Journal of Food Engineering 106 (2011) 290–297

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Journal of Food Engineering

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

Effect of salt and sucrose content on dielectric properties and microwavefreeze drying behavior of re-structured potato slices

Rui Wang a,b, Min Zhang a,⇑, Arun S. Mujumdar c, Hao Jiang a

a State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, Chinab Foshan Haitian Flavouring & Food Co., Ltd., 528000 Foshan, Guangdong, Chinac Department of Mechanical Engineering & Engineering Science Program, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore

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

Article history:Received 17 October 2010Received in revised form 9 May 2011Accepted 11 May 2011Available online 23 May 2011

Keywords:Dielectric propertiesSaltSucrosePotato pureesRe-structured potato slicesMicrowave freeze drying

0260-8774/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.jfoodeng.2011.05.015

⇑ Corresponding author. Address: School of FooJiangnan University, 214122 Wuxi, Jiangsu Province, C

E-mail addresses: min@jiangnan.edu.cn, min@sytu

Potato puree was used to prepare re-structured potato slices. The effect of salt and sucrose content onmicrowave freeze drying (MFD) and the variation of the dielectric properties of the potato slices werestudied. Results showed that the dielectric constant (e0) and loss factor (e00) of the potato purees decreasedsignificantly after freezing. Both salt and sucrose addition increased e0 and e00 of the samples at low tem-peratures (�25 to �5 �C). The salt content had greater influence relative to sucrose content on dielectricproperties of the samples. Sucrose addition shows insignificant influence on dielectric properties of thethawing samples; while for the salted samples, e0 decreased gradually with increasing salt content ande00 increased significantly with increasing salt content and temperature. MFD drying rate was significanthigher after adding salt and sugar.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Re-structured potato slices and potato chips are often madefrom potato granules or flour, it provides higher material utiliza-tion efficiency when processing. The re-structured product also of-fers different taste or flavor options, and can be made into differentshapes. Re-structured potato slices produced by freeze drying haveadditional advantages of high rehydration rate and better quality.

Freeze drying can produce the highest quality dried foods. How-ever, a major problem with conventional freeze drying (FD) is thelong drying time, which in turn leads to high energy consumptionand high capital costs (Hammami and Rene, 1997; Ratti, 2001).This is partly due to the poor heat transfer rate associated withthe conventional electrical heating shelf method which transfersheat for drying by conduction. To overcome this drawback, micro-wave heating instead of the traditional conduction or radiant heat-ing on FD has proven to give better heat and mass transfer rate(Sunderland, 1980; Sochanski et al., 1990; Lombraña et al., 2001;Wang and Chen, 2003; Tao et al., 2005). This technique is calledmicrowave freeze drying (MFD). Recently, MFD has been used suc-cessfully for drying beef (Wang and Shi, 1999), skim milk (Wanget al., 2005), cabbage (Duan et al., 2007), sea cucumber (Duan

ll rights reserved.

d Science and Technology,hina. Fax: +86 510 580 7976..edu.cn (M. Zhang).

et al., 2008a,b, 2010), vegetable soup (Wang et al., 2009, 2010a),potato (Wang et al., 2010b,c), apple (Wu et al., 2010) and banana(Jiang et al., 2010).

The microwave heating rate dependents on dielectric propertiesof the material. Dielectric properties are commonly represented bya complex number, the relative complex permittivity e⁄ = e0 � e00,where the real part e0 is the dielectric constant and the imaginarypart e00 is the dielectric loss factor. Here e0 indicates the material’sability to store electrical energy and e00 is a measure of its abilityto dissipate the electrical energy in the form of heat (Venkateshand Raghavan, 2004; De los Reyes et al., 2007; Coronel et al.,2008). When water is frozen, both the dielectric constant anddielectric loss factor decrease sharply compared with the liquidstate (Venkatesh and Raghavan, 2004). To enhance the MFD dryingrate markedly, Wang and Chen (2003, 2005) and Wang et al. (2005)investigated dielectric material-assisted MFD of aqueous mannitolsolution, aqueous pharmaceutical excipient and skim milk. The re-sult showed that the MFD process can be enhanced significantly byaddition of dielectric cores with high e00 to the materials. Becausethe dielectric properties of food materials are affected by theirchemical composition such as moisture, sugar, salt, protein andash contents (Ryynänen, 1995; Venkatesh and Raghavan, 2004;Coronel et al., 2008), some studies based on improving dielectricproperties of materials were also carried out. The research per-formed by Wang et al. (2010a) showed that the MFD rate of instantvegetable soup could be improved by adding food seasoning

