Journal of Food Engineering 101 (2010) 131–139

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Journal of Food Engineering

journal homepage: www.elsevier .com/ locate / j foodeng

Effects of vacuum and microwave freeze drying on microstructure and qualityof potato slices

Rui Wang a, Min Zhang a,*, Arun S. Mujumdar b

a State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, Chinab Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore

a r t i c l e i n f o

Article history:Received 16 April 2010Received in revised form 27 May 2010Accepted 28 May 2010Available online 2 June 2010

Keywords:QualityFreezingSublimation dryingDesorption dryingPotato

0260-8774/$ - see front matter � 2010 Published bydoi:10.1016/j.jfoodeng.2010.05.021

* Corresponding author. Address: School of Food SKey Laboratory of Food Science and Technology, JiangnJiangsu Province, China. Fax: +86 510 580 7976.

E-mail address: min@jiangnan.edu.cn (M. Zhang).

a b s t r a c t

Potato slices immersed in 0.5% CaCl2 solution for 10 min were examined using light microscopy for theirmicrostructure in frozen state before drying, during the sublimation drying stage, in the desorption dry-ing stage and in the final dried form. Additionally, the final dried product was also tested for vitamin Ccontent, color, starch content, texture and sugar content. Experiments were carried out using conven-tional vacuum freeze dryer as well as a microwave freeze dryer. Results for both unblanched andblanched potato tissues, crystal growth during sublimation drying stage was observed to cause structuraldamage to the cell walls. Blanched tissue suffered more damage during the freezing process. Interest-ingly, microwave freeze drying yielded product similar in quality to that obtained in vacuum freeze dry-ing with conductive heating.

� 2010 Published by Elsevier Ltd.

1. Introduction

Vacuum freeze drying (FD) is a method of dehydration of frozenmaterials by sublimation under vacuum. It is well known that FDproduces the highest-quality dried foods. However, a major prob-lem with conventional FD is the long drying time needed, whichin turn leads to high energy consumption and high capital costs.This was partly due to the poor heat transfer rate associated withthe conventional electric heating method which transfers heatfor drying by conduction. Application of a volumetric heat sourcesuch as microwave field is a viable option that has attracted muchattention over the last decade (Zhang et al., 2006). Most of the pub-lished work deals with modeling of microwave freeze drying(MFD) (Lombraña et al., 2001; Wang and Chen, 2003; Wu et al.,2004; Tao et al., 2005; Wang et al., 2005), which shows that MFDcan give better heat and mass transfer rate. Only several MFD stud-ies of food materials have been reported. For instance, Wang andShi (1999) studied MFD characteristics of beef and effect of dielec-tric material on MFD of skim milk. Duan et al. (2007) carried outMFD experiments on cabbage, and later MFD was successfully usedby them to dry sea cucumber (Duan et al., 2008a,b). Wang et al.(2009) investigated the drying characteristics of an instant vegeta-ble soup. They have shown that MFD is one of the most promising

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cience and Technology, Statean University, 214122 Wuxi,

techniques to accelerate drying and to enhance overall quality offood products. However, there is still insufficient literature onchanges of the microstructure and other quality attributes in theMFD process of food materials as well as the comparison with FDprocess.

A viable new dehydration technology should not only exhibithigher efficiency and lower cost, but also should have little or noloss in the quality of dried products. The microstructure of thefruits and vegetables may be damaged during food processing.The damage included loss of integrity of the cell membranes, lossof turgor and deterioration of cell wall structure (Carbonell et al.,2006). All these changes have dramatic effects on the texture ofthe product, which is an important attribute in the quality of prod-ucts. This may influence the quality of the dried products. Theknowledge of the microstructural changes of materials during thedrying is useful for understanding the various process mechanismsand improving final product quality. It also provides reference datafor further research and development of the production processesas well as process optimization.

Potatoes are highly nutritious vegetable containing high energy,dietary fiber, biologically active photochemicals, vitamins, andminerals which offer great benefit for use as functional food ingre-dient (Brinley et al., 2008). For different consumption purposes, thedehydration process of potatoes can produce two types of prod-ucts, namely, dehydrated vegetable and snack food. This may re-quire different pre-treatment methods, which could have animportant influence on the final quality of the products. Blanchingof fruits and vegetables has been widely used in food processing.

