ORIGINAL PAPER

Effect of Air-Drying Conditions on Physico-chemicalProperties of Osmotically Pre-treated Pomegranate Seeds

Brahim Bchir & Souhail Besbes & Romdhane Karoui &Hamadi Attia & Michel Paquot & Christophe Blecker

Received: 10 August 2010 /Accepted: 3 November 2010 /Published online: 18 November 2010# Springer Science+Business Media, LLC 2010

Abstract The drying of pomegranate seeds was investigat-ed at 40 °C, 50 °C and 60 °C with air velocity of 2 m/s.Prior to drying, seeds were osmodehydrated in 55 °Brixsucrose solution for 20 min at 50 °C. The drying kineticsand the effects of osmotic dehydration (OD) and air-dryingtemperature on antioxidant capacity, total phenolics, colourand texture were determined. Analysis of variance revealedthat OD and air-drying temperature have a significantinfluence on the quality of seeds. Both anthocyanin andtotal phenolic contents decreased when air-drying temper-ature increased. The radical diphenylpicril-hydrazyl activityshowed the lowest antioxidant activity at 60 °C. Bothchromatic parameters (L*, C* and h°) and browning indexwere affected by drying temperatures, which contributed tothe discolouring of seeds. The final product has 22%, 20%and 16% of moisture; 0.630, 0.478 and 0.414 of aw; 151,141 and 134 mg gallic acid equivalent/100 g fresh matter(FM) of total phenolics; 40, 24, 20 mg/100 g FM ofanthocyanins and 46%, 39% and 31% of antioxidant

activity, for drying temperatures of 40 °C, 50 °C and60 °C, respectively. In view of these results, the tempera-ture of 40 °C is recommended as it has the lowest impact onthe quality parameters of the seeds. Differential scanningcalorimetry data provided complementary information onthe mobility changes of water during drying. Glasstransition temperature (Tg′) depends on moisture contentand as consequence, on drying conditions. In fact, Tg′ ofseeds dried at 60 °C (Tg′=−21 °C) was higher than thosedried at 50 °C (Tg′=−28 °C) or 40 °C (Tg′=−31 °C) andosmodehydrated seeds (Tg′=−34 °C). During OD anddrying process, the texture of seeds changed. The thicknessof seeds shrank by 55% at 60 °C.

Keywords Pomegranate . Osmotic dehydration . Drying .

Antioxidant activity . Differential scanning calorimetry .

Texture

Introduction

Pomegranate (Punica granatum L.) presents a virtualexplosion of interest as a medicinal and nutritional product.Recently, more than 475 new products containing pome-granate (food and drinks) were created in United States,including a chewing gum named “pomegranate power”, achicken sauce containing pomegranate, etc. (Storey 2007).

The edible part of the fruit (seeds) contains a consider-able amount of sugars, vitamins, polysaccharides, mineralsand polyphenols (Espiard 2002). Due to their polyphenolcompounds (e.g. anthocyanins), condensed tannins(e.g. proanthocyanidins) and hydrolysable tannins (e.g.ellagitannins and gallotannins), pomegranate seeds possessanti-oxidative properties (Hernandez et al. 1999; Jaiswal etal. 2010). In fact, these compounds are able to reduce the

B. Bchir (*) : R. Karoui :C. BleckerGembloux Agro-Bio Tech, University of Liege,Passage des Déportés, 2,5030, Gembloux, Belgiume-mail: b.bchir@student.ulg.ac.bee-mail: bchbrahim@yahoo.fr

S. Besbes :H. AttiaLaboratory of Food Analyses,National Engineering School of Sfax,Route de Soukra,3038, Sfax, Tunisia

M. PaquotDepartment of Industrial Biological Chemistry,Gembloux Agro-Bio Tech, University of Liege,Passage des Déportés, 2,5030, Gembloux, Belgium

Food Bioprocess Technol (2012) 5:1840–1852DOI 10.1007/s11947-010-0469-3

formation of free radical compounds that cause oxidationreactions associated with biological complications suchas aging, cardiovascular disease and carcinogenesis(Rosenblat et al. 2006).

Despite all these advantages, the consumption ofpomegranate seeds is limited to the crop season due toproblems of preservation (Defilippi et al. 2006). Indeed, themajor cause limiting the potential of pomegranates isthe development of decay, which is often caused by thepresence of fungal inoculum in the blossom end of the fruit.During long-term storage, rind scald symptoms appear as asuperficial browning (Defilippi et al. 2006).

Preservation methods can be used to increase the shelf-life of fruits; among them, there are drying, pasteurization,osmotic dehydration, etc. (Raoult-Wack et al. 1991).Freezing is also a preservation method; however, thistreatment involves modifications of the texture and cellstructures (Bchir et al. 2010a; b). As consequence, frozenfruit cannot be consumed directly after thawing. Neverthe-less, freezing could be an excellent pre-treatment forosmotic dehydration of fruit and vegetable, improvingsignificantly mass transfer during osmotic process. Ourprevious investigations showed that freezing before osmoticdehydration provided 1.4 and 3.5 times more water loss andsolids gain, respectively, than an untreated pomegranateseeds (Bchir et al. 2010a). Osmotic process has receivedconsiderable attention as a pre-drying treatment to reduceenergy consumption and improve food quality (El-Aouar etal. 2003; Ruiz-Lopez et al. 2010). According to Pokharkaret al. (1997) and Uribe et al. (2010), the main advantages ofthe osmotic dehydration process are: retention of naturalcolour without addition of sulphites and high retention ofvolatile compounds during subsequent drying.

After osmotic process, water activity of sample wasfound to be higher than 0.900, allowing development ofmicroorganisms (e.g. bacteria, fungi), and some undesirablereactions such as enzymatic and non-enzymatic browningreactions, fat oxidation, vitamin degradation and proteindenaturation during storage (Bchir et al. 2009, 2010c). As aconsequence, a complementary treatment such as dryingmay enable better conservation of pomegranate seeds.

