Evaluation of the drying methods and conditions with respect to drying kinetics, colour quality and specific energy consumption of thin layer pumpkins

Transcription

1 Bulgarian Chemical Communicaions, Volume 48, Number3 (pp ) Evaluaion of he drying mehods and condiions wih respec o drying kineics, colour qualiy and specific energy consumpion of hin layer pumpkins O. İsmail 1 *, Ö. G. Kocabay 1, 1 *Deparmen of Chemical Engineering, Faculy of Chemical and eallurgical Engineering,YildizTechnical Universiy,Davupaşa Campus, 3410 İsanbul, Turkey T.R. inisry of Culure and Tourism, Direcorae of Cenral and Laboraory for Resoraion and Conservaion, İsanbul, Turkey Received Augus 3, 015, Revised Sepember 1, 015 In his sudy, he effecs of dry air emperaure and power levels when drying pumpkin (Cucurbia pepo L.) slices by five differen mehods was invesigaed experimenally. Pumpkin slices were dehydraed by five differen drying mehods: open-sun, vacuum, microwave, infrared and ho air drying. In paricular, he experimens were carried ou a wo differen microwave power levels (90 and 180 W), infrared power levels (83 and 15 W) and ho air emperaures (50 and 70 o C) o invesigae he effecs of hese facors on he microwave, infrared and ho-air drying, respecively. The vacuum drying experimen was carried ou in one vacuum oven dryer a a consan emperaure of 50 C and a pressure of 0.1 kpa. The experimenal moisure daa was fied o some models (namely Lewis, Henderson and Pabis, Page, Logarihmic, Aghbashlo e al.,verma e al. and idilli e al. models) available in he lieraure and according o he resuls, he idilli e al. model is superior o he ohers in explaining he drying behavior of pumpkin slices. The energy efficiency and diffusion coefficiens increased wih he increase in microwave power. In erms of colour crieria he bes values were obained by he ho-air and open-sun drying mehods. Keywords:colour, drying models, mahemaical models. INTRODUCTION In accordance wih he boanical classificaion Cucurbiaceae is par of he Dicoyledoneae class, Cucurbiales eam, Cucurbiaceae genus. Approximae 118 kinds and 85 species are presen in he Cucurbiaceae genus. In he genus of Cucurbiaceae, Cucurbia pepo L. is a species wih a high economic value [1]. Pumpkin (Cucurbia pepo L.) is one of he mos imporan vegeables grown in Turkey. In 01, pumpkin s world producion reached ons meanwhile in Turkey is producion was ons. The five major pumpkin producing counries in he world are China, India, Ukraine, Egyp and he Unied Saes []. Pumpkins, which grow in he differen regions of Turkey, are a seasonal crop and for his reason a processing sep is ofen used o preserve pumpkin producs. Depending upon he processing possibiliies, inadequaeness of seasonal fresh vegeables and fruis, Turkey as well as in many counries are experiencing big economic losses. I was repored ha in he developing counries approximaely 30 o 40 percen of he seasonal fresh vegeables and fruis are cas away due o spoilage [3]. * To whom all correspondence should be sen: ismail@yildiz.edu.r The excess quaniy of agriculural produce can be consumed immediaely and is life span is oo shor. These producs can be kep fresh afer special processing. Grea numbers of preserving echniques such as freezing, hea reamen, drying ec. are used o increase he endurance of foodsuffs. In food producs wih respec o proecion of he viamin value, mainenance of a good ouward appearance, ase preservaion, he emergence of a decreasing mass advanage, he improvemen of sorage and ransporaion faciliies, drying is he mos appropriae mehod [4, 5]. The mos common drying mehod used for drying fruis and vegeables in he world and in Turkey is open-sun and oudoors drying. Because he solar energy is renewable, clean and cheap, open-sun drying is carried ou commonly in he ropical counries. As here are no energy requiremens and a mainenance expense, open-sun drying is a cos-effecive mehod. The only disadvanages of drying under he sun in accordance wih oher processes are ha he drying rae is slow and he ime is prolonged [6]. Convecive drying by ho air is a widely used drying mehod in he lieraure. In hese kinds of dryers specifical, air speed is praciced by produc, he produc has a shor drying ime. Having a simple design, manufacured localy, having a small mainenance and operaional cos, drying differen 016 Bulgarian Academy of Sciences, Union of Chemiss in Bulgaria