Fig. 1. Schematic diagram of the microwave freeze dryer.

132 R. Wang et al. / Journal of Food Engineering 101 (2010) 131–139

To produce a snack food using freeze drying, potatoes should beblanched prior to drying to provide a final product ready-to-eat.Blanching by heating can result in loss of soluble solids, enzymedenaturation, air removal from the tissue, hydrolysis and solubili-zation of structural polymers such as protopectin, and gelatiniza-tion of starch granules (Maté et al., 1998). These consequencesmake the internal structure of blanched potato different from thatof the unblanched one. The structural changes likely influence thedrying process as well as the quality of the dried product.

The pre-treatment with Ca2+ has been applied successfully toenhance the firmness of heat-treated fruit and vegetables in foodprocessing. The calcium plays a role in increasing cell rigidity hasbeen related to its ability to bind with pectin. In this way, the firm-ness of the flesh of fresh-cut fruit products may be improved iftreated with calcium compounds (Moraga et al., 2009). Prior todrying, the pre-treatment with Ca2+ has been reported by someresearchers to minimize tissue damage and improve dried productrehydration quality (Sham et al., 2001; Deng and Zhao, 2008; Gon-zález-Fésler et al., 2008). This work choose potato as a model mate-rial, to investigate microstructural changes of potato slices withand without blanching in MFD and FD process and evaluate thedried products quality (i.e., vitamin C content, starch content, sugarcontent, color, texture, and rehydration).

2. Materials and methods

2.1. Raw materials

Fresh potatoes employed in the experiments were purchasedfrom the local market. They were stored at a temperature of4 ± 0.5 �C until the drying experiments. The potatoes were manu-ally peeled and cut into 4 mm slices. A circular tool was used toprovide slices with a diameter of 40 mm. As the slices were cut,they were held in water until the entire batch was prepared.

2.2. Sample pre-treatments

Control: The prepared raw potato slices without any pre-treat-ment were removed from the water, blotted with tissue paper toremove superficial water and then frozen at �80 �C in a refrigera-tor (U410, New Brunswick Scientific Co., USA).

Calcium treatment: In order to observe the effect of calciumtreatment on microstructure of potato tissues, raw potato sliceswere dipped in solution of 0.5% calcium chloride for 10 min, andthen the samples were removed, rinsed with water and blottedwith tissue paper to remove superficial water and then frozen inthe freezer as mentioned earlier.

Blanching: In order to investigate the microstructural changes ofblanched potato tissues during drying process, the potato sliceswere blanched in boiling water for 5 min. After blanching, the sam-ple was cooled to room temperature under running cold water, andthen blotted with tissue paper to remove superficial water, and fro-zen in the freezer for use.

2.3. Experimental equipment

Experiments were performed in a lab-scale microwave freezedryer (YT2S-01, Nanjing Yatai Microwave Power Technology Re-search Institute, China). A schematic diagram of the equipment ispresented in Fig. 1. As shown in Fig. 1, there are two drying cavitieswhere FD and MFD tests were carried out. When materials weredried in the FD cavity, they were heated by the electrically heatedshelf. When the samples were dried in the MFD cavity, appliedmicrowave field supplied the required energy. During drying, thepressure was maintained at 100 Pa by a vacuum pump, and the

temperature of the cold trap and the temperature (�40 to�45 �C) of the cold trap was low enough to condense vapor. Themicrowave frequency was 2450 MHz and the power could be reg-ulated continually from 0 to 2000 W. The material temperaturewas detected using an optical fiber probe. The equipment was con-trolled by a Program Logic Control (PLC) system, which could con-trol the microwave power level, material temperature, vacuumpressure and cold trap temperature.

2.4. Drying experiments

Experiments were carried out using different drying methods(FD and MFD). In order to determine the sublimation and desorp-tion drying stage of different pre-treatment potato slices in FD andMFD process, the drying kinetics were investigated. The pre-frozensamples were transferred to the microwave freeze drying machineand dried at 1.6 W/g microwave power (MFD process) or 55 �Cheating shelf temperature (FD process). Moisture loss during dry-ing was measured by periodically taking out and weighing the dishon a digital balance (JH2102, Shanghai Precision & Scientific Instru-ment Co. Ltd., China) with 0.01 g precision. Drying continued untilthe moisture content of the samples dropped to 0.06 g/g on drybasis.