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9101112

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1. Microwave freeze drying chamber 2. Optical fiber temperature sensor 3. Vacuum breakage valve, for MFD 4. Sample supporting plate 5. MFD Sample 6, 7. Microwave source 8. Pressure sensor, for MFD chamber 9. Freeze drying chamber 10, 12. Heating plate 11. FD sample 13. Pressure sensor, for FD chamber 14. Vacuum breakage valve, for FD 15. Temperature sensor 16. FD vacuum va lve 17. MFD vacuum valve 18. Cold trap19.Vacuum pump 20. Draining valve 21.Refri geration compressor 22.Control system

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

R. Wang et al. / Journal of Food Engineering 106 (2011) 290–297 291

ingredient. A recent study on effect of osmotic dehydration com-bined with MFD showed that osmotic treatment is effective inaccelerating MFD drying rate (Wang et al., 2010b). These studiesfocused on finding the optimum processing parameters to achieveshorter drying time, higher final product quality, and better energyefficiency.

The dielectric properties of some agricultural materials havebeen reported for different frequency ranges, temperatures, andmoisture contents (Sosa-Morales et al., 2010). However, mostdielectric properties of the materials are studied at temperaturesabove 0 �C. Very limited data is reported on dielectric propertiesof food materials below 0 �C. The objectives of this study were(1) to investigate the effect of temperature and water content onthe dielectric properties of potato purees, and (2) to study the ef-fort of salt and sucrose content on the dielectric properties andMFD characteristics of re-structured potato slices.

2. Materials and methods

2.1. Sample preparation

Fresh potatoes used in the experiments were purchased fromthe local market. They were stored at 4 ± 0.5 �C prior to the exper-iments. The ingredients including salt and, sugar were purchasedfrom a local supermarket.

Potatoes were washed, boiled, peeled, and mashed using a foodprocessor. The puree was for preparing re-structured potato slicesafter molding. To investigate effect of salt and sucrose content ondielectric properties and drying behavior of re-structured potatoslices, the required amounts of salt and sugar were added to makethe potato purees. The initial moisture content of the preparedpuree without additives was 81.5% (w.b.). For the drying experi-ments, the puree samples were filled in plastic moulds, and placedin a freezer at �15 �C for pre-cooling. After pre-cooling, the potatopurees were cut into 3 mm slices by a knife. The slices were pre-

frozen at �80 �C in a freezer (U410, New Brunswick ScientificCo., USA) for at least 5 h prior to the drying experiments.

2.2. Experimental equipment

Experiments were performed with a lab-scale microwave freezedryer (YT2S-01, Nanjing Yatai Microwave Power Technology Re-search Institute, China). A schematic diagram of the equipment isgiven in Fig. 1. There are two drying cavities where the FD andMFD tests can be carried out. When materials are dried in the FDcavity, they are heated by conduction through the electricallyheated shelf. When materials are dried in the MFD cavity, appliedmicrowave field supplied the required energy. During drying, thepressure was maintained at 100 Pa by a vacuum pump. The tem-perature (�40 to �45 �C) of the cold trap was low enough to con-dense vapor. The microwave frequency was 2450 MHz. The powercould be regulated continuously varying from 0 to 2000 W. Thematerial temperature was monitored and recorded using an opticalfiber probe. The equipment was controlled by a Program Logic Con-trol (PLC) system. During drying, the microwave heating systemcould be turned on or off automatically to control the materialtemperature.

2.3. Drying experiments

The pre-frozen samples were quickly transferred to the dryingchamber. Before moving the frozen potato slices into the dryingchamber, the cold trap was set at a temperature of �40 �C. Thenthe vacuum pump was turned on. Microwave heating was startedwhen the vacuum level reached an absolute pressure of 100 Pa.Moisture loss of samples was measured by periodically takingout and weighing the dish on a digital balance (JH2102, ShanghaiPrecision & Scientific Instrument Co., Ltd., China) with 0.01 g preci-sion. Drying continued until the moisture content of the samplesdropped to 0.06 g/g on dry basis.

