PaPer Ital. J. Food Sci., vol. 27 - 2015 459 - Keywords: eggplant, air drying, infrared - Determination of Drying CharaCteristiCs anD Quality ProPerties of eggPlant in Different Drying ConDitions gözDe Bayraktaroğlu urun1*, Ünal riza yaman2 and ergun köse1 1Department of Food Engineering, Faculty of Engineering, Celal Bayar University, Muradiye, Manisa, Turkey 2Department of Fruit-Vegetable Processing and Engineering Tire-Kutsan Vocational Training School, Ege University, Tire, İzmir, Turkey *Corresponding author: gozdebayraktarurun@yahoo.com AbstrAct Drying is the most traditional process used for preserving eggplant a long time. the aim of this study was to determining drying characteristics and quality properties of eggplant dried by sun drying, hot air convective drying and infrared assisted convective drying. convective drying and infrared assisted convective were carried out in a convective dryer at three different temperatures (40°, 50°, 60°c) and air velocity at 5 m/s. the increasing of temperatures during the drying of eggplant led to a significant reduction of the drying time. However loss of nutrition was observed in eggplant samples dried at higher temper- ature. the biggest change in colour parameters was observed in samples dried with sun drying. so it was thought that sun drying had a negative effect on quality properties of eggplant samples. mailto:gozdebayraktarurun%40yahoo.com?subject= 460 Ital. J. Food Sci., vol. 27 - 2015 IntroDuctIon Eggplant (Solanum melongena L.) is a com- mon annual vegetable crop grown in the sub- tropics and tropics (concELLon, 2012). Eggplant is an important market vegetable of Asian and Mediterranean countries and has a very limited shelf life for freshness (Wu et al., 2007; bouLE- KbAcHE-MAKHLouF et al., 2013). Its shelf-life at temperature of 10–15°c is about 10 days (Hu et al. 2010). the limited shelf-life consti- tutes a heavy drawback for commercial purpose (brAsIELLo et al., 2013). Drying which is the process of removal of most of the moisture present in the food is the oldest preservation method applied since an- cient times (AYHAn and ALİbAŞ, 2005; Er and AKbuLut, 2011; ALİbAŞ, 2012). the removal of moisture from the food materials prevents the growth and reproduction of spoilage microor- ganisms, slows down the action of enzymes and minimizes many of the physical and chemical reactions (cEYLAn et al., 2006; Wu et al., 2007; GuInE et al., 2012a). nowadays drying process of product is carried out by various methods such as sun drying, contact, convective, radi- ation, dielectric, vacuum, freeze drying and os- motic drying (KArAbAYIr, 2006). natural sun drying is practiced widely in the World and also turkey, but has some problems related to the contamination by dirt and dust and infestation by insects, rodents and other animals (KocA- bIYIK and DEMIrtÜrK, 2008). therefore, the convective drying process carried out in closed equipments is preferred (ErtEKIn and YALDIZ, 2004). convective drying is the most traditional de- hydration method used to preserve foods; it mainly consists of forcing air through the prod- uct to be dried. the surface area of the prod- uct to be dried, the drying time, drying tem- perature, air velocity, moisture content of air and atmospheric pressure determine drying ef- ficiency (cEMEroĞLu, 2004). convective dry- ing processing effectively extends the shelf life of agricultural products, however this drying process involves chemical, physical, structur - al and nutritional changes, linked to the water loss and the high temperatures applied, which affect the product quality (GArcIA-PErEZ et al., 2012). Loss of sensory and nutritive quali- ties is considered inevitable during traditional drying process due to the undesirable textur- al and biochemical changes (Wu et al., 2007). the expansion of dehydrated food market de- mands high quality products that maintain at a very high level the nutritional and sensorial properties of the initial fresh product (russo et al., 2013). Infrared radiation has significant advantages over conventional drying. these advantages are higher drying rate, energy