Journal of Food Engineering 115 (2013) 347–356

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Journal of Food Engineering

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A multi-stage combined heat pump and microwave vacuum drying of green peas

M. Zielinska a,⇑, P. Zapotoczny a,1, O. Alves-Filho b,2, T.M. Eikevik b,2, W. Blaszczak c,3

a Department of Systems Engineering, University of Warmia and Mazury in Olsztyn, ul. Heweliusza 14, 10-718 Olsztyn, Polandb Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), Kolbjørn Hejes vei 1D, NO-7491 Trondheim, Norwayc Division of Food Science, Institute of Animal Reproduction and Food Research, Polish Academy of Sciences, ul. Tuwima 10, 10-747 Olsztyn, Poland

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 August 2012Received in revised form 17 September2012Accepted 27 October 2012Available online 5 November 2012

Keywords:Green peasHeat pumpFluidized bedAtmospheric freeze dryingMicrowave vacuum drying

0260-8774/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.jfoodeng.2012.10.047

⇑ Corresponding author. Tel.: +48 895233413.E-mail addresses: m.zielinska@uwm.edu.pl (M. Zie

(O. Alves-Filho), w.blaszczak@pan.olsztyn.pl (W. Blasz1 Tel.: +48 8952334132 Tel.: +47 73594250.3 Tel.: +48 895234615.

The effect of multi-stage heat pump fluidized bed atmospheric freeze drying (HP FB AFD) and microwavevacuum drying (MVD) on the drying kinetics, moisture diffusivities, microstructure and physical param-eters of green peas was evaluated. The results were compared with those obtained for microwave vac-uum drying (MVD) and hot air convective drying (HACD). In case of combined method, the initialdrying rate was about 0.04 l/min. The application of MVD increased the drying rate to the values0.08 l/min. The drying rates of green peas dried by MVD and HACD were 0.59 and 0.20 l/min, respectively.MVD samples were characterized by a structure with minimal changes in respect to fresh samples. How-ever, HP FB AFD and MVD satisfied important requirements, such as high product quality (due to lowmaterial temperature during AFD and low pressure during MVD), and increased drying rates in the finalstage due to application of microwave heating.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Green peas are among the most common and popular vegeta-bles in the world. For industrial purposes they are usually driedwith hot air in static bed or thin layer (Jayarman and Gupta,1992). The method itself is a low-cost one, but the disadvantageis that it is an entailing time-consuming process (Senadeeraet al., 2003). Additionally, drying operations that use hot air are en-ergy intensive. Therefore, seeking alternatives for green peas dry-ing is highly important. To increase drying rates, hot air has beenreplaced by superheated steam (SS) and used to dry a number offood products (Pronyk et al., 2004; Zielinska et al., 2009).

The vacuum freeze-drying (VFD) is used as a benchmark regard-ing product quality. It produces highly valued products with thehighest nutritional value. Nevertheless, the VFD is time-consumingand its application involves problems such as low productivity,high fixed and operational costs or technical inconveniences (Songand Yeom, 2009). In order to reduce manufacturing costs, atmo-spheric freeze drying (AFD) has been developed. Claussen et al.(2007) investigated the drying kinetics and material propertiesduring AFD of apple, turnip cabbage and cod and found superiorquality of AFD products compared to VFD products. There has been

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linska), odilio.alves@ntnu.noczak).

a significant growth in the potential market for heat pump dryers,aided by the impact of new designs under development or recentlyintroduced to the market. Heat pumps used in drying satisfyimportant requirements, such as product quality control, low en-ergy consumption, and reduced environmental impact in industrialdrying (Alves-Filho, 2002). They enable drying under low temper-ature conditions (Uddin et al., 2004).

Application of microwave to remove water from the materialhas many advantages. Energy supply to the whole volume of thematerial results in considerable shortening of drying time andtherefore it shortens the time of contact between the dried mate-rials with oxygen at elevated temperature. As a result there is areduction of negative biochemical effects while maintaining appro-priate color and nutritive value of final product (Kelen et al., 2006).Application of microwave heating at lower pressure significantlyincreases drying efficiency and improves the quality of the product.It also produces favourable conditions for the occurrence of thepuffing phenomenon, which is opposed to drying shrinkage (Shamet al., 2001). The potential for achieving a high-quality product ofan attractive texture has directed the attention of many investiga-tors to the applicability of the microwave vacuum drying (MVD)method to the drying of cranberries, potatoes, tomatoes, bananasand garlic (Bondaruk et al., 2007; Durance and Wang, 2002;Maskan, 2000; Figiel, 2006; Clary et al., 2005). Regretfully, themethod requires strict control, as at the final stage of drying anabrupt rise in the temperature of the material is likely to occur(Drouzas and Schubert, 1996). Additionally, the necessity of usinga vacuum raises the cost of the drying process.