Table 1The dielectric properties of potato puree as affected by temperature and moisturecontent.

Temperature(�C)

Water content(%, w.b.)

e0 e0 0 Dp (mm)

20 81.5 60.79 ± 1.32 17.36 ± 0.41 8.84 ± 0.1679.0 60.56 ± 1.21 18.12 ± 0.62 8.46 ± 0.2576.0 58.61 ± 0.73 18.78 ± 0.67 8.04 ± 0.5567.5 50.52 ± 1.03 18.96 ± 0.73 7.43 ± 0.5754.4 33.82 ± 0.52 15.61 ± 0.65 7.44 ± 0.5442.2 15.18 ± 0.75 7.42 ± 0.51 10.52 ± 0.4534.5 5.83 ± 0.41 2.11 ± 0.23 22.65 ± 0.4121.1 3.41 ± 0.35 0.47 ± 0.02 76.76 ± 0.92

5.7 1.79 ± 0.13 0.12 ± 0.01 217.43 ± 0.60

65 81.5 58.32 ± 1.05 16.1 ± 0.45 9.33 ± 0.2279.0 58.31 ± 1.36 17.01 ± 0.55 8.84 ± 0.2376.0 56.05 ± 1.12 17.49 ± 0.76 8.44 ± 0.3567.5 47.23 ± 1.21 17.32 ± 0.67 7.86 ± 0.2254.4 30.92 ± 0.76 13.39 ± 0.45 8.27 ± 0.1042.2 13.25 ± 0.53 5.12 ± 0.52 14.10 ± 0.3134.5 7.67 ± 0.32 2.89 ± 0.30 19.00 ± 0.4521.1 4.78 ± 0.22 0.82 ± 0.12 52.16 ± 1.56

6.0 2.66 ± 0.11 0.33 ± 0.02 96.51 ± 1.38

�20 81.5 3.71 ± 0.13 0.29 ± 0.01 129.55 ± 0.8579.0 3.48 ± 0.06 0.32 ± 0.02 113.74 ± 0.5776.0 3.42 ± 0.10 0.41 ± 0.03 88.07 ± 0.7867.5 3.36 ± 0.03 0.44 ± 0.02 81.37 ± 0.6554.4 3.01 ± 0.06 0.37 ± 0.03 91.56 ± 1.1242.0 2.65 ± 0.12 0.28 ± 0. 02 113.47 ± 1.2321.1 1.69 ± 0.02 0.09 ± 0. 02 281.63 ± 1.35

5.7 1.65 ± 0.02 0.08 ± 0. 01 313.04 ± 1.25

Mean and Standard deviation of 3 replicates were reported.

292 R. Wang et al. / Journal of Food Engineering 106 (2011) 290–297

The effect of salt and sucrose content on microwave freeze dry-ing of re-structured potato slices was investigated in this experi-ment. All drying processes were carried out at ultimate materialtemperature of 55 �C and microwave power of 1.6 W/g.

The dielectric properties of samples were also measured period-ically during drying. All experiments were performed in triplicateand the mean values were used for analysis.

2.4. Dielectric properties measurement

An Agilent 85070E dielectric probe kit was used to measure thedielectric properties of potato samples. The probe was connectedto a network analyzer (E5062A, Agilent Technologies, Malaysia).The network analyzer measured the reflection coefficient at theprobe-sample interface. Calibration of the system was done usingair, metallic short-circuit and de-ionized water before experi-ments. Once the calibration was made, the de-ionized water wasmeasured again to check validity. Beakers containing the sampleswere placed in a temperature controlled water-bath, and thedielectric probe and samples were allowed to equilibrate at the re-quired temperature prior to taking the measurements. The dielec-tric properties were measured by attaching the probe to thesurface of the samples or inserting them into the puree, ensuringthat no air gaps were present so as to minimize error. The samplewas heated from 10 to 80 �C and dielectric properties were mea-sured in the frequency range 500–3000 MHz (at every 25 MHz).To measure the dielectric properties of samples below 0 �C, thedielectric probe was frozen together with the sample in the freezer.The probe can work well over the temperature range from �40 to+200 �C.