sav- ing, and uniform temperature distribution giv- ing a better quality product. At present, many driers use infrared radiator to improve drying efficiency, save space and provide clean work- ing environment, etc. therefore infrared drying can become popular as an energy saving dry- ing method (WAnG and sHEnG, 2006). Drying times of carrot pomace dried at the in- frared power levels of 83, 125, 167 and 209 W were studied. According to the results, it was de- termined that drying rate increased and drying time decreased with increasing infrared power level (DoYMAZ, 2013). the effect of harvest time and drying tech- niques on the quality characteristics, which are specifically important for maize crop were in- vestigated. Energy expenses of the drying tech- niques were calculated for all harvest periods and it was found out that the expenses to re- duce the moisture level from 15 to 13% with hot air drying are higher than the expenses to reduce moisture level from 29 to 13% with in- frared-hot air drying combination (YILMAZ and tunÇEL, 2008). AbDELMotALEb et al. (2009) investigated thin layer drying of garlic slices under convection and combined infrared–convection heating modes and observed increases in drying rate, thermal efficiency, rehydration ratio, flavor strength and colour difference and decreases in drying time and specific energy consumption for the com- bined (infrared–convection) heating mode in comparison with convection only. Effects of infrared power, air temperature and air velocity on drying rate and quality of onion slices dried in infrared-convective dryer were studied. It was found that drying time of onion slices increased with increas- ing air velocity, and decreased with increas- ing temperature and infrared power (sHAr- MA et al., 2005). the objectives of this study were to investi- gate the drying characteristics of the eggplant samples, to examine the effect of drying condi- tions on the drying process, and to choose op- timum drying method for quality of dried egg- plant samples. MAtErIALs AnD MEtHoDs Sample preparation Fresh eggplants were obtained from Öcal Ag- ricultural Product Limited company (turgutlu, Manisa, turkey). Vegetables were washed and sliced (30 mm diameter and 6 mm thickness). Eggplant slices were placed over a metal grating in a convective oven operating at constant tem- perature (russo et al., 2013). Drying experiments Eggplant slices were subjected to drying with three methods. these drying methods were: Ital. J. Food Sci., vol. 27 - 2015 461 sun drying, hot air convective drying and in- frared assisted convective drying. convective drying and infrared assisted convective drying of eggplants were carried out in a drying sys- tem consisting of solar collector, carbon fiber infrared heaters, drying chamber, condenser, heat exchanger, hot water tank, and PLc (Pro- grammable logic controller) panel at three dif- ferent temperatures (40°, 50°, 60°c) and air ve- locity of 5 m/s. Drying kinetics In our study drying time was defined as the time passing from initial moisture content of the samples until final moisture content of sam- ples. Drying rate was described as the amount of water removed from the sample per unit of time. Effect of temperature on drying time and drying rate of eggplant samples was determined (nAsIroĞLu and KocAbIYIK, 2007). Specific energy consumption specific energy consumption is amount of en- ergy required for removing unit amount of wa- ter from samples during drying of samples. spe- cific energy consumption of eggplant samples dried in different conditions was calculated as follows Eq. (1): E s = E T /W R (1) E s : specific energy consumption (MJ/kg), E T : total energy (MJ), W R : Amount of water removed during drying (kg) (sHArMA and PrAsAD, 2006). Shrinkage shrinkage, which occurred during drying as a result of water evaporation, was evaluated by determination of the relative volume of dried ma- terial. the relative volume was the ratio of egg- plant slices volume after drying to that before drying as follows Eq. (2): V s = V/V 0 (2) V s = shrinkage V = Volume of dried samples V 0 = Volume of