Drying is the most commonly used method for fooddehydration since it is the most rapid process; it inhibitsenzymatic degradation, limits microbial growth and produ-ces a uniform dried product (Harbourne et al. 2009; Uribeet al. 2009). In this context, various fruits and vegetablessuch as onions (Singh and Sodhi 2000), red pepper(Doymaz 2007), garlic cloves (Sharma et al. 2003), earcorn (Friant et al. 2004), apricots (Doymaz 2007) andmulberry (Doymaz 2007) have been dried, despite severalnegative reactions such as shrinkage, loss of colour, textureand nutritional–functional properties (Arabhosseini et al.2009; Miranda et al. 2009).

The aim of the present study was to: (a) investigate thekinetics and influence of air-drying temperature on masstransfer and (b) determine the impact of drying temper-atures on antioxidant activity, phenolic, anthocyanin con-tent, colour development and texture of pomegranate seeds.

Materials and Methods

Materials

Fresh pomegranate fruits (P. granatum L.) of El Gabsi varietywere grown and harvested in Gabes during autumn (2009).Tunisia pomegranate fruits were collected at full ripenessstage (weight, ∼500 g; skin colour: red; juice colour: pink;juice pH, ∼4.4; °Brix, 15 g/100 g; skin thickness, ∼4 mm).Pomegranate is composed of a non-edible part composed of30% skin (external part) and 13% internal lamel and anedible part composed of 50–70% seeds. The seeds arecomposed of about 15% pips (woody part), which deter-mines hardness, and 85% pulp, which contains juice (Espiard2002). The investigated seeds presented the followingcharacteristics: shape, ellipsoids; length, 13±1 mm; breadth,7±1 mm; pip thickness, 2±0.2 mm; average weight of anindividual seed, 0.504±0.04 g; bulk density, 628±2 kg/m3.

Twenty kilogrammes of pomegranate were frozen at−50 °C for 1 month. Before osmotic dehydration process,pomegranates were thawed at room temperature for 1 h. Adigital thermometer BT20 (Bresso, Italy) was placed in thepomegranate core to measure the temperature elevationduring 1 h of thawing at room temperature. Temperature ofpomegranate core reached −7.5 °C, after thawing. Seedswere immediately separated manually prior to the osmoticdehydration process.

Osmotic Dehydration Treatment

About 100 g of frozen seeds were osmodehydrated insucrose solution (55 °Brix) for 20 min at 50 °C using ashaking water bath (GFL instrument D 3006, Germany;oscillation rate 160 rpm). The time and temperaturecombination was selected on the basis of our previousfindings, which showed that osmotic dehydration of pome-granate seeds for 20 min using sucrose solution at 50 °Cgives higher mass transfer rate (Bchir et al. 2009, 2010a).Sucrose solution was heated at 50 °C before adding the seedsto the bottles (Schott, Saint-Gallen, Switzerland) of 500 ml.The volume ratio between the seeds and the sugar solutionwas kept at one part of seeds and four parts of solution (1:4;Bchir et al. 2009). After osmotic dehydration process, seedswere removed from the solution, quickly rinsed (withdistilled water, 20 min) and the excess of solution at thesurface was removed with absorbent paper.

Food Bioprocess Technol (2012) 5:1840–1852 1841

Air-Drying Experiment

Approximately 200 g of osmodehydrated seeds wereuniformly spread on perforated stainless trays and dried atthree temperatures 40 °C, 50 °C and 60 °C for 240 min.These temperatures have been selected according to thosemostly used for fruit and vegetable in the literature (Kingslyand Singh 2007; Erbay and Icier 2009).

Dried seeds were taken out from dryer at different timeintervals (30, 60, 120, 180 and 240 min). Drying experi-ments were carried out with a laboratory scale drier byoperating it at an air velocity of 2.0±0.1 m/s. The dryingcabinet (Memmert tcp 800, Schutzart, Germany) wasequipped with an electrical heater, a fan, and temperatureindicators.

All analytical determinations were performed in triplicate.Values were expressed as the mean±standard deviation.

Physico-Chemical Analysis

Dry Matter, Moisture Contents and Water Activity

The dry matter (DM) was calculated according to AOACmethod 934.01 (1990). For the different time intervals,approximately 5 g of seeds were oven dried at 103 °C±2 °C until constant weight. Moisture content was estimatedby difference of mean values, 100% of DM (Chenlo et al.2007). Water activity (aw) was measured using an aqualab(Switzerland) instrument at 20 °C.

Surface Colour Measurement

The CIELAB coordinates (L*, a*, b*) were directly readwith a spectrophotocolorimeter Mini Scan XE (Germany)with a lamp (D 65). In this coordinate system, L* value is ameasure of lightness, ranging from 0 (black) to +100(white); a* value ranges from −100 (greenness) to +100(redness) and b* value ranges from −100 (blueness) to+100 (yellowness). The total colour difference (ΔE*) wasdetermined by using the following equation (Romano et al.2008):

ΔE» ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiL» � L0

»� �2 þ a» � a»0

� �2 þ b» � b»0

� �2qð1Þ

Where L*, a*, and b* and L0*, a0*, and b0* are thecurrent and the initial CIELAB coordinates, respectively.

The Hue angle (h*ab) and chroma or intensity (C*) werecalculated according to the following equations:

h» ¼ arctan b»=a»ð Þ ð2Þ

C» ¼ a»2 þ b»2� �1=2 ð3Þ

For Hue colour index, 0° or 360° represents red and 90°,180° and 270° represent yellow, green and blue, respectively.

Browning Index

The methodology applied for determination of browningindex was that proposed by Vega-Galvez et al. (2009).Pomegranate seeds were placed in distilled water at 40 °Cfor 6 h, using a solid to liquid ratio of 1:50. Then, waterwas first clarified by centrifugation (Beckman coulter J-E,Indianapolis, USA) at 3,200×g for 10 min. The supernatantwas diluted with an equal volume of ethanol at 95% andcentrifuged again at 3,200×g for 10 min. The browningindex (absorbance at 420 nm) of the clear extracts wasdetermined in quartz cuvettes using a spectrophotometer(Shimadzu UV 240, Cambridge, USA).