348 M. Zielinska et al. / Journal of Food Engineering 115 (2013) 347–356

Each drying method has its own advantages and limitations.Nowadays, hybrid drying techniques are being developed to max-imize the benefits of different drying techniques, i.e. produce bet-ter quality of foodstuffs, reduce drying time and increase dryingrate (Huang and Zhang, 2012). Mejia-Meza et al. (2008) report thatthe nutritional properties of berries may be retained to a greaterextent when combined microwave-vacuum, hot-air drying andfreeze drying technologies are employed. Pre-drying of the mate-rial using hot air before MVD may reduce the total cost of dehydra-tion and guarantee high quality of material (Durance and Wang,2002; Figiel et al., 2010). Claussen et al. (2007) report that heatpump atmospheric freeze drying in the first step of the drying pre-serves the product quality with minimal changes. After removingthe loosely bound water, the material can be transferred to differ-ent type of dryer to increase the drying rate. A combination ofmicrowave and heat pump may provide the desire condition ofdrying to achieve fast drying rate, lower shrinkage, better appear-ance of the product and least cost (Uddin et al., 2004).

The literature contains references to the convective drying ofgreen peas in fluidized bed as well as under atmospheric freezedrying conditions (Senadeera et al., 1999; Alves-Filho et al.,2004), but as yet no reports are available on the multi-stage com-bined heat pump and microwave vacuum drying of green peas.This generates a necessity for further studies on drying kineticsof green peas dehydrated by combination of HP FB AFD andMVD. Furthermore, more profound knowledge of the changes inthe properties of green peas that occur during drying is neededfor the better design of drying methods preserving the desirablecharacteristics and minimizing or eliminating the undesirableones. The multi-stage combined HP FB AFD and MVD, when prop-erly applied, can be used for achieving a high-quality product.Therefore, the present work is concerned to study of combinedheat pump fluidized bed atmospheric freeze drying and microwavevacuum drying (HP FB AFD + MVD) of green peas and the influenceof drying methods on the drying kinetics and product quality char-acteristics. The properties studied are bulk density, particle den-sity, bulk porosity, shrinkage, internal porosity, microstructure,hardness, size, shape and color.

2. Material and methods

2.1. Material

The work was carried out using green peas (Pisum sativum) sup-plied by NTNU in Trondheim, Norway. The initial moisture contentof the material was about 3.23 ± 0.02 kg/kg dry basis (d.b.). Greenpeas were dried under different drying conditions to the equilib-rium moisture content of 0.09 ± 0.04 kg/kg dry basis (d.b.). Thecontrol sample composed of raw green peas was not subjected tothe drying processes.

2.2. Drying apparatus, drying test conditions and procedures

Green peas were subjected to the three different protocols: (i)hot air convective drying (HACD), (ii) microwave vacuum drying(MVD), and (iii) combined heat pump fluidized bed atmosphericfreeze drying and microwave vacuum drying (HP FB AFD + MVD).

The HACD and MVD experiments were carried out at theDepartment of Systems Engineering, University of Warmia andMazury in Olsztyn, Poland. The air temperature during hot air con-vective drying (HACD) was 60 �C. The moisture was evaporatedfrom the sample by oven drying in a heating chamber with forcedconvection (FED53 127 Binder, US) according to the standards(AOAC, 1975).

The MVD system comprised a motor (1,8), a regulating valve(2), a condensation unit (3), a drying container (4), a control unit

(5), a microwave generator (6), a microwave circulator (7), a tem-perature and vapour pressure measuring unit (9). Each time theportion of about 100 g of green peas was subjected to pulsedmicrowave vacuum drying (MVD) at 100 (50) W microwave powerin order to maintain the temperature of the material inside thedrying chamber below 50 �C. The absolute pressure in the vacuumwas 3 kPa in the drum that was rotating at 6 rev/min.

The heat pump drying (HPD) system was developed at theDewatering Laboratory, Department of Engineering Science andTechnology, Norwegian University of Science and Technologyand SINTEF Energy Research Laboratory in Trondheim, Norway.The scheme of a heat pump fluidized bed atmospheric freeze dry-ing (HP FB AFD) system is given in Fig. 1. Green peas sampleswere dried under atmospheric freeze-drying conditions usingair as the drying medium. The samples were placed in a freezerat �20 �C prior to the tests. Samples of 1.5 kg of green peas wereused for all drying experiments. HP FB AFD tests were performedat the temperature �5 �C, which is below the freezing point ofthe frozen green peas. Inlet relative air humidities were kept atlow (RH L = 20%) and high (RH H = 50%) level. The flow rate ofthe drying air was maintained at the constant level of4.5 ± 0.1 m/s during all drying experiments. The height of the sta-tic layer was 0.090 ± 0.005 m, while the height of the fluidizedbed was 0.180 ± 0.015 m. The infrared heater source had a250 W maximum power and 18.5 X emitter. The distance be-tween the heater and a bed of fluidized green peas was main-tained constant at 30 cm throughout the experiments. Thesemidry product of moisture content about 2.07 ± 0.11 kg/kgdry basis (d.b.) was transferred to the MVD to shorten the timeof drying operation and aiming to increase the overall efficiencyof the drying process.

The drying processes were stopped when there was no variationin the mass for two consecutive measurements at 5 min intervalsindicating that the samples reached equilibrium moisture content(Sharma and Prasad, 2004). All the drying tests were performed intriplicate. Table 1 gives the experimental design and parametersfor MVD, HACD and combined HP FB AFD and MVD drying, suchas temperature, relative humidity, infrared radiation power, micro-wave power, pressure and drying time.