fresh samples (FIGIEL, 2010). Rehydration rehydration kinetics study was carried out for dried eggplant slices. the samples were placed in water at 45°c and waited for 5 h. the rehy- drated samples were spread on absorbent pa- per for the removal of free water on the surface of vegetable. the change in weight was record- ed after a regular interval of time. the rehydra- tion capacity was calculated from the ratio of sample weight after and before the rehydration as follows Eq. (3): r r = M r /M D (3) r r = rehydration ratio M r = Weight of rehydrated samples M D = Weight of dried samples (russo et al., 2013). Colour parameters colour of dried and fresh samples was evaluat- ed by means of a Minolta chroma Meter cr-300 (Minolta co. Ltd., osaka, Japan). Instrumental colour data were expressed as cIE L*, a*, b* co- ordinates, which define the colour in a three- dimensional space: L* (dark–light), a* (redness– green) and b* (yellowness–blueness). total col- our difference (∆E), chroma (c), hue angle (h) and r (a/b) values were calculated by using L*, a*, b* values in Eqs. (4)-(7). Eggplant slices were placed in container without space. colour meas- urements were performed twice (DEMIr and AK- buLut 2010; nAsIroĞLu and KocAbIYIK, 2007). (4) (5) (6) (7) Textural properties For determining the textural properties of fresh and rehydrated eggplant slices, texture profile analysis (tPA) was performed using a tex- ture Analyser (model tA.Xt.Plus). the texture profile analysis was carried out by two compres- sion cycles between parallel plates performed on cylindrical samples (diameter 10 mm, height 3 mm) using a flat 35 mm diameter plunger, with a 5 s of time between cycles. the parameters that have been used were the following: 50 kg force load cell and 0.5 mm s−1 test speed (nAYAK et al., 2007; GuİnE and bArrocA, 2012b; rus- so et al., 2013). the textural properties: hardness, springi- ness, cohesiveness, gumminess and chewiness were calculated after Eqs. (8)-(12): Hardness, H=F1 (8) springiness, s=∆t2/∆t1 (9) cohesiveness, c=A2/A1 (10) Gumminess, G=H x c (11) chewiness=H x s x c (12) 462 Ital. J. Food Sci., vol. 27 - 2015 Total Dry Matter Dry matter of dried and fresh samples was de- termined by drying the samples cut into small piec- es at 105°c to constant weight. total dry matter content of the samples was calculated from the dif- ference in mass before and after the drying process (cEMEroĞLu, 2007; ÖZtÜrK and ÇAPur, 2010). Water activity Water activity measurement set was used for determination of water activity values of all the samples. In this system, the product to mea- sured water activity was cut into small pieces, placed in a hermetic steel chamber. When hu- midity of air inside the container reached equi- librium with the product, equilibrium moisture content of samples was measured by probe in the container (HAstÜrK-ŞAHİn and ÜLGEr, 2010). Ascorbic acid (Vitamin C) A spectrophotometric method was used to de- termine the total amount of vitamin c in the egg- plant slices. the absorbance value of samples was measured by means of a uv-Visible spectro- photometer (shimadzu corp., Kyoto, Japan) with wavelength at 518 nm. Ascorbic acid of samples were calculated from standard curve showed absorbance values to concentrations of ascor- bic acid and expressed as microgram of ascor- bic acid per 100 gram of sample (HIŞIL, 2010). Statistical analysis In our study drying methods in different con- ditions were designed as applications and a com- pletely randomized design was used for statis- tical analysis. these applications were sun dry- ing, hot air convective drying (40°, 50°, 60°c) and infrared assisted convective drying (40, 50, 60°c). Effect of different drying methods and dry- ing conditions on drying characteristics, chemi- cal, physical and textural properties of eggplant samples was determined. number of replica- tion was two. In order to determine the differ- ences between applications, analysis of variance (AnoVA) was carried out using statistical Analy- sis software (sAs, 2001). Data found important in result of AnoVA were evaluated with Proc MIXED procedure. For