All drying processes were carried out at 100 Pa cavity pressureand �40 �C cold trap temperature. All experiments were repeatedtwice and the average of results was used for analysis.

2.5. Light microscopic analysis

Structural changes in potato during drying were studied usinglight microscopy (LM). The paraffin method was used. Small cubesof about 5 mm were removed from the internal zone of the sam-ples for microscopic examination. The sample cubes were fixedin formol-aceto-alcohol (FAA, formaldehyde 5%, glacial acetic acid5%, 70% ethanol 90%) fixative solution for 24 h. Dehydration wasperformed with a series of gradually increasing ethanol concentra-tions, then samples were cleared in xylene and embedded in paraf-fin (melting point 56–58 �C). Sectioning was done with a rotarymicrotome (YD-202, Zhejiang Jinghua Yidi Medical Appliance Co.Ltd., China) at 15 lm thickness. After deparaffinage with xylene,the sections were rehydrated with a series of decreasing ethanol

R. Wang et al. / Journal of Food Engineering 101 (2010) 131–139 133

steps, and then the Heidenhain’s iron-alum hematoxyling methodwas employed for staining. Finally, the samples were examinedunder a light microscope (XSP-8C, Shanghai Optical InstrumentFactory, China) equipped with a digital camera (WV-CP240, Suz-hou Matsushita Communication Industrial Co. Ltd., China). Micro-graphs were taken at 100 times magnification.

For the pre-frozen samples and samples of the sublimation dry-ing stage, the specimen required for observation were obtained at�20 �C in a freezer. Inside this chamber, small cubes of about 5 mmwere removed from the sample for microscopic examination. Thefixation was carried out at �20 �C to avoid structure changes byice thawing, and the remaining steps were conducted followingthe above procedure at room temperature.

The histological procedures were performed in duplicate. Tis-sues from each of the drying processes were listed in Table 1(according to the drying experiments).

2.6. Analysis of sample

2.6.1. Moisture contentThe moisture content of potato was determined gravimetrically

in triplicate by drying 3 ± 0.5 g samples at 105 ± 1 �C until a con-stant mass was achieved. The moisture content of samples in thedrying process was calculated using the following equation:

Xt ¼mt �md

mdð1Þ

Here, Xt is moisture content at t on dry basis (g/g, d.b.), mt is theweight of material at t, md is the dry matter weight of the material,and t is the drying time (h).

2.6.2. Vitamin C, sugar and starch contentThe ascorbic acid content was determined by 2,6-dichlorophe-

nol-indophenol titration method (Li, 1997). Data were calculatedon a dry basis and expressed as mg/100 g d.b. The analyses weredone in triplicate and the averages of these measurements werereported.

The sugar content was determined by direct titration method(Zhang, 2004). An alkalin solution of copper salt was subjected tohot reduction by direct titration of a protein-free solution, in thepresence of methylene blue, without inversion. Total sugar wasdetermined after inversion with concentrated hydrochloric acid.Further procedures were the same as in the case of reducing sug-ars. Data were calculated on a dry basis. The analyses were done

Table 1The type of potato tissues observed.

SampleNo.

Pre-treatment SampleNo.

Pre-treatment

1 Fresh (raw) 10 FD unblanched potatoslices

2 Fresh + 0.5% CaCl2 soaking 11 FD blanched potato slices3 Fresh + 0.5% CaCl2

soaking + freezing12 MFD unblanched potato

slices for 2 h4 Blanched 13 MFD blanched potato

slices for 2 h5 Blanched + freezing 14 MFD unblanched potato

slices for 5 h6 FD unblanched potato

slices for 3 h15 MFD blanched potato

slices for 5 h7 FD blanched potato slices

for 3 h16 MFD unblanched potato

slices8 FD unblanched potato

slices for 8 h17 MFD blanched potato

slices9 FD blanched potato slices

for 8 h

in triplicate and the averages of these measurements werereported.

