) of papayas during dryingwas calculatedas:

MR ¼ ðXt � XeÞðX0 � XeÞ (1)

where Xt is the moisture content (kg kg�1 d.b.) at time t duringdrying, X0 is the initial moisture content (kg kg�1 d.b.) and Xe is theequilibrium moisture content (kg kg�1 d.b.). In this study, the X ofthe samples was determined by volumetric Karl Fischer titration(model 758 KFD Titrino, Metrohm GmbH and Co., Herisau,Switzerland) carried out at 50 �C with the addition of 10 mL offormamide as a solubilizing agent. Samples were subjected todouble determination.

The modified Halsey equationwas used to compute Xe of papayaat the respective conditions of drying air. Model parameters wereapplied as previously established in a study performed by the au-thors (Udomkun, Argyropoulos, Nagle, Mahayothee, & Müller,2015):

Xe ¼"�expð�1:8960� 0:0047$TÞ

ln�

RH100

�#1=1:2010

(2)

where Xe is the equilibrium moisture content (kg kg�1 d.b.), RH isthe relative humidity (%) and T is the temperature (�C). The eval-uation of moisture transfer during drying was described by a firstorder kinetic model (the Newton model):

MR ¼ expð�ktÞ (3)

whereMR is the moisture ratio (dimensionless), t is the drying time(h) and k is the drying constant (dimensionless). The effect ofconditions of drying air (T, x and v) on k value was incorporated byan Arrhenius-type equation:

k ¼ a$exp�bT

�$xc$vd (4)

where k is the drying constant (dimensionless), T is the dryingtemperature (�C), x is the specific humidity (g kg�1 dry air), v is theair velocity (m s�1) and a, b, c and d are the model coefficients.

The DR of papaya slabs during experiments was calculated usingthe following equation:

DR ¼ �dXt

dt¼ �Xt � XtþDt

Dt(5)

where XtþDt is moisture content at time tþ Dt (kg kg�1 d.b.), t is thedrying time (h) and Dt is time difference (h).

2.5. Statistical analysis

Coefficients from the drying model were determined using non-linear least square regression with OriginPro software (Ver. 9.0,OriginLab Corporation, USA). The correlation coefficient (R2) andmean absolute percentage error (MAPE) were used to determinethe quality of fit. MAPE was calculated as follows:

MAPE ¼ 100N

XN

i¼1

��MRexp �MRpre��

MRpre(6)

where MRexp is the experimentally observed moisture ratio, MRpreis the moisture ratio predicted by the drying model and N is thenumber of observations.

3. Results and discussion

3.1. Airflow distribution

The effect of airflow mode on the air distribution in the dryingchamber was simulated during drying of papaya as shown in Fig. 1.A more uniform air distribution of vertical up-flowing air wascomputed for through-flowdrying. In over-flowmode, more spatialdifferences of v were noticed. Higher DR occurred at the left of thedrying chamber (inlet), while the product towards the middle andright side (outlet) remainedmoist for longer due to lower initial DR.This could be explained by deflection of the airstream by the firstrow of papaya slabs.

3.2. Product temperature and drying rate

Curves illustrating average temperature inside the product (Tp)during through-flow and over-flow drying at T ¼ 70 �C,x ¼ 10 g kg�1 and v ¼ 0.5 m s�1 is shown in Fig. 2a. Tp was alwayslower than the set drying air temperature (Ta). This temperaturedifference can be explained by the heat diffusion effect due to thecontinuous evaporation of water from the wet material. As dryingprogressed, the external surface of the product became dry and theTp slowly started to reach Ta. In through-flow mode, Tp from thedifferent sampling positions increased faster during initial dryingstages in comparison with over-flow mode. In contrast, high vari-ations in Tp from three different rows of samples were obvious inover-flow mode, particularly during the initial drying stage. How-ever, after 4 h, the variation decreased and the difference in Tpbetween the two airflow modes only ranged 2.2e4.1 �C.

