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Analysis of the Drying of Broccoli Florets in a FluidizedPulsed BedAlejandro Reyes a , Andrea Mahn a , Carolina Guzmán a & Dafne Antoniz aa Department of Chemical Engineering, Universidad de Santiago de Chile, Santiago, Chile

Version of record first published: 17 Aug 2012

To cite this article: Alejandro Reyes, Andrea Mahn, Carolina Guzmán & Dafne Antoniz (2012): Analysis of the Drying ofBroccoli Florets in a Fluidized Pulsed Bed, Drying Technology: An International Journal, 30:11-12, 1368-1376

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Analysis of the Drying of Broccoli Florets in aFluidized Pulsed Bed

Alejandro Reyes, Andrea Mahn, Carolina Guzman, and Dafne AntonizDepartment of Chemical Engineering, Universidad de Santiago de Chile, Santiago, Chile

The effect of air temperature and flow rate, plate rotationalspeed, and initial broccoli particle size in a fluidized pulsed bed dryeron moisture content, diffusivity, antioxidant capacity, polyphenolscontent, and selenium concentration of conventional and selenium-enriched broccoli was investigated.

Adjustment of the drying curves with the simplified constantdiffusivity model was performed by intervals, in order to considershrinkage, resulting in an average diffusivity equal to 4.9 E-09(m2/s), which is 82% higher than that observed in drying of broccoliin a tunnel dryer. In addition, the drying curves were adjusted toPage’s model, resulting in R2 values higher than 0.9.

The conditions that maximize the antioxidant properties ofconventional broccoli florets were air temperature of 60�C, air flowrate in the dryer inlet of 3m/s, and initial particle size of 1 cm. Thedisk rotational speed had no statistically significant effect on theantioxidant properties in the range from 60 to 100 rpm. These oper-ating conditions were then used to dehydrate selenium-enrichedbroccoli. The antioxidant properties in selenium-enriched broccoliafter drying were comparable to those obtained for conventionalbroccoli. The conditions that minimized selenium loss were air tem-perature of 53�C and air flow rate of 2m/s.

Keywords Broccoli; Drying; Pulsed fluidized bed

INTRODUCTION

Broccoli (Brassica oleracea var. italica) has a great poten-tial to prevent some types of cancer[1] and cardiovasculardiseases.[2] Consuming this vegetable improves the generalhealth status, due to the presence of some bioactive com-pounds that positively affect the defense against oxidativestress. Among these compounds, glucosinolates, sulfora-phane, polyphenols, and minerals such as selenium are ofmajor interest.[3] Selenium acts as a cellular antioxidant,and a deficient metabolic status of this element leads to ahigher risk of developing cardiovascular diseases and sometypes of cancer.[4] Selenium (Se) fertilization of broccolicrops results in a significant increase of Se concentrationin the vegetable,[5] converting it into a functional food.

On the other hand, broccoli is mostly consumed as a pro-cessed food, and then its functional properties, as well as its

physical characteristics, can be affected to different extents.Temperature, humidity, or light may induce some reactionsduring storage, sometimes leading to a loss of quality.[6]

Blanching, cooking, and freezing affect the content of glu-cosinolates, sulforaphane, and polyphenols in broccoli.[7]

The effect of dehydration processes on the bioactivecompounds of broccoli has been poorly studied. Mrkicet al.[8] investigated the effect of temperature (50–100�C)and air flow rate (1.20–2.25m=s) in a pilot tray dryer onthe bioactive compounds content and antioxidant activityof broccoli. The main conclusion of the authors was thathigh-temperature, short-time drying processes resulted inthe highest antioxidant activity of broccoli. Recently, Mahnet al.[9] optimized the convective drying of broccoli andfound that the maximum antioxidant activity was obtainedat 60�C, air flow rate of 4m=s, and 1.5 cm particle diameter.A 70% reduction in free radical scavenging ability and a29% increase in total reductive capability, compared tothe fresh vegetable, was achieved. Icier et al.[10] performedan exergy analysis of broccoli dehydration by differentprocesses, such as fluidized bed and heat pump drying. Theyconcluded that the highest exergy efficiency was obtained ina fluidized bed dryer.

No studies addressing the effect of operating conditionsin fluidized bed dryers on the bioactive compounds ofbroccoli, which would be useful in establishing the mostappropriate processing conditions that allow convertingbroccoli into a functional food, have been published so far.

The pulsed fluidized bed dryer is a modified conventionalfluidized bed in which gas pulses cause movement of theparticle bed.[11,12] The pulsed fluidized bed allows easy flui-dization for irregularly shaped particles or particles with awide size distribution; fluidization with 30–50% less air;improved fluidization uniformity (reduced channeling);fluidization of fragile particles; and lower pressure drop.