To investigate the effect of salt and sucrose addition on dielec-tric properties of potato purees, different contents of salt (1%, 3%,5%, and 7%, w/w) and sucrose (3%, 7%, 11% and 15%, w/w) wereadded to the purees during the blending process. The concentra-tion range was selected because the dried-product showed accept-able quality according to our preliminary experiments. Thedielectric properties were determined in the temperature rangefrom �25 to 80 �C.

To investigate the effect of water content on dielectric proper-ties of potato purees, samples were freeze-dried to various mois-ture contents for the dielectric properties determination. Alldeterminations were made in triplicate and the mean values wereused for analysis.

2.5. Calculation of penetration depth

The penetration depth is the distance into a material at whichthe microwave power has decayed to 1/e (e = 2.7183) of its originalvalue, i.e. 62.2% of the power is absorbed between the surface andthis depth. The penetration depth was calculated using the follow-ing equation (Holtz et al., 2010):

Dp ¼k00

2pffiffiffiffiffiffiffiffi2e00p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1þ e00e0

qr� 1

ð1Þ

Here Dp is the penetration depth in meters, k0 is the free space wave-length in meters, e0 is the dielectric constant, and e00 is the dielectricloss factor.

2.6. Determination of moisture content

The moisture content of potato was determined gravimetricallyin triplicate by drying 3 ± 0.5 g samples at 105 ± 1 �C until a con-stant mass was achieved. The moisture content of the samples inthe drying process was calculated from:

Xt ¼mt �md

mdð2Þ

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

The dimensionless moisture content of samples was calculatedusing the following equation:

MR ¼ Xt � Xe

X0 � Xeð3Þ

Here, MR is the dimensionless moisture ratio; Xt is the moisturecontent at time t (g/g, d.b.), X0 is the initial moisture content (g/g,d.b.), Xe is the equilibrium moisture content (g/g, d.b.), and t is thedrying time (h). The values of Xe are much lower than Xt or X0, hencethe error involved in the simplification by assuming that Xe is equalto zero is negligible (Kiranoudis et al., 1997; Soysal et al., 2006; Reyeset al., 2007; Contreras et al., 2008; Song et al., 2009).

2.7. Statistical analysis

The ANOVA analysis was carried out using Statistical AnalysisSystem software (SAS, version 8.1, SAS Institute Inc., Cary, NC).The least Significance Difference test was used to determine differ-ence between the means. Significance was assumed at P < 0.05.

3. Results and discussion

3.1. Dielectric properties of potato purees

The dielectric properties of potato puree affected by tempera-ture and moisture content are shown in Table 1. Both the dielectricconstant and loss factor increased with increasing water content, asexpected. For example, the value of e0 increased from 1.79 to 60.79at 20 �C as moisture content increased from 5.7% to 81.5%. The valueof e00 also increased from 0.12 to 17.36 over the same moisture

-30 -20 -10 0 10 20 30 40 50 60 70 80 9005

101520253035

60

80

100

120

Die

lect

ric

loss

fac

tor

Temperature (oC)

control1% NaCl3% NaCl5% NaCl7% NaCl

Fig. 3. Dielectric loss factor of potato purees with different salt contents at differenttemperatures.

R. Wang et al. / Journal of Food Engineering 106 (2011) 290–297 293

content. Water is a strongly polar and hence strongly interacts bythe applied MW field; the greater the moisture content of thematerial, the higher the dielectric constant and loss factor. Fordehydrated samples, the dielectric constant and loss factors arevery small due to their low moisture content which is also mostlybound moisture. Moreover, the dielectric loss factor of bound wateris very low since it is not free to rotate under the influence of theelectromagnetic field. Furthermore, it was seen that the dielectricconstant and loss factor decreased with temperature increase athigher moisture content while the opposite trend was observedwhen the moisture content was lower than 34.5%. In the finaldrying stage, the moisture content is very low and the temperatureis relatively high, so the positive correlation between the dielectricloss factor and temperature would lead to overheating of thematerial. Thus, it was very important to control microwave powerand temperature to avoid thermal ‘‘runaway’’ in this period.