2 producs according o he season, are among he advanages of hese drying sysems [7, 8]. During boh convenional drying processes, considerable loss in food conen akes place. To reduce he food conen losses, o preserve he qualiy of he dried food and o shoren he drying ime a low emperaures, he vacuum drying process is successfully applied insead of he convenional mehods [9]. icrowave and infrared echnology, which has become common in recen years, akes is place in he food indusry by shorening he drying ime associaed wih providing qualiy food. Using microwave driers in combinaion wih ho air live driers increases produc qualiy and energy efficiency [10]. The fundamenal principle of microwave heaing is provided wih he conversion of elecromagneic energy o hermal energy by affecing he polar molecules in he maerial [11]. icrowave heaing sysems are successfull drying sysems for fruis, cereal crops and many food producs which have a high moisure conen. In brief, drying wih microwaves has he characerisics of 4 imporan properies; fas processing, energy efficiency, low cos and a high qualiy of he dried produc [1]. As saed in [13], various infrared hea sources could be used for drying applicaions. IR energy is a form of elecromagneic energy or hea energy. The peneraion properies of IR radiaion ino a given maerial direcly increases he energy flux wihou burning he maerial s surface and so he required heaing ime of he convenional heaing mehod decreases. The advanages of infrared heaing by convenional heaing can be lised as follows: providing regular heaing in a shor ime, decreasing deerioraion and nuriional loss, he equipmen has simple and flexible usage areas and considerably economizes he energy consumpion. Drying is also he mos energy consuming process in he food indusry. New drying mehods and dryers mus be designed and invesigaed o minimise he energy cos of he drying process [14]. Alhough here are some invesigaions focused on he drying characerisic of he pumpkin, here is oo much informaion on he drying characerisics of he Cucurbia pepo L., which is a subgroup of pumpkin. The objecives of his sudy are o evaluae and compare he drying kineics, produc qualiy and specific energy consumpion during he drying of pumpkin slices (Cucurbia pepo L.) by five drying mehods: (1) ho-air convecion drying, () open-sun drying (3) he vacuum drying process, (4) IR drying and (5) microwave drying. Two differen drying condiions were applied for he microwave, IR and ho-air drying mehods. In addiion o his, o obain he bes model for he drying kineics of pumpkin slices; Lewis, Henderson & Pabis, Page, Logarihmic, Aghbashlo e al., Verma e al. and idilli e al. models were fied o he experimenal daa. Also effecive moisure diffusiviy values were calculaed. ATERIALS AND ETHODS aerial Fresh pumpkin (Cucurbia pepo L.) samples were obained from a local supermarke in İsanbul, Turkey and sored in a chamber a 15 0 o C unil processing. The iniial moisure conen of he fresh pumpkin samples was deermined using he drying oven (emmer U-400, Germany) a 105 o C for 4 h [15]. These experimens were run hrice o obain a reasonable average. The average iniial moisure conen of he pumpkins was found o be 9 % w.b. Before he drying process he pumpkin samples were washed, heir op and boom pars were cu and hen he pumpkin samples were cu ino 0.5 ± 0.03 cm sized cylindrical slices using a knife. Drying equipmen and IR drying procedure Drying experimens were carried ou in a moisure analyzer wih a 50 W halogen lamp (Snijders oisure Balance, Snijders b.v., Tilburg, Holland). During he infrared drying process, a sample was separaed over he enire pan evenly and homogeneously. The power level was se in he conrol uni of he equipmen. The drying experimen was performed a he infrared power levels of 83 W and 15 W. The pumpkin samples (approximaely 40 ± 0. g) were aken from he dryer a 30 min ime inervals during he drying process and heir weighs were measured wih a digial balance (Precisa, model XB0A, Precisa Insrumens AG, Dieikon, Swizerland) wih an accuracy of g. When he samples moisure conen reached approximaely 0.08 g waer/g of dry maer (dry basis) he drying process was erminaed. W The drying experimens were carried ou in a Rober Bosch Hausgerae GmbH (Germany) model microwave oven which has a maximum oupu power of 800 W a 450 Hz. The microwave oven has he capabiliy of operaing a differen microwave sages while is power range is 90 o 800 W. The area subjeced o microwave drying is 530 mm x 500 mm x 3 mm in size and consiss 481