The starch content was determined by the acid-hydrolysismethod (Zhang, 2004). After removal of fat and soluble sugars,the starch was hydrolyzed to monosaccharides with concentratedhydrochloric acid, and then the obtained reducing sugars weredetermined and converted to the starch content. Data were calcu-lated on a dry basis. The analyses were done in triplicate and theaverages of these measurements were reported.

2.6.3. ColorThe color of the dried chips was measured using a Chroma Me-

ter (Konica Minolta Sensing Inc., Japan). The colorimeter was cali-brated with a certified standard white plate. For each treatment,one measurement was performed at random locations on a slicewhile the data were taken for five slices. The color parameterswere reported in CIE-lab scale as L*, a* and b*, where L* is the lumi-nance or lightness component, and parameters a* (from green tored) and b* (from blue to yellow) are the two chromatic compo-nents. The total color difference (DE) was calculated using the fol-lowing equation:

DE ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðL� � L�0Þ

2 þ ða� � a�0Þ2 þ ðb� � b�0Þ

2q

ð2Þ

Here, L�0, a�0 and b�0 are the color parameters of raw potatoes beforedrying.

2.6.4. RehydrationThe rehydration ability was evaluated by immersing about 5 g

potato slices in 500 ml distilled water at 20 �C, the potato sliceswere withdrawn periodically, drained and weighed. The rehydra-tion ratio (RR) of the sample was then calculated by

RR ¼ Mt

M0ð3Þ

Here, Mt and M0 are the samples mass (g) at time t (rehydrated sam-ples) and zero (dried samples), respectively. All tests were per-formed in triplicate and the average values were reported.

2.6.5. TextureTexture properties of the dried samples were measured using a

TA-XT2i texture analyser (Stable Micro System, Surrey, UK) with a5 mm diameter cylinder probe. The probe was passed through thesample at a test, pre-test and post-test speed of 2, 2 and 3 mm/s,respectively. The penetration distance was set to 3 mm and triggerforce was 20 g. Texture properties were derived from the force–deformation curve (force versus time). The hardness is the amountof maximum force required to break the sample. For the same dry-ing treatment, measurements were taken individually on five dif-ferent slices and the average values were reported.

2.6.6. ShrinkageShrinkage ratio (SR) was used to evaluate volume changes of

dried samples. SR of the dried sample was defined as:

SR ¼ Vd

V0ð4Þ

Here, V0 and Vd are the volume of the original sample (cm3) and thedried sample (cm3), respectively.

The volume of sample was measured by the volumetric dis-placement method using glass beads with a diameter in the rangeof 0.105–0.210 mm as a replacement medium (Thuwapanichaya-nan et al., 2008). The measurements were carried out five timesfor the same treatment and the average values were reported.

30

40

50

60

)

MFD (blanched)

FD (blanched)

MFD (unblanched)

FD (unblanched)

134 R. Wang et al. / Journal of Food Engineering 101 (2010) 131–139

2.7. Statistical analysis

Data were analysed using the Statistical Analysis System soft-ware (SAS, version 8.1, SAS Institute Inc., Cary, NC). Analyses of var-iance were performed by the ANOVA procedure. Mean values wereconsidered significantly different when p 6 0.05.

-40

-30

-20

-10

0

10

20

0 1 2 3 4 5 6 7 8 9 10 11 12

Drying time (h)

Tem

pera

ture

(

Fig. 3. Variation of material surface temperature with time of the potato slicesduring MFD and FD.

3. Results and discussion

3.1. Drying characteristics of blanched and unblanched potato slices inMFD and FD process

To determine the sublimation and desorption stages of drying,FD and MFD drying curves of blanched and unblanched potatoslices were presented in Fig. 2. The moisture content decreasedwith drying time for all drying process. It was observed thatblanched potatoes had higher initial moisture content than un-blanched ones indicating a water uptake during blanching. It wasshown that the total drying time and drying rate were significantlydifferent for MFD and FD process while the blanching did not sig-nificantly influence the rate of the drying process. Nevertheless, itwas found that the blanched samples dried by FD a slight fasterthan the unblanched one. This behavior was probably due to struc-ture softening due to blanching that might facilitate water removal(Severini et al., 2005). Contrary to the FD process, the drying rate ofthe blanched samples dried by MFD is slightly slower than the un-blanched one. This could be explained by the dielectric propertiesof the potatoes changes due to leaching out of chemical composi-tion and starch gelatinization during blanching. The required dry-ing times for MFD and FD process were about 6.3 and 10.0 h,respectively. MFD could reduce drying time by 37%. The drying ratewas high in the primary drying stage, and it decreased in the finaldrying process. It was because there was considerable free mois-ture in the material and it was removed by ice sublimation. Forthe MFD process, the sublimation stage lasted about 4 h and fol-lowed by the desorption stage. While for the FD process, the sub-limation stage lasted about 7 h.