Polyphenol Oxidase Extraction and Activity Measurement

A portion of pulp (10 g) was dipped in a McIlvaine buffersolution (1:1) at pH=6.5. The buffer contained NaCl 1 Mand 5% polyvinylpolypyrrolidone. The homogenate wascentrifuged (8,000 rpm, 30 min) at 4 °C. The solid residuewas discarded, and the supernatant was filtered through aWhatman # 1 paper. The resulting liquid constituted thecrude enzyme extract.

Polyphenol oxidase activity was determined by placing3 ml of 0.05 M cathechol and 75 μl enzyme extract in a1-cm path cuvette. Assays were carried out at 410 nm usinga shimadzu UV 240 spectrophotometer (Cambridge, USA;Robert et al. 2002). A change in absorbance at 410 nm perminute and millilitre of enzymatic extract correspond to1 unit of PPO activity. The initial rate of the reaction wascomputed from the linear portion of the plotted curve.Results were expressed as relative activity (RA, %)calculated by Eq. 4

RA ¼ 100A

A0ð4Þ

Where A and A0 are the current and the initial PPOactivity, respectively (Robert et al. 2002).

Hydroxylmethylfurfural Analysis

The analysis of hydroxymethylfurfural (HMF) was carriedout by high pressure liquid chromatography (HPLC).Approximately, 1 g of pulp was placed in 25 ml flask;2 ml each of Carrez I and II reagents were added withstirring for 30 min and the volume made up with Milli-Qwater. After standing for 30 min, the supernatant wasfiltered through a filter of 0.45 μm and then injected in tothe chromatograph (Rada-Mendoza et al. 2002).

1842 Food Bioprocess Technol (2012) 5:1840–1852

HPLC determination was carried out, following themethod of Vinas et al. (1992), using a ZorBax 300SB-C18 column (4.6×150 mm; waters) at 30 °C. Mobil phasewas a mixture of methanol/water (10/90, v/v) with isocraticelution with 1 mlmin−1 flow rate and 20 μl injectionvolume. The UV detector PDA was set at 280 nm.Quantification was carried out by the external standardmethod, using a commercial standard of HMF (Sigma, NewJersey, USA). A standard curve was obtained by usingHMF standard dissolved in distilled water at variousconcentrations (ranging from 0 to 104 μg/ml).

Antioxidant Activity

Antioxidant activity was determined using pomegranateseeds extract solution. Approximately 5 g of pomegranateseeds were crushed and mixed with 15 ml methanol–watersolution (4:1, v/v) at room temperature (20 °C) for 5 hunder stirring. The extracts were then filtered and centri-fuged (Beckman coulter J-E, Indianapolis, USA) at4,000×g for 10 min, and the supernatant was concentratedunder reduced pressure at 40 °C for 1 h using a rotaryevaporator (Buchi B-461 water Batch, Switzerland) toobtain the crude extract. The crude extract was kept indark glass bottles inside the freezer until use (Biglari et al.2008).

Antioxidant activity of pomegranate seeds was deter-mined using the 2,2,-diphenyl-2-picryl-hydrazyl (DPPH)method (Vega-Galvez et al. 2009). Two millilitres of DPPHradical (0.15 mM in ethanol) was added to a test tube with1 ml of the crude extract. The reaction mixture was vortex-mixed for 30 s and left to stand at room temperature in thedark for 20 min. The absorbance was measured at 517 nmusing a spectrophotometer (Shimadzu UV 240, Cambridge,USA). The spectrophotometer was equilibrated with 80%(v/v) ethanol. Control sample was prepared without addingextract. Total antioxidant activity (TAA) was expressed asthe percentage inhibition of the DPPH radical and wasdetermined by the following equation:

TAA %½ � ¼ 1� Abssample

Abscontrol

� �� 100 ð5Þ

Where TAA is the total antioxidant activity and Abs isthe absorbance.

Phenolic Content

Total phenolic were determined using Folin-Ciocalteaureagents. Crude extract (40 μl) or gallic acid standard weremixed with 1.8 ml Folin-Ciocalteu reagent (predilutedtenfold with distilled water) and allowed to stand at roomtemperature for 5 min, and then 1.2 ml of sodium

bicarbonate (7.5%) was added to the mixture. Afterstanding for 60 min in darkness at room temperature,absorbance was measured at 765 nm.

A standard curve was obtained by using gallic acidstandard solution at various concentrations (ranging from 0to 2 mg/100 g). The absorbance of the reaction samples wascompared to that of the gallic acid standard, and the resultswere expressed as mg gallic acid equivalents/100 g sample(Biglari et al. 2008).

Anthocyanin Content

Anthocyanin content was determined using the pH-differential method described by Kirca et al. (2007).Aliquot (1 g) of crush pulp was mixed with 80 ml ofdistilled water. The mixture was sonicated (15 min) andcentrifuged (1,500×g for 10 min), and the upper phase wasused for assay. Two samples of 1 ml were taken from theupper phase, and each one was placed in 25 ml flask. Thefirst flask was diluted with buffer solution pH 1 (1.49 gKCl/100 ml and 0.2 N HCl) and the second one with buffersolution pH 4.5 (1.64 g sodium acetate/100 ml). Afterstanding for 30 min at room temperature, absorbance wasmeasured at 510 and 700 nm, using spectrophotometer(Shimadzu UV 240, Cambridge, USA). Pigment contentwas calculated, based on cyanidin-3-glucoside using thefollowing equation (Kirca et al. 2007):

Anthocyanin cyanidin� 3� glucoside equivalents;mg=100gð Þ

¼ Abs

eL�Mw � D� V

G� 100

ð6ÞWhere Abs (absorbance) = (Abs510 nm − Abs700 nm)

pH1 − (Abs510 nm − Abs700 nm) pH4.5; Mw (molecularweight)=449.2 g/mol, for cyanidin-3-glucoside; D =dilution factor; L = path length in centimetres; e=26 900molar extinction coefficient of cyanidin-3-glucoside[L×mol−1×cm−1]; V = final volume [ml] and G = sampleweight [mg].

