SB-MW Vacum dry

This article was downloaded by: [Central U Library of Bucharest]On: 06 February 2013, At: 10:38Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Drying Technology: An International JournalPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/ldrt20

Study of Drying Uniformity in Pulsed SpoutedMicrowaveVacuum Drying of Stem Lettuce Slices withRegard to Product QualityYuchuan Wang a , Min Zhang a , Arun S. Mujumdar b , Kebitsamang Joseph Mothibe a & S. M.Roknul Azam aa State Key Laboratory of Food Science and Technology, Jiangnan University, Wuxi, Jiangsu,Chinab Department of Mechanical Engineering, National University of Singapore, SingaporeVersion of record first published: 10 Jan 2013.

To cite this article: Yuchuan Wang , Min Zhang , Arun S. Mujumdar , Kebitsamang Joseph Mothibe & S. M. Roknul Azam (2013):Study of Drying Uniformity in Pulsed Spouted MicrowaveVacuum Drying of Stem Lettuce Slices with Regard to Product Quality,Drying Technology: An International Journal, 31:1, 91-101

To link to this article: http://dx.doi.org/10.1080/07373937.2012.721431

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form toanyone is expressly forbidden.

The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses shouldbe independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly inconnection with or arising out of the use of this material.

Study of Drying Uniformity in Pulsed SpoutedMicrowaveVacuum Drying of Stem Lettuce Slices withRegard to Product Quality

Yuchuan Wang,1 Min Zhang,1 Arun S. Mujumdar,2 Kebitsamang Joseph Mothibe,1

and S. M. Roknul Azam11State Key Laboratory of Food Science and Technology, Jiangnan University, Wuxi, Jiangsu, China2Department of Mechanical Engineering, National University of Singapore, Singapore

Drying uniformity, shrinkage, rehydration, and textural proper-ties were measured to evaluate the quality of pulsed spouted micro-wavevacuum-dried stem lettuce slices. Drying was carried out in a5-cm (od) vacuum drying chamber at 710 kPa and microwavepower level of 2.4 Wg1. Pulsed spouted microwavevacuum-driedproducts were found to be more uniform compared to those obtainedin a conventional rotating turntable microwavevacuum dryer. Thepulsed spouted mode also resulted in dried stem lettuce slices withlow discoloration and high rehydration capacity as well as highhardness after rehydration. The total drying time required forpulsed spouted bed microwavevacuum-dried products was approxi-mately 60min, reduced by 50% compared to conventional rotatingturntable microwavedried ones.

Keywords Drying uniformity; Pulsed spouted microwavevacuum dryer; Quality; Stem lettuce slices

INTRODUCTION

Stem lettuce is a highly valuable vegetable, especially inChina, and is noted for its edible stems. Large amounts ofhot airdried stem lettuce are produced each year and usedas a food ingredient in fast foods such as instant noodles. Poorquality and high energy consumption are serious concerns.Due to their heat sensitivity, vacuum drying methods providea good solution to obtain high-quality dried stem lettuce.

Conventional vacuum drying (VD) is a time-consumingand highly energy-intensive unit operation in postharvestand food preservation processes.[1,2] To reduce dryingtime and thus reduce net energy consumption, microwavedrying is highly recommended. Microwavevacuum dryingcombines the advantages of both vacuum drying andmicrowave drying because it can improve energy efciencyas well as product quality. The benets and drawbacks ofmicrowavevacuum drying in the food and pharmaceuticalindustries are well known. Though microwave drying can

offer unique advantages,[3,4]the inherent problem preventingits widespread use is the inherent nonuniform temperaturedistribution caused by uneven spatial distribution of theelectromagnetic eld inside the drying cavity. This resultsin hot and cold spots in the dried product. Moreover, it isdifcult to monitor or control the electromagnetic eld dis-tribution and its effect. The nonuniform temperature distri-bution can not only inuence the quality of the driedproduct but also poses the issue of food safety.[57]

The uniformity of microwavevacuum drying of pro-ducts is inuenced by many factors, such as vacuum cavityeffects, product attributes, and their spatial location in themicrowave cavity. In the last few decades, many researchershave studied the drying characteristics of food materials,both experimentally[811] and theoretically using analyticaland numerical methods.[1214] Moreover, many experimentalresearch studies have been conducted using a rotating turn-table microwave cavity in which product uniformity hasrarely been investigated using mechanical moving modestirrers.[15] Some investigators suggested that more uniformdrying could be achieved by using mechanical moving modestirrers or waveguide rotating joints or by simple agitation ofthe workload, as well as by using a cylindrical-shaped cavityand several adjustable magnetrons.[16,17] Although somesolutions have been provided, the results are conned tospecic conditions and cannot be generalized.[4,6]

The objectives of this study were to (a) improve micro-wavevacuum drying uniformity by introducing a pulse-pneumatic agitation in a laboratory system and (b) com-pare pulsed spouted microwavevacuum drying (PSMVD)with conventional rotating turntable microwavevacuumdrying (MVD) based on the quality and drying character-istics of stem lettuce slices.

MATERIALS AND METHODS

Material Preparation

Fresh stem lettuce (Lactucasativa L.) obtained from HaiTong Group farm in Ning Bo, China, were washed, peeled,

Correspondence: Min Zhang, School of Food Science andTechnology, Jiangnan University, 1800 Lihu Avenue, Wuxi214122, Jiangsu Province, China; E-mail: [email protected]

Drying Technology, 31: 91101, 2013

Copyright # 2013 Taylor & Francis Group, LLCISSN: 0737-3937 print=1532-2300 online

DOI: 10.1080/07373937.2012.721431

91

Dow

nloa

ded

by [C

entra

l U L

ibrary

of B

ucha

rest]

at 10

:38 06

Febr

uary

2013

cut into slices (12mm diameter and 5mm thickness; seeFig. 1a) with a cutting machine and then kept at 4C and95% relative humidity in a refrigerator. Two batches of pre-treated stem lettuce slices were used. The rst batch wasused to study the drying uniformity based on temperature,moisture content (MC), color difference, shrinkage, anddrying rate during MVD and PSMVD. The second batchwas used to study the effects of different drying methods(VD, MVD, and PSMVD) on the quality factors of driedstem lettuce slices, such as color, apparent density, rehydra-tion capacity (RC), and texture. Table 1 shows the experi-mental plan for pretreated samples used in this study. TheMC, color, and apparent density of the fresh stem lettuceslices were measured before drying tests.