The difference in Tp as a result of different airflow directions wassubsequently reflected by the DR. At the beginning of experiments,the DR was higher in through-flow mode in comparison to over-flow mode. This indicated that through-flow conditions increasedheat transfer potential between the surrounding air and theproduct due to more uniform airflow distribution and acceleratedmoisture diffusivity, which enhanced the evaporation of waterfrom samples. After 8 h, the DR was similar for both airflow di-rections as shown in Fig. 2b, most likely because surface moistureevaporation was complete and only hygroscopic water remained.During the initial drying period, the evaporated water came fromparenchyma cells within the sample and needed be transported tothe surface. The slower DR at the final stage indicated increased

Fig. 1. Airflow distribution inside the chamber during drying of papaya with (a) through-flow and (b) over-flow mode. Arrows indicate airflow direction, colours indicate airvelocity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

P. Udomkun et al. / LWT - Food Science and Technology 64 (2015) 67e7370

resistance to mass transfer through the inner cells, whereas heattransfer was less because of the lower driving force. To�grul andPehlivan (2003) and Singh and Gupta (2007) stated thatdecreased DR in the later period of the drying process might be

Fig. 2. Dynamics of (a) product temperature and (b) drying rate as a function of dryingtime during drying of papaya with different airflow direction at air temperature of70 �C, specific humidity of 10 g kgDynamics of �1 and air velocity of 0.5 m s�1.Through-flow mode and over-flow mode are indicated by and - - -, respectively.

associated with volume reduction (case hardening) and resultingshrinkage, which increase the resistance tomovement of water andlead to further decreases in DR. Moreover, Nieto, Castro, andAlzamora (2001) also attributed the changes in moisture diffu-sivity to solid uptake and/or starch gelatinization in mango. In thisstudy, same result regarding the influence of airflow direction onthe DR was observed for all the applied drying conditions.

3.3. Drying kinetics

Samples of papayawith average initialX¼5.01±0.19 kg kg�1 d.b.were dried to X¼ 0.14e0.18 kg kg�1 d.b. For both airflow directions,the drying curves for T ¼ 50e80 �C, x ¼ 10 and 25 g kg�1, andv¼ 0.7m s�1, are presented in Fig. 3. As drying progressed, X steadilydecreased, which corresponded to a reduction in DR. The total trequired to reach the targetXwas found tobe stronglydependent onT (Fig. 3a). A reduction of t of approximately 50e60% was observedwhenTwas raised from 50 to 80 �C for both airflow directions at thesame x and v. Results from modelling of drying behaviour are pre-sented inTable 1 and showed that k increasedwith increasing T. Thiscan be explained by the effect of T on evaporation, diffusivity andheat transfer during drying, which subsequently accelerates thewater migration via diffusion mechanism inside the product andwater uptake by the air (Singh&Gupta, 2007; Xiao, Pang,Wang, Bai,& Gao, 2010). The finding was also in agreement with Sablani,Rahman, and Al-Habsi (2000), Saravacos and Maroulis (2001) andPhupaichitkun et al. (2008), who stated that k is a function of T.Moreover, a higher DRwas observed for through-flow drying whencomparedwith over-flowmode under the same conditions. Hossainand Bala (2007) also reported that through-flow drying of greenchilli resulted in a higher k value when compared with over-floweunderflow drying. This result might be associated with thedeflection of the airflowby the inlet facing rowof papaya slabswhensubjected over-flow drying mode (see Section 3.2).

The effect of x on the drying behaviour of papaya at T ¼ 50 and80 �C at v ¼ 0.5 m s�1 is illustrated in Fig. 3b. Although a slightly

Fig. 3. Drying kinetics of papaya as affected by air conditions: (a) temperature; (b) specific humidity; (c) velocity, ( ) Newton model. TF and OF indicate through-flowmode andover-flow mode, respectively. Temperature is indicated by (C) 50, (-) 60, (:) 70 and (A) 80 �C for TF and (B) 50, (▫) 60, (▵) 70 and (>) 80 �C for OF. Specific humidity isindicated by (C) 10 and (-) 25 g kg�1 for TF and (B) 10 and (▫) 25 g kg�1 for OF. Velocity is indicated by (C) 0.2, (-) 0.5 and (:) 0.7 m s�1 for TF and (B) 0.5, (▫) 0.7 and (▵)1.0 m s�1 for OF.