3 of a roaing glass plae which is 300 mm in diameer a he base of he oven. The adjusmen of he microwave oupu power and processing ime was done wih he aid of a digial conrol apparaus locaed on he microwave oven. The drying experimens were se a wo differen microwave power levels of 90 W and 15 W. During drying, he experimens were carried ou using he sliced pumpkins known weigh of approximaely 34± g wih he hin layer placed on he roaable plae fied inside he microwave oven cabin. The roaing glass plae was removed from he oven every min during he drying period and he moisure loss was deermined by weighing he plae using a digial balance. The microwave drying process coninued unil he moisure conen reduced o approximaely 0.09 g waer/g of dry basis of he iniial moisure conen. 48 Cabine dryer The drying experimens were performed in a cabine ype dryer (APV & PASILAC Limied of Carlisle, Cumbria, UK). I was made up of sainless seel shees and i consised of a recangular unnel 0.54 m x 1.4 m x 1.0 m in size. The dryer consised of a cenrifugal fan o supply he air flow, an elecrical heaer and an air filer. The dryer is operaed a dry bulb emperaures of 0 00 o C. The desired drying air emperaure was aained by elecrical resisance and conrolled by he heaing conrol uni. The velociy of he air passed hrough he sysem was measured by an anemomeer in he range of ms -1 (model A-401, Luron Elecronic, Taipei, Taiwan). The air flow was measured direcly in he drying chamber. The samples were dried in he perforaed square chamber, which had a flow cross-secion of 30 cm 30 cm. Weigh loss of he samples was recorded by using a digial balance (eler-toledo AG, Grefensee, Swizerland, model BB3000) wih a sensiiviy of 0.01 g. The pumpkin slices of abou 100g were disribued uniformly as a single layer a he sample ray and hen were dried in he ho air dryer. The ho air drying was carried ou by drying he pumpkin samples a 50ºC and 70ºC air emperaures and wih a consan air velociy of 1 m/s. Pumpkin samples moisure loss was measured by a balance and recorded a 30 min inervals. The drying process was finished when he moisure conen of he samples achieved approximaely 0.07 g waer/g of dry maer. Open air-sun drying To clean he samples from dus and foreign maerials, he seleced pumpkin samples were washed wih ap waer. Open air - sun drying experimens were carried ou during he monh of Augus 014 (from a.m. o 0.00 p.m.) in Greece. The pumpkin slices of abou 50 g were disribued uniformly as a single layer on he sample ray and hen were exposed o sunligh for 1 hours daily. oisure loss and he ambien air s emperaure was measured by a porable digial balance during he drying process a 30 min inervals (Alfais, I000-1, which has g measuremen range wih an accuracy of ± 0.1 g). When he drying ime ook more han 1 h o reduce he effec of he increase in moisure conen, samples were packed overnigh. The ambien air s emperaure during he drying experimens was beween 36 o 49 C. The highes air emperaure was reached beween 10:30 a.m. and 14:30 p.m. The drying process coninued unil he sample reached he desired moisure level of 9 % (w.b.). The dried samples were packed in low densiy polyehylene bags. Vacuum drying oven Vacuum drying reamen was performed in he laboraory ype vacuum oven (Nuve EV 0180, Turkey) wih he echnical feaures ~0 V, 50 Hz, 3.5 A and 800 W. The vacuum oven s emperaure, which has a sensiiviy of 1 C, is a maximum of 50 C. The area of vacuum drying was 30 cm x 0 cm x 5 cm in size. A laboraory ype vacuum pump (Carpanelli DE80B4, Ialy) was used in he vacuum drying operaion. Is operaing condiions were ~0/40 V, 50/60 Hz and 5.1/4.8 A. The adjusmen of he vacuum value and processing emperaure was done wih he aid of a digial conrol faciliy locaed on he vacuum drying oven. In he drying experimens pumpkin slices wih a weigh approximaely of 30 ± g and a consan emperaure of 50 C and pressure of 0.1 kpa were used in he vacuum oven dryer. The moisure loss in he pumpkin slices was measured wih a balance and i was recorded a 30 min inervals. Good resuls as a consequence of drying of he pumpkin slices by vacuum drying were no achieved. ahemaical modelling oisure raio The moisure raio (R) and drying rae were calculaed using he following equaions: e R, (1) o e where R is he moisure raio,, o and e are he moisure conen (g waer/g dry maer) on a dry

4 basis a any ime, iniial and equilibrium, respecively. The equilibrium moisure conen (e) was assumed o be zero for microwave, infrared drying ec. and he R equaion (Equaion 1) was simplified as Equaion [16]: R, () o Drying Rae The drying rae during he experimens was calculaed using he following formula: d d d, (3) d where is he drying ime (min), and +d are he moisure conen a and + d (g waer/g dry maer) respecively. Effecive moisure diffusiviy The effecive moisure diffusiviy is herefore calculaed by he following equaion [17]: R 0 e e 8 n0 1 (n 1) (n 1).. D exp 4L eff. (4) where, D eff is he effecive moisure diffusiviy (m /s); L is he half-hickness of he slab in he samples (m); and n is a posiive ineger. In pracice, only he firs erm of Eq. (4) is used o yield [18]: 8 D e R exp 0 e 4L eff (5), The effecive moisure diffusiviy (D eff) was also ypically calculaed by using