every data LsMEAns val- ues were determined and least significant dif- ferences (LsD) between data were calculated. rEsuLts AnD DIscussIon Drying kinetics of eggplant samples In our project it was observed that drying method affected drying time of eggplant slic- es. Drying time of infrared assisted convec- tive drying and convective drying was shorter than drying time of sun drying. Infrared appli- cation decreased drying time of eggplant sam- ples during convective drying at air tempera- ture of 50° and 60°c. Also uMEsH-HEbbAr et al. (2004) reported that the combined infrared and hot air dryer reduced the processing time dramatically (48%), in addition to consuming less energy (63%) for water evaporation com- pared to hot air drying. In convective drying, drying time of the egg- plant samples showed reduction with increas- ing temperature. However drying rate of egg- plant samples increased at higher tempera- ture (Fig. 1). similarly the effect of temperature on the drying kinetics and quality attributes of apple (var. Granny smith) slices during drying was investigated and the experimental results of study showed that dehydration were faster Fig. 1 - Drying curves of eggplant samples dried with convective drying at three different temperatures. Ital. J. Food Sci., vol. 27 - 2015 463 when air temperature increased (VEGA-GALVEZ et al., 2012). Effect of air temperature on dry- ing time of cornelian cherry fruits dried in con- vective dryer was investigated, it was observed that increasing air temperature reduced drying time by 34% (KAYA and AYDIn, 2008). similar result were described by ErtEKIn and YALDIZ (2004) working about drying characteristics of eggplants dried using heated ambient at air temperatures from 30° to 70°c and it was stat- ed that drying time decreased with increasing drying air temperature. During the infrared assisted convective dry- ing, drying time of the eggplant samples showed reduction with increasing temperature. Howev- er drying rate of eggplant samples increased at higher temperature (Fig. 2). toĞruL et al. (2005) studied drying characteristics of banana slic- es dried in infrared dryer at drying tempera- ture ranging from 50° to 80°c and it was found that drying rate increased with increasing dry- ing temperature. When eggplant samples were dried with sun drying, it was determined that sun drying took a longer time than convective drying and infra- red assisted convective drying (Fig. 3). Fig. 2 - Drying curves of eggplant samples dried with infrared-convective drying at three different temperatures. Fig. 3 - Drying curves of eggplant samples dried with sun drying. 464 Ital. J. Food Sci., vol. 27 - 2015 table 1 - specific energy consumption values of dried egg- plant samples (kj/kg). Drying methods Eggplant CD (40°C) 0,0023±0,001 CD (50°C) 0,0040±0,004 CD (60°C) 0,0033±0,000 ICD (40°C) 0,0018±0,001 ICD (50°C) 0,0036±0,001 ICD (60°C) 0,0038±0,0001 SD 0±0,000 p=0.3071 CD: Convective drying, ICD: Infrared assisted convective drying, SD: Sun drying. table 2 - rehydration ratios of dried eggplant samples. Drying methods Eggplant CD (40°C) 5,27±0,23c CD (50°C) 5,71±0,26cb CD (60°C) 5,81±0,35ab ICD (40°C) 5,42±0,07cb ICD (50°C) 5,92±0,10ab ICD (60°C) 6,33±0,08a SD 5,50±0,31cb p=0.0283 LSD=0.539 Fig. 4 - change of rehydration for eggplant samples dried at different conditions. Specific energy consumption of dried eggplant samples In our study amount of energy required for drying of samples was calculated. specific en- ergy consumptions of dried eggplant samples are showed in table 1. As a result of statistical analysis, it was determined that there were not important differences between specific energy consumption values of dried eggplant samples at different drying methods (p>0.05). the lowest specific energy consumption of eggplant slices was measured during infrared assisted convec- tive drying at air temperature of 40°c. According to results obtained from previous study, specific energy for drying of mushroom slices in a hot air flow-infrared combination dryer increased with increasing temperature, while the specific ener- gy for mushroom