Application of microwaves dielectric spectroscopy for controlling osmotic dehydration of kiwifruit (Actinidia deliciosa cv Hayward)

Innovative Food Science and Emerging Technologies 12 (2011) 623–627

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Innovative Food Science and Emerging Technologies j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i f s e t

Application of microwaves dielectric spectroscopy for controlling osmotic dehydration of kiwifruit (Actinidia deliciosa cv Hayward) M. Castro-Giráldez a, P.J. Fito a,⁎, M. Dalla Rosa b, P. Fito a a b

Instituto Universitario de Ingeniería de Alimentos para el Desarrollo, Universidad Politécnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain Department of Food Science, Alma Mater Studiorum, University of Bologna, Piazza Goidanich 60, 47521 Cesena (FC), Italy

a r t i c l e

i n f o

Article history: Received 4 January 2011 Accepted 24 June 2011 Editor Proof Receive Date 09 August 2011 Keywords: Osmotic dehydration Dielectric spectroscopy Dielectric properties Microwaves Kiwifruit

a b s t r a c t Dielectric spectroscopy studies have been performed on fresh and osmotically dehydrated kiwifruits (Actinidia deliciosa cv Hayward). The osmotic treatment consisted on the immersion the samples into 65% (w/w) sucrose aqueous solution at 30 °C during different treatment times from 5 to 1440 min. Some physical– chemical parameters were measured in fresh, treated and reposed (24 h at 30 °C) samples. Dielectric spectra were measured in the frequency range from 500 MHz to 20 GHz by an Agilent 85070E Open-ended Coaxial Probe connected to an Agilent E8362B Vector Network Analyzer in the fresh, treated and reposed samples. It has been demonstrated that the dielectric technique is a good method to control the osmotic treatment in kiwifruit. Industrial relevance: The results of this research article are demonstrated to be useful for controlling candying of kiwifruits in bakery industries. Thus, the industrial relevance is clear in order to optimize the osmotic dehydration times and the final quantity of sugars added by using a non destructive technique that can be implemented in process line. Dielectric spectroscopy, which can be considered an emerging technology, has the advantage of being an objective and a rapid technique. For all these reasons we are sending to this journal “Innovative Food Science and Emerging Technologies” our results. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Currently, food industry is looking for non-destructive and accurate methods to control the quality of foods. In this context, sensors based on microwave radiation can determine some physical and chemical properties of foods, predicting the final quality of the product. Dielectric spectroscopy determines the dielectric properties of a medium as a function of frequency. Complex permittivity (ε*) (Eq. 1) is the dielectric property that describes food behaviour under an electromagnetic field (Metaxas & Meredith, 1993; Nelson & Datta, 2001). The dielectric constant (ε′) is related with the material ability to store energy, and the dielectric loss factor (ε″) is related to the absorption and dissipation of the electromagnetic energy in other kinds of energy such as the thermal one.


0

ε = ε + iε

00

ð1Þ

Foods are heterogeneous materials which contain water, organic molecules, macromolecules, ions, and insoluble matrix. In some foods, these constituents are highly organized in cellular and subcellular

⁎ Corresponding author. E-mail address: pedfi[email protected] (P.J. Fito). 1466-8564/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ifset.2011.06.013

structures forming the macroscopic complex structure of animal or plant tissues. Dielectric properties of food tissues reflect the contribution of some polarizations at different levels (Gabriel, 2006). It includes basically four dispersions: α, β, δ, γ. The γ- is the main dispersion at the microwaves frequency range. It is due to the orientation and induction of dipoles, mainly water molecules (Metaxas & Meredith, 1993). It is also important to analyze the energy dissipation of these relaxation phenomena in terms of loss factor spectra. Loss factor can be expressed by Eq. (2), which reflects the different contribution phenomena to the loss factor spectrum in the frequency range of the present study. The relaxation frequency can be defined as the frequency at which the loss factor is maximum in a given dispersion. In this work, it represents the relaxation frequency in the range of the dipolar effect. ;;

;;

ε = εd +

σ ε0 ω

ð2Þ

where: εd″ represents the loss factor caused by the dipolar orientation or dipolar relaxation; σ/ε0ω represents the loss factor due to effect of ionic conductivity, where σ, ε0 and ω are the conductivity of the material, the dielectric constant in vacuum and the angular frequency, respectively.

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Notation aj G Gj Jj Lj Mr M n P R S T t V x z ε’ ε”

activity of the chemical specie j, (–) Gibbs free energy (J) partial Gibbs molar free energy of the specie j, (J/mol) molar flux of the specie j, (mol/m 2s) phenomenological coefficient of the specie j, (mol 2/ J s m 2) molecular weight, (g/mol) Mass, (g) Number of moles, (mol) absolute pressure (atm) Ideal gases universal constant, (J/mol K) Surface area, (m 2) Temperature, (K) Time, (min) Volume, (m 3) Mass or molar fraction (–) Mass or molar fraction of the liquid fraction (–) dielectric constant (–) loss factor (–).