2.3. Rehydration

Rehydration was done by immersing dry samples for 10 mininto a hot water bath filled with water of temperature about 95 �C.

2.4. Physical properties

The moisture content of fresh and processed green peas wasmeasured by the air-oven drying method (FED53 127 Binder,US) according to the AOAC standards. The oven temperatureand heating period were set at 105 �C and 24 h, respectively.The result was the mean value of two replications. The bulk den-sity of a deep-bed of green peas was determined using the stan-dard test weight procedure (Singh and Goswami, 1996). Theparticle density and the volume of sample were determined bystandard liquid pycnometric method. Water-insoluble liquid,which was toluene of density 0.867(1) g/cm3 and a calibratedglass pycnometer of approx. 50 ml in volume (LG-3838-3658,Chemland Ltd., Poland) were used for the experiment. The parti-cle density of the sample was calculated using the formula givenby Vilche et al. (2003). The volume of green peas was determinedas a difference of the initial volume of liquid in a pycnometer andthe volume of the liquid with immersed green peas andcalculated using the formula given by Vilche et al. (2003). Thepycnometric volume of one solid object was determined fromthe pycnometric volume of the sample that contained the known

Fig. 1. Scheme of a heat pump fluidized bed drying system: 1 – air heater; 2 – air blower; 3 – heat exchanger; 4 – a surplus heat exchanger; 5 – fluid bed drying chamber; 6 –a compressor; 7 – air filter; 8 – air cooler; 9 – infrared heat source; 10 – expansion valve; 11 – humidifier.

Table 1Experimental design for microwave vacuum drying, hot air convective drying, combined heat pump fluidized bed atmospheric freeze drying and microwave vacuum drying.

Sample no. First drying step Second drying step

T (�C) RH (%) IR (W) MW (W) P (kPa) t (min) T (�C) MW (W) P (kPa) t (min)

1 – – – – – – – – – –2 50 – – 100 3 145 – – – –3 60 45 – – – 300 – – – –45 �5 50 – – 200 55 100 3 10067 �5 20 – – 200 55 100 3 10089 �5 50 250 – 200 55 100 3 100

(1) Control; (2) MVD; (3) HACD; (4) HP FB AFD (RH L); (5) HP FB AFD (RH L) + MVD; (6) HP FB AFD (RH H); (7) HP FB AFD (RH H) + MVD; (8) HP FB AFD (RH H + IR); and (9) HPFB AFD (RH H + IR) + MVD; T – temperature, �C; RH – relative humidity, %; IR – infrared radiation power, W/m2; MW – microwave power, W; P – drying chamber pressure,kPa; t – time, min; EMC – equilibrium moisture content, % dry basis.

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number of green peas. Volumetric shrinkage was evaluated bydetermination of the ratio of the actual volume of green peasphere to its initial volume (Moreira et al., 2000). Followed byWang and Brennan (1995), porosity was calculated from bulkdensity and particle density. A spectrophotometer Miniscan XEPlus for a standard illuminant D65, observer 10� and 8� dia-phragm was used to describe color parameters of raw and driedgreen peas. The display was set to CIE L�a�b� color coordinates,where L� is lightness or darkness, a� is redness (+) or greenness(�), and b� is yellowness (+) or blueness (�). The measurementswere taken directly on the top surface of one object. The indices:DE� (total color difference), DC�(total saturation difference), andDH� (total hue difference) were calculated and the results wereanalyzed followed by Zielinska and Markowski (2012). The crite-rion established by the International Commission on Illumination(CIE) was applied during data analysis (Hutchings, 1999). The cal-culated indices of color were averaged over 30 measurements.

2.5. Morphology

The image acquisition and image analysis workstation consistedof an Epson Perfection 4490 Photo flat scanner. The analytical pro-cedure involved a series of the following successive steps: scannercalibration, green peas arrangement in the matrix, matrix removal,scanning, image saving (2673 � 4031 resolution, 400 DPI, 24-bitcolor depth, TIFF format). The computer-aided image analysiswas performed by MaZda 4.3 software (Szczypinski et al., 2009).Two morphological features, i.e. length and width were extractedto describe the changes in size of green peas during processing.Additionally, circular shape factor was calculated to numericallydescribe the shape of a particle, independent of its size. Zapotoczny(2011) reports that circle presents a circularity reference valueequal to 1 and circularity of an object, which is bigger than 1, isassociated with the degree at which the shape of a particle issimilar to the shape of a circle.

350 M. Zielinska et al. / Journal of Food Engineering 115 (2013) 347–356

2.6. Textural properties

Quasi-static compression tests of a single sample were per-formed on raw (control), dried and rehydrated samples. All thetests were carried out using a texture analyzer (Stable Micro Sys-tems, TA.XD. Plus, Surrey, UK) operating in the compression mode,fitted with a parallel plate fixture for uniaxial compression and a100 N load cell. During compression experiments the force was ap-plied to the sample by a 35-mm flat probe at a constant speed of1 mm/s until 80% of sample deformation (Moreno-Perez et al.,1996). The results were averaged over 15 measurements.