Pulse generation can be achieved by relocation of thegas stream through the use of a (1) rotating distributorvalve that makes the gas stream sweep across specifiedchambers of a rectangular bed; (2) perforated rotating disclocated below the circular bed; or (3) rotating slottedhorizontal cylinder that directs the gas stream to differentsections of the base of a circular bed.[12–14]

Correspondence: Alejandro Reyes, Department of ChemicalEngineering, Universidad de Santiago de Chile, Bernardo OHig-gins 3363, Santiago, Chile; E-mail: alejandro.reyes@usach.cl

Drying Technology, 30: 1368–1376, 2012

Copyright # 2012 Taylor & Francis Group, LLC

ISSN: 0737-3937 print=1532-2300 online

DOI: 10.1080/07373937.2012.686548

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Drying processes are usually divided into a constantrate drying period (the drying rate is only determined byexternal conditions) and falling rate drying period (thedrying rate diminishes continually and is determined bythe internal flow of liquid and=or vapor in response to theexternal conditions). In both periods, drying takes placethrough different types of moisture diffusion. The usualapproach to modeling mass transfer in foods employs theconcept of effective diffusivity (Deff),

[15] which allowsdescribing moisture diffusion by Fick’s second law.

@ X�

@t¼ r � ðDeffrX

�Þ ð1Þ

Equation (1) can be integrated for different geometries,boundary=initial conditions, and special physical consid-erations,[16] giving origin to different mathematical modelsfor the drying process. In this work, it is assumed that dur-ing the drying process the effective diffusivity (Deff) remainsconstant, as well as the conditions of the drying air, and thesolids are symmetrical with respect to a unidirectional dif-fusion coordinate. Although the particles of broccoli floretsdo not exhibit a regular geometry, they were consideredspherical for the modeling purpose. Equation (1) was solvedwith the following initial and boundary conditions:

I :C: X�¼ Xo; t ¼ 0; 8R � Ro ð2Þ

B:C:1@ X

�

@R¼ 0; R ¼ 0; 8t > 0 ð3Þ

B:C:2 X�¼ X �; R ¼ Ro; t > 0 ð4Þ

The solution of Eq. (1) for the falling drying rate periodunder conditions 2, 3, and 4, in terms of the average moist-ure content (X) instead of the local moisture content, isgiven by Eq. (5), corresponding to the constant diffusivitymodel[17]:

X ðtÞ � X �

Xo � X � ¼ 6X1n¼1

1

ðnpÞ2exp �ðnpÞ2Deff � t=R2

o

h ið5Þ

By omitting the factor 6=p2, and considering only thefirst term of the series, a simplified constant diffusivitymodel (SCDM) is obtained, which is given by Eq. (6).The SCDM allows a semi-empirical estimation of Deff:

XðtÞ � X �

Xo � X � ¼ exp �p2Deff

R2o

� t� �

ð6Þ

in which the external resistance to mass transfer is negli-gible (which is justified given the movement of the particles

and the characteristic of a pulsed fluidized bed), so theexternal surface of the solid is in equilibrium with thedrying gas. We assumed that X� was very close to zeroand it was thus neglected in the model.

Additionally, many empirical models have been usedto represent the dehydration curves of agriculturalproducts.[18] The most commonly used is the modified Pagemodel, given by Eq. (7), where k (min�1) is a dimensionlessvalue and t (min) is the drying time.

X ðtÞXo

¼ expð�k � tnÞ ð7Þ

MATERIALS AND METHODS

Fluidization Equipment

The equipment (Fig. 1) used was made of stainless steeland consisted of a drying chamber, an electric heater withcontrollable power up to 8,000W, a 10-HP centrifugalexhauster, and a system to collect the product (a cyclonewith a bag filter as a backup unit). The drying chamberconsisted of a 1-m-high cylindrical column connected to atruncated conical base section with a 0.25-m bottom diam-eter and a 0.4-m upper diameter. Air enters the chamber ofthe dryer through a slotted rotary plate that has 6% of freesection area, resulting in pulsed fluidization. A particle sam-pling system was located on the lower section of the dryer(0.07m above the rotary plate). This system consisted of a30-mL sampling spoon that was introduced through a sam-pling port, so it was possible to sample in different positionsat the same horizontal level inside the bed. Once the

FIG. 1. Pulsed fluidized bed dryer: (A) heater, (B) conical trunk drying

chamber, (C) sampling system, (D) rotary slotted plate rate control, (E)

cyclone, and (F) centrifugal exhauster.