As shown in Table 1, the dielectric properties of potato pureeare significantly affected by temperature. When the material is fro-zen, both e0 and e0 0 show a large decrease. For instance, the values ofe0 falls from 60.79 to 3.71 and e0 0 decreased from 17.36 to 0.29 asthe temperature decreased from 20 to �20 �C. The results are inaccordance with previous reports on dielectric properties of somefruits and vegetables (Ryynänen, 1995; Venkatesh and Raghavan,2004). It is also noted that e0 and e0 0 of the frozen samples and theirvariations with water content are very small. This is expected sincethe dielectric properties of food materials are mainly influenced byconcentrations of mobile water and ions (Ryynänen, 1995). Thevery low values of the dielectric constant and loss factor of potatopuree at low temperature result in low microwave absorption abil-ity during microwave heating. Conversely, the smaller e0 and e0 0 ofsamples at low temperature results in larger penetration depth,which is beneficial for uniformity of microwave heating and henceimproving the quality.

3.2. Effect of salt content on dielectric properties of potato purees

The dielectric properties of potato purees with different saltcontents at 2450 MHz were measured in the temperature rangefrom �25 to 80 �C, as shown in Figs. 2 and 3. For temperaturesabove 0 �C, e0 decreases with increasing salt content, while e0 0 in-creases with increasing salt content. The increase in salt contentresults in increasing e0 0 of mashed potatoes was also reported byGuan et al. (2004). The decrease in e0 at higher salt content is prob-ably due to the reduction in water activity. In high moisture foods,e0 0 is influenced by two factors; dipole and ionic losses (S. Wang

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70

Die

lect

ric

cons

tant

Temperature ( oC)

control 1% NaCl 3% NaCl 5% NaCl 7% NaCl

Fig. 2. Dielectric constant of potato purees with different salt contents at differenttemperatures.

et al., 2003; Y. Wang et al., 2003). The sharp increase in e0 0 is a re-sult of increased ionic loss due to salt addition.

It is noted that the dielectric properties of frozen and thawedpotato purees are significantly different. These results agreed withprevious reports on mashed potatoes by Regier et al. (2001). Forsamples with salt addition, e0 decreases with increasing in temper-ature and e0 0 increases with increasing in temperature for temper-ature above 0 �C, while both e0 and e0 0 increase with increase intemperature for temperatures below 0 �C. It was observed thatthe dielectric properties increase sharply with temperature duringthawing, which is in accordance with the observation of Farag et al.(2008) for beef meat blends.

The effect of temperature on e0 depends on the proportion offree water and bound water in the wet material. These effects varyinversely with temperature. It has been known that the e0 of freewater decreases and the e0 of bound water increases with temper-ature (Sipahioglu and Barringer, 2003). Most of the water in the po-tato puree is free water, hence, e0 decreases when temperaturerises from 0 to 80 �C. The e0 0 of salted samples increases signifi-cantly with temperature increasing (P < 0.05). This is due to thefact that the temperature dependent behavior of e0 0 is determinedby the amounts of free water and the dissolved ions. Dipole loss re-sults from water dipole rotation, and its value decreases with tem-perature increasing at 2450 MHz, while ionic loss results frommigration of ions and increases with temperature. For sampleswith salt, the increase of ionic loss with temperature exceeds thedecrease of dipole loss with temperature, so the net loss factor in-creases with temperature increasing.

The dielectric properties of material in the frozen state areimportant in affecting drying rate. From Figs. 2 and 3 shows thate0 and e0 0 of potato purees increases significantly with increasingsalt content. Consequently, it can be deduced that the salt additionsignificantly improves the MFD drying rate.

In frozen state, the dielectric loss factors are smaller relative tothe un-frozen state, and both e0 and e0 0 increase with temperature.Furthermore, salt increased both e0 and e0 0. In earlier studies, it isgenerally agreed that high moisture content results in lower e0

and e0 0 of frozen foods. However, it has been proven here to bean inaccurate viewpoint. The dielectric properties are also affectedby the chemical composition of the food e.g. salt, sugar, amino acidand other components. The influence of temperature on the dielec-tric properties of foods depends on many factors as well, includingfood composition. At temperatures below 0 �C, ionic conductionslows down but does not stop; thus the dielectric loss factor ofthe frozen food material is larger than pure ice, therefore MFD

0.0

0.1

0.2

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0.5

0.6

0.7

0.8

0.9

1.0

0 1 2 3 4 5 6 7 8

Drying time (h)

control1% NaCl3% NaCl5% NaCl7% NaCl

Fig. 6. Effect of salt contents on microwave freeze drying curves of re-structuredpotato slices.