Experimental Apparatus

A newly developed experimental apparatus, which hasbeen explained in the recent patent,[18] was used forPSMVD tests (Figs. 2 and 3).The system consisted of thefollowing six basic systems: (1) a cylindrical multimodemicrowave cavity (stainless steel, 40 cm od,200 cm high)with four microwave generators(at 2,450MHz) distributedsymmetrically along with the microwave cavity height; (2) acircular duct vacuum drying chamber (Teon, 5.0 cm od,0.5 cm wall thickness, 200 cm high); (3) a pulsed spoutedsystem equipped with a set of adjustable air ow and dis-tributive unit as well as a set of air handing units of1m3=min capacity (modelBST-1HTF,Shanghai BstairIndustrial Development Co., Ltd., Shanghai, China); (4)a heat supply system. Each magnetrons power outputcould be regulated between 0.1 and 1.0 kW by a GPA-1800W microwave power controller (Gospell ElectricTechnology Co., Ltd., Shenzhen, China); (5)a water loadsystem. The water load system was added to prevent themagnetron from overheating, using a cooling=heatingwater unit (model HAAKE DC10-K10, Thermo ElectronCorporation, Karlsruhe, Germany). The circulating watertemperature could be monitored and the power absorbedby the water load could be calculated accordingly; and (6)a vacuum system equipped with a cooler and a water-ringvacuum pump with a pumping rate of 1m3=min (modelLD-60A, Nantong Zenith Machinery Manufacturing Co.,Ltd., Nantong, China). The pressure inside the drying cham-ber could be regulated between 3.5 and 100 kPa. The vac-uum system was used for all drying methods (VD, MVD,and PSMVD). In order to prevent particulates from escap-ing from the top of drying chamber during spouting, a screenplate valve was xed on the top of the drying chamber.

A modied experimental apparatus, originally designedby Cui et al.,[19] was used for MVD tests (Fig. 4).The sys-tem consists of the following ve basic sections:(1) a squaremultimode microwave drying cavity (stainless steel,36 34 26 cm); (2) a glass rotating turntable (5 rpm,rotation speed) and a cylindrical plastic dish (25 cm od

FIG. 1. Pictures of stem lettuce slices dried using VD, MVD, and

PSMVD compared to pretreated stem lettuce slices: (a) pretreated sample;

(b) VD sample; (c) MVD sample; and (d) PSMVD sample (color gure

available online).

TABLE 1Experimental plan of the study

Pretreatedsample Applications Measurements

First batch Microwavevacuum drying (MVDand PSMVD)

Drying uniformity (temperature, moisture content, colordifference and shrinkage)

Drying characteristicsSecond batch Vacuum drying (VD, MVD and

PSMVD)Apparent densityRehydration capacityTextureMicrostructure

92 WANG ET AL.

Dow

nloa

ded

by [C

entra

l U L

ibrary

of B

ucha

rest]

at 10

:38 06

Febr

uary

2013

and 2 cm deep) containing the samples being dried. The bot-tom of the dish was made of 6-mm-diameter mesh to allowremoval of water from the bottom of the sample. The dishwas supported by a plastic tri-foot stand on top of a turn-table; (3) a microwave generator. The magnetrons (at2,450MHz) output power could be regulated between 0.1and 1.0 kW; (4) a ber-optic temperature on-line measure-ment system. The ber-optic temperature probe was xedon a glass bar to avoid movement; and (5) a vacuum system.

Experimental Procedure

For PSMVD, 200 0.5 g of pretreated samples (perbatch) was put into the drying chamber from the top whilethe bottom of the drying chamber was blocked with a sili-con rubber stopper together with a gas distributor andber-optic temperature probe (Fig. 3). For MVD, the sameamount of sample was spread on a cylindrical plastic dishand then placed on the turntable (Fig. 4). Microwave heat-ing was started when the pressure inside the dryingchamber=square microwave cavity reached 7 kPa absolutefor both PSMVD and MVD.

During PSMVD, the samples were spouted in the timeinterval by allowing air to ow periodically into the ductdrying chamber by use of electromagnetic valve, whichwas turned on for 2 s and off for 3 s. In the PSMVD pro-cess, air temperature, relative humidity, and velocity werepreset as 20 1C, 30 5%, and 3.5m=s, respectively.The air ow rate was regulated by a manual ux adjustingvalve (Fig. 3), ensuring that the pressure uctuated withinthe drying chamber from 7 to 10 kPa. During MVD, thedrying process was performed at 7 kPa. Both drying pro-cesses were conducted at a microwave power level of480W. An electric heating vacuum oven (modelDZF-6050,Shanghai Jing Hong Laboratory Equipment Co., Ltd.,Shanghai, China) was used for VD as a control. The samesample load (200 g) and absolute pressure (7 kPa) were usedat a heating temperature of 60C for 270min.

The average initial MC of the pretreated stem lettuceslices was 96.4 1.1% (wb). They were dried to a nalMC of 6.5%.During the drying process, samples were with-drawn at 15 and 30min time intervals in PSMVD andMVD, respectively, for determination of the weight loss,shrinkage, and color changes. After each batch of measure-ments, the samples were discarded. Another batch of pre-treated sample was dried until preset time intervals andmeasurements were performed. The same procedure wasdone until the desired MC was achieved. The dried sampleswere immediately heat sealed in polyethylene bags afterdrying for storage until further analyses. All of the dryingexperiments were performed in triplicate.