Texture Analysis

Texture analysis were carried out using a texture profileanalyzer (TA.XT2; Stable Micro Systems, UK), with75-mm compression probe as described by Bchir et al.(2010a). The operating conditions of the instrument were asfollows: 1.5 mm/s pre-test speed, 0.5 mm/s test speed,10.0 mm/s post-test speed, 0.10 N trigger force and 85%sample deformation. The hardness and toughness of seedswere the means of 20 single seed measurements. Hardness[N] of seed was taken as the force in compression, whichcorresponded to the breakage of samples, while the

Food Bioprocess Technol (2012) 5:1840–1852 1843

toughness [Nmm] is the energy required to crush thesample completely.

Differential Scanning Calorimetry

Differential scanning calorimetry (DSC) was performed onthe pulp previously separated from pip. A 2920 TAInstruments (New Castle, Delaware, USA) with a refriger-ated cooling assessory and modulated capability was used.The cell was purged with 70 mlmin−1 of dry nitrogen andcalibrated for baseline using an empty oven and fortemperature and enthalpy using two standards (indium,Tonset, 156.6 °C; ΔH, 28.7 Jg−1; eicosane, Tonset, 36.8 °C;ΔH, 247.4 Jg−1). Specific heat capacity (Cp) was calibratedusing a sapphire. The empty sample and reference panswere of equal mass to within ±0.10 mg. DSC curves wererecorded during heating from −50 °C to 40 °C at a scan rateof 5 °C/min. All these DSC experiments were made usinghermetic aluminum pans. The analysed sample mass wasabout 3.50±0.25 mg.

Drying Rate and Effective Diffusion Coefficients

The drying rate (DR) was calculated using the equation (Shiet al. 2008);

DR ¼ M0 �Mt

tð7Þ

Where DR is overall drying rate [g water/g dry mattermin−1]; M0 is moisture content of seeds at time 0 [g water/gdry matter]; and Mt is moisture content of seeds at timet [g water/g dry solid].

Diffusion coefficients (Deff) were calculated using Fick’ssecond law equation applied to a sphere, by modifying theFourier number F0 ¼ Deff t=R2 using shape factor, due to anellipsoids shape of pomegranate seeds as has been reportedin our previous investigation (Bchir et al. 2009).

Statistical Analyses

Statistical analyses were carried out using a statisticalsoftware programme (SPSS for windows version 11.0).The data sets were subjected to analysis of variance usingthe general linear model option (Duncan test) in order toinvestigate differences between samples (P<0.05).

Results and Discussion

Chemical composition of pomegranate seeds and osmodehy-drated seeds (Table 1) showed a predominance of carbohy-drate in pomegranate seeds (84.93±0.25 g/100 g DM) anda high amount of protein (7.79±0.86 g/100 DM), in

agreement with previous findings of Espiard (2002) andFadavi et al. (2005). Pomegranate seeds were found tocontain low levels of ash (2.87±0.19 g/100 g DM) and lipid(4.55±0.40 g/100 g DM). The DM and water activity wereabout 16% and 0.989, respectively.

After osmotic dehydration process, carbohydrate andtotal soluble solids in osmodehydrated seeds increased by10% and 62%, respectively. This increase is due to thediffusion of sucrose from osmotic solution (high sucrosecontent) to the seeds. On the contrary, protein and ash contentsdecreased from 7.8 and 2.9 to 0.5 and 1.0 g/100 g DM,respectively. The amount of lipid was found to vary slightly(i.e. 4.5% to 4.0% DM).

DM of osmodehydrated seeds increased by 27% andwater activity slightly decreased to 0.954±0.002. Asconsequence, complementary treatments such as dryingwould be required to reduce water activity to 0.650 and tocontrol bacteria, fungi, and yeast development (Fabiano etal. 2008; Pinho et al. 2009; Miranda et al. 2009).

Drying Kinetics

The effect of drying time on DM water activity (aw) anddrying rate (DR) was studied in osmodehydrated seeds atdifferent temperatures (40 °C, 50 °C and 60 °C). FromFig. 1, DM increased from 42% to 78%, 80% and 84%after 240 min of the process time, for drying temperaturesof 40 °C, 50 °C and 60 °C, respectively. Moisture contentdecreased by 26% and 64% after osmotic dehydration anddrying compared to untreated seeds, respectively. Wateractivity decreased from 0.954 to 0.700, 0.565 and 0.430 in180 min for drying temperatures of 40 °C, 50 °C and 60 °C,respectively. After 180 min, a slight decrease was observed(40 °C, 0.630; 50 °C, 0.478; 60 °C, 0.414). Under the samecondition, DR decreased (from 2.21×10−2, 2.00×10−2 and1.20×10−2 to 0.50×10−2, 0.35×10−2 and 0.30×10−2 g water/g dry mattermin−1, for drying temperatures of 60 °C, 50 °Cand 40 °C, respectively), during the first 180 min, tending tobe stable at the end of the process. Statistical analysis(analysis of variance, ANOVA) did not show a significantdifference (P>0.05) between 180 and 240 min for all theinvestigated parameters. Similar findings have been previ-ously reported in many works (Kingsly and Singh 2007;Falade and Onyeoziri 2010; Fathi et al. 2010).

The drying kinetics of seeds could be subdivided in twophases. The first period (until 180 min) corresponds to aneasy diffusion of water from the inside to the surface ofseeds and the evaporation of free water on the seeds surfaceduring drying; the second one (from 180 to 240 min)corresponds to a difficult diffusion of water. This could bedue to the modifications in seed surface during the drying.In fact, many authors showed that after a few hours ofdrying, the product becomes denser, tougher and often

1844 Food Bioprocess Technol (2012) 5:1840–1852

leathery in nature with a case hardened surface, whichdoes not facilitate moisture diffusion (Doymaz 2007;Marquez and De-Michelis 2009). This behaviour could befavoured by the pre-treatment (osmotic dehydration).Indeed, Mandala et al. (2005) showed that sugar surfaceimpregnation during osmosis favours sugar crystallization,in some parts of the outer layers of apple tissue, forming abarrier to the movement of water during drying.