he slope of Eq. (5). A sraigh line wih a slope of k o was obained when ln(r) was ploed versus he ime: Table 1. Lis of models. Deff ko (6) 4L, Using he slope value of (Eq. 6), he effecive moisure diffusiviy could be deermined. The saisical modelling procedure In order o deermine he moisure raio as a funcion of drying ime, seven differen hin-layer drying models, namely Lewis, Henderson & Pabis, Page, Logarihmic, Aghbashlo e al., Verma e al. and idilli e al. models were used (Table 1). Saisical analysis The saisical analysis of he experimenal daa was deermined using he STATISTICA compuer program. Three crieria of saisical analysis were used o evaluae he adjusmen of he experimenal daa o he differen models; he coefficien of deerminaion (R ), reduced chi-square ( ) and roo mean-square error (RSE). The parameers can be calculaed as follow:, (7), (8) where R exp,i and R pre,i are he experimenal and prediced dimensionless R, respecively, N is he number of he daa values and z is he number of consans of he models. Higher R, smaller χ and RSE values indicaed a beer fi of he experimenal daa o he model [6]. odel name odel Reference Lewis R exp( k) [19] Henderson and Pabis R aexp( k) [0] Page n R exp( k ) [1] Logarihmic R a k c Aghbashlo e al. exp [] k1 R exp 1 k R aexp( k) (1 a)exp( g Verma e al. ) [3] [4] idilli e al. n R a k ) b exp( [5] 483

5 Colour measuremen The sample colour before and afer he drying process was measured using a Chromameer CR (inola, Japan). For he dried samples hree parameers L, a and b, which indicae he brighness (on a lighness darkness scale), greenness-redness and blueness-yellowness respecively, were used o sudy he changes in colour. The Chroma was deermined using he following equaion [7]. C * a b, (9) Energy consumpion Energy consumpion in he infrared In he IR dryer, sum of he energy was consumed by he IR lamp o dry he pumpkin samples. E is he oal energy consumpion during he infrared drying ha is calculaed using he following equaion (10). E P*, (10) where E is he oal energy consumpion (kwh), P is he infrared power level (kw), is he drying ime (h). Energy consumpion in he microwaves The energy consumpion value required for drying pumpkin slices in he microwaves was calculaed wih Equaion (11) [8]. dryer ( C), D is he oal drying ime of each sample (h). Calculaion of he specific energy consumpion The oal energy consumpion of he drying process was evaluaed hrough he Specific Energy Consumpion (SEC). Elecrical energy was consumed during he drying process. The specific energy consumpion, which is a measure of he energy needed o evaporae a uni mass of waer from he produc, was calculaed using he following equaion [30]: Q Qs m w, (13) where Q s is he specific energy consumpion in kwh *kg -1 [H O], Q is he consumed energy in kwh, m w is he mass of vaporized waer in kg [H O]. RESULTS AND DISCUSSION Drying curves Figure 1 shows he moisure raio as a funcion of drying ime for he differen drying mehods and condiions. E P*, (11) where E is he oal energy consumpion (kwh), P is he microwave power oupu (kw), is he drying ime (h). 484 Energy consumpion in he ho air In ho air drying, he oal energy consumpion was due o he drying and blowing of air by an elecric heaer and fan, respecively. The oal energy consumpion value was calculaed from Equaion (1) [9]: E a AvcpT * D, (1) where E is he oal energy consumpion (kwh), a is he air densiy (kg/m 3 ), A is he cross secional area of he conainer (m ), herein a sample is placed, υ is he air velociy (m/sec), c p is he specific hea (kj/kg C), T is a emperaure difference beween he air inle and oule of he Fig. 1. Drying curves of pumpkin slices a differen drying mehods and condiions. As seen from Figure 1, he drying ime was quie differen for four drying mehods o reach he final moisure raio. The drying ime of he dried pumpkin slices wih differen drying mehods ranged from 14 o 1140 minues. The drying curves are ypical and similar o fruis and vegeables. In Fig. 1 i is discerned ha he increase in he microwave power level [31, 3], air emperaure [33, 34] and infrared power level [35] shorened he drying period in microwave drying, air-drying and infrared drying, respecively. The moisure raio decreases gradually wih he increases in drying ime, exhibiing a downward curve. The figure also indicaes ha he drying ime for microwave drying is much shorer han he ho air, infrared and opensun drying. The ime required o reduce he