Analysis Methods

Drying Uniformity

Drying uniformity is dened as the relative standarddeviation (RSD; ratio of standard deviation to meanmeasurement value) of temperature, MC, color, andshrinkage. In this study, 12 stem lettuce slices were selectedper batch for measurements.

Temperature uniformity during microwavevacuumdrying was examined by measuring surface=central tem-perature of individual stem lettuce slices from differentlocations. A ber-optic thermometer (model MAD-A,Neoptix Inc., Quebec, Canada) was used for on-line tem-perature measurements of stem lettuce slices at the centraland edge locations of the air distributor during PSMVD(Fig. 3). The center temperature of stem lettuce slices abovethe central and edge locations of the rotating sample dishwas also measured on-line during MVD (Fig. 4).

In addition, an infrared thermal imaging camera (IRI4010 Multi-Purpose Imager, IRISYS, Northampton, UK)was used to determine the temperature distributions ofstem lettuce slices during MVD and PSMVD at differenttime intervals (20min for PSMVD and 30min for MVD).For PSMVD, the samples were taken out of the drying

FIG. 3. Schematic diagram of pulsed spouted microwavevacuum dry-

ing system (color gure available online).

FIG. 2. (A) Laboratory vacuum dehydration equipment and (B)

PSMVD system (color gure available online).

DRYING OF STEM LETTUCE SLICES 93

Dow

nloa

ded

by [C

entra

l U L

ibrary

of B

ucha

rest]

at 10

:38 06

Febr

uary

2013

chamber and immediately put into an insulated cylindricalplastic vessel (15 cm od, 10 cm depth) for determination ofthe temperature distributions. For MVD, the temperaturedistribution was measured directly on the sample trayimmediately after it was taken out of the drying chamber.After each batch of measurements, the samples were dis-carded. Another batch of samples was put into the dryerand thermal imaging of the samples was noted until thenext time point.

Randomly chosen stem lettuce slices (12 slices) of pre-treated samples from each batch were used to measure uni-formity (MC, color, and shrinkage). These samples wererst measured for weight, color, and volume individuallyand tied with threads of different colors. For PSMVDthe samples were placed in the drying chamber and forMVD they were placed on the rotating sample dish andarranged in a radial position. The weight of the sampleswas measured using a PL203 analytical balance with pre-cision of 0.0001 g (Mettler-Toledo Instrument Co. Ltd.,Shanghai, China). All measurements were performed intriplicate and the average weight loss values were recorded.

Color Measurement

Sample surface color measurements were conductedusing a CR-400 Chroma Meter (Konica Minolta SensingInc., Tokyo, Japan). The color difference (DE) was used todescribe the color change during drying and was calculatedusing the color values of fresh stem lettuce as a standard.

DE L0 L2 a0 a2 b0 b2

q1

where L0, a0, and b0 are the color readings of fresh stem

lettuce slices.

Apparent Density

The shrinkage ratio (SR) was measured in order to esti-mate volume changes in the samples being dried. The SR of

the dried sample was expressed as:

SR VdV0

2

where V0 and Vd are the volume of the sample (cm3) before

and after drying, respectively.The method used to determine the apparent relative

density (ARD) of coal[20] was applied to measure theARD of stem lettuce slices in this study. The sample wasrst weighed and then dipped in melted wax (at 6070C)for 2min. Before measuring the weight, the wax-coatedsample was cooled to room temperature. Subsequently,the sample was put in a 60-mL density bottle with a neckdiameter of 1.6 cm and then lled with 1gL1 sodiumdodecyl sulfate solution. Before the third reading wastaken, the relative density measuring bottle with capillarystopper was held at 20 0.5C for 1 h. Finally, a fourthreading was recorded by weighing the density bottle lledwith distilled water. The ARD of the fresh and dried sam-ples at 20Cwasthen calculated as:

ARD m1m2m4m3

ds

h i m2m1dwaxh i

d20w3

where m1 and m2 are the mass (g) of the sample before andafter coating wax, respectively; m3 refers to the density bot-tle plus 1 gL1 sodium dodecyl sulfatedistilled water sol-ution and the wax-coated sample; m4 refers to the densitybottle plus distilled water only; and ds, dwax, and dw20 arethe density of the 1 gL1 sodium dodecyl sulfatedistilledwater solution at 20C (0.99847 kgdm3), wax, and dis-tilled water (about 1.00000 kgdm3), respectively.

The dwax was measured using a buoyant forcemethod.[21] Before weighing, the wax was melted and madeinto a 15 g wax cake by a standard module, and then addedto a 1 gL1 sodium dodecyl sulfate solution attached to a15 g brass rod using a thin string. Second readings werethen taken with the sample and brass rod submerged.Finally, third readings were taken when only brass rodwas submerged into the solution. The wax density was thencalculated as:

dwax m1m1 m2 m3=ds 4

where m1 is the mass (g) of the wax, and m2 and m3 are thewax plus brass rod and the brass rod only in the 1 gL1

sodium dodecyl sulfate solution, respectively. The value forthe density of wax used in this study was 0.9126kg dm3.

The apparent volume of the fresh and dried stem lettuceslices was calculated from the measured of density andmass values. The weight of the samples was measured withan analytical balance (0.0001 g). All measurements wereperformed three times and the average values were used.

FIG. 4. Schematic diagram of rotating turntable microwavevacuum

drying system.

94 WANG ET AL.

Dow

nloa

ded

by [C

entra

l U L

ibrary

of B

ucha

rest]

at 10

:38 06

Febr

uary

2013

Moisture Content

The initial and nal MCs were measured using the vac-uum oven method at 60 5C and 4053 kPa.[22] The MCof the samples taken from the drying chamber at presettime intervals top lot the drying curve was calculated bytheir weight loss as well as the MC of the samples beforeand after drying. The MC of individual slices from 12 stemlettuce slices was also calculated using this method. Theaverage values of three measurements were recorded.