Variation of material surface temperature with time of the po-tato samples during MFD and FD is shown in Fig. 3. It can be seenfrom Fig. 3 that the heating rate during MFD was faster than that ofFD process. The temperature changes in blanched and unblanchedpotatoes for the same drying process were not significantly differ-ent. The temperature curves had a slow temperature rising stageand a rapid temperature rising stage, indicating the sublimationdrying stage and the desorption drying stage. In the sublimationdrying stage, the most energy absorption by materials was used

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 1 2 3 4 5 6 7 8 9 10 11 12Drying time (h)

Moi

stur

e co

nten

t (g/

g d.

b.)

MFD (blanched)

FD (blanched)

MFD (unblanched)

FD (unblanched)

Fig. 2. MFD and FD curves of blanched and unblanched potato slices.

for ice sublimation so the temperature increased slowly. As dryingprogresses, most of the free water (ice crystals) was removed, theremainder was non-frozen free water and bound water, indicatingthe desorption step. When all the ice crystals are sublimated, theproduct temperature increases sharply until reaching the shelftemperature, which can be considered as an indicator of the subli-mation end-point.

According to the drying curves and material temperaturecurves, the type of potato tissues observed during drying is shownin Table 1.

3.2. Microstructural changes of unblanched potatoes in MFD and FDprocess

The LM microphotographs of potatoes in MFD and FD processare presented in Fig. 4. It can be seen that the fresh potato cellsare intact and in perfect contact. The arrangement of polyhedralpotato cells was orderly. The potato starch granules are embeddedwithin the potato cells. After soaking with CaCl2, a thickening ofthe cell wall can be observed due to the absorption of Ca2+ andthe formation of calcium pectate, making the cell wall more resis-tant and stable. Calcium treatment can increase the thickness ofthe cell wall, making potato tissue can become firmer as well(Vega-Gálvez et al., 2008).

After freezing, deformation of the cells occurs and some cellwall was disrupted. The cell breakage was presumably caused byice crystals. During the sublimation stage, the orderly polyhedralarrangement was lost and more cell wall was disrupted. In desorp-tion stage, the cells were found to be further damaged, which wascaused by the growth of ice crystals due to the temperature in-crease during sublimation stage. The microstructure of the samplesof the MFD/FD desorption stage were similar as the final product. Itwas concluded that there were no significant changes in cell struc-ture during desorption stages of FD and MFD for unblanched pota-toes. The most microstructure changes occurred in pre-frozen andsublimation stage.

Fig. 4 also showed that both MFD and FD process did not causesignificant changes to the starch structure. However, MFD and FDcaused deformation and degradation of cell wall. It could be an ef-fect of the free water removing from the parenchyma cell. It wasstated that heating of starch granules at 65 �C initiates the gelatini-zation process, amorphous regions hydration, and hydrated amy-lase leakage from granules (Markowski et al., 2009). However, forfreeze drying process, the material temperature was controlled be-low 55 �C, which would not cause starch gelatinization but pre-serve their granular shape during the drying.

Fig. 4. The light micrographs for the unblanched potato tissues in MFD and FD process. Magnification: 100�.