6 moisure raio from 1 o ranged beween 14 and 36 min a wo power levels wih microwave drying, while i ranged beween 70 and 480 min a wo emperaures wih ho air drying, while i ranged beween 10 and 405 min a wo power levels wih infrared drying. To reach he desired final moisure conen, 14 minues drying ime indicaed he high efficiency of he microwave drying mehod (180 W), which was abou 8 imes faser han open-sun drying. The drying ime of pumpkin slices dried under he sun was deermined as 19 h. The drying ime was much faser han for he oher drying mehods as compared o he condiions by microwave drying of pumpkin slices. For his reason, he drying rae (g waer)/(g dry maer*min) curves are shown in he wo differen figures as Figure 1 and in order o clearly see he drying rae curves. Fig.. Drying rae versus moisure conen of pumpkin slices a wo differen microwave power levels. Since he iniial moisure conen of he pumpkin slices was relaively consan (11.5 g waer/g dry maer), he difference in he drying ime requiremens was considered mainly due o he difference in he drying raes. The drying rae curves for pumpkin slices dried a wo microwave oupu power levels (90 o 180 W) are given in Figure. Depending on he drying condiions, he average drying raes of pumpkin slices ranged from o (g waer)/(g dry maer*min) for he oupu power beween 90 and 180 W, respecively. The moisure conen of he pumpkin (9% w.b.) was very high during he iniial phase of he drying process which resuled in a higher absorpion of microwave power level and higher drying raes due o he higher moisure diffusion. As he drying ime progressed, he moisure loss in he produc caused a decrease in he absorpion of microwave power and resuled in a fall in he drying rae. Higher drying raes were deermined a higher microwave oupu power levels. Thus, he microwave oupu power level had a crucial effec on he drying rae. The variaion of drying rae wih moisure conen for he differen drying condiions of ho air, infrared and open-sun drying are shown in Figure 3. As expeced, he drying rae would decrease as he moisure conen decreases. As can be seen from hese figures, no consan rae period exiss. All he drying processes occurred in he falling rae period. In he infrared drying experimens (83 and 15 W) iniially rising bu afer a while falling he drying rae period is presen. The experimen, in which he infrared power level is 15 W, he rising drying rae period is clear by far. During he falling drying rae period, he predominan mechanism is he inernal mass ransfer. The open sun drying experimen lased for approximaely 19 h. Consequenly, a small rising drying rae period formed in he figure ha originaed as a resul of he drying process coninued he nex day. Similar resuls were repored by [36, 14, 37]. ahemaical odelling of Drying Daa The values of R, χ and RSE were calculaed and given in Table. Fig. 3. Drying rae versus moisure conen of pumpkin slices a differen drying mehods and condiions. 485

7 Table. Saisical resuls obained from he seleced models. odels Drying mehods Drying condiions R χ RSE Lewis Infrared 83 W Infrared 15 W icrowave 90 W icrowave 180 W Sun 36 o 49 C Ho - air 50 o C Ho - air 70 o C Henderson Pabis & Infrared 83 W Infrared 15 W icrowave 90 W icrowave 180 W Sun 36 o 49 C Ho - air 50 o C Ho - air 70 o C Page Infrared 83 W Infrared 15 W icrowave 90 W icrowave 180 W Sun 36 o 49 C Ho - air 50 o C Ho - air 70 o C Logarihmic Infrared 83 W Infrared 15 W icrowave 90 W icrowave 180 W Sun 36 o 49 C Ho - air 50 o C Ho - air 70 o C Aghbashlo e al. Infrared 83 W Infrared 15 W icrowave 90 W icrowave 180 W Sun 36 o 49 C Ho - air 50 o C Ho - air 70 o C Verma e al. Infrared 83 W Infrared 15 W icrowave 90 W icrowave 180 W Sun 36 o 49 C Ho - air 50 o C Ho - air 70 o C idilli a al. Infrared 83 W Infrared 15 W icrowave 90 W icrowave 180 W Sun 36 o 49 C Ho - air 50 o C Ho - air 70 o C

8 Table 3. Colour parameers of fresh and dried pumpkin slices. Drying mehods Colour parameers Drying condiions C* L a b Fresh İnfrared 83 W W icrowave 90 W W Ho - air 50 o C o C pen - sun o C Table 4. Effecive moisure diffusiviy values for he various drying mehods. Drying mehods Drying condiions D eff (m s -1 ) İnfrared 83 W 3.4 x W 6.09 x icrowave 90 W 0.85 x W.48 x 10-7 Ho - air 50 o C.78 x o C 9.38 x pen - sun o C.96 x Thin-layer drying models, in oher words he Lewis, Henderson and Pabis, Page, Logarihmic, Aghbashlo e al., Verma e al. and idilli e al. were used o describe he drying process during he drying of pumpkin slices. In order o describe he moisure raio as a funcion of drying ime wih differen drying mehods, 7 differen drying models were fied o he experimenal daa and heir coefficien of deerminaion (R ), reduced chi-square ( ) and roo mean-square error (RSE) were calculaed. R, RSE and χ saisical daa wih respec o 7 differen drying models are given in he Table. The grade of fiing was deermined by he lowes and RSE and he highes R values. From Table, he saisical daa wih respec o 7 differen drying models used for explaining