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10121450

55

60

65

Die

lect

ric

cons

tant

control 3% sucrose 7% sucrose 11% sucrose 15% sucrose

Temperature (oC)

Fig. 4. Dielectric constant of potato purees with different sucrose contents atdifferent temperatures.

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1

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5

14

16

18

20

22

Die

lect

ric

loss

fac

tor

Temperature (oC)

control 3% sucrose 7% sucrose 11% sucrose 15% sucrose

Fig. 5. Dielectric loss factor of potato purees with different sucrose contents atdifferent temperatures.

294 R. Wang et al. / Journal of Food Engineering 106 (2011) 290–297

can improve the heating rate of the material and shorten the dry-ing time.

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0 1 2 3 4 5 6 7 8Drying time (h)

Tem

pera

ture

()

control1% NaCl3% NaCl5% NaCl7% NaCl

Fig. 7. Variation of material surface temperature with time of the re-structuredpotato slices with different salt contents during MFD.

3.3. Effect of sucrose content on dielectric properties

The dielectric properties of potato purees at 2450 MHz with dif-ferent sucrose content were measured over �25 to 80 �C, as shownin Figs. 4 and 5. The increase of sucrose content had a smaller effecton e0 and e0 0 relative to that of the salt contents. Sucrose additionhas no significant influence on the dielectric properties for temper-atures above 0 �C. This is due to the low ionic conductivity of su-crose. For samples with sucrose, both e0 and e0 0 decreased onlyslightly with increasing temperature for above 0 �C although theeffect of temperature on e0 and e0 0 is also insignificant. However,e0 and e0 0 increase marginally with increasing temperature forsub-zero temperature zones; they also increase with increasing su-crose content.

Over the range from �25 to �10 �C, both temperature and su-crose content was found to have no significant effect on e0 ande0 0. For temperature above �10 �C, the dielectric properties showa significant increase (P < 0.05) with sucrose content and tempera-ture increasing due to increasing of mobile water. The addition ofsucrose improves dielectric properties of potato purees probablydue to freezing-point depression, which leads to more water being

in liquid phase. Furthermore, sucrose also binds with water mole-cules, reducing the proportion of free water. The increase in e0 ande0 0 of potato purees of different sucrose content in the frozen statewas smaller relative to that of the salt addition.

3.4. Effect of salt content on MFD characteristics

MFD drying rate curves of re-structured potato slices of differ-ent salt content are shown in Fig. 6. MFD drying rate increased sig-nificantly (P < 0.05) with the increasing of salt content. As shown inFig. 6, Re-structured potato slices with higher salt content resultedin shorter drying time. The drying time for the samples withoutseasoning ingredient was about 5.4 h, while the required dryingtimes for the four salted samples (i.e., 1, 3, 5 and 7% NaCl content)were 4.5, 3.9, 3.0 and 2.5 h, respectively. A 17–54% reduction indrying time was found when the salt content increased from 1%to 7% respectively. It has been reported that dielectric propertiesof salt solutions show a dependence on concentration of salt(Venkatesh and Raghavan, 2004; Coronel et al., 2008). The effectsof salt content on enhancing drying rate is due to the addition ofsalt which increases the dielectric loss factor of the re-structuredpotato slices, thus improving microwave absorption ability of foodmaterials.

Because the dielectric properties of the re-structured potatoslices of different salt content are different, the variations of tem-perature with time during drying are also different. As seen inFig. 7, the heating rate of samples with higher salt content was

0

5

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25

Drying time (h)

Die

lect

ric

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control1% NaCl3% NaCl5% NaCl 7% NaCl

0

5

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30

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

Drying time (h)

Die

lect

ric

loss

fac

tor

control1% NaCl3% NaCl5% NaCl 7% NaCl

Fig. 8. Variation of dielectric properties with time of the re-structured potato sliceswith different salt contents during MFD.

0.0

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0.7

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40 1 2 3 5 6 7 8

Drying time (h)

control3% sucrose7% sucrose11% sucrose15% sucrose

Fig. 9. Effect of sucrose contents on microwave freeze drying curves of re-structured potato slices.

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0 1 2 3 4 5 6 7 8Drying time (h)

Tem

pera

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()

control3% sucrose7% sucrose11% sucrose15% sucrose

Fig. 10. Variation of material surface temperature with time of the re-structuredpotato slices with different sucrose contents during MFD.