From the results showed in Fig. 1, it can be concludedthat increasing temperature of drying from 40 °C to 60 °Cresulted in quicker removal of water and shorter dryingtimes to reach aw of 0.650. In fact, using a temperature lessthan 60 °C resulted in a higher water activity and a lowerdrying rate. The increase of temperature at 50 °C inducedthe same evolution of aw and DM as with 40 °C. At 60 °C,significant difference was observed after 60 min and 30 minfor aw and DM, respectively. Moreover, using a dryingtemperature of 60 °C caused a reduction in the drying timeby four times, in order to reach a water activity (aw) of0.650 as compared with that at 40 °C. These findings are inagreement with previous studies reported for various driedfruits and vegetables (Miranda et al. 2009; Gokhale andLele 2010). Park et al. (2002) and Shi et al. (2008) foundthat the increase of air-drying temperature (from 40 °C to80 °C) induced an increase of heat energy, which speededup the movement of water molecules and resulted in highermoisture diffusivity.

The calculated values of effective diffusivity (Deff) atdifferent temperatures are presented in Table 2. It can beseen that the values of Deff greatly increased with theincreasing air-drying temperature. Effective diffusivityvalues for dried pomegranate seeds are similar to thoseestimated by different authors for other vegetables(Madamba et al. 1996; Ahrné et al. 2003; Doymaz, 2007).Table 2 showed that effective diffusivity values andexperimental data of Peleg’s equation parameters (K1 andK2) presented a good fit, showing average correlationcoefficients (R2) close to 0.99.

The investigation of the effect of air-drying temperatureon the mobility changes of water in dried seeds by DSCconfirmed the previous results regarding aw and Deff. Fromthe results obtained, it was possible to determine asignificant decrease in water mobility after osmotic dehy-dration (OD) and drying process. Indeed, DSC resultsshowed that after 20 min of OD, the % of frozen water (freewater) decreased from 70% to 28% (determined by dividingthe enthalpy of fusion of sample by the enthalpy of fusionof pure water). After 240 min of drying, free water in seedswas eliminated. In fact, Fig. 2 showed a disappearance ofthe endothermic peak after 240 min of drying at different

0 50 100 150 200 250

Time [min]

Wat

er a

ctiv

ity

60 ˚C 50 ˚C 40 ˚C

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250

Time [min]

Dry

mat

ter

[%]

60 ˚C 50 ˚C 40 ˚C

1.2

0.8

1

0.6

0.4

0.2

a

b

Untreated seeds Osmodehydrated seeds

Dry matter [%] 16.00±0.05 42.75±0.33

Protein [g/100 g DM] 7.79±0.86 0.51±0.02

Lipid [g/100 g DM] 4.55±0.40 4.03±0.81

Ash [g/100 g DM] 2.87±0.19 1.04±0.04

Carbohydrate [g/100 g DM] 84.93±0.25 94.41±0.97

°Brix 15.50±0.09 41.50±0.50

aw 0.989±0.002 0.954±0.002

Table 1 Chemical characteristicof pomegranate seeds

DM dry matter

Food Bioprocess Technol (2012) 5:1840–1852 1845

Fig. 1 Variation of a water activity and b dry matter of pomegranateseeds as a function with time and temperature (40 °C, 50 °C and60 °C)

air-drying temperatures, compared to osmodehydratedseeds. This is due to the loss of total free water fraction inseeds. In fact, endothermic peak could be attributed to themelting point of crystallized water. During the cooling, onlyfree water was crystallized to give ice, while duringheating, frozen water undergoes a fusion of ice.

On other hand, DSC thermograms (Fig. 2) showed aconsiderable increase in glass transition temperature (Tg′)as the air-drying temperature increased. In fact, Tg′ of seedsdried at 60 °C (Tg′=−21 °C) was higher than those dried at50 °C (Tg′=−28 °C) or 40 °C (Tg′=−31 °C) and to theosmodehydrated seeds (Tg′=−34 °C). The increasing of Tg′could be induced by a progressive loss of non-freezingwater (tightly bound water) of seeds during the dryingprocess. Sá et al. (1999) found that Tg′ for polysaccharideswater systems reach to a maximum with decreasing watercontent, inducing the decreased mobility of the polymerchains. Glass transition temperature was determined fromthe change in heat capacity (ΔCp). ΔCp can be related to theglass transition temperature (Tg′) due to the presence ofsucrose, protein, fibre (pectin, lignin, hemicellulose andcellulose) and water in the sample. As reported in suchproducts, carbohydrates and proteins can be described asamorphous food polymers constituted by not arrangedchains (Roos 1995).

Hot air-drying temperature is very important for thedehydration, but it is limited by the heat sensitivity of seeds

and the expected quality of the final product (Erbay andIcier 2009; Jaya and Das 2009; Mujumdar and Law 2010).Therefore, the physico-chemical properties of seeds atdifferent air-drying temperature were analysed.

Physico-Chemical Properties

Antioxidant Activity

Antioxidant compounds are considered as an indicator of thequality of food processing due to its low stability duringthermal process (Biglari et al. 2008; Saxena et al. 2010).Antioxidant activity (AA) was determined in terms of stablefree radical DPPH according to the method described byVega-Galvez et al. (2009). Antioxidant activity of pome-granate seeds (84%), cultivated in Tunisia, was found to beslightly higher compared to other pomegranate seeds (62–72%) cultivated in India (Kulkarni and Aradhya 2005). Afterosmotic dehydration process, seeds showed a rapid decreaseof antioxidant activity (i.e. until 58%). This value remainedinteresting compared to other fruits and vegetables (Biglari etal. 2008; Miranda et al. 2009; Kuljarachanan et al. 2009).