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particles were removed from the bed, the spoon was takenaway and the port was closed.

A fine metallic mesh on the rotary plate prevents thesolids from dropping down below it. The slotted plate isrotated by a vertical shaft driven by a 1=4-HP electric motorprovided with a frequency regulator. The inlet air velocitywas determined with a handheld vane anemometer (accu-racy of 0.1m=s) in a duct of 0.10m diameter. The tempera-ture inside the chamber was measured with a thermocouplewith an accuracy of 0.1�C.

Materials

The drying experiments in the first stage were carried outwith broccoli floret particles purchased in a local market.The broccoli florets had an irregular form, consisting of astalk and a small florets in different proportions. The mainstem was cut off and discarded, and the florets were kept at�20�C in a freezer. Prior to processing, the samples weredefrosted and then blanched in 90�C water for 2min tominimize the enzymatic reactions and reduce the microbialcharge. Immediately after blanching, the samples wereimmersed in an ice bath to lower their temperature. Afterthis procedure, the water was drained off and the floretswere cut into 1- or 3-cm pieces. Batch sizes of 1.6 kg of eachsize of broccoli pieces were used, and the size was as homo-geneous as possible. The values 1 and 3 cm corresponded tothe largest dimension of the particles and were used asreference values in order to distinguish between the twodifferent particle sizes.

This procedure was repeated with selenium-enrichedbroccoli in order to investigate the effect of the operatingconditions in a pulsed fluidized bed dryer on some bioac-tive compounds.

Experimental Method

Selection of the experimental factors considered air tem-perature (60 and 80�C) due to its direct influence on dryingtime; initial broccoli size (1 and 3 cm) because it affects thediffusion path of moisture; air flow rate (1 and 3m=s) anddisk rotational speed (60 and 100 rpm), given that bothaffect the fluid dynamics of the particles in the bed andthe contact time between the particles and air; as well asoxidation of some broccoli compounds. A factorial experi-mental design, with the above-mentioned factors in twolevels (24), was used to analyze the drying process. Theresponses of the experimental design were moisture contentat 40min drying, antioxidant capacity, and total polyphe-nols content. Table 1 shows the experimental matrix in stan-dard order. Runs were carried out in a random order.

Drying Kinetics

A batch of 1.6 kg of blanched broccoli florets with aninitial moisture content of Xo¼ 0.9 kg=kg (wb) was driedunder the selected operating conditions, with samples taken

every 10min at the beginning and every 5min in the laterstages of the drying process in order to determine moisturecontent and particle volume. For determination of particleshrinkage, 3 to 5 particles were immersed in a hexane-containing burette to determine their volume. The moisturecontent determinations were carried out according toAOAC 920.151[19] by drying at 80�C in a vacuum oven(model 60061, Cole & Palmer, IL) until constant masswas attained, and the average of three measurements wasused for calculations.

Antioxidants Extraction

The extraction of antioxidant compounds was performedas reported by Vinson et al.[20] A 200-mg aliquot of fresh,blanched, or freeze-dried broccoli was accurately weighedin a vial; 4mL of 80% methanol–water was added andultrasound-processed for 10min at 8V and then incubatedat room temperature in an orbital shaker for 4 h. Then thesamples were centrifuged at 12,000 � g for 5min to removethe solids. The supernatant was recovered and 80% meth-anol was added to complete the 5mL extract volume.

Free Radical Scavenging Ability

The antioxidant capacity was determined based on thefree radical sequestering ability of the compounds presentin the fruit extracts, using the stable radical 2,2-diphenyl-1-picryhydrazyl (DPPH).[21] Forty microliters of methanol-diluted extract (at six dilutions) was mixed with 1.960 mLDPPH solution (6� 10�5M in methanol). The absorbancedecrease at 515 nm was recorded until a plateau wasreached. The DPPH � concentration in the reaction mixturewas calculated by means of a calibration curve and the

TABLE 1Drying conditions of conventional broccoli

Run n (m=s) Size (cm) T (�C) x (rpm)

E1 3 1 60 60E2 4 1 60 60E3 3 3 60 60E4 4 3 60 60E5 3 1 80 60E6 4 1 80 60E7 3 3 80 60E8 4 3 80 60E9 3 1 60 100E10 4 1 60 100E11 3 3 60 100E12 4 3 60 100E13 3 1 80 100E14 4 1 80 100E15 3 3 80 100E16 4 3 80 100

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remaining DPPH � concentration was calculated. Theresults were reported as anti-radical power (ARP¼ 1=EC50). Measurements were made in duplicate, and thearithmetical average is reported.