2.7. Microstructural observations

The microstructure of dried green peas was measured accordingto the procedure described by Blaszczak et al.(2005). The greenpeas were cut along the cotyledons (raphe) axis and placed in a fix-ative containing glutardialdehyde (2.5 g/100 g in 0.1 mol/L phos-phate buffer, pH = 7.2) for 48 h at 4 �C. The specimens were thenrinsed in MQ water and dehydrated in a serial ethanol solutioncontaining 30 ml/100 ml, 50 ml/100 ml, 70 ml/100 ml, 90 ml/100 ml and alcohol anhydrous reagent for 15 min in each solution.After the critical point drying (BAL-TEC CPD 030, Liechtenstein), across-section through the dried cotyledons was carried out. Finally,the specimen fragments were mounted on aluminium stubs usingsilver paste, coated with gold and photographed by using an scan-ning electron microscope SEM (Jeol 5200, Japan) using an acceler-ating voltage of 10 kV.

2.8. Drying kinetics

The drying kinetics were determined on the basis of mass lossesof green peas. The moisture ratio of green peas was computed byusing the initial moisture content and equilibrium moisture con-tent. The equilibrium moisture content was determined at the finalstage of drying as an asymptotic value of the function fitted to theexperimental points (Figiel, 2010). Mathematical differentiation ofthe drying kinetics allowed for the determination of drying rates(Pabis et al., 1998). The effect of drying methods and conditionson drying rates was investigated. For spherical materials, whenassumptions of negligible shrinkage and constant temperatureand effective diffusivity can be considered, the solution of Fick’slaw can be written in the following form (Pabis et al., 1998):

MR ¼ M �Me

Mo �Me¼ 6

p2

X1

n¼1

1n2 expð�n2p2F0mÞ

� �ð1Þ

The effective diffusivity was determined from the solution ofthe Fick’s equation written in the form of Fourier series and calcu-lated based on the Fourier mass transfer number. This series con-verged with small differences between neighbouring points when30 terms were used in the calculations.

Fig. 2. Moisture ratio of green peas dehydrated by: (a) MVD, HACD and (b) HP FBAFD + MVD.

2.9. Statistical analysis

One-way analysis of variance has been used to test differencesbetween the samples dehydrated by different drying techniques.The significance of differences between the samples was deter-mined by the Duncan multiple range test at p 6 0.05. The parame-ters of the model were estimated by non-linear least squares andthe goodness of fit of the solution to the experimental data wasindicated by the coefficient of determination (R2). The calculationswere done using STATISTICA 9.0 (StatSoft Inc., Tulsa, USA)software.

3. Results and discussion

3.1. Drying kinetics

The changes in moisture content over drying time during HACDand MVD are shown in Fig. 2a. As the drying progressed the mois-ture content decreased exponentially and asymptotically to theequilibrium moisture content. The moisture ratio changes vs. dry-ing time indicate that residence times to achieve the same mois-ture losses are much shorter for MVD than for HACD of greenpeas. The drying times for reaching the equilibrium moisture con-tent were 145 and 300 min for MVD and HACD, respectively. It wasfound that the application of microwave power at reduced pres-sure resulted in statistically significant shortening drying time by52% in comparison with HACD. Fig. 2b shows the drying curvesfor the green peas dehydrated by combined HP FB AFD and MVD.Samples pre-dried using HP FB AFD until moisture content of2.07 ± 0.11 kg/kg dry basis (d.b.) were additionally subjected toMVD until moisture content of 0.09 ± 0.04 kg/kg dry basis (d.b.).In HP FB AFD, the drying times were considerably longer than inMVD and HACD due to the low temperature used and internalresistance to mass transfer. Pre-drying step in HP FB AFD was con-ducted until 200 min and the slopes of the kinetic curves for AFDsamples were much lower than that observed for HACD andMVD samples where the slope sharply rose. Then, finish MVD

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was applied to reduce drying times, especially in the last dryingperiod. The drying times for reaching the equilibrium moisturecontent in combined HP FB AFD and MVD were about 300 min(Fig. 2b). The moisture ratio changes vs. drying times indicate thatthere were no statistical differences between the drying times forgreen peas dried by HACD as well as combined HP FB AFD andMVD. Furthermore, no statistical differences (p 6 0.05) were ob-served among the three trials conducted for different conditionsat AFD.

Experimental results of moisture variation with drying timewere fitted to the theoretical diffusion model. In all cases, the val-ues of R2 greater than 0.9 indicated a good fit of theoretical Fick’smodel to the experimental data. It was concluded that Fick’s sec-ond low equation is a useful theoretical model for correlatingmoisture content with drying time.

Mathematical differentiation of the drying curves shown inFig. 2a and b allowed the calculation of the drying rates (Pabiset al., 1998). No constant drying rate period was observed forany of the experiments performed. Statistically significant differ-ences were found between drying rates obtained during MVDand HACD. During the initial phase of drying the drying rates ofgreen peas were 0.59 and 0.20 l/min for MVD and HACD, respec-tively (Fig. 3a).