Antioxidant activity continues to be reduced duringdrying, regardless the considered drying temperature. Infact, AA reached 46%, 39% and 31%, after 240 min fordrying temperatures of 40 °C, 50 °C and 60 °C,respectively (Table 3). In spite of this decrease, AA%

Drying temperature Deff [m2s−1] R2 [%] K1 K2 R2 [%]

40 °C 2.85×10−10 97.57 5.94×103 2.00 99.54

50 °C 3.74×10−10 99.67 9.43×103 1.86 99.31

60 °C 4.49×10−10 98.92 17.82×103 1.48 99.78

Table 2 Effective diffusivitiescalculated by Fick’s model andvalues of Peleg’s equationparameters (K1 and K2)

T˚: 40˚CT˚: 50˚CT˚: 60˚C

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Hea

t F

low

(W

/g)

-60 -40 -20 0 20 40

Temperature (˚C)Exo Up Universal V3.9A TA Instruments

OD

Fig. 2 DSC thermogramsobtained for osmodehydrated(OD) and dried pomegranateseeds at different temperature(40 °C, 50 °C and 60 °C)

1846 Food Bioprocess Technol (2012) 5:1840–1852

remained higher than those observed in date and close totea and coffee antioxidant activity value (Biglari et al.2008). As shown, the lowest antioxidant activity wasrecorded using a higher air-drying temperature (60 °C).ANOVA analysis showed a significant difference (P<0.05)of AA% as a function of air-drying temperatures. Similarresults have been reported by Miranda et al. (2009) andKuljarachanan et al. (2009) during the increase of air-drying temperature from 50 °C to 90 °C of Aloe Vera andlime. This reduction of AA% could be explained due to lossof different components (i.e. phenolics acids, flavonoid andascorbic acid), which are responsible for the antioxidantactivity of pomegranate seeds, during heating (Nicoli et al.1999; Kulkarni and Aradhya 2005). Vega-Galvez et al.(2009) concluded that although natural antioxidants are lostduring heating, the overall antioxidant properties of foodscould be maintained or enhanced by the formation of newantioxidant compounds with enhanced antioxidant proper-ties. In fact, increase in AA% following thermal treatmenthas been reported in many vegetables (Choi et al. 2006;Kang et al. 2006). As consequence, in this study, thedestruction rate of antioxidants during heating was higherthan the formation rate of these compounds.

Total Phenolic Content

Pomegranate seeds’ phenolic content is 326.7±1.4 mggallic acid equivalent/100 g FM (Table 3). This value is inagreement with previous finding in pomegranate seeds,which varied between 230 and 510 mg gallic acidequivalent/100 g FM (Kulkarni and Aradhya 2005).During osmotic dehydration treatment, a decrease of40% (184 mg/100 g) compared to the initial phenoliccontent was observed (Table 3). This value was lower tothat found in fruits (apple and cherry, 500 mg/100 g;strawberry, 330 mg/100 g) and higher compared tovegetable (25–100 mg/100 g; Yang et al. 2006).

Moreover, pomegranate seeds showed a regress in totalphenolic during the drying from 184 mg/100 g FM(osmotic dehydrated seeds) to 151, 141 and 134 mg gallicacid equivalent/100 g FM for drying temperatures of 40 °C,50 °C and 60 °C, respectively (Table 3), in agreement with

other findings of Nicoli et al. (1999) and Erbay and Icier(2009). In spite of this reduction during drying, valuesremained higher compared to those observed in vegetables(Yang et al. 2006).

The reduction of total phenolic compounds after osmoticprocess was due to the migration of phenolic compoundsfrom pulp to osmotic solution induced by a large osmoticdriving force. This fact was due to the higher difference inconcentration between dilute seeds sap (15 °Brix) and thesurrounding hypertonic medium (55 °Brix; Raoult-Wack etal. 1991). This behaviour has been reported in the osmoticdehydration of pomegranate seeds (Bchir et al. 2009,2010a). During drying, total phenolic compounds signifi-cantly decreased indicating the negative effect of highertemperature on total phenolics compounds. This could beascribed to thermal degradation of naturally occurring anti-oxidative compounds present in pomegranate seeds asflavonoids (anthocyanins) and phenolic acid (Madrigal-Carballo et al. 2009; Devic et al. 2010). This resultcorroborates the findings of Jukunen-Tiitto (1985) andHarbourne et al. (2009) who reported that an increase intemperature from 40 °C to 70 °C caused a decrease of theflavonoid content in willow leaves and meadowsweet.Moreover, enzymatic and non-enzymatic reaction could bea responsible for the decrease of phenolic compounds inseeds supported by the increase of the temperature (Jeantetet al. 2006). In fact, phenolic compounds are the substratefor polyphenol oxidase enzyme. Also, Maillard reaction(non-enzymatic reaction) use phenolic compounds havingcarbonyl functions like a substrate (Jeantet et al. 2006).

Total Anthocyanin Pigments Content

Similar to antioxidant activity and total phenolic, anthocy-anin pigment decreased from 82 to 68 mg/100 g during thefirst 20 min of osmotic dehydration process (Table 3). Thisfact could be due to the migration of anthocyanin pigmentfrom pulp to the osmotic solution induced by the drivingosmotic force. Antioxidant activity was lower to thoseobserved in strawberry (450–700 mg/100 g) and in rangecompared with blueberry (25–495 mg/100 g) and mulberry(67–107 mg/100 g) (Cisse et al. 2009).