Total Polyphenols Content

The total polyphenols content was determined spectro-photometrically using the Folin-Ciocalteau method.[22]

One hundred eighty microliters of extract and 90 mL Folin-Ciocalteau reagent (diluted 1:1) were added to 360 mL dis-tilled water. The mixture was homogenized and left indarkness for 5min. Then, 450 mL of a 20% (w=v) sodiumcarbonate solution was added and left in darkness for30min. Then the samples were centrifuged at 12,000 � gfor 5min to remove the precipitate, and absorbance at750 nm was measured. The results were expressed as milli-grams of gallic acid equivalents per gram of dry matter (mgGAE=g DM). The measurements were made in duplicateand average values are reported.

Selenium-Enriched Broccoli Culture

Broccoli (Brassica oleracea var. italica) was grown in indi-vidual 9-L pots containing organic soil in a greenhouse.Seedlings (10 cm height) were purchased from a local nurserygarden. Selenium fortification consisted of adding 30mL of a30mmol=L sodium selenate solution to yield a final concen-tration of 100mmol=L sodium selenate in the pot. This pro-cedure began 4 weeks after transplantation and was repeatedonce a week for 10 weeks. Broccoli florets were harvestedwhen they reached an appropriate size and were kept at�20�C in plastic sealed bags before processing and analysis.

The total selenium content in fresh and dried broccoliwas determined using inductively coupled plasma atomicemission spectrometry after microwave digestion of thesamples. Free radical scavenging ability and total polyphe-nols content of selenium-enriched broccoli (fresh anddried) were determined as described above.

Statistical Analyses

The statistical analyses, including calculation of thestandardized effects on the moisture content after 40minof drying and nutritional properties, were carried outaccording to standard procedures[23] using the softwareStatgraphics Plus 5.1 (Statistical Graphics Corp.). A 95%confidence interval was considered. Drying model adjust-ment was performed by means of Newton’s method, usingprogressive derivatives and linear estimation. Goodness offit was determined by the coefficient of determination (R2).

RESULTS

Fluid Dynamics of the Bed

Broccoli particles are relatively fragile, especially afterblanching, and therefore fluidization must be mild in the

primary drying stage. For the chosen air flow rates, inthe first 10–15min, the particles were in slow motion, andthis motion increased with time; thus, at the end of the pro-cess there was a vigorous fluidization.

Characterization of Particles

Drying of agricultural products is usually accompaniedby significant volume contraction caused by changes intheir microstructures due to moisture gradients. Shrinkagedepends on the particle structure, dryer type, and dryingconditions. This phenomenon has been described by differ-ent models.[13,24,25]

An analysis of the samples collected during the dryingruns yielded an exponential relation between the volumeof the particles and their moisture content:

V ¼ a1 � expða2 � XwbÞ ð8Þ

This relation was affected only by the initial particle size,as shown in Fig. 2, where a1¼ 0.1887 and a2¼ 3.2435 foran initial size 1 cm (R2¼ 0.982) and a1¼ 0.0495 anda2¼ 4.0686 for an initial size of 3 cm (R2¼ 0.975). Then, thisvolume was considered as the volume of a sphere and fromit the equivalent diameter was obtained using Eq. (9):

dsphere ¼6 � Vp

� �1=3

¼ 2 � Ro ð9Þ

Figure 2 shows that at moisture content (wb) higherthan 0.5 kg=kg the standard deviation was considerablylarger than those observed for lower moisture contents.The falling rate period was adjusted, and only the particlevolume for moisture content (wb) lower than the criticalmoisture content (Xc¼ 0.4) was used to estimate particlesize in this period.

FIG. 2. Experimental and estimated particle volume vs. moisture

content.

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Experimental Drying Curves

The drying experiments were carried out according to thestatistical design shown in Table 1, and resulting dryingcurves are shown in Fig. 3. A high noise level in the experi-mental points of some curves was observed, which can beattributed to the larger particle size (resulting in a low num-ber of particles in a sample) and to the heterogeneous par-ticle shape, which gave rise to different behaviors relatedto shrinkage and consequently different drying kinetics. Inaddition, the bed may not have been absolutely homo-geneous due to the fluid dynamics behavior of the bed,especially during the constant drying rate period. Despitethis, in the drying curves the expected effect of temperatureand particle size could be qualitatively assessed. In addition,it was observed that the highest drying rates during the first30–50min of drying corresponded to the saturated surfacedrying (first drying period). Figure 4 shows a Pareto dia-gram for moisture content after 40min of drying. Thisanalysis shows that a temperature increase resulted in a stat-istically significant reduction of moisture content, whereaslarger particle size resulted in significantly higher moisturecontent, due to the fact that larger particle size means alower specific surface area.