R. Wang et al. / Journal of Food Engineering 101 (2010) 131–139 135

136 R. Wang et al. / Journal of Food Engineering 101 (2010) 131–139

3.3. Microstructural changes of blanched potatoes in MFD and FDprocess

Fig. 5 shows the microstructure of blanched potato tissues priorto drying and in the MFD and FD process. It was seen that blanch-ing caused starch gelatinization. The cells of blanched potato werewell coherent, and the shape was more regular, while the cellswere filled with starch gel. It was also observed that blanched po-tato is more vulnerable to freezing damage compared to the un-blanched ones. That is, the microstructural damage of blanched-then-frozen potatoes is far worse than the damage of the frozenunblanched ones. The results were consistent with the reported

Fig. 5. The light micrographs for the blanched potato ti

microscopic structure observations on blanched-then-frozen car-rots by others (Préstamo et al., 1998; Roy et al., 2001). As shownin Fig. 5, freezing resulted in irregular cell arrangement and cellswelling of blanched potatoes, including some cell separation anddeformation which occurs as well. This was because of the volumeincrease of the ice. The cell arrangement became more irregularduring sublimation (i.e., MFD 2 h or FD 3 h), and the separationand cavities increased which indicates the growth of ice crystals.In the desorption stage (i.e., MFD 5 h or FD 8 h), more intercellularvoids appeared, and this was due to ice crystals formed duringfreezing which pushed the gelled starch towards the cell wall.The further dehydration of parenchyma cells during the desorption

ssues in MFD and FD process. Magnification: 100�.

R. Wang et al. / Journal of Food Engineering 101 (2010) 131–139 137

drying stage also caused some relaxation and shrinkages of the cellwalls and starch gel, although the significant shape change was notfound in the final dried products. It was concluded that the finalstructure of FD potato was mainly formed during freezing.

The reason for the more damages in microstructure of blanchedpotatoes during drying is due to the expanding starch granules thatcaused an increase in pressure inside the cells. The blanching pro-cess results in an uptake of water and a structure change of starchgranules after blanching. Furthermore, the blanching process re-sulted in the softening of potato issues which is associated withthe loss of turgor due to membrane disruption and solubilizationand depolymerization of pectic polymers that are involved incell–cell adhesion (Buggenhout et al., 2009). So, more impairmentand disruption of cell walls actually occurs during the frozen pro-cess for the blanched samples.

Table 3The color parameters of MFD and FD potato slices.

Sample L* a* b* DE

Raw material 71.77 ± 1.12b �2.04 ± 0.20a 15.15 ± 0.51b –Blanched 59.76 ± 0.62a �5.30 ± 0.23c 3.51 ± 0.49a 16.37FD (unblanched) 91.93 ± 0.52d �2.33 ± 0.34ab 14.42 ± 0.55b 20.18MFD (unblanched) 91.62 ± 0.61d �2.58 ± 0.36ab 14.16 ± 0.62b 19.88FD (blanched) 88.59 ± 0.45c �3.05 ± 0.52b 18.63 ± 0.77c 17.21MFD (blanched) 88.28 ± 0.67c �3.37 ± 0.57b 18.20 ± 1.05c 16.84MFD (raw) 89.57 ± 0.73cd �2.78 ± 0.46ab 15.05 ± 0.65b 17.82

a,b,c,d Different letters in the same column indicate a significant difference(p 6 0.05).

3.4. Vitamin C, sugar and starch content

The content of sugars, starch, and vitamin C in raw, blanchedand dried potato slices is presented in Table 2. Ascorbic acid isan important nutrient that is present in vegetables. It is water sol-uble and sensitive to pH, light and heat. It is usually selected as anindex of the nutrient quality because of its labile nature comparedto other nutrients in foods (Goula and Adamopoulos, 2006). If vita-min C is retained well, other nutrients are also likely to be pre-served. Blanching was often used as a pre-treatment prior todrying to reduce the loss of vitamins during drying. However, vita-mins losses in fruits and vegetables are inevitable during thermalblanching itself. Lin et al. (1998) reported that vitamin C contentin carrots reduced from 770 lg/g solid to 443 lg/g solid duringthe blanching process (90 �C, 7 min). For potatoes in this work, itwas observed that blanched potatoes prior to drying reduced thevitamin C by 27.8%. It seems that vitamin C in potatoes is more sta-ble during blanching owing to the protective effect of starch gel.The retention of vitamin C in FD samples was found to have no sig-nificant difference with the MFD ones. A comparison of MFD and/or FD blanched and/or unblanched potato slices showed that boththe drying processes provide a high vitamin C retention. Comparedto the samples before drying, about 92.5% and 93.1% of the vitaminC in the unblanched potatoes were retained during FD and MFD,respectively; while about 93.5% and 94.4% of the vitamin C in theblanched potatoes were retained in the FD and MFD samples. Thatwas because the low temperature and vacuum pressure duringdrying did not result in loss of vitamin C. Furthermore, it was notedthat the blanched samples had less vitamin C loss during drying.