R. Wang et al. / Journal of Food Engineering 106 (2011) 290–297 295

higher than that of the lower salt content ones. Higher heating ratecould provide more energy for ice sublimation, thus improving thedrying rate.

Variations of dielectric properties with time for different saltcontent samples during MFD are shown in Fig. 8. It is seen that dur-ing the sublimation drying stage, e0 and e00 of the samples are rela-tively small and increasing gradually with time. This occursbecause most water in the material is frozen during this period.Both e0 and e00 increase with salt content, indicating that it canabsorb more microwave energy and convert it into heat. So thesamples with higher salt content display a faster drying rate. Asdrying progresses, most ice crystals are removed and a sharp in-crease in e0 and e00 was observed; this indicates the beginning ofdesorption drying stage. The relatively high e00 contributes to waterremoval. In the final drying stage, e0 and e00 decrease sharply to verylow values due to low water content.

3.5. Effect of sucrose content on MFD characteristics

MFD drying curves of re-structured potato slices with differentsucrose content are shown in Fig. 9. As seen in Fig. 9, the MFD dry-ing rate of re-structured potato slices increases slowly with in-crease of sucrose content. Sucrose Addition increases e0 0 of thefrozen sample, while e0 0 increases slightly due to its sucrose con-tent. The required drying times for the four sucrose samples (i.e.,3%, 7%, 11% and 15% w/w sucrose content) were 4.8, 4.3, 4.0 and3.6 h, respectively. An 11–33% reduction in drying time was foundas the sucrose content increased from 3% to 15% compared to 5.4 hfor the control sample.

Variations of material temperature during MFD with time at dif-ferent sucrose content are shown in Fig. 10. As expected, the heat-ing rate for higher sucrose content samples was faster than that ofthe lower sucrose content ones. It was also observed that the heat-ing rate of the re-structured potato slices with sucrose additionwas slower relative to the corresponding salted samples.

Variations of the dielectric properties with time for different su-crose content samples of MFD are shown in Fig. 11. Similar to thesalted samples, it is observed that e0 and e0 0 are relatively small andgradually increasing with time in the sublimation drying stage. Asharp increase in e0 and e0 0 was observed during initial stage of dry-ing, which decreased to very low level towards the end of drying. Itwas noted that for re-structured potato slices with sucrose, the va-lue of e0 0 is larger relative to the salted samples in the final dryingstage. The high sucrose content samples could absorb more micro-wave energy even towards the end of drying, which leads to rapidtemperature rise of the samples. Thus, these samples are prone toscorching. Samples of high sucrose content require close control ofthe material temperature to assure product quality. In fact, char-ring of the samples was observed in our experiments towardsthe end of drying.

4. Conclusions

Dielectric properties of potato purees were found to beinfluenced by water content, salt content, sucrose content and

0

2

4

6

8

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Drying time (h)

Die

lect

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control3% sucrose7% sucrose11% sucrose15% sucrose

0

1

2

3

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5

6

7

8

0 1 2 3 4 5 6 7

Drying time (h)

Die

lect

ric

loss

fac

tor

control3% sucrose

7% sucrose11% sucrose15% sucrose

Fig. 11. Variation of dielectric properties with time of the re-structured potatoslices with different sucrose contents during MFD.

296 R. Wang et al. / Journal of Food Engineering 106 (2011) 290–297

temperature. It was dominant influenced by water and salt con-tent. The dielectric properties of frozen and non-frozen potatopurees showed large differences as expected. The e0 and e0 0 valuesdecreased with increasing of water content in the non-frozen state.Both e0 and e0 0 values of frozen samples were low and their varia-tions with water content were also very small.

e0 and e0 0 values was increased by both salt and sucrose additionat low temperatures (�25 to �5 �C); salt content had the greaterinfluence. Sucrose addition showed insignificant influence ondielectric properties of the thawing samples. For salted samples,the e0 decreased with the increase of salt content, while e00 in-creased significantly with the increase of salt content as well astemperature. MFD experiments showed that the drying rate ofre-structured potato slices with salt and sucrose addition was sig-nificantly higher. A 17–54% reduction in drying time was noted asthe salt content increased from 1% to 7%, while 11.1–33.3% reduc-tion in drying time was found as the sucrose content increasedfrom 3% to 15%.

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 of theequipment.

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