Adjustment of Drying Kinetics

The critical moisture content values on a wet basis (Xc)were estimated from the experimental drying curves whenthe slope of the experimental curves changed. The valuesobtained for the 16 runs fluctuated within the range of0.35–0.46 kg=kg wet basis, without a clear statistical

tendency. As a consequence, a mean critical moisture con-tent of 0.4 kg=kg was considered for all experiments.

The SCDM (Eq. (6)) was used to adjust the experimentaldrying curves below the critical moisture content (Xc¼ 0.4),which were divided into moisture content intervals using Ro

values calculated by Eq. (9) in order to consider particleshrinkage. The adjustment parameter was the effectivemoisture diffusivity (Deff). Accordingly, Ro was considereddependent on moisture content. The Deff values arepresented in Table 2. The SCDM provided a reasonablefit to the experimental data, resulting in average R2 equalto 0.952. The Deff obtained for broccoli florets fluctuatedbetween 5.57� 10�10 and 1.78� 10�9 (m2=s), which com-pared well with values obtained in a tunnel dryer[9] and withvalues reported in the literature for different vegetables.[15]

However, these results differed from those reported byMrkic et al.[26] who estimated a Deff of 1.0-cm convectivelydried broccoli florets on the order of 10�8 (m2=s), whichdiffered considerably from reported values for vegetables.This could be attributed to the different methodology usedto estimate Deff.

On the other hand, Page’s empirical model (Eq. (7)) wasused to describe the entire drying period. Parameter adjust-ment was carried out by minimizing the sum of squares withthe Solver application of Microsoft Excel. The R2 were usedto depict the quality of adjustment, resulting in valueshigher than 0.96. Table 2 shows the parameters of the Pagemodel and Figs. 5 and 6 compare experimental values withpredictions by the Page model and SCDM for two runs.

Antioxidant Properties

Because the disk rotational speed in the investigatedrange from 60 to 100 rpm had no statistically significanteffect on drying time, this factor was not further considered.Therefore, the effect of drying conditions on the nutritionalproperties of broccoli was analyzed using a 23 factorialdesign, whose factors were air flow rate, particle size, anddrying air temperature.FIG. 3. Experimental drying kinetics.

FIG. 4. Pareto chart for moisture content at 40min of drying (color

figure available online).

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The antioxidant capacity of fresh and dehydratedbroccoli florets was determined as the free radical scaveng-ing ability (expressed as ARP) and the total polyphenolscontent. Table 3 shows the results obtained for dehydratedbroccoli after blanching. The maximum ARP was32.9 mmol=g DM and was obtained with air temperatureof 60�C, air flow rate of 3m=s, and 1.0 cm initial particlesize. In fresh broccoli, the ARP was 100.7 mmol=g DM,

and in blanched broccoli the ARPwas dramatically reducedto 32.8 mmol=g DM. The main loss of antioxidant capacitywas due to blanching (77% reduction with respect to freshbroccoli), and after fluidized bed drying this property wasonly slightly affected. These numbers agree with valuesreported by Mahn et al.[9] for a tunnel dryer. In addition,Tanongkankit et al.[27] investigated the effect of hot air dry-ing of white cabbage leaves (Brasica oleracea var. capitata)on antioxidant properties and found that after processing,the product maintained 70% of the original antioxidant

TABLE 2Adjustment of drying kinetics to Page’s model and the SCDM

Page SCDM

Run K n R2 Deff (m2=s) R2 Xwb (t¼ 40min)

E1 2.02E-03 1.7195 0.955 6.44E-10 0.955 0.3415E2 1.36E-03 1.7290 0.992 5.57E-10 0.974 0.3696E3 6.70E-04 1.7793 0.990 8.84E-10 0.990 0.5909E4 5.11E-03 1.3827 0.994 1.03E-09 0.958 0.4151E5 4.39E-04 2.3718 0.993 1.45E-09 0.994 0.0500E6 1.41E-02 1.2784 0.981 6.78E-10 0.853 0.1951E7 7.87E-03 1.4663 0.991 1.73E-09 0.955 0.1489E8 3.46E-03 1.6157 0.995 1.65E-09 0.973 0.2222E9 2.40E-03 1.7147 0.962 6.63E-10 0.917 0.1852E10 7.15E-04 1.9598 0.988 7.80E-10 0.926 0.2969E11 6.99E-04 1.9039 0.986 1.38E-09 0.917 0.3448E12 1.81E-03 1.6020 0.996 1.01E-09 0.944 0.4528E13 1.31E-03 2.0047 0.977 1.23E-09 0.956 0.1200E14 1.94E-03 1.8894 0.997 1.03E-09 0.955 0.1176E15 2.41E-03 1.7343 0.991 1.78E-09 0.961 0.2063E16 2.34E-03 1.7071 0.982 1.69E-09 0.998 0.2500

FIG. 5. Comparison of experimental values with prediction by SCDM

and Page models (run E5).