The application of microwave power at a reduced pressure in-duced a three times higher initial drying rate than observed for

Fig. 3. Drying rates of green peas dehydrated by: (a) MVD, HACD and (b) HP FBAFD + MVD.

hot air drying, which is in agreement with the data reported byother authors (Sharma and Prasad, 2004; Bondaruk et al., 2007).During the initial phase of HP FB AFD the drying rates of green peaswere about 0.04 l/min for all of the trials conducted under AFDconditions (Fig. 3b). It means that initial drying rate for AFD wasfifteen or five times (not shown in the plots) lower than in caseof MVD and HACD, respectively. The drying rates were enhancedby the application of microwave power at reduced pressure inthe final stage of drying. Thus, two times higher drying rates wereobserved in the finish MVD in comparison with pre-drying stepusing HP FB AFD (Fig. 3b).

The values of effective moisture diffusivity obtained in thisstudy were in the range from 10�11 to 10�9 that are within the val-ues reported by Panagiotou et al. (2004) for different food materi-als. The values of effective moisture diffusivity of green peas driedat HACD and MVD were 1.43 � 10�9 and 1.92 � 10�9 m2/s, respec-tively. Low values of the effective moisture diffusivity, which werebetween 6.94 � 10�11 and 8.78 � 10�11 m2/s were found for HP FBAFD samples. Santacatalina et al. (2011) also reported the low val-ues of the effective moisture diffusivities for AFD eggplant, appleand carrot, which were 5.15 � 10�11, 1.83 � 10�11 and1.07 � 10�11 m2/s, respectively. Alves-Filho et al. (2004) foundmuch lower values of moisture diffusivity for HP FB AFD samplesand they were even one order of magnitude lower than that foundin this study. In the final stage of combined HP FB AFD and MVD,microwave vacuum application raised the values of moisture diffu-sivity to the range between 3.39 � 10�9 and 3.66 � 10�9 m2/s.

3.2. Morphological parameters

The non-dried samples length and width were found to be8.90 ± 0.03 mm and 8.00 ± 0.04 mm, respectively. Statistically sig-nificant differences between the dimensions of control (non-dried)and dried materials were observed for all of the drying methods.The length of dried material was found to be in the range between6.69 ± 0.05 mm and 6.96 ± 0.06 mm, while width was found in therange between 5.87 ± 0.03 mm and 6.38 ± 0.03 mm, respectively. Itwas found that different drying conditions at HP FB AFD had nostatistical effect on the changes in morphological parameters suchas length and width. This means negligible shrinkage in all HP FBAFD tests. HP FB AFD caused minimal changes in morphologicalparameters of green peas. The changes in linear dimensions ofsamples pre-dried using HP FB AFD until moisture content of2.07 ± 0.11 kg/kg dry basis (d.b.) were in the range between 0.3%and 3.3%. After HACD, MVD and HP FB AFD followed by MVD thechanges in linear dimensions were in the range between 20% and27% depending on experimental conditions. The dominant attri-bute of control (non-dried) sample was round shape and the circu-larity index about 1.01 ± 0.04. In case of HACD as well as combinedHP FB AFD and MVD green peas, the circularity was about1.30 ± 0.02. HACD green peas were found to be wrinkled, due toshrinkage that occurred at elevated temperature, while in case ofcombined HP FB AFD and MVD samples, the shape was nearlyround. It was observed that during MVD green peas changed theirshape and a batch of MVD particles was found to be irregular andmore elongated than before drying. Circularity of MVD sampleswas about 1.42 ± 0.03.

3.3. Bulk density, particle density, bulk porosity and shrinkage of greenpeas

During HACD, MVD as well as combined HP FB AFD and MVD ofgreen peas changes in structural properties were observed as waterwas removed from the moist material. Thus, the effect of dryingmethod on structural properties such as bulk density, particle den-sity and bulk porosity of dehydrated products was examined. Fig. 4

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presents the variation of bulk density, particle density and bulkporosity for green peas dried by different drying techniques. Theparticle density of raw material was 0.95 ± 0.01 g/cm3. The valuesbetween 0.66 ± 0.03 and 0.76 ± 0.02 g/cm3 were found for particledensity of green peas dried by combined HP FB AFD and MVD aswell as HACD. Much lower values, i.e. about 0.59 ± 0.02 g/cm3 werefound for MVD green peas (Fig. 4a). The bulk density of raw mate-rial was about 0.62 ± 0.02 g/cm3. All of the values of bulk density ofdehydrated green peas were found to be significantly lower thanthat obtained for control sample. It was found that bulk densitywas strongly affected by dehydration process. More specifically,the bulk density of MVD samples was significantly lower thanthose obtained by other drying techniques. MVD produced the lessdense product with bulk density about 0.28 ± 0.02 g/cm3. The val-ues of 0.32 ± 0.01 and 0.34 ± 0.01 g/cm3 were found for combinedHP FB AFD and MVD as well as HACD, respectively. The final bulkporosity of dehydrated products strongly depended on the dryingmethods as well as the material structure created under differentdrying conditions. Porosity of all the dried samples changed be-tween 0.43 ± 0.03 and 0.63 ± 0.03. It was found that bulk porosityof HACD materials was significantly higher in comparison to that