P. Udomkun et al. / LWT - Food Science and Technology 64 (2015) 67e73 71

higher DRwas observed at 50 �Cwhen x¼ 10 g kg�1 as compared tox ¼ 25 g kg�1, no differences were observed at 80 �C (Table 1). Thismight be associated with the influence of T on the RH of the air. Forexample, x ¼ 10 and 25 g kg�1 corresponds to RH of 8 and 10% atT ¼ 80 �C, whereas at T ¼ 50 �C, RH is much higher at 20 and 31%,respectively. However, with respect to the influence of x on dryingkinetics, the reduction in MR was generally faster for through-flowas compared to overflow-drying. This was in agreement withZlatanovi�c, Komatina, and Antonijevi�c (2013), who showed that

increased RH reduced the DR of apple cubes. Nevertheless, the in-fluence of x became less accentuated at higher T (especially atT ¼ 70 and 80 �C) in both through-flow and over-flow drying.

The reduction in MR for different v at T ¼ 50 and 80 �C andx ¼ 10 g kg�1 are depicted in Fig. 3c. For both airflow modes, theeffect of v on MR was evident. The results indicated that a shortert was achieved with higher v. At T ¼ 50 �C and x ¼ 10 g kg�1, the trequired to reach target X was reduced by 10 and 23%, when the vincreased from 0.2 to 0.7 m s�1 for through-flow mode, and

Through­flow R2¼0:9718� �

: k¼2:3388$exp�102:9440

T

� �$x�0:0270$v0:2641 (7)

Over­flow R2 ¼ 0:9821� �

: k ¼ 3:0563$exp�115:7210

T

� �$x�0:1238$v0:3408 (8)

Table 1Drying constant and statistical results (R2, MAPE) for through-flow and over-flowdrying of papayas at different drying air parameters as indicated by code (airflowdirection/temperature/specific humidity/velocity).

Through-flow Over-flow

Code k (e) R2 MAPE Code k (e) R2 MAPE

TF/50/10/02 0.2167 0.9959 3.56 OF/50/10/05 0.1846 0.9981 2.21TF/60/10/02 0.2727 0.9965 3.09 OF/60/10/05 0.2535 0.9990 2.06TF/70/10/02 0.3371 0.9977 3.76 OF/70/10/05 0.3521 0.9984 2.26TF/80/10/02 0.4111 0.9979 2.59 OF/80/10/05 0.4318 0.9923 3.08

TF/50/25/02 0.1708 0.9983 3.82 OF/50/25/05 0.1717 0.9974 2.09TF/60/25/02 0.2429 0.9988 3.87 OF/60/25/05 0.2461 0.9993 1.94TF/70/25/02 0.3069 0.9990 2.95 OF/70/25/05 0.2907 0.9985 1.66TF/80/25/02 0.3741 0.9968 4.31 OF/80/25/05 0.3762 0.9980 2.90

TF/50/10/05 0.2591 0.9846 5.67 OF/50/10/07 0.2005 0.9934 1.43TF/60/10/05 0.2951 0.9893 5.01 OF/60/10/07 0.2724 0.9986 1.68TF/70/10/05 0.3999 0.9939 2.01 OF/70/10/07 0.3842 0.9945 2.54TF/80/10/05 0.4945 0.9974 1.16 OF/80/10/07 0.5023 0.9930 1.15

TF/50/25/05 0.2327 0.9837 6.34 OF/50/25/07 0.1973 0.9935 1.22TF/60/25/05 0.3354 0.9909 4.28 OF/60/25/07 0.2605 0.9993 3.71TF/70/25/05 0.3940 0.9965 1.27 OF/70/25/07 0.3277 0.9989 0.95TF/80/25/05 0.5028 0.9972 4.23 OF/80/25/07 0.4430 0.9988 1.19

TF/50/10/07 0.2650 0.9796 7.78 OF/50/10/10 0.2450 0.9959 2.41TF/60/10/07 0.3320 0.9857 4.35 OF/60/10/10 0.3216 0.9982 2.19TF/70/10/07 0.4689 0.9939 1.74 OF/70/10/10 0.4358 0.9984 1.78TF/80/10/07 0.5610 0.9976 1.80 OF/80/10/10 0.5378 0.9992 2.06

TF/50/25/07 0.2668 0.9873 6.21 OF/50/25/10 0.2211 0.9985 2.44TF/60/25/07 0.3261 0.9906 1.38 OF/60/25/10 0.2833 0.9987 0.43TF/70/25/07 0.4276 0.9961 1.44 OF/70/25/10 0.3858 0.9996 1.58TF/80/25/07 0.5822 0.9971 0.57 OF/80/25/10 0.4944 0.9979 2.96