As for sugar and starch, the results showed that all these con-tent were also mainly lost in the blanching process. The reducingsugar loss was about 8.8% during blanching, while it was not fur-ther loss in the subsequent drying process. The total sugar andstarch content of blanched MFD and FD potatoes had no significantdifferences with the raw material, but a slight decrease happeneddue to leaching losses. The results showed that MFD process canpreserve the nutritional quality of potatoes the same as FD process.

Table 2Vitamin C, sugar, and starch content of MFD and FD potato slices.

Sample Vitamin C (mg/100 g d.b.) Reducing

Raw material 153.56 ± 7.52a 2.85 ± 0.1Blanched 110.81 ± 5.65b 2.60 ± 0.1FD (unblanched) 142.10 ± 5.38a 2.83 ± 0.0MFD (unblanched) 142.98 ± 6.40a 2.85 ± 0.1FD (blanched) 104.65 ± 5.72b 2.60 ± 0.0MFD (blanched) 103.67 ± 5.19b 2.61 ± 0.0

a,b Different letters in the same column indicate a significant difference (p 6 0.05).

3.5. Color

Color parameters of potato slices after different treatment arepresented in Table 3. The L* (i.e., the luminance or lightness) valueof the potato slice decreased after blanching. This can be due to thestarch gelatinization and the uptake of water during blanchingwhich changed the optical properties of potatoes. The lightnesswas reduced due to the clarity-like characteristic of gelatinizedstarch (Pimpaporn et al., 2007). Furthermore, some browning reac-tions might occur during blanching. The results showed that dryingcaused an increase in lightness in comparison with the referencesample. The dried samples were all significantly lighter than theoriginal potatoes. The lightness increased since the clarity-likecharacteristics of the samples became opaque. All color parametersof MFD and FD unblanched potatoes slices had no significant dif-ference as well as for dried blanched ones. It was proved that theMFD and FD potato slices had the similar color for the same pre-treatment. However, dried blanched potatoes had lower L*, a* val-ues and higher b* values than the unblanched ones. The total colorchanges (DE) were greater for unblanched dried samples than forblanched potatoes. In fact, the unblanched dried samples had aslight white color due to dehydration of raw starch. Ca2+ pre-treat-ment did not significantly influence the color parameters of driedproducts in comparison with raw products.

3.6. Rehydration

The rehydration curves of unblanched (with calcium chloridetreatment) and blanched potato slices dried in MFD and FD processare shown in Fig. 6. RR of MFD raw potatoes without any pre-treat-ment was also plotted for comparison. The rehydration curvesshowed an initial high rate of water absorption followed by aslower absorption rate in the latter stages of rehydration. This typ-ical rehydration behavior has been observed by many researchers(Markowski et al., 2009; Setiady et al., 2009). It was shown thatthe dried blanched potato slices had much higher RR than the un-blanched ones regardless of FD or MFD samples. The maximum RRof blanched FD and MFD samples were 6.123 ± 0.25 and5.802 ± 0.32, respectively, while the RR of unblanched FD andMFD samples were 2.492 ± 0.15 and 2.504 ± 0.13, respectively.

sugar (%, d.b.) Total sugar (%, d.b.) Starch (%, d.b.)

3a 6.57 ± 0.32a 73.41 ± 2.12a

0b 6.30 ± 0.27a 71.52 ± 2.60a

9a 6.57 ± 0.30a 72.41 ± 1.98a

2a 6.56 ± 0.29a 73.13 ± 2.31a

7b 6.31 ± 0.23a 72.61 ± 1.85a

8b 6.30 ± 0.25a 73.10 ± 2.02a

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0 2 4 6 8 10

Time (min)

Reh

ydra

tion

ratio

FD blanched) MFD (blanched) FD (unblanched)MFD (unblanched) MFD (raw)

Fig. 6. Effect of pre-treatment and drying methods on rehydration characteristics ofpotato slices.