FIG. 6. Comparison of experimental values with prediction by SCDM

and Page models (run E2).

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capacity. These values were compared with values obtainedfor broccoli florets, and they differed considerably. Thisdifference was attributed to the different cellular structuresand chemical composition of broccoli florets compared withcabbage leaves.

Figure 7 shows Pareto diagrams for ARP and total poly-phenols content. Air flow rate had a significantly negativeeffect on ARP, which implies a decrease in ARP whenhigher air flow rates are used. This can be attributed tothe occurrence of oxidation reactions due to the highoxygen flow. In addition, the interaction between air flowrate and particle size had a significant positive effect.

The maximum polyphenols content in dehydrated broc-coli was equal to 309 GAE=100 g DM (run E5). This repre-sents a 78% reduction with respect to fresh broccoli (1,423GAE=100 g DM). Such a drastic reduction can mainly beattributed to blanching, because the value for blanchedbroccoli was 330 GAE=100 g DM, which represents a 77%reduction in polyphenols content with respect to fresh broc-coli. As a consequence, the drying conditions in a fluidizedbed dryer can be chosen to have a minimal detrimentaleffect on this property.

Air flow rate and the interaction between particle sizeand air temperature had a significantly negative effect onthe total polyphenols content of broccoli, whereas the

interaction between air flow rate and both particle sizeand air temperature had significantly positive effects onthis response.

Selenium-Enriched Broccoli

In order to investigate the effect of drying conditions ontotal selenium content and to evaluate the effect on antiox-idant properties of broccoli, a 32 design was executed. Thesame pulsed fluidized bed dryer was used, and the factorsand their levels were chosen based on the results obtainedfor conventional broccoli (see previous section). Dryingair temperature (50, 60, and 70�C) and air flow rate (2, 3,and 4m=s) were the experimental factors, initial particle sizewas fixed at 1.0 cm, and the plate rotational speed was set on60 rpm.

Figure 8 shows Pareto diagrams for ARP and total poly-phenols content. Air flow rate had a significantly negativeeffect on ARP, which agrees with the results obtained forconventional broccoli. In addition, the interaction betweenair flow rate and temperature had a significantly positiveeffect. Finally, the interaction between temperature andair flow rate had a significantly positive effect on the totalpolyphenols content of selenium-enriched broccoli. Thelosses of antioxidant properties in selenium-enriched broc-coli after drying were comparable to those obtained forconventional broccoli.

TABLE 3ARP and total polyphenols content of dried broccoli

RunARP

(mmol=g DM)Total polyphenols(GAE=100 g DM)

E1 32.9� 6.5 295.0� 15.2E3 10.4� 0.4 274.5� 13.0E5 19.8� 3.0 309.1� 14.5E6 12.1� 1.9 231.2� 13.4E8 12.0� 2.1 269.3� 6.0E10 14.1� 1.3 194.2� 1.8E12 19.2� 2.7 286.1� 18.1E15 16.4� 1.9 229.9� 10.8Fresh broccoli 100.7� 3.6 1423.3� 10.1Fresh blanchedbroccoli

32.7� 0.1 329.6� 6.9

FIG. 7. Pareto charts for ARP and total polyphenols content of conven-

tional broccoli subjected to blanching and fluidized bed drying (color

figure available online).

FIG. 8. Pareto charts for ARP and total polyphenols content of

selenium-enriched broccoli subjected to blanching and fluidized bed drying

(color figure available online).

FIG. 9. Estimated response surface for total polyphenols content of

selenium-enriched broccoli subjected to blanching and fluidized bed drying

(color figure available online).

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Figure 9 shows the response surface obtained for totalpolyphenols in dried selenium-enriched broccoli. Amaximum polyphenols content was observed, correspondingto air temperature of 50�C and air flow rate equal to 2.8m=s.

Table 4 shows the drying conditions and total seleniumcontent of selenium-enriched broccoli. Fortification ofbroccoli culture with sodium selenate resulted in a totalselenium content 5,000 times higher than that found inconventionally grown broccoli (0.0025mg Se=g DM). Thisagrees with values reported in the literature.[28] Drying pro-duced a maximum decrease of total selenium content ofbroccoli of 35% with respect to the fresh selenium-enrichedvegetable. Blanching caused only a slight decrease inselenium content compared with fresh broccoli.