Fig. 4. Bulk density, particle density (a) and bulk porosity (b) of control (1) and dehydratAFD (RH L); (5) HP FB AFD (RH L) and MVD; (6) HP FB AFD (RH H); (7) HP FB AFD (RH

dried by all other dehydration processes (Figs. 4b). It seems thatthe changes in final bulk porosity were strongly related to theshrinkage occurring during green peas processing under differentdrying conditions. The volumetric shrinkage values are shown inTable 2. Comparing the shrinkage of green peas dehydrated byHACD with the samples fully dried by MVD or combined HP FBAFD and MVD significant differences were found among them.HACD generated the highest volumetric shrinkage of 59.7 ± 0.3%,which can be explained by the fact that long drying time gives along time for the product to shrink. Probably during HACD casehardening of the surface occurred and the volume of the samplebecame fixed at an earlier stage, preventing higher shrinkage.The three different combinations of HP FB AFD and MWD gave sig-nificantly less volumetric shrinkage, which was between46.7 ± 0.2% and 50.0 ± 0.7%, depending on drying conditions. Itseems that the solid state of water during HP FB AFD, with re-stricted movement in comparison to liquid water, protected theprimary structure, and preserved the original structure and theshape of the food material with about 10% or even lower reductionin volume, comparing with HACD. Contrary, MVD green peasdeveloped the lowest bulk porosity due to a limited volumetric

ed green peas (2–9). Drying methods and conditions: (2) MVD; (3) HACD; (4) HP FBH) and MVD; (8), HP FB AFD (RH H, IR); and (9) HP FB AFD (RH H, IR) and MVD.

Table 2Estimation of volumetric shrinkage (SV) of green peas dried under different dryingmethods.

No. SV (%) MC (% d.b.)

1 – –2 43.3 ± 0.1 0.07 ± 0.023 59.7 ± 0.3 0.05 ± 0.014 20.0 ± 0.4 1.96 ± 0.065 50.0 ± 0.7 0.09 ± 0.026 20.0 ± 0.4 2.18 ± 0.047 46.7 ± 0.2 0.13 ± 0.018 13.3 ± 0.3 2.07 ± 0.069 50.0 ± 0.5 0.11 ± 0.01

Drying methods and conditions: (1) control; (2) MVD; (3) HACD; (4) HP FB AFD (RHL); (5) HP FB AFD (RH L) + MVD; (6) HP FB AFD (RH H); (7) HP FB AFD (RH H) + MVD;(8) HP FB AFD (RH H + IR); and (9) HP FB AFD (RH H + IR) + MVD.

Fig. 5. Photographs of raw, pre-dried and finish-dried green peas samples: (1)control sample, (2) MVD; (3) HACD; (4) HP FB AFD (RH L); (5) HP FB AFD (RHL) + MVD; (6) HP FB AFD (RH H); (7) HP FB AFD (RH H) + MVD; (8) HP FB AFD (RH H;IR); and (9) HP FB AFD (RH H; IR) + MVD.

Fig. 6. Macrostructure observations of the cross sections of green peas. Dryingmethods: (2) MVD; (3) HACD; (5) HP FB AFD (RH L) + MVD; (7) HP FB AFD (RHH) + MVD; and (9) HP FB AFD (RH H; IR) + MVD.

M. Zielinska et al. / Journal of Food Engineering 115 (2013) 347–356 353

shrinkage, which was 43.3 ± 0.1% (Table 2). Thus, MVD sampleswere characterized by a structure with minimum deformation inrespect to fresh samples. The effect of drying methods on shrinkageof green peas can also be seen from Fig. 5. The shrinkage of samplesdried by combined drying method was more than that of samples

dried by MVD. However, it can be seen from Fig. 5 that the samplesstill show the flat external appearance without marked warping,which is desirable in case of dried green peas. Contrary, green peasdehydrated by HACD possessed a wrinkled seed coat and concaveappearance. Surface cracking was not observed neither duringHACD, MVD nor combined HP FB AFD and MVD.

3.4. Internal porosity and cotyledon structure changes

The influence of combined HP FB AFD and MVD as well as HACDand MVD on the internal porosity and shrinkage of cotyledon wasinvestigated. Cross section observations of macrostructure of greenpeas processed by MVD, HACD and combined HP FB AFD and MVDare illustrated in Fig. 6. Macrostructure observations revealed thepresence of pores on the cross section area of MVD and combinedHP FB AFD and MVD samples (Fig. 6). It is also evident from Fig. 6that samples processed by MVD had much more uniform structurethan the samples dried by combination of HP FB AFD and MVD.Some cavities and disrupting of the continuity of the cellular struc-ture was observed for the samples dried by combined HP FB AFDand MVD. As an explanation of the observed changes of structuralproperties during drying, it could be presumed that during MVDthe energy of microwaves is absorbed by water located in thewhole volume of the material being dried (Figiel, 2010). MVD cre-ated a large vapor pressure in the centre of the material, allowingrapid transfer of moisture to the surrounding vacuum and prevent-ing structural collapse. Furthermore, the puffing phenomenon, thataccompanied MVD, created a porous structure of the green peasand facilitated obtaining a desired product texture, and in thisway it reduced the product’s density as well as shrinkage. How-ever, the MVD dried samples show highly segregated skin, whichcan be further released resulting in product losses in packing andtransport. It was also found that HP FB AFD and MVD preservedthe advantages of freeze-drying (small shrinkage) and led to por-ous structure created by sublimation of frozen water. Due to theabsence of liquid water and the lower temperature required forthe process, most deterioration reaction rates were very low,which gave a final product of high porosity. Porosity developmentduring HACD drying was relatively small comparing with the sam-ples dried by MVD or combined HP FB AFD and MVD. In case ofHACD samples, much more shrunken cotyledon could be observed.The structure of the interior part of the dried green peas was com-pact and the cells were tightly packed. In fact, after HACD dryingcompletion, cotyledon, which is part of the embryo within theseed, occupied less than half of a space available inside the particle