Table 3 Value of total phenolic, anthocyanin and antioxidant activity of untreated, osmotic dehydrated and dried seeds

Total phenolic [mg/100 g] Total anthocyanin [mg/100 g] Antioxidant activity [%]

Untreated seeds 326.68e±1.40 82.30d±1.42 84.23e±0.31

Osmotic dehydrated seeds 184.39d±1.15 68.43c±0.30 57.88d±1.07

Dried seeds 40 °C 151.76c±1.93 40.11b±1.53 46.23c±0.56

50 °C 141.14b±1.23 24.03a±0.14 39.04b±0.80

60 °C 134.58a±1.14 20.10a±0.28 31.17a±1.16

Means in column with different letters are significantly different (P<0.05)

Food Bioprocess Technol (2012) 5:1840–1852 1847

During heating (from 40 °C to 60 °C), a decrease in theanthocyanin pigment concentration was also observed forpomegranate seeds (Table 3). In spite of this decrease ofantioxidant activity, values remained closer to those observedin plum (30 mg/100 g), grapes (30–750 mg/100 g) andblueberry (25–495 mg/100 g; Yang et al. 2006). The highestconcentration of anthocyanin (40 mg/100 g FM) wasrecorded by using the lower temperature (40 °C). In fact,after drying, anthocyanin content was reduced by 41%, 64%and 70% for drying temperatures of 40 °C, 50 °C and 60 °C,respectively. Similar trends were observed for pomegranateseeds anthocyanins (Jaiswal et al. 2010) and black carrotanthocyanins (Kirca et al. 2007) during heating. Thedegradation of anthocyanins could be due to enzymatic(polyphenol oxidase) reaction. In fact, Raynal et al. (1989)reported that polyphenol oxidase playing important role inoxidative degradation of anthocyanins during the drying ofplums. Moreover, Cemeroglu et al. (1994) founded that thedegradation rate of anthocyanins in sour cherry increasedwith increasing heating temperature (e.g. 60 °C, 80 °C). Infact, the increase in the temperature enhanced the modifica-tion rate of the anthocyanin chemical structure favouring itsdegradation (Jackman and Smith 1996). The decrease ofanthocyanin content contributes to the decline of the colour-ful appearance of seeds (Jaiswal et al. 2010).

Relation Between Antioxidant Activity, Total Phenolicand Total Anthocyanin Pigments

Phenolic compounds, including anthocyanins, displaystrong antioxidant activity, contributing significantly to theantioxidant capacity of fruits (Nicoli et al. 1999; Jeantet etal. 2006; Fathi et al. 2009). In fact, the decrease of phenoliccompound by 17% involved a decrease of the antioxidantactivity by 20% at 40 °C. The percentage of loss inantioxidant activity remained slightly higher than thatobserved with total phenolic at different air-drying temper-ature. Contrary to other studies, these results showed thatthe production of new antioxidant compounds duringdrying was very weak (Shi et al. 2008; Vega-Galvez et al.2009).

Colour

The effect of osmotic dehydration and air-drying tempera-ture on seeds colour was illustrated in Table 4. Fivechromatics coordinates was used to characterise thechanges of seeds colour during these processes. Seedscolour was found to be dependent on air-drying temperatureand osmotic process. After osmotic dehydration hue angle(h°) and lightness (L*) values increased, while an oppositetrend was observed for chroma (C*) values. Furthermore,a* and b* colour parameters showed a slight decrease

during osmotic process. These variations indicated thatseeds become less dark during OD. This could be explaineddue to the migration of pigment from pulp to the osmoticsolution inducing by osmotic driving force.

During drying, hue angle and lightness values decreasedwith the increase of air-drying temperature from 84° and 29to 69° and 23, respectively. This changes indicated thereduction of colour from a more green (when hue >90°) toan orange–red (when hue <90°) and seeds turning darker(decreasing of L*). Chroma values, increased with theincrease of air-drying temperature showing that seedscolour became more saturated. Moreover, a* and b* colourparameters showed a slight increase during drying (Table 4).This modification in seeds colour is mainly due to the effectof temperature on heat-sensitive compounds such ascarbohydrates, proteins and vitamins, which cause colourdegradation during drying process. According to Mandalaet al. (2005), the decrease of “L*” values and the increaseof “a*” values correspond to the increase of fruit browning.To better understand the effect of air-drying on seedscolour, browning index and total colour difference weredetermined. It can be observed that an increase oftemperature led to a significant formation of brownproducts. Indeed, the maximum browning index wasoccurred at 60 °C (0.075) as compared to 50 °C (0.064)and 40 °C (0.051). Similar observations were reported byMiranda et al. (2009) and Vega-Galvez et al. (2009) usingaloe vera and red pepper, respectively. The total colourdifference (ΔE*) value increased slightly with the increaseof air-drying temperature (40 °C, 3.0±0.5; 50 °C, 5.1±0.2and 60 °C, 9.8±0.8). This indicated that seeds becamebrownish (Romano et al. 2008). However, ΔE* was lowerto those observed in many dried fruits (Pereira et al. 2007;Chong et al. 2008). In addition, browning index was verylow indicating that air-drying temperature does not have agreat influence on the browning of seeds. This could be dueto the osmotic dehydration pre-treatment. Indeed, Ponting(1973) and Krokida et al. (2001) showed that dehydrationof foodstuffs (e.g. potato) by immersion in osmoticsolutions before convective air-drying improves the qualityof the final product since it prevents oxidative browning.The formation of brown compounds in seeds may berelated to both enzymatic and essentially non-enzymatic(Maillard reaction) reactions (Miranda et al. 2009).

Enzymatic Browning

Browning colour could be induced by polyphenol oxidase(PPO) present in pomegranate seeds. PPO was extractedfrom the pulp and the relative activity was measured as afunction of air-drying temperature. Results showed arelative activity of 27% for PPO in osmotically dehydratedseeds.

1848 Food Bioprocess Technol (2012) 5:1840–1852

The increase in air-drying temperature involved a declinein PPO relative activity. The relative activity decreased by4% and 5% at air-drying temperatures of 50 °C and 60 °C,respectively, as compared with that at 40 °C. Our resultswere in agreement with previous findings showing thatPPO is a heat-labile compound (Mandala et al. 2005; El-Aouar et al. 2003; Jaiswal et al. 2010).

The presence of PPO in seeds could be responsible forphenolic compounds (flavonoids, tannins, lignins, phenolicacids) degradation involving colour modification (Jaiswalet al. 2010). In fact, Saxena et al. (2010) showed that tissuebrowning is mainly due to the oxidation of phenoliccompounds into quinine compounds under aerobic con-ditions by PPO, then the quinine compounds undergoespolymerization forming brown polymeric pigments, leadingto browning. However, Lenart (1996) found that thepresence of sugar on the surface of the osmodehydratedsample is a barrier for the contact with oxygen thusreducing the oxidative reactions and the resultant discolour-ing of the fruit.