A Pareto chart for total selenium content (Fig. 10)shows that only drying air flow rate had a significant effecton this property, and higher rates reduced the selenium

content. This can be attributed to some volatile seleniumcompounds being picked up by the drying air. The optimaldrying conditions, estimated from the experimental design,were air temperature of 53�C and air flow rate of 2m=s.

CONCLUSIONS

The pulsed fluidized bed dryer was adequate for batchdrying of broccoli floret particles when the moisturecontent was lower than 0.75 kg=kg. A marked shrinkagewas observed, resulting in a final particle volume equal toabout 4% of the initial volume.

The adjustment of the drying curves, which were dividedinto intervals in order to consider shrinkage, using theSCDM resulted in Deff values between 5.57� 10�10 and1.78� 10�9 (m2=s), which was on the same order of magni-tude than the values found in tunnel dryer for broccoli flor-ets and agreed with values reported for different vegetables.The adjustment of the drying kinetics over the entire dryingprocess to the empirical Page’s model resulted in R2 higherthan 0.96.

The main loss of antioxidant capacity and total polyphe-nols content for conventional and selenium-enriched broc-coli was due to blanching, and after drying these propertieswere only slightly affected.

The conditions used to obtain maximum polyphenolscontent were 50�C and 2.8m=s, whereas the optimal dryingconditions that minimized selenium loss were air tem-perature of 53�C and air flow rate of 2m=s. Dryingdecreased the total selenium content of broccoli by amaximum of 35% with respect to the fresh selenium-enriched vegetable.

NOMENCLATURE

ai Empirical constant in Eq. (8)b Empirical constantDeff Effective diffusivity (m2=s)dsphere Particle equivalent diameter (m)k Page constant (min�1)n Page constantR Equivalent radius of particle (m)Ro Initial equivalent radius of particle (m)R2 Coefficient of determinationT Air temperature (�C)t Time (min)V Particle volume (m3)v Air flow rate (m=s)X� Equilibrium moisture content (wb) (kg=kg)

X � Local moisture content (kg=kg)Xc Critical moisture content (wb) (kg=kg)Xo Initial moisture content (wb) (kg=kg)Xwb, X Mean moisture content (wb) (kg=kg)

Greek Symbol

x Plate rotational speed (rpm)

TABLE 4Drying conditions and total selenium content of

selenium-enriched broccoli

Temperature(�C)

Air flowrate (m=s)

Total selenium content(mg=g DM)

50 2 12.7� 0.660 2 13.4� 2.270 2 10.9� 0.750 3 12.9� 0.360 3 10.2� 0.170 3 10.9� 0.050 4 10.8� 0.560 4 10.7� 1.070 4 10.5� 0.7Fresh (Se-enriched) 15.7� 0.1Blanched (Se-enriched) 14.9� 0.1

Values are given as mean� standard deviation.

FIG. 10. Pareto chart for total selenium content of selenium-enriched

broccoli subjected to blanching and fluidized bed drying (color figure

available online).

DRYING OF BROCCOLI FLORETS 1375

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ACKNOWLEDGMENT

The Fondecyt Program (Grant No. 1100437) is grate-fully acknowledged.

REFERENCES

1. Abdulah, R.; Miyazaki, K.; Nakazawa, M.; Koyama, H. Chemical

forms of selenium for cancer prevention. Journal of Trace Elements

in Medicine and Biology 2005, 19, 141–150.

2. Mukherjee, S.; Gangopadhyay, H.; Das, D.K. Broccoli: A unique

vegetable that protects mammalian hearts through the redox cycling

of the thioredoxin superfamily. Journal of Agriculture and Food

Chemistry 2008, 56, 609–617.

3. Moreno, D.A.; Carvajal, M.; Lopez-Berenguer, C.; Garcıa-Viguera, C.

Chemical and biological characterization of nutraceutical compounds

of broccoli. Journal of Pharmaceutical and Biomedical Analysis 2006,

41, 1508–1522.

4. Witte, K.K.; Clark, A.L.; Cleland, J.G. Chronic heart failure and

micronutrients. Journal of the American College of Cardiology 2001,

37, 1765–1774.

5. Finley, J.W.; Davis, C.; Feng, Y. Selenium from high selenium

broccoli protects rats from colon cancer. Journal of Nutrition 2000,

130, 2384–2389.