354 M. Zielinska et al. / Journal of Food Engineering 115 (2013) 347–356

(shown in Fig. 6) and most of the volume was empty space occu-pied by air (the second half of a particle not shown in Fig. 6).

3.5. Textural properties

Fig. 7 shows that there were no statistical differences in maxi-mum breaking force between the samples dried by different dryingmethods and control sample. HACD green peas had mechanicalresistance comparable with MVD, as well as combined HP FBAFD and MVD samples (Fig. 7). After drying and rehydration, statis-tically lower mechanical resistances were found for all of the sam-ples dried in comparison with the control. It means that due toextensive changes during drying and rehydration, green peas weremuch softer and did not recover their original hardness. Summa-rizing, mechanical resistance of green peas has not been influencedby drying method. However, macrostructure observations (Fig. 6)revealed significant differences between the internal structure ofcontrol and differently treated samples.

Fig. 8. Microstructure observations of the green peas cotyledons. Drying methods:(2) MVD; (3) HACD; (8) HP FB AFD (RH H; IR); and (9) HP FB AFD (RH H; IR) andMVD.

3.6. Microstructure changes

The pea starch shows a wide range of gelatinization tempera-tures. Polesi et al. (2011) report three temperatures: 56.1, 74.8and 89.7 for onset, peak and conclusion temperature. As above, itcan be presumed that MVD and combined HP FB AFD and MVDwere conducted at the temperatures below pea starch gelatiniza-tion, while the temperature during HACD was high enough to sat-isfy the conditions for starch gelatinization. To confirm abovestatements, SEM observations of green peas processed by MVD,HACD and combined HP FB AFD and MVD are illustrated inFig. 8. Fig. 8 additionally shows the microstructure of samplespre-dried by HP FB AFD. It can be seen that the non-gelatinizedstarch granules were still present in the samples dried by MVDand combined HP FB AFD and MVD and the internal structure ofstarch granules remained almost intact or only partially gelati-nized. Fig. 8 shows that whereas MVD or combined HP FB AFDand MVD maintained starch granules in non-gelatinized or par-tially gelatinized form, HACD caused full or almost complete starchgranules gelatinization. During HP FB AFD, the drying chamberoperated at atmospheric pressure while performing mode of lowtemperature. Thus, the product temperature has not reached thestarch gelatinization temperature. In this study, the samples pre-dried by HP FB AFD lost about 40% wb of their initial moisture con-

Fig. 7. The maximum breaking force for control (1), dried and rehydrated green peas (2,methods and conditions: (2) MVD; (3) HACD; (5) HP FB AFD (RH L) + MVD; (7) HP FB A

tent. It means that uncompleted starch gelatinization occurredduring finish MVD due to limited water content. If such productswere processed by HACD at 60 �C, the inner structure of green peascomposed of a compact starch–protein matrix containing dilutedcell walls. It is in agreement with the data reported by Bondaruket al. (2007). Additionally, starch granules heated beyond a criticaltemperature (up to 60 �C) in the presence of water caused full oralmost complete starch gelatinization in final product. Thus, theirorganized molecular structure was destroyed and a melting andloss of granule crystallinity occurred (Fig. 8, Sample 3). It seemsthat the application of MVD or combined HP FB AFD and MVDfor green peas processing created less rigorous conditions compar-ing with HACD. In case of MVD, lower temperature in the chamber,comparing with HACD prevented complete starch gelatinization(Fig. 8, Sample 2). However, a violent evaporation of water andmelting of starch granules probably weakened the starch–proteinmatrix.

3, 5, 7, 9). Symbols d and r mean dried and rehydrated samples, respectively. DryingFD (RH H) + MVD; and (9) HP FB AFD (RH H; IR) + MVD.

Fig. 9. The total difference in color DE�, saturation DC� and hue DH� betweencontrol (non-dried) and dried green peas. Drying methods and conditions: (2) MVD;(3) HACD; (5) HP FB AFD (RH L) and MVD; (7) HP FB AFD (RH H) and MVD; and (9)HP FB AFD (RH H; IR) and MVD.

M. Zielinska et al. / Journal of Food Engineering 115 (2013) 347–356 355

3.7. Color

Statistically significant differences (p 6 0.05) between the colorof control (non-dried) and dried green peas were observed for all ofdrying methods. All of the samples turned lighter during drying.Probably, due to heat treatment chlorophyll was degraded to unde-sirable gray–brown compounds such as pheophorbide or pheophy-

tin and further metabolized to colorless compounds (Heaton andMarangoni, 1996).