drying in the convection dry- er decreased with increasing temperature (Mİ- nAEİ et al., 2011). Rehydration ratio of dried eggplant samples In our project rehydration ratio values of egg- plant samples dried with different drying meth- ods were determined. Data obtained as a re- sult of analysis were given in table 2. Increas- ing air temperature increased rehydration ra- tio values of eggplant samples. similarly rus- so et al. (2013) did scientific study about dried and rehydrated eggplant and state that samples dried at higher temperature showed faster wa- ter uptake during rehydration because of wrin- kled structure. It was determined that drying methods had a significant effect on rehydration ratio values of dried eggplant samples (p<0.05). Especially it was observed that there was important dif- ference between the rehydration ratio values of eggplant samples dried with convective drying at 40°c and the rehydration ratio values of egg- plant samples dried with infrared assisted con- vective drying at 60°c. change in rehydration ratio of dried eggplant samples was given in Fig. 4. rehydration ratio of dried eggplant samples reached to maximum value in five hour. Drying method did not affect rehydration time of dried eggplant samples. Ital. J. Food Sci., vol. 27 - 2015 465 table 3 - shrinkage of dried eggplant samples. Drying methods Eggplant CD (40°C) 0,225±0,05 CD (50°C) 0,239±0,02 CD (60°C) 0,278±0,024 ICD (40°C) 0,217±0,032 ICD (50°C) 0,254±0,076 ICD (60°C) 0,271±0,012 SD 0,230±0,034 p=0.6868 Shrinkage of dried eggplant samples shrinkage values of dried eggplant were de- termined with analysis and showed in table 3. Increasing of air temperature in convective dry- er caused increasing of shrinkage values of egg- plant slice. As a result of statistical analysis it was found that drying conditions did not have an important effect on shrinkage values of dried eggplant samples (p>0.05). However LEWIcKI and JAKubcZYK (2004) investigated mechanical properties of apples dried at drying temperature ranging from 50° to 80°c in a laboratory con- vection dryer and found that the increasing dry- ing temperature caused the gradual decrease of shrinkage values. Chemical properties of eggplant samples In our project total dry matter, water activity, ascorbic acid and loss of ascorbic acid of egg- plant samples were detected. these data were showed in table 4. Eggplant samples were dried to 90% dry matter content. the highest ascor- bic acid loss in dried samples was determined in eggplant samples dried with infrared assist- ed convective drying at 60°c. the lowest ascor- bic acid loss was observed in samples dried with sun drying. Effect of different drying methods on chemical properties of eggplant samples was examined as statistical, it was determined that drying meth- ods had a significant effect on total dry matter, water activity, ascorbic acid and loss of ascor- bic acid of eggplant samples (p<0.05). Textural properties of eggplant samples In our thesis project hardness, springiness, cohesiveness, gumminess and chewiness val- ues of fresh and rehydrated samples were mea- sured for determining textural properties of egg- plant samples. textural properties of eggplant samples were given in table 5. As a result of statistical examination of textur- al properties of rehydrated samples, rehydration process was found important in terms of hard- table 4 - chemical properties of eggplant samples. Samples Total Dry Matter Water Activity Ascorbic Acid Loss of Ascorbic (mg/100g) Acid (%) Fresh 8,81±0,516a 0,991±0,010a 16,22±0,08a - CD (40°C) 90,73±2,520b 0,594±0,013b 12,36±0,625bc 23,76±4,20cd CD (50°C) 92,12±0,177b 0,501±0,026d 11,44±0,577c 29,46±3,24bc CD (60°C) 90,25±1,582b 0,555±0,033bcd 9,74±0,481d 39,92±3,25ab ICD (40°C) 90,27±1,117b 0,568±0,002bc 12,53±0,577bc 22,73±3,21cd ICD (50°C) 91,43±4,582b 0,584±0,040bc 11,37±0,866c 29,86±5,03bc ICD (60°C) 89,21±4,509b 0,591±0,050b 9,54±0,962d 41,18±5,65a SD 87,75±0,080b 0,522±0,005cd 13,76±0,962b 15,18±5,57d p<0.0001 p<0.0001 p=0.0002 p=0.0053 LSD=5.864 LSD=0.064 LSD=1.6135 LSD=10.465 table 5 - textural properties of fresh and