P. Udomkun et al. / LWT - Food Science and Technology 64 (2015) 67e7372

0.5e1.0 m s�1 for over-flow mode, respectively. Nonetheless, anincrease in v from 0.5 to 0.7 m s�1 at T ¼ 50 �C did not influencethe total t for both airflow modes. As is presented in Table 1, k alsoincreased with increasing v as well as decreasing external resis-tance. This phenomenon could be explained by the fact that thewater flux inside the fruits as well as at the surface depends onthe mass flow of drying air. El-Aouar, Azoubel, and Murr (2003),working with fresh and osmo-dehydrated papaya cubes treatedwith a sucrose solution of 70 �Brix containing appropriateamounts of sodium lactate (24 g L�1) and lactic acid (0.1 mol L�1),also reported that v had some influence on k. However, as shownfor through-flow drying in Fig. 3c, the changes in drying curve

characteristics at T ¼ 50 �C when v was increased from 0.5 to0.7 m s�1 was negligible. A similar result was observed for over-flow drying when v increased from 0.7 to 1.0 m s�1. The dryingcurve at T ¼ 60 �C showed a similar tendency (data not pre-sented). On the contrary, v clearly influenced the drying curves athigher T, namely at 70 and 80 �C, and especially in the case ofthrough-flow drying. Here the effect of v was most pronouncedduring initial drying stages, presumably since evaporation by themoisture diffusion process first took place at the sample surface.Towards the end of the drying process, the effect of v on dryingbecame negligible, due to greater internal resistance to moisturetransfer (caused in part by structure changes, also when consid-ering the effect of T). In drying, various factors can interfere tomake moisture transfer more difficult, such as concentration ofsolutes, crust formation and shrinkage (Chkir et al., 2015). Theseresults were in strong agreement with earlier observations of thedrying process in some other fruits such as figs (Babalis,Papanicolaou, Kyriakis, & Belessiotis, 2006), peaches (Zhu &Shen, 2014) and quince (Tzempelikos, Vouros, Bardakas, Filios,& Margaris, 2014).

Noticeably, the increase in k observed in these experiments,as well as the increase in DR, confirmed the aforementionedresults that efficiency of dried papaya production can beenhanced by increasing T and v and decreasing x. Therefore, theeffects of drying properties were taken into account in applyinga first order kinetic rate equation as mentioned in Eq. (3).

The k values were found to be Arrhenius-type functions of T, xand v, which was confirmed by strong R2 values. The equation todescribe the k values of papaya samples for through-flow and over-flow drying are given as:

According to the equation for both airflow modes, T is the mostrelevant factor as compared to v and x, respectively. This could be anindication that heat diffusion accelerates the drying process morethan mass flow of the drying air.

4. Conclusions

The mode of airflow influenced drying behaviour of papayasamples. Drying in through-flow mode had a more favourableimpact on airflow distribution and heat uniformity when comparedwith over-flow mode. At the initial period of drying, higher Tp andDR were observed under through-flow mode. MR was found to be

P. Udomkun et al. / LWT - Food Science and Technology 64 (2015) 67e73 73

influenced by T and v, whereas x in the applied range did not show aconsiderable effect on drying behaviour, especially in through-flowdrying at high temperatures. Nonetheless, the effect of v on dryingbehaviour was negligible, particularly at low X towards the end ofdrying. The results also revealed that increasing T and v anddecreasing x had a distinct influence on k values. The change ofgeneralized k as a function of T, x and v was successfully describedby an Arrhenius equation. Conclusively, the mode of airflow wasonly significant in terms of reduction t in some of the studiedranges of drying parameters. Further research should also focus onoperating cost and final quality of dried papayas.

Acknowledgements

This research is the result of a scholarship from the Food Secu-rity Center (FSC) of Universit€at Hohenheim, which is part of theDAAD (German Academic Exchange Service) Program “Exceed”. It isalso supported by DAAD and the German Federal Ministry forEconomic Cooperation and Development (BMZ). Acknowledge-ment is given to Mrs. Dorothea Hirschbach-Müller, Mr. SebastianRomuli and Mr. Sebastian Reyer, Institute of Agricultural Engi-neering (440e), for their technical contributions. In addition, we arealso grateful to the FSC for their supporting to Asst. Prof. Dr.Busarakorn Mahayothee as a visiting professor.

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