No significant differences (p < 0.05) in color were observed dur-ing HP FB AFD (Fig. 9a). The more visible changes between the col-or of control and dried green peas were noticed for MVD and HACDsamples in comparison with combined HP FB AFD and MVD greenpeas and they were in the range of 9.0 and 11.4, respectively. Theresults demonstrate that an exposure to high temperature over along time caused bigger changes in color between control and finalproduct. Less changes in DE� were observed for combined HP FBAFD and MVD samples and DE� was in the range between 7.8and 8.8. It means that better color is obtained by applying HP FBAFD alone or as the first step of the drying operation since it pre-served the color with minimal changes. Fig. 9b shows that the sat-uration of dehydrated green peas was seen as much different fromthat of raw ones and it corresponded with different values of DC�,which were about 6.0 and 7.5 for MVD and HACD, respectively. Thesmallest DC� was observed for combined HP FB AFD and MVD andit was in the range between 4.4 and 5.9, depending on the dryingconditions. The positive values of DC� mean that green peas driedby HP FB AFD were characterized by more intense color after dry-ing completion. Small differences in hue of dried and non-driedgreen peas were observed. Dried samples were yellower and thetotal difference in hue was in the range between 1.9 and 2.5 forall of the dried samples (Fig. 9c). However, the samples dried bycombined HP FB AFD and MVD exhibited the smallest changes incolor, saturation and hue, which can be attributable to the highestcontent of chlorophyll.

4. Conclusions

The multi-stage combined drying of green peas has been devel-oped to maximize the benefits of two different drying techniques:HP FB AFD and MVD. The quality of dried green peas was deter-mined in terms of bulk density, particle density, bulk porosity,shrinkage, internal porosity, microstructure, compression force,size, shape and color. The drying time for reaching the equilibriummoisture content in MVD was 145 min, while in combined HP FBAFD and MVD as well as HACD the process was carried out for300 min. The drying rates of green peas dried in MVD and HACDwere 0.59 and 0.20 l/min, respectively. In case of combined HPFB AFD and MVD the initial drying rate was about 0.04 l/min. How-ever, two times higher values were observed in final stage of dry-ing, when microwave power was applied. The values of effectivemoisture diffusivity of green peas dried at HACD and MVD were1.43 � 10�9 and 1.92 � 10�9 m2/s, respectively. In case of HP FBAFD, the values of effective moisture diffusivity were about two or-ders of magnitude lower, i.e. between 6.94 � 10�11 and8.78 � 10�11 m2/s. The application of microwave power at lowpressure in the final stage of combined HP FB AFD and MVD raisedthe values of moisture diffusivity to the range between 3.39 � 10�9

and 3.66 � 10�9 m2/s.MVD samples were characterized by a structure with minimum

deformation in respect to fresh samples. MVD produced the lessdense product, followed by combined HP FB AFD and MVD as wellas HACD. A limited shrinkage, i.e. 43.3 ± 0.1% was developed duringMVD of green peas. The shrinkage of samples dried by combinedHP FB AFD and MWD was found to be significantly higher (be-tween 46.7 ± 0.2% and 50.0 ± 0.7%) than that observed for MVDsamples. HACD generated the highest volumetric shrinkage ofabout 59.7 ± 0.3%, Furthermore, combined HP FB AFD and MWDsamples showed the flat external appearance without markedwarping, while HACD green peas possessed a wrinkled seed coatand concave appearance. Mechanical resistance of green peas hasnot been influenced by drying method. However, macrostructure

356 M. Zielinska et al. / Journal of Food Engineering 115 (2013) 347–356

observations revealed significant differences between the internalstructure of control and differently treated samples. The structureof the interior part of HACD green peas was compact and the cellswere tightly packed, while in case of MVD and combined HP FBAFD and MVD samples, number of pores on the cross section areawere observed. Also, much more shrunken cotyledon could be ob-served for HACD samples. SEM observations revealed that HACDcaused full or almost complete starch granules gelatinization,whereas MVD or combined HP FB AFD and MVD maintained starchgranules in non-gelatinized or partially gelatinized form, HP FBAFD preserved the color of green peas with minimal changes, whilemore visible changes were noticed for MVD and HACD.

It was found that multi-stage combined HP FB AFD and MVDsatisfied important requirements, such as high product quality(due to low material temperature during AFD and low pressureduring MVD), and increased drying rates in the final stage due toapplication of microwave heating. Summarizing, to respond thecurrent demand of high quality products, combined HP FB AFDand MVD represents an interesting technique for green peasprocessing.

Acknowledgements

The study was financially supported by the Scholarship andTraining Fund established within the EEA Financial Mechanismand Norwegian Financial Mechanism through the Projects FSS/2011/V/D3/W/0103. The improvement of new techniques for fooddrying with emphasis on energy and quality aspects and FSS/2011/V/D3/W/0102. Evaluation of the quality of agri-food products pro-cessed with innovative drying technologies.

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