Rehydration

The rehydration ratio of dried stem lettuce slices wasdetermined by immersing about 2 g of dried samples in200mL of water at 95C for 5min (according to instruc-tions for cooking instant noodles). Before weighing, thesample was immediately drained and cooled for 5min byblowing with a fan until there was no visible water onthe surface of the samples. The following formuladescribed by Lewicki[23]was used to calculate the RC:

RC mass of water absorbed during rehydrationmass of water removed during drying

1005

The RC measurements were conducted in triplicate forall tests and average values were reported.

Texture

The textural properties of the dried samples after rehy-dration were measured using a texture analyzer (modelTA-XT2, Stable Micro System Ltd., Leicestershire, UK).A forcetime curve was developed and analyzed using thesoftware Texture Exponent 32 (Stable Micro SystemLtd., Surrey, UK). The instrument was calibrated with a1-kg load cell and tted with a at-ended aluminum probeof diameter 36mm (Code P=36R, Stable Micro SystemLtd., Surrey, UK). The parameters were preset as follow:pretest speed 1mm=s, test speed 0.5mm=s, posttestspeed 10mm=s, strain 60%, and trigger force 5g,respectively. The average values of rmness from ve mea-surements were calculated for each experimental condition.

Scanning Electron Microscopy

Microstructures of stem lettuce slices dried with VD,MVD, and PSMVD were measured using a scanningelectron microscope (S-4800, Hitachi, Tokyo, Japan) atan accelerating voltage of 1.0 kV. Dried samples werecoated using a goldpalladium alloy coater (Baltec Co.,Manchester, NH), and the samples were observed at400magnication. Stem lettuce slices dried by freezedrying (FD) were used as standard reference samples toobserve the microstructure of fresh stem lettuce slices.

Data Analysis

MATLAB (ver.7.01) was used to analyze the statisticalsignicance of the collected data and analyses of variancewere performed. Mean values were considered signicantlydifferent at p 0.05.

RESULTS AND DISCUSSION

Drying Curve

Drying curves of the microwavevacuum-dried stem let-tuce slices with pulsed spouted and rotating turntable modeswere compared (Fig. 5). The total drying time required forstem lettuce slices dried with PSMVD was 60min and forMVD it was less than 120min. The two drying curvesshown in Fig. 5 can roughly be divided into two dryingstages. In the rst drying stage including a short initial dry-ing stage, the MC decreased from 26.93 to lower than 2 g=g(db). In the second drying stage, the MC decreased fromabout 2 to a nal MC of 0.07 g=g (db). In the rst dryingstage there was a sharp decrease in both MC and dryingrate, as well as shorter drying time (30min) for PSMVDand a slow decrease in MC and drying rate and long dryingtime (75min) for MVD. In the second drying stage, thedrying rate and MC for PSMVD and MVD exhibited aslow decrease. The time required to dry stem lettuces slicesusing the turntable method in this study was longer thanthat of previous studies reported by Cui et al.,[19] whoshowed that the total microwave drying time requiredwas 35min for 120 g of fresh garlic slices dried to 10% ofmoisture (wb) using a magnetron (750W) working in pulsemode. The reason for this difference might be due to thefact that improved experimental apparat uses are able toregulate the microwave power according to a preset pro-duct temperature program, avoiding excessive microwavepower supply to the dried products, resulting in greateruniformity of the dried products. Another reason for thisphenomenon may be that throughout the drying process

FIG. 5. Drying curves of stem lettuce slices dried using MVD and

PSMVD.


Dow

nloa

ded

by [C

entra

l U L

ibrary

of B

ucha

rest]

at 10

:38 06

Febr

uary

2013

no vapor condensation occurred on the chamber wall andsample dish surface. Although condensed water on theglass turntable surface increases turntable temperatureand accelerates drying process, it may cause more nonuni-form temperature distribution and consume more micro-wave energy.

Drying Uniformity

Temperature

Temperature curves of the typical rotating turntable andpulsed spouted microwavevacuum drying tests are shownin Figs. 6 and 7. These temperature curves were partitionedroughly into three stages. In the rst stage, microwaveheating was relatively intense due to the high MC of thesample. As a result, the sample temperature increased rap-idly and surpassed the saturated vapor temperature (39C)corresponding to the vacuum pressure (7 kPa) in 5min. Thesample temperature was actually lower than the saturatedvapor temperature because of heat loss due to moistureevaporation, especially in the pulsed spouted mode. In thesecond stage, the sample central=surface temperature con-tinued to increase due to intense microwave heating andthen reached a plateau. In the nal stage, the sample tem-perature remained stable because the energy due to micro-wave heating was balanced by cooling caused byevaporation. A slight temperature reduction occurredtoward the end of drying. It is likely that the sample lossfactor was sharply reduced as a result of low MC left inthe dried samples. A similar temperature prole wasreported by Feng and Tang[16] in apple slices microwavedrying tests in a spouted bed and by Mousa and Farid[8]

for microwavevacuum drying of banana slices at 30 kPausing a pulsed microwave power. However, they observedthat the temperature increased again in the nal stagedue to the sensible heating of the dried banana. However,

a signicant difference in the central temperature of thesamples occurred at three different locations during rotat-ing turntable drying. The center temperature of the samplesat the turntable central location was lower in the rst andthe second stages, and the time to reach a plateau wasless for about 30min compared to those located at thetwo edges. The maximum sample temperature differencebetween the central and edge positions of the rotating sam-ple dish was 41C in the second stage and 5C in the nalstage, which corresponds to the maximum standard devia-tions of sample temperature distribution of 20.8C and2.6C for the average temperature at the central and edgepositions of the rotating sample dish, respectively. In con-trast, there was no signicant difference in the surface tem-peratures of the sample in the pulsed spouted mode at thecentral and edge locations throughout drying process. Themaximum standard deviations of surface temperature dis-tribution at the central and edge positions of the dryingduct were less than 2.1C in three drying stages. Similarresults were reported by Feng and Tang,[16] who found thatafter 2.5min of drying, the maximum center temperaturedifference values were 127.5C in apple pieces dried in astationary bed, whereas the maximum temperature vari-ation was 1.4C in apple pieces dried using microwavespouted bed drying and reached 4C at the nal dryingstage. Geedipalli et al.[24]observed that after 35 s of micro-wave heating, the temperature difference and standarddeviation values were 37.7 and 26.9C in potato heatedby stationary mode and 25.5 and 9.31C in potato heatedby rotating turntable mode, respectively. In addition, itwas found that the surface temperature of the pulsedspouted dried sample was far less than that for rotatingturntable drying. Results from thermal image changes inthe temperature distributions of the samples duringPSMVD and MVD (Fig. 8) indicated the same results.