The inactivation of PPO by drying prevents the brown-ing reaction in seeds. However, in precedent paragraph, wefound that the browning colours increased slightly asfunction of air-drying temperature. Therefore, there isanother reaction that induced browning reaction. Manyauthors found that during drying non-enzymatic browning(Maillard reaction, caramelisation) was responsible forbrowning of fruits (Maskan 2001; Lewicki 2006; Mirandaet al. 2009).

Non-enzymatic Browning

Maillard reaction, also called sugar–amino browningreaction, which is a form of non-enzymatic browning, is achemical reaction between an amino acid and reducingsugar under heating conditions (Rada-Mendoza et al. 2002).The reactive carbonyl group of the sugar interacts with thenucleophilic amino group of the amino acid to create

hundreds of different compounds. 5-HMF is one of themajor intermediate products in the Maillard reaction (Rada-Mendoza et al. 2002).

It was observed that increasing the air-drying temperatureleads to enhanced HMF content (40 °C, 0.017 mg/100 g FM;50 °C, 0.019 mg/100 g FM and 60 °C, 0.024 mg/100 g FM)compared to osmodehydrated seeds (0.011 mg/100 g FM).However, HMF values of different seeds were very lowshowing that air-drying temperatures do not have a greatinfluence on the formation of HMF. This could be due to thelow content of protein, in osmodehydrated seeds. In fact,protein is a necessary substrate for the Maillard reaction(Rada-Mendoza et al. 2002). That is confirming the lowerbrowning index and the decrease of AA% as function oftemperature. In fact, low HMF content and the decrease ofAA% induced through the enhancing of the temperatureshow that the rate of destruction of antioxidant compoundswas higher than the rate of formation of these compounds.Indeed, many authors found that Maillard reaction let theformation of many antioxidant compounds (e.g. melanoidins;Shi et al. 2008; Vega-Galvez et al. 2009).

Texture Analysis

Texture analysis of osmodehydrated and dried pomegranateseeds were studied over time periods of up to 20 min and4 h, respectively (Table 4). Two textural parameters(hardness and toughness) were used to characterise seedstexture modification. Based on the results, hardness andtoughness were affected by osmotic process and air-dryingtemperature. In fact, after OD, seeds hardness and tough-ness increased by 17% and 13%, respectively, comparedwith untreated seeds. During drying, hardness increased by38, 55 and 60 N, while toughness also increased by 20, 25and 36 Nmm at drying temperatures of 40 °C, 50 °C and60 °C, respectively. This behaviour could be explained bythe structural collapse of seeds induced by the increasedwater loss during osmotic and drying process (Mandala et

Table 4 Effect of air-drying temperature on chromatics coordinates and on textural properties of seeds

Untreated seeds Osmotic dehydrated seeds Dried seeds

40 °C 50 °C 60 °C

Chromatics coordinates h° 63.50±2.30 84.60±3.40 77.44±3.10 73.33±3.00 69.47±1.00

C* 15.29±0.10 11.71±0.50 14.30±0.06 15.54±0.48 19.08±0.17

L* 26.31±1.10 28.91±0.17 27.79±1.10 25.90±0.33 22.95±0.01

a* 12.44±0.14 7.80±0.77 10.20±0.41 11.50±0.70 14.65±0.25

b* 8.90±0.03 8.60±0.02 9.91±0.11 10.40±0.06 12.21±0.01

Hardness [N] 46.73±2.47 63.46±3.04 101.54±4.06 118.61±3.47 123.63±4.91

Toughness [Nmm] 54.55±3.96 67.21±5.55 87.01±4.52 92.33±3.24 103.38±4.12

Pip crush [mm] 4.51±0.34 3.70±0.34 2.10±0.17 1.80±0.12 1.65±0.11

Food Bioprocess Technol (2012) 5:1840–1852 1849

al. 2005). As a consequence of this exchange, the productswill more or less lose weight and will shrink eventually(Aversa et al. 2009). Indeed, the peaks of pip crushingdecreased after osmotic process and drying (Table 4). Thus,compared with untreated seeds, those that were onlyosmodehydrated reduced thickness by 18% and those thatwere also dried at 40 °C, 50 °C and 60 °C lost 43%, 51%and 55% thickness, respectively. Similar results have beenreported by Mandala et al. (2005) and Bchir et al. (2010a)using the textural changes during the drying of apple andchempedack and osmotic dehydration of pomegranateseeds, respectively.

Conclusion

OD and drying process could be used for the conservation ofpomegranate seeds. Indeed, OD followed by drying allowedreduction of water activity until a value less than 0.650 after60, 120 and 240 min at drying temperatures of 60 °C, 50 °Cand 40 °C, respectively. To reduce energy consumption andimprove food quality, it would be interesting that dryingstopped after these times. From the obtained results, it isrecommended to use 40 °C since the low influence on thequality parameters of seeds was observed.

The determination of PPO activity and HMF contentafter drying showed that enzymatic and non-enzymaticreactions (Maillard reaction) have no market effect onbrowning index, showing the benefit effect of pre-treatment(osmotic dehydration) on colour stability.

During drying, not only the composition of the tissue ischanged but also the textures, since seeds reduce theirthickness to maximum 55% using 60 °C. Differentialscanning calorimetry data showed a relation between Tg′and texture parameters. In fact, water loss of seeds inducedan increase of hardness and toughness and also an increaseof Tg′.

These processes permit a microbiological stability butalso a degradation of the nutritional quality of the fruit thatremained slightly lower compared to other fruits andvegetables. It should be interesting to use seeds asingredients in food formulations.

Acknowledgment The authors (B.B) acknowledge the financialsupport of Gembloux Agro-Bio Tech, University of Liege (Belgium)and National Engineering School of Sfax (Tunisia).

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