he drying circumsance occurred in he falling rae drying period was examined individually and using he idilli e al. model and provided he minimum error for a separable moisure rae. A drying he process of pumpkin slices sandard error (RSE) of predicion conduced by his model ranged from o In addiion o his as seen from he able, he chi-square ( ) values ranged from o which are close o zero. Adequacy of modelling ranged from o Effec of drying mehods and condiions on he colour of pumpkin slices Before and afer he drying process, he L (lighness), a (greenness), b (yellowness) and C* (Chroma) values of pumpkin slices were measured and hese resuls are given in Table 3. One of he qualiy of he parameers of food and agriculural produc is he colour parameer. Too much colour changes influence he markeing negaively by affecing he qualiy of he produc. I is an index of he inheren good qualiies of a food and he associaion of colour wih he accepabiliy of a food is universal. Among he several basic qualiy characerisics of dried pumpkin slices he colour is an imporan one which indicaes he effec levels of differen drying mehods or condiions. As saed previously, he L erm sands for brighness, he a erm is for a green-red balance, he b erm is for a blue-yellow balance, he Huner colour raio and he chroma are measures for he colour puriy. Values for he L, a, b and chroma (C*) coordinaes of he fresh pumpkin slices were 67.75, 11.06, 1.77 and 4.41, respecively. 487

9 In general, he infrared drying a 83 and 15 W produced no remarkable changes in he colour parameers of he pumpkin slices as compared wih he fresh pumpkin. However, he increase of power level from 83 W o 15 W caused an increase of he a and b values and a decrease of he L value. The microwave irradiaion drying a 90 W allowed he obaining of a produc which was more similar o he fresh sample when he lighness was considered since he L value varied only from o When comparing he values obained for he opposing colour parameers beween he fresh sample and he pumpkin slices dried a 90 W, he a decreased from o 0.59, showing ha he red colour decreased wih he drying process. As for he b, i increased slighly from 1.77 o 7.8, he dried samples were more yellow. These observaions are corroboraed wih he increase from 4.41 o 7.8 in he value of he chroma. The dried pumpkin a 180 W urns ino he final produc as much ligher, much less red and more yellow wih L, a and b values for which he values are 57.11, 3.89 and 6.31, respecively. The colour crieria obained from he air drying experimens using 50 and 70 C emperaures are given in Table 3. According o his he colour is closes o he fresh produc when he air drying process s emperaure is 50 C and 75 C. When he drying air emperaure increased from 50 o 75 C; he L, a values decreased and b values increased. These resuls were consisen wih he observaions made by differen auhors on he drying of pumpkin (Cucurbia pepo L.) slices [38, 39]. As seen from Table 3, he bes colour values were achieved during he open sun drying and hese values are he closes o he fresh maerial. This is followed by ho air-drying, microwave drying and infrared drying successively. Similar findings are also available in he lieraure [40, 41]. Deerminaion of Effecive oisure Diffusiviy The effecive moisure diffusiviy was calculaed using he mehod of slopes. Effecive diffusiviies are ypically deermined by ploing he experimenal drying daa in erms of ln (R) versus ime. From he Eq. (6), a plo of ln (R) versus ime gives a sraigh line wih a slope (k o). This slope is he measure of he diffusiviy. The effecive diffusiviy values for various drying mehods and condiions are presened in Table 4. Among he four drying mehods, microwave drying offered he highes values of D eff for microwave power levels of 90 and 180 W. In microwave drying, he D eff values increased wih he increasing drying microwave power. If samples were dried a higher microwave power, increased heaing energy would increase he aciviy of he waer molecules leading o a higher moisure diffusiviy. The value of D eff for infrared and ho-air drying was slighly higher han he open sun drying. In drying pumpkin slices by he infrared and ho-air drying mehods, he effecive moisure diffusiviy value increases wih increasing power and emperaure. In foods he effecive moisure diffusiviy values are in he range of 10-1 o 10-6 m /s and he accumulaion of values is in he region o 10-8 m /s (75%) [4, 43]. In he lieraure here is no sudy associaed wih he drying of pumpkin slices (Cucurbia pepo L.) wih infrared and open sun drying. Some sudies dealing wih he drying of pumpkin slices wih microwaves are available in he lieraure bu daa abou he effeciveness of moisure diffusiviy does