FIG. 6. Temperature curves of stem lettuce slices during MVD at the

central and edge positions of the rotating turntable at 2,450MHz (color

gure available online).

FIG. 7. Temperature curves of stem lettuce slices at the central and edge

positions of the duct drying chamber during PSMVD at 2,450MHz (color

gure available online).

96 WANG ET AL.

Dow

nloa

ded

by [C

entra

l U L

ibrary

of B

ucha

rest]

at 10

:38 06

Febr

uary

2013

However, the temperature distribution was found to be sig-nicantly altered with the height of the sample dish duringMVD. The best sample temperature uniformity obtained inthe pulsed spouted drying model can be explained by theimproved cavity effects (cylindrical microwave cavity, cir-cular tube drying chamber, and microwave source sym-metrical distribution) and changing spatial positions(three dimensions) of the sample in the drying chamberto ensure that all sample slices receive equal electromag-netic eld intensity over a period of time.[4,16] Moreover,the lower sample surface temperature during PSMVDmight be caused by low inlet air temperature (1921C);water load self-regulating microwave eld intensity,especially in the nal drying stage; the use of a cooler(10C); and the high evacuation rate of the water-ringpump, which resulted in rapid removal of vapor from thedrying samples and avoided condensation of vapor intowater on the chamber wall and sample dish surface.

Moisture

Tables 2 and 3 show the average MC and RSDs of 12chosen stem lettuce slices dried by pulsed spouted andthe rotating turntable modes. Moisture content standard

deviations or RSDs of the 12 stem lettuce slices duringMVD and PSMVD were found to have the same trend,in which they increased up to a certain point then decreasedwith decreasing MC. The maximum RSD values (5.06% forPSMVD and 24.25% for MVD) occurred at an MC ofabout 30% where the third drying stage began. Thisphenomenon can be explained by the fact that free watercontent was almost completely removed as the drying pro-cess approached the last stage of the falling rate period.Some stem lettuce slices temperatures were close to thehighest temperature (60C) producing few changes in theirmicrostructures, which affected the release of water fromthe matrix to the exterior due to the decrease in porosityor intracellular spaces.[16] As expected, with the pulsedspouted drying method the measured RSD value of theMC of the nal dried sample slices was 1.5% less than thatof the rotating turntable drying mode. In addition, it wasobserved that the measured MC values of individual stemlettuce slice during MVD were clearly related to its position(Fig. 9). The sample slices at the central location of therotating sample dish had higher MCs compared to thoseat the edges. For example, the difference between theMCs of sample slices at the center and at the edge was9.1% after 60min of drying and 21.9% after 90min of dry-ing (Table 3). This further conrmed that although theturntable moves constantly within the square microwavecavity in MVD, sample slices cannot achieve an averageelectromagnetic eld intensity over a period of time. In

TABLE 2Changes in mean MC, SD and RSD of 12 stem lettuce

slices taken from the pulsed spouteddrying chamber during drying

Drying time (min) MC (%) SD (%) RSD (%)

0 96.42 0.82 0.24 0.01 0.25 0.0115 91.96 1.27 0.60 0.03 0.65 0.0330 69.68 1.89 0.69 0.03 0.98 0.0445 35.56 1.06 1.80 0.09 5.06 0.2160 6.5 6 0.31 0.14 0.01 2.15 0.10

TABLE 3Changes in mean MC, SD and RSD of 12 stem lettuceslices taken from the rotating turntable during drying

Drying time (min) MC (%) SD (%) RSD (%)

0 96.42 0.87 0.24 0.01 0.25 0.0130 90.96 2.34 0.87 0.04 0.95 0.0460 81.31 2.12 3.18 0.13 3.91 0.1390 29.99 1.13 7.27 0.32 24.25 1.21120 6.51 0.32 0.53 0.02 8.14 0.35

FIG. 8. Thermal imagery of stem lettuce slices dried using MVD for

(a) 30min; (b) 60min; and (c) 90min and PSMVD for (d) 30min; (e)

60min; and (f) 90min (color gure available online).


Dow

nloa

ded

by [C

entra

l U L

ibrary

of B

ucha

rest]

at 10

:38 06

Febr

uary

2013

contrast, during PSMVD the difference between stem let-tuce slice MCs was far lower compared to that for MVD.A similar idea for improving uniformity during micro-wavevacuum drying was also performed by Venner-strum,[15] who designed a device with mechanical movingstirrers to splatter product during microwavevacuum dry-ing. Huet al.[9] also used a microwavevacuum drier withsix rotating plates to move edamame during drying. How-ever, these experimental equipment still resulted in productnonuniformity due to a simple product plane movementrather than spatial motion.

Color

From Tables 4 and 5, it can be seen that the changevalues of the mean color and DE of the samples dried usingPSMVD and MVD rst reached higher absolute values inthe initial stage of drying and then mostly maintained con-stant values until the nal stage for PSMVD; conversely, arapid decrease occurred in the MVD samples in the nalstage. This may be explained by the fact that the gas exist-ing in the intercellular space of the stem lettuce wasexpelled, as well as the changes in the distribution ofdifferent components of chloroplast in the stem lettuce,caused by microwave heating.[25]

From Tables 4 and 5, it can be observed that the RSDvalues of the samples dried using both drying methodsshowed the same trend; these values increased in the initialstage of drying, subsequently decreased with drying time,and then increased in the nal stage of drying. This maybe related to chlorophyll species. Because chlorophyll-acontains less positive charges in the porphyrin rings com-pared tochlorophyll-b, under microwave heating,chlorophyll-a decomposed rst, followed bychlorophyll-b.[26] As expected, the RSDs of the samplesdried by the rotating turntable method were signicantlyhigher than those for pulsed spouted drying.