rehydrated eggplant samples. Samples Hardness (N) Springiness Cohesiveness Gumminess (N) Chewiness (N) Fresh 148,8±2,73a 0,670±0,026 0,577±0,005e 85,90±2,28a 57,17±0,160a RCD (40°C) 50,78±8,23b 1,877±1,428 0,708±0,023dc 36,00±4,29b 32,12±2,56b RCD (50°C) 47,27±3,88b 0,816±0,003 0,693±0,019d 32,81±3,47b 27,49±3,56b RCD (60°C) 44,65±7,71b 1,895±1,465 0,705±0,017dc 31,48±6,02b 27,19±4,28b RICD (40°C) 52,59±3,67b 1,331±0,792 0,754±0,043ab 39,22±0,814b 32,39±2,13b RICD (50°C) 47,77±9,51b 0,878±0,084 0,686±0,010d 32,55±5,77b 28,77±7,84b RICD (60°C) 42,08±8,68b 0,889±0,013 0,747±0,016abc 31,21±7,27b 27,69±5,71b RSD 41,63±7,04b 0,881±0,033 0,774±0,009a 32,26±5,10b 28,48±3,56b P<0.0001 p=0.6138 p=0.0003 p<0.0001 p=0.0015 LSD=15.879 LSD=0.048 LSD=11.07 LSD=9.9053 RCD: Rehydrated Convective drying. 466 Ital. J. Food Sci., vol. 27 - 2015 ness, cohesiveness, gumminess and chewiness values of samples (p<0.05). However it was de- termined that there was not an important dif- ference between the springiness values of egg- plant samples (p>0.05). results obtained from research indicated that hardness, gumminess and chewiness values of rehydrated samples were smaller than those of fresh samples. VEGA-GALVEZ et al. (2008) stud- ied with red pepper samples (Capsicum annuum L.) dried at four air inlet temperatures from 50° to 80°c and rehydrated in water at 30°c and found that firmness was significantly affected by the temperature used during drying. Colour parameters of eggplant samples In our project L*, a*, b* values of fresh and dried eggplant samples were measured during determining colour parameters of samples. chroma, hue angle, r(a/b) and total colour dif- ference (∆E) values were calculated by using L*, a*, b* values of samples. colour parameters of samples were showed in table 6. As a result of statistical examination, it found that drying method did not affect to a* and ∆E values of eggplant samples (p>0.05), however it was determined that there were significant difference between L*, b*, chroma, hue angle, r(a/b) values of samples (p<0.05). so it was concluded that drying process had a significant effect on colour parameters of fresh samples. ErtEKIn and YALDIZ (2004) also investigated the effect of drying air temperature on colour pa- rameters of the eggplant samples and suggested that increasing drying air temperature decreased the colour lightness and raised the saturation. From the results of the present work it was concluded that drying process increased a*, r(a/b) values of eggplant samples, while de- creased L*, b*, chroma and hue angle values of eggplant samples. the biggest change in colour parameters was observed in samples dried with sun drying during the examination of colour pa- rameters of fresh and dried eggplant samples. therefore it was possible to determined that sun drying had a negative effect on quality proper- ties of eggplant samples. table 6 - colour parameters of eggplant samples. Samples L* a* b* Chroma Hue angle R (a/b) ΔE Fresh 61,64±1,23a 3,74±0,02 19,55±0,648a 19,91±0,636a 79,27±0,197a 0,190±0,004b - CD (40°C) 50,10±0,108b 6,97±1,72 17,96±0,616ab 19,91±0,308a 69,69±4,98b 0,378±0,107a 12,53±2,12 CD (50°C) 47,52±2,52bc 7,20±0,101 16,98±0,062bc 18,45±0,015ab 66,95±0,372b 0,426±0,008a 14,84±1,37 CD (60°C) 47,85±2,28bc 6,09±0,318 15,45±0,847c 16,61±0,670c 68,48±2,16b 0,395±0,044a 14,72±1,34 ICD (40°C) 47,81±0,986bc 5,67±1,02 15,31±0,554c 16,34±0,872c 69,77±2,62b 0,369±0,052a 14,26±0,522 ICD (50°C) 47,91±1,64bc 6,48±0,621 17,87±1,37ab 19,01±1,50ab 70,05±0,325b 0,363±0,006a 13,93±2,40 ICD (60°C) 50,17±1,00b 5,30±0,477 17,04±1,25bc 17,86±1,05bc 72,58±2,66b 0,315±0,051a 12,10±0,055 SD 44,77±4,01c 5,85±1,03 16,79±0,268bc 17,80±0,082bc 70,79±3,41b 0,349±0,067a 17,27±5,16 p=0.001 p=0.0591 p=0.0126 p=0.0121 p=0.0343 p=0.0491 p=0.496 LSD=4.7391 LSD=1.8826 LSD=1.832 LSD=6.0647 LSD=0.1246 concLusIons In our thesis project, the drying characteris- tics of the eggplant slices dried by sun drying, hot air convective drying (40°, 50°, 60°c) and in- frared assisted convective drying (40°, 50°, 60°c) were studied. 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