138 R. Wang et al. / Journal of Food Engineering 101 (2010) 131–139

The RR of MFD and/or FD blanched potatoes were twice as muchthan that of the unblanched ones, while the MFD raw samples(without Ca2+ treatment) had the lowest RR (2.121 ± 0.21).Whether the potatoes were blanched or not, it was observed theMFD and FD products had similar rehydration properties for thesame pre-treatment. However, the RR of MFD blanched potatoslices were slightly lower than the FD ones though they were foundto have no significant difference (p > 0.05). The slight decrease inRR of the blanched MFD potato chips were associated with the rel-ative high drying rate and non-uniform heating by MW. The differ-ences in RR of three different pre-treated samples showed that Ca2+

treatment has contributed to increase rehydration capacity ofdried potatoes, whereas blanching was more effective in improvingthe rehydration capacity. It was due to that the infusion of calciumions into the potato tissues bound with pectin to increase cellrigidity (Buggenhout et al., 2009), resulting in less structure dam-age during drying. The low RR of unblanched potato slices can bedue to cell wall damage and low water absorbability of raw starch.

3.7. Texture and shrinkage

The texture (hardness) and shrinkage of the samples are shownin Fig. 7. It was seen that blanching could significantly increase thehardness of potato slices. This was due to the rigidity structure ofstarch gel. The hardness values of MFD and FD unblanched sampleshad no significant differences. However, the MFD blanched sam-

0

500

1000

1500

2000

2500

(unblanched) (unblanched)MFD (raw) FD MFD FD (blanched) MFD

(blanched)

Har

dnes

s (g

)

0

0.2

0.4

0.6

0.8

1

1.2

SR

Hardness SR

Fig. 7. Hardness and SR of FD and MFD potato slices.

ples were significantly harder than the FD blanched ones. Thiswas caused by the relative high drying rate and uniformity ofmicrowave heating. It was noted that the MFD raw sample washarder than the unblanched (i.e., with Ca2+ treatment) ones. Thiswas because the dense structure of sample due to shrinkage, whichled to a harder texture. However, for the samples pre-treated withCa2+, the role of Ca2+ on increasing cell rigidity reduced the tissuedamage during drying, and the samples could preserve the porousstructure.

The SR was used to estimate the volume changes of the driedproduct. The lowest SR values namely the highest volume changesof potato slices. It was seen that the MFD raw potato slices had thelowest SR, indicating the lowest volume and largest deformation ofthe dried slices. In fact, it was also observed that the raw driedslices showed some warp in our experiment. However, after pre-treated with Ca2+, the volume of the samples can be well preserveddue to the improved cell structure strength. It was seen from Fig. 7that the SR of the blanched and unblanched potato slices driedwith MFD and FD was found to have no significant difference. Re-sults showed that potatoes pre-treated by calcium chloride solu-tion or blanching had excellent shape retention in both MFD andFD process. It was concluded that both calcium ions treatmentand blanching were useful methods to avoid shape changes duringdrying.

4. Conclusions

The drying rate for MFD process was relatively faster than theFD process, and about 37% of the total drying time could be re-duced by MFD process in comparison with the FD process. Micro-structural investigation showed that most microstructure changesof both unblanched and blanched potatoes in MFD and FD processoccurred in pre-frozen and sublimation stage, and there were nosignificant changes in the cell structure during desorption stages.For the raw potatoes, the polyhedral cells were intact and orderly,and after Ca2+ soaking, a thickening of the cell wall was observed.Blanching also causes starch gelatinization. The regular and swol-len cells of blanched potatoes were easier to be damaged by freez-ing than the unblanched ones. Thus, the blanched dried samplesunderwent more microstructural changes than the unblanchedsamples.

Quality evaluation showed that, no significant vitamin C, sugarand starch losses occurred in both MFD and FD process. Ca2+

treatment contributed to increase rehydration capacity of driedpotatoes, whereas blanching was more effective in improve rehy-dration capacity. Both calcium ions treatment and blanching wereuseful ways to avoid changes in shape during microwave freezedrying.

Acknowledgements

The authors express their appreciation to Frito-lay Inc., Plano,TX, USA, for the financial support of this study. Discussions withDr. Ted Farrington were very valuable during the course of thiswork. His support and contribution is gratefully acknowledged.The authors also thank the National Natural Science Foundationof China (No. 20776062) for the financial support.

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