Table 6 shows mean color values of fresh and rehy-drated stem lettuce slices dried using various vacuum dry-ing methods. As expected, products dried using PSMVDexhibited the lowest lightness, L; redness, a; yellowness,b; and color difference, DE, values, follow by MVD, andVD had the highest. This might be due to the fact thatthe samples dried by the pulsed spouted method retainedmore chlorophyll due to the shorter drying time and lowerdrying temperature. Hence, after rehydration, pulsedspouted mode products exhibited lower color variationcompared to those of the rotating turntable mode.

Shrinkage

From Tables 7 and 8, it can be observed that the sam-ples dried by the two drying modes had the same shrinkagetrend, in which there was a rapid decrease in the initial dry-ing stage and slow decrease with drying time. This might bedue to the fact that removal of large amounts of water fromthe tissues in the microwave-dried sample during the initialdrying period. In addition, there was no signicant differ-ence in the SR for samples dried by these two drying meth-ods. The measured SR value in the samples dried usingPSMVD was slightly lower than that of the samples driedusing MVD. This result can be seen in Figs. 1b and 1c. Thisphenomenon for no signicant difference in the SR of sam-ples dried by PSMVD and MVD can be explained by thefact that no signicant pufng effects occurred in the twodrying processes due to optimum distribution of the micro-wave power. The size of the dried samples shrank to almostsame size at the end of drying close to that of the samples

FIG. 9. Moisture content variations of 12 stem lettuce slices taken from

the pulsed spouted drying chamber and the rotating turntable after drying

for 30 and 60min, respectively.

TABLE 4Changes in mean color (L,a, b) and color difference (DE), (SD), and (RSD) of 12 stem lettuce slices taken from the

pulsed spouted drying chamber during drying compared fresh stem lettuce slices

Drying time (min) DL Da Db DE SD RSD (%)

15 9.25 0.45 6.75 0.32 11.79 0.56 16.43 0.81 2.05 0.09 12.47 0.5730 4.11 0.19 6.10 0.28 9.97 0.48 12.39 0.59 1.28 0.05 10.36 0.4945 5.01 0.23 6.50 0.31 10.45 0.51 13.29 0.64 0.77 0.03 5.76 0.2860 6.34 0.31 6.34 0.31 13.35 0.64 16.08 0.76 1.38 0.06 8.59 0.41

98 WANG ET AL.

Dow

nloa

ded

by [C

entra

l U L

ibrary

of B

ucha

rest]

at 10

:38 06

Febr

uary

2013

dried by VD method (Figs. 1b1d). However, the collisionbetween the sample slices and the drying chamber wall dur-ing PSMVD might have resulted in a decreased shrinkagevolume compared to MVD.

Results from changes in the RSD values of the sampleSR during PSMVD and MVD (Tables 7 and 8) indicatedthat the RSD values of PSMVD samples were always lowerthan those of MVD samples in three drying stages. Inaddition, the shape of dried stem lettuce slices dried using

PSMVD was nearly circular (Fig. 1d). It was also con-rmed that the uniformity in SR of the samples dried usingthe pulsed spouted method was far higher than that of sam-ples dried using the rotating turntable due to sample spout-ing in spatial eld rather than constant movement in therotating turntable, as expected.

Microstructure and Apparent Density

Structural properties are important for the characteriza-tion of the quality of a dehydrated product. The dryingmethod signicantly affects the microstructure and appar-ent density of dried products.[27] The microstructure of thecross section of the dried stem lettuce slices was examinedto explore the effect of different drying dried methods onthe samples (Fig. 10). From the scanning electron micro-graphs shown in Fig. 10, it can be observed that the cellsin three vacuum-dried stem lettuce slices were tightly linkedand the cell boundary disappeared compared to those driedby FD, which had a clear edge and smooth surface, indicat-ing that greater shrinkage occurred during vacuum drying.Moreover, wrinkles were found on the surface of the sam-ples dried using VD and MVD methods, whereas the cellsof the samples dried using PSMVD were not only tightlylinked but the cell surface was more compact with no obvi-ous wrinkles. This might be because the pulsed inlet air cancause the samples spatial movement in the drying chamber,which resulted in collision between the sample slices andthe drying chamber wall during PSMVD. In addition, anintermittent pressure change (between 7 and 10 kPa) in thedrying chamber may form an alternative pressure differenceinside and outside the drying sample and lead to extrusionbetween cells, resulting in a compact microstructure.

TABLE 5Changes in mean color (L, a, b) and color difference (DE), SD and RSD of 12 stem lettuce slices taken from the

rotating turntable during drying compared fresh stem lettuce slices

Drying time (min) DL Da Db DE SD RSD (%)

30 13.76 0.66 7.21 0.35 9.20 0.43 18.06 0.81 2.78 0.11 15.37 0.6860 13.12 0.63 7.24 0.31 9.42 0.44 17.70 0.85 1.5 8 0.06 8.91 0.0490 16.26 0.78 8.23 0.34 9.74 0.43 20.66 0.95 2.33 0.09 11.30 0.49120 9.21 0.43 2.89 0.14 2.93 0.13 10.08 0.45 1.55 0.07 15.34 0.67

TABLE 6Color (L, a, b) and color difference (DE) for rehydrated stem lettuce slices dried using different methods compared to

fresh stem lettuce slices

Drying method L a b DE

Fresh 40.55 2.02 10.21 0.49 15.68 0.77VD 53.61 2.26 12.39 0.63 23.77 1.13 15.52 0.78MVD 48.21 2.36 12.80 0.64 23.40 1.16 11.17 0.55PSMVD 47.29 2.13 11.50 0.56 22.61 1.06 9.75 0.49