no exis. The D eff value of pumpkin slices (Cucurbia pepo L.) [8] undergoing ho-air drying a 50 and 60 o C was in he range of o m /s, respecively. Energy consumpion The energy consumpion values obained in he drying rials carried ou wih hree differen drying mehods are given in Figure 4. When hree differen drying mehods were compared wih he energy consumpion values, he lowes energy consumpion occurred in he microwave drying mehod and his was followed by he infrared and ho-air drying mehods. Energy consumpion is zero for he open sun drying so his drying mehod isn accouned for in Figure 4. As seen in Figure 4, he oal energy consumpion decreases wih he increasing air emperaure and power level. The bes resul wih regard o energy consumpion was obained for microwave drying a he 180 W power level. The energy consumpion a his level was 0.04 kwh. Among all he drying mehods he highes value wih respec o he energy consumpion was obained for he ho-air drying process a a emperaure of 50 o C and 1.35 kwh. Th energy consumpion was , and kwh for microwave, infrared and ho-air drying, respecively. As a resul, he energy consumpion in he drying processes carried ou a low emperaure and power levels which yielded a longer drying period was deermined o be a higher raes. These resuls agree wih he observaions of previous researchers [44, 45]. 488

10 Fig. 4. Energy consumpion versus differen drying mehods of pumpkin slices during he drying process. Fig. 5. Specific energy consumpion versus differen drying mehods and condiions for drying of 1 kg we produc. Specific energy consumpion The specific energy consumpion was deermined by considering he oal energy supplied o dry pumpkin samples from an iniial moisure conen of abou 11.5 g waer/g dry maer o a final moisure conen of approximaely 0.05 o 0.10 g waer/g dry maer in all hree dryers. The specific energy consumpion of he drying process under he differen drying mehods and condiions was calculaed by Equaion (13) and his graph is given in Figure 5. As can be undersood from Figure 5, a minimum hea energy (1.6 kwh/kg) is needed by he microwave drying mehod o dry 1 kg of pumpkin slices. The maximum energy (16.08 kwh/kg) is needed for he ho-air drying mehod. Because of a minimum of hea energy consumpion (1.6 kwh/kg) and less drying ime (14 min) i can be said from Figure 5 ha he microwave drying mehod mus be seleced for drying he fresh pumpkin samples. Again as seen from Figure 5, reducing he specific energy consumpion was observed by he increase in power level and emperaure. These resuls are similar o hose repored by he researchers for he oher producs [46, 47, 8, 48]. CONCLUSION The pumpkin (Cucurbia pepo L.) has an imporan place for our counry s vegeable producion. Obaining new producs by drying pumpkin slices will increase he income gained from pumpkin producion/processing and his siuaion will allow for he consumpion of pumpkin all he year round. Researchers were moivaed o prospec using differen combinaions of drying echnologies because of he increasing rends in energy cos, produc qualiy and produc quaniy. Based on he conduced experimens, we can draw he following conclusions. The vacuum drying process is no suiable for he drying of pumpkin slices. The bes resul based on he drying period, coefficien of diffusion and specific energy consumpion was obained by he microwave drying mehod a he 180 W oupu power level. 489

11 In drying pumpkin slices by his mehod, he drying period was found o be 14 min, he coefficien of diffusion was.48 x 10-7 m s -1, he energy consumpion was found o be 0.04 kwh and he specific energy consumpion was found o be 1.6 kwh/kg. For he foodsuffs high L, b and low a values are imporan parameers. The measured colour parameers of he dried samples compared o fresh, he bes colour qualiy was obained in he pumpkin slices dried by he ho-air and open-sun drying mehods. The experimenal daa was obained as a consequence of drying pumpkin slices by five differen drying mehods and he condiions were modelled wih seven differen hin layer drying models available in he lieraure. The idilli e al. model, which will be used for deermining he changing of he produc s moisure conen, has a high modelling efficiency. Therefore i is possible o obain resuls very close o he experimenal values. The opinion is ha he microwave drying mehod can be easily applied indusrially and offers uniform high qualiy producs o he consumer. 490 REFERENCES 1. R.W. Robinson, D.S. Decker-Walers, Cucurbis, CAB Inernaional, New York, NY, (1997).. FAO, FAO Saisical Daabase (01), available from:hp:// 3..A. Karim,.N.A. Hawlader, In. J. Hea ass Transfer., 48, 4914 (005). 4. D. Arslan,.. Ozcan, LWT-Food Sci and Technol., 43, 111 (010). 