6. Sanjuan, N.; Bon, J.; Clemente, G.; Mulet, A. Changes in the quality

of dehydrated broccoli florets during storage. Journal of Food

Engineering 2003, 62, 15–21.

7. Gebczynski, P.; Lisiewska, Z. Comparison of the level of selected

antioxidative compounds in frozen broccoli produced using tra-

ditional and modified methods. Innovative Food Science and Emerging

Technologies 2006, 7, 239–245.

8. Mrkic, V.; Cocci, E.; Rosa, M.D.; Sacchetti, G. Effect of drying con-

ditions on bioactive compounds and antioxidant activity of broccoli

(Brassica oleracea L.). Journal of the Science of Food and Agriculture

2006, 86, 1559–1566.

9. Mahn, A.; Antoine, P.; Reyes, A. Optimization of drying kinetics and

quality parameters of broccoli florets. International Journal of Food

Engineering 2011, 7(2), Art. 14.

10. Icier, F.; Colak, N.; Erbay, Z.; Kuzgunkaya, E.H.; Hepbasli, A.

A comparative study on exergetic performance assessment for drying

of broccoli florets in three different drying systems. Drying Technology

2010, 28, 193–204.

11. Gawrzynski, Z.; Glaser, R.; Kudra, T. Drying of powdery materials

in a pulsed fluid bed dryer. Drying Technology 1999, 17(7&8),

1523–1532.

12. Kudra, T.; Mujumdar, A.S. Advanced Drying Technologies, 2nd Ed.;

CRC Press: Boca Raton, FL, 2009.

13. Reyes, A.; Moyano, P.; Paz, J. Drying of potato slices in a pulsed

fluidized bed. Drying Technology 2007, 25, 581–590.

14. Reyes, A.; Vega, R.; Garcıa, G. Drying sawdust in a pulsed fluidized

bed. Drying Technology 2008, 26, 476–486.

15. Sablani, S.; Rahman, S.; Al-Habsi, N. Moisture diffusivity in foods—

An overview. In Drying Technology in Agriculture and Food Sciences,

Mujumdar, A., Ed.; Science Publishers: Enfield, NH, 2000; 35–59.

16. Crank, J. The Mathematics of Diffusion; Oxford University Press:

London, 1975.

17. Alvarez, P.I.; Legues, P. A semi-theoretical model for the drying of

Thomson seedless grapes. Drying Technology 1986, 4, 1–17.

18. Togrul, H. Simple modeling of infrared drying of fresh apple slices.

Journal of Food Engineering 2005, 71, 311–323.

19. Association of Official Analytical Chemists. Official Methods of

Analysis of the AOAC; Association of Official Analytical Chemists:

Arlington, VA, 1990.

20. Vinson, J.A.; Su, W.; Zubik, L.; Bose, P. Phenol antioxidant quantity

and quality in foods: Fruits. Journal of Agricultural and Food Chemis-

try 2001, 49, 5315–5321.

21. Brand-Williams, W.; Cuvelier, E.; Berset, C.M. Use of free radical

method to evaluate antioxidant activity. Lebensmittel Wissenschaft

und Technologie 1995, 28, 25–30.

22. Faller, A.L.K.; Fialho, E. The antioxidant capacity and polyphenol

content of organic and conventional retail vegetables after domestic

cooking. Food Research International 2009, 42, 210–215.

23. Box, G.E.P.; Hunter, W.G.; Hunter, J.S. Statistics for Experimenters.

An Introduction to Design, Data Analysis, and Model Building;

Reverte: Barcelona, Spain, 1989 (in Spanish).

24. Raghvan, G.S.V.; Silveira, A.M. Shrinkage characteristics of straw-

berries osmotically dehydrated in combination with microwave

drying. Drying Technology 2001, 19(2), 405–414.

25. Suzuki, K.; Kubota, K.; Hasegawa, T.; Osaka, H. Shrinkage in

dehydration of root vegetables. Journal of Food Science 1976, 41,

1189–1194.

26. Mrkic, V.; Ukrainczyk, M.; Tripalo, B. Applicability of moisture

transfer Bi Di correlation for convective drying of broccoli. Journal

of Food Engineering 2007, 79(2), 640–646.

27. Tanongkankit, Y.; Chiewchan, N.; Devahastin, S. Effect of processing

on antioxidants and their activity in dietary fiber powder from

cabbage outer leaves. Drying Technology 2010, 28, 1063–1071.

28. Whanger, P.D. Selenocompounds in plants and animals and their

biological significance. Journal of the American College of Nutrition

2002, 21, 223.

1376 REYES ET AL.

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nloa

ded

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vers

ity o

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h] a

t 23:

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4 A

ugus

t 201

2