TABLE 7Changes in mean SR, SD, and RSD of 12 stem lettuceslices taken from the pulsed spouted drying chamber

during drying

Drying time (min) SR (%) SD (%) RSD (%)

15 60.11 2.96 1.68 0.07 2.79 0.1230 20.38 0.95 1.99 0.09 9.77 0.4445 11.25 0.51 1.22 0.05 10.89 0.5160 8.46 0.39 0.81 0.03 9.57 0.41

TABLE 8Changes in mean SR, SD, and RSD of 12 stem lettuceslices taken from the rotating turntable during drying

Drying time (min) SR (%) SD (%) RSD (%)

30 59.85 2.89 3.44 0.14 5.74 0.2460 31.44 1.47 5.12 0.21 16.30 0.7990 9.14 0.44 1.73 0.08 18.93 0.91120 8.77 0.39 1.39 0.06 15.83 0.71


Dow

nloa

ded

by [C

entra

l U L

ibrary

of B

ucha

rest]

at 10

:38 06

Febr

uary

2013

The ARD measurements of the samples dried usingthree different vacuum drying methods are shown inTable 9. The ARD values of stem lettuce slices dried withVD, MVD, and PSMVD were 1.387, 1.296, and1.417 kgdm3, respectively, far higher than that of the freshstem lettuce slices (1.027 kgdm3). As expected, the ARDvalue of the sample dried by rotating turntable mode wasthe lowest compared to the other two drying methods. Thismight be explained by the fact that an outward ux of rap-idly escaping vapor caused by microwave energy can helpto prevent the collapse (shrinkage) of tissue structure. Simi-lar results were reported by Huang et al.,[10] who also foundthat the bulk density of mixed chips (apple potato) driedusing MVD was higher than that of mixed chips driedusing VD. However, the samples dried by pulsed spoutedmode had the highest ARD value. This might be due tothe formation of a compact microstructure during PSMVD(Fig. 10d). MVD has been reported to provide higherporosity products[13,28] due to the pufng effect caused bythe development of greater internal stresses as well as

physical damage such as scorching due to excessive heatingcaused by higher microwave power and uneven drying. Inthis study, no prominent pufng was visually observed.This may be due to the optimal microwave power level sup-ply and higher pumping vapor rate as well as a the rationaldesign of the microwavevacuum drying system.

Rehydration and Texture

The rehydration characteristics of dried products arewidely used as a quality index and can indicate the physicaland chemical changes in the structure and composition ofplant tissue caused by drying and treatments.[16,23] As seenin Table 9, the rehydration capacities for stem lettuce slicesdried with VD, MVD, and PSMVD were 24.365, 15.449,and 17.510%, respectively. This indicates that for all threemethods the dehydrated products did not recover theirstructural properties after rehydration. This may be dueto the fact that irreversible physicochemical changesoccurred during drying and the solutes leaking fromdamaged cells migrated to the surface to form a crustand resulted in a relatively closed surface structure.[16,21]

The RC of the samples dried by VD was higher than thatof microwave-dried samples. These results showed thatmicrowave energy might result in greater changes in thestructure and composition of stem lettuce tissue duringdrying compared to conventional vacuum drying. Duringmicrowave drying, especially in the initial drying stage,apart from the heat required for drying, a signicantincrease in the electrical conductivity of stem lettuce sliceswas observed,[29] indicating that there were more mobileions formed in the samples being dried due to interactionsof the ions with the electromagnetic eld. It was suggestedthat the larger interactions of ions with polymers duringMVD and PSMVD processes stiffened the structure andrestricted polymer hydration and swelling during rehydra-tion compared to VD processes.[23] As expected, the RCof the samples dried using PSMVD was higher that ofthe samples dried using MVD. It is inferred that smallinjuries occurred in the structure and composition of stemlettuce tissue due to lower sample temperature and betterdrying uniformity during PSMVD compared to MVD.

After rehydration, the surface hardness of stem lettuceslices dried using VD, MVD, and PSMVD was 0.848,0.657, and 2.166 kg (Table 9), respectively. The hardnessforce (HF) value of the rehydrated stem lettuce slices driedwith pulsed spouted mode was higher than that of the sam-ple dried by VD and almost four times higher than that ofthe sample dried by MVD. This indicates that the stem let-tuce slices dried by PSMVD had the highest elastic beha-vior after rehydration, and the sample dried by MVDwas the softest. Similar results were reported by otherresearchers. Cui et al.[19] observed that microwavevacuum-dried garlic slices were softer than air-dried garlicslices. Higher RC and rmness of the rehydrated samples

TABLE 9ARD, RC, and HF of stem lettuce slices dried using

different drying methods

Dryingmethod

ARD(kg dm3) RC (%) HF (kg)

VD 1.387 0.051 24.365 1.12B 0.848 0.038MVD 1.296 0.043 15.449 0.762 0.657 0.029PSMVD 1.417 0.052 17.510 0.835 2.166 0.081

FIG. 10. Scanning electronmicrographs of stem lettuce slices dried using dif-

ferent methods: (a) FD; (b) VD; (c) MVD; (d) PSMVD. Magnication 400.

100 WANG ET AL.

Dow

nloa

ded

by [C

entra

l U L

ibrary

of B

ucha

rest]

at 10

:38 06

Febr

uary

2013

dried by PSMVD might be due to the high apparentrelative density created by the interaction of stem lettuceslices and low sample temperature during drying, aswell as shorter drying time, resulting in less structuraland functional changes compared to the turntable dryingmode.

CONCLUSIONS

1. The quality of dried stem lettuce slices mainly dependson the drying uniformity caused by the style of themovement of the samples in the microwave cavity.The pulsed spouted mode within a circular tubevacuum drying chamber resulted in much more uniformdrying within the microwave cavity, as indicated bymore uniform temperature, moisture, color, and shrink-age distribution among sample particles duringPSMVD.