5.. Zielinska,. arkowski, Chem Eng Process., 49, 1 (010). 6. D.B. Jadhav, G.L. Visavale, N. Suar, Drying Technol., 8, 600 (010). 7. F.. Berrui,. Klaas, C. Briens, F. Berrui, J Food Eng., 9, 196 (009). 8. İ. Doymaz, J Food Eng., 79, 43 (007). 9. A. Arevalo-Pinedo, F.E.X. urr, J Food Eng., 80, 15 (007). 10. G.P. Sharma, S. Prasad, J Food Eng., 50, 99 (001). 11. R. Vadivambal, D.S. Jayas, Biosys Eng, 98, 1 (007). 1.. Zhangz, J. Tang, A. J. ajumdar, S. Wang, Trends Food Sci Tech., 17, 54 (006). 13. C. Srumillo, T. Kudra, Drying: Principles, Applicaions and Design., Topics in Chemical Engineering. R.Hughes (ed.), Vol. 3. New York; Gordon and Breach Science Publishers, p. 371, (1986). 14. H. Kocabiyik, D. Tezer, In J Food Sci Tech., 44, 953 (009). 15. AOAC., Official mehod of analysis, Arlingon, USA, (1990). 16. R.A. Chayjan,. Kaveh, S. Khayai, J Food Process Preserv., 1745, 4549 (014). 17. J. Crank, The ahemaics of Diffusion, second ed. Oxford Universiy Press, London, UK, (1975). 18. K. Sacilik, J. Food Eng., 79(1), 3 (007). 19. J.S. Robers, D.R. Kidd, O. Padilla-Zakour, J. Food Eng., 89, 460 (008). 0. S.. Henderson, S. Pabis, J Agr Eng Res., 6, 169 (1961). 1. G. Page, [.S. hesis], Facors influencing he maximum raes of air-drying shelled corn in hin layer, Deparmen of echanical Engineering, Purdue Universiy, Wes Lafayee, IN R.P. Kingsly, R. K. Goyal,. anikanan, S.. Ilyas, In J Food Sci Tech., 4, 65 (007). 3.. Aghbashlo,.H. Kianmehr, S. Khani,. Ghasemi, In Agrophys., 3, 313 (009). 4. L.R. Verma, R.A. Bucklin, J.B. Endan, F.T. Wraen, T ASAE, 8, 96 (1985). 5. A. idilli, H. Kucuk, Z. Yapar, Drying Technol., 0(7), 1503 (00). 6. K.O. Falade, O.S. Ogunwolu, J. Food Process., 38, 373 (014). 7. S. Phoungchandang, S. Saenaweesuk, Food Bioprod Process., 89 (4), 49 (011). 8. A. oevali, S. inaei,. H. Khoshagaza, Energ Convers anage., 5(), 119 (011). 9.. Aghbashlo,.H. Kianmehr, H. Samimi- Akhijahani, Energ Convers anage., 49, 865 (008). 30. W. Jindara, P. Raanadecho, S. Vongpradubcha, Exp Therm Fluid Sci., 35(4), 78 (011). 31. L.V. ana, T. Orikasa, Y. uramasu, A. Tagawa, J Food Process Technol., 3, 10 (01). 3. J. Wang, J.S. Wang, Y. Yu, In J Food Sci Tech., 4, 148 (007). 33. T.Y. Tunde-Akinunde, G. O. Ogunlakin, J Food Sci Tech., 50 (4), 705 (013). 34. P.K. Wankhade, R.S. Sapkal, V. S. Sapkal, Procedia Eng., 51, 371 (013). 35. İ. Doymaz, J. Agric. Sci., 19, 44 (013). 36. D. Evin, Food Bioprod Process., 90, 33 (01). 37. J. Wang, K. Sheng, LWT., 39, 47 (006). 38. T. Rakcejeva, R. Galoburda, L. Cude, E. Srauniece, Procedia Food Sci., 1, 441 (011) Shafafi Zenoozians, H. Feng, S.. A. Razavi, F. Shahidi, H.R. Pourreza, J Food Process Pres, 3, 88 (008). 40. I. Alibas, LWT., 40, 1445 (007). 41. R.P.F. Guine, S. Pinho,. J. Barroca, Food Bioprod Process., 89, 4 (011). 4. Z. Erbay, F. Icier, Cri Rev Food Sci Nur., 50 (5), 441 (010). 43. P.S. adamba, R.H. Driscoll, K. A. Buckle, J. Food Eng., 9(1), 75 (1996). 44. I. Alibas, Biosysems Eng., 96(4), 495 (007). 45. H.S. El-esery, G. wihiga, Afr. J. Agric. Res., 7(31), 4440 (01). 46. A.R. Celma, F.L. Rodriguez, F. C. Blazquez, Food Bioprod Process., 87, 47 (009). 47. I. Das, S.K. Das, S. Bal, J Food Eng., 6, 9 (004). 48. G.P. Sharma, S. Prasad, Energy., 31, 191 (006).

12 ОЦЕНЯВАНЕ НА МЕТОДИТЕ И УСЛОВИЯТА НА СУШЕНЕ СПОРЕД КИНЕТИКАТА, КАЧЕСТВАТА НА ЦВЕТОВЕТЕ И СПЕЦИФИЧНАТА ЕНЕРГИЯ НА СУШЕНЕ НА ТЪНКИ СЛОЕВЕ ОТ ТИКВА О. Исмаил 1, Й.Г. Коджабай 1, 1 Департамент по химично инженерство, Факултет по химично и металургично инженерство, Кампус Давут паша, Технчески университет Йилдиз, 3410 Истанбул, Турция Регионална лаборатория по възстановяване и консервация в Истанбул, Турция Постъпила на 3 август 015 г.; коригирана на 1 септември 015 г. (Резюме) В тази работа са изследвани експериментално температурата на сушене на резени от тиква (Cucurbia pepo L.) и нивата на мощността. Образците са дехидратирани по пет различни метода: на открито, под вакуум, микровълни, инфрачервени лъчи и горещ въздух. Две нива на мощността са изпитани както следва: микровълни (90 и 180 W), инфрачервени лъчи (83 и 15 W) и горещ въздух (50 и 70 o C), за да се изследва ефекта на тези методи. Вакуумното сушене е изследвано при постоянна температура от 50 C и налягане от 0.1 kpa. Опитните данни за влагата са обработени по различни модели, известни в литературата (на Lewis, Henderson Pabis, Page, логаритмичен, Aghbashlo и др., Verma и др. и idilli и др.). Според тези резултати моделът на idilli и др. превъзхожда останалите в хода на сушенето на образците. Енергийната ефективност и дифузионните коефициенти нарастват с нарастване на мощността на микровълните. От цветова гледна точка най-добри резултати се получават при сушене с горещ въздух и на открито. 491