2. PSMVD greatly reduced the drying time by more than50% compared to conventional MVD.

3. PSMVD resulted in high-quality dried stem lettucecompared to MVD; after rehydration, products driedusing pulsed spouted mode were of better quality; thatis, they showed the least discoloration, highest elasticbehavior, and higher RC compared to products driedusing the rotating turntable or conventional vacuumdrying.

4. These results show that pulsed spouted mode in a tubedrying chamber substantively improved the micro-wavevacuum-dried product quality, drying uniformity,and drying characteristics.

ACKNOWLEDGMENTS

The authors are grateful to the 863-HI-TECH Researchand Development Program of China for supporting thisresearch under contract No. 2011AA100802.

REFERENCES

1. Mujumdar, A.S. Handbook of Industrial Drying, 3rd Ed.; CRC Press:

Boca Raton, FL, 2007.

2. Mujumdar, A.S.; Law, C.L. Drying technology: Trends and applica-

tions in postharvest processing. Food and Bioprocess Technology

2010, 3, 843852.

3. Zhang, M.; Tang, J.; Mujumdar, A.S.; Wang, S. Trends in

microwave-related drying of fruits and vegetables. Trends in Food

Science & Technology 2006, 17(10), 524534.

4. Li, Z.Y.; Wang, R.F.; Kudra, T. Uniformity issue in microwave

drying. Drying Technology 2011, 29(6), 652660.

5. Zhang, M.; Jiang, H.; Lim, R.-X. Recent developments in

microwave-assisted drying of vegetables, fruits, and aquatic pro-

ductsDrying kinetics and quality considerations. Drying Technology

2010, 28(11), 13071316.

6. Vadivambal, R.; Jayas, D.S. Non-uniform temperature distribution

during microwave heating of food materialsA review. Food and Bio-

process Technology 2010, 3, 161171.

7. Jangam, S.V. An overview of recent developments and some R&D

challenges related to drying of foods. Drying Technology 2011,

29(12), 13431357.

8. Mousa, N.; Farid, M. Microwave vacuum drying of banana slices.

Drying Technology 2002, 20, 20552066.

9. Hu, Q.; Zhang, M.; Mujumdar, A.S.; Xiao, G.; Sun, J. Drying of eda-

mames by hot air and vacuum microwave combination. Journal of

Food Engineering 2006, 77(4), 977982.

10. Huang, L.L.; Zhang, M.; Mujumdar, A.S.; Sun, X.L. Comparison of

four drying methods for re-structured mixed potato with apple chips.

Journal of Food Engineering 2011, 103, 279284.

11. Nahimana, H.; Zhang, M. Shrinkage and color change during

microwave vacuum drying of carrot. Drying Technology 2011, 29(7),

836847.

12. Giri, S.K.; Prasad, S. Drying kinetics and rehydration characteristics

of microwave vacuum and convective hot-air dried mushrooms. Jour-

nal of Food Engineering 2007, 78, 512521.

13. Han, Q.-H.; Yin, L.-J.; Li, S.-J.; Yang, B.-N.; Ma, J.-W. Optimization

of process parameters for microwave vacuum drying of apple slices

using response surface method. Drying Technology 2010, 28(4),

523532.

14. de Jesus, S.S.; Filho, R.M. Optimizing drying conditions for the

microwave vacuum drying of enzymes. Drying Technology 2011,

29(15), 18281835.

15. Vennerstrum, S. Microwave Vacuum Dryer. U.S. Patent No.

4,856,203, 1989.

16. Feng, H.; Tang, J. Microwave nish drying of diced apples in a

spouted bed. Journal of Food Science 1998, 63(4), 679683.

17. Balakrishnan,M.; Raghavan, G.S.V.; Sreenarayanan, V.V.; Viswanathan,

R. Batch drying kinetics of cardamom in a two-dimensional spouted bed.

Drying Technology 2011, 29(11), 12831290.

18. Zhang, M.; Wang, Y. A Device and Its Application for Uniform

Microwave Vacuum Drying in a Spouted Mode. China Patent No.

ZL 2010 1 0572843.0, April 18, 2012.

19. Cui, Z.; Xu, S.; Sun, D. Dehydration of garlic slices by combined

microwavevacuum and air drying. Drying Technology 2003, 21(7),

11731184.

20. Determination of apparent relative density of coal. National Standard

of China. GB=T 69491998; Standardization Administration of the

P.R. China: Peking, 1999.

21. Mohsenin, N.N. Physical Properties of Plant and Animal Materials;

Gordon and Breach Science Publishers: New York, 1986.

22. National Standard of China. Determination of moisture in foods.

GB=5009.32010; Standardization Administration of the P.R. China:

Peking, 2010.

23. Lewicki, P.P. Some remarks on rehydration of dried foods. Journal of

Food Engineering 1998, 36, 8187.

24. Geedipalli, S.S.R.; Rakesh, V.; Datta, A.K. Modeling the heating uni-

formity contributed by a rotating turntable in microwave ovens. Jour-

nal of Food Engineering 2007, 82(3), 359368.

25. Hutchings, J.B. Food Color and Appearance; Blackie: London, 1994.

26. Hendry, G.A.F.; Haughton, J.D. Natural Food Colorants; Van Nos-

trand Reinhold: New York, 1992.

27. Krokida, M.K.; Maroulis, Z.B. Structural properties of dehydrated

products during rehydration. International Journal of Food Science

and Technology 2001, 36(5), 529538.

28. Mounir, S.; Besombes, C.; Al-Bitar, N.; Allaf, K. Study of instant

controlled pressure drop DIC treatment in manufacturing snack and

expanded granule powder of apple and onion. Drying Technology

2011, 29(3), 331341.

29. Wang, Y.; Zhang, M.; Mujumdar, A.S.; Mothibe, K.J.; Roknul

Azam, S.M. Effect of blanching on microwave freeze drying of stem

lettuce cubes in a circular conduit drying chamber. Journal of Food

Engineering 2012, 113, 177185.


Dow

nloa

ded

by [C

entra

l U L

ibrary

of B

ucha

rest]

at 10

:38 06

Febr

uary

2013

Documents

SB-MW Vacum dry