Greek alphabet μj chemical potential of the specie j, (J/mol) μ 0j chemical potential of reference of the specie j, (J/mol) ν specific volume (L/mol) Δ variation of a variable Subscript 0 24 LP max s w t OS e

y superscript initial time time 24 h after the treatment liquid phase maximum sucrose water Time, (min) sucrose solution external liquid phase

Osmotic dehydration is a preservation technique which consists in the immersion of foodstuffs in hypertonic solutions, giving rise to a high water flow out of the food into the solution and a simultaneous transfer of solute from the solution into the food (Salvatori, Andrés, Chiralt, & Fito, 1999). This technique is very common nowadays in the transformation and conservation of alimentary products (Ferrando & Spiess, 2001). The objective of this work was to study the viability of using dielectric properties measurements at microwaves frequencies to analyze the kiwifruit (Actinidia deliciosa cv Hayward) osmotic dehydration process. 2. Materials and methods

Preliminary kinetic studies were done at the same working conditions in order to select the osmotic dehydration times. Osmotic dehydration was conducted over several time periods: 5, 10, 15, 20, 30, 45, 60, 90, 120, 180, 250, 320, 400, 720, 1440 min. Three samples were analyzed at each dehydration time. After the treatment, the samples were reposed at 30 °C for 24 h, on Decagon containers, closed with parafilm®, in order to eliminate the concentration profiles in samples. Mass, volume, surface water activity and dielectric properties were analyzed for each fresh, treated and reposed sample. Representative fresh samples and reposed samples were used to determine the initial moisture and sugar content (ºBrix). At each osmotic time, an aliquot of sucrose solution was also taken from the vessel. Water activity and ºBrix of the solution were measured at each time. Mass was determined by using a Mettler Toledo Balance (±0.0001) (Mettler Toledo, Inc., U.S.A.). Volume was measured by using a photographic treatment of the samples and the software Adobe Photoshop® (Adobe Systems Inc., San Jose, CA, U.S.A.). Surface water activity was measured in the structured samples with a dew point hygrometer Aqualab® series 3 TE (Decagon Devices, Inc., Washington, USA). Moisture was determined by drying in a vacuum oven at 60 °C till constant weight was reached (AOAC method 934.06 (2000)). Sugar content was determined in a refractometer (ABBE, ATAGO Model 3-T, Japan). Analytical determinations described above were obtained by triplicate.

2.1. Dielectric properties measurement The system used to measure dielectric properties consists on an Agilent 85070E Open-ended Coaxial Probe connected to an Agilent E8362B Vector Network Analyzer. The software of the Network Analyzer calculates the dielectric constant and loss factor as a reflected signal function. For these measurements the probe was fixed to a stainless steel support, and an elevation platform brings the sample near the probe. The system was calibrated by using three different types of loads: air, short-circuit and 30 °C Milli®-Q water. Once the calibration was made, 30 °C Milli®-Q water was measured again to check calibration suitability. The dielectric properties were measured by attaching the probe to the surface of the samples. The mean values of three replicates of kiwifruit samples are reported in this article. All determinations were made at 30 °C from 500 MHz to 20 GHz.

0,1

ΔM ΔV

0 0

Sucrose solution (65% w/w, 30 °C), prepared with commercial sugar and distilled water, was used as an osmotic agent. Before the experiment, kiwi fruits (Actinidia deliciosa cv Hayward) with the same size and ripeness were bought from a local supermarket and kept refrigerated until use. The kiwifruits were cut with a calimeter in 45 half slices (1 cm thickness) and the core was eliminated. The samples were immersed in a vessel containing the osmotic solution with continuous stirring. The relation between the fruit and the solution was of 1:20 (w/w) to avoid significant changes in the sugar concentration of the solution during the process. The system was maintained at 30 °C in a constant-temperature chamber. To prevent evaporation the vessel was covered with a sheet of plastic wrap.

200

time (min) 400

600

800

-0,1

0,1

-0,2

-0,1

1000

1200

1400

1600

0 0

50

100

150

200

-0,2 -0,3

-0,3

-0,4 -0,5

-0,4 -0,5 -0,6

ΔV

ΔM

-0,7 Fig. 1. Evolution of overall mass (◆) and volume (○) variation through the osmotic treatment.

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35

35

ε'' 30

ε'' (fR)

33

Treatment time

625

31 29

25

27 20 25 23

15

21 10 19 Osmotic solution

5

17 15

0 0.1

0 1

10

200

400

600

800

1000

1200

100

1400

time (min)

f (GHz) Fig. 2. Dielectric loss factor spectra from fresh (―) and treated samples at different times of osmotic dehydration: 5 min (....), 10 min (- - -), 15 min ( ), 20 min ( ), 30 min ( ), 45 min ( ), 60 min ( ), 90 min ( ), 120 min ( ), 180 min ( ), osmotic solution at 0 min ( ),osmotic solution at 1440 min ( ).

3. Results The osmotic dehydration operation with 65ºBrix solution produces compression and relaxation phenomena (Fito & Chiralt, 1997). These mechanisms could be observed by analyzing the mass and volume variation. Fig. 1 shows the overall mass and volume variation through the osmotic treatment. In the figure, the decrease in total mass and volume is appreciated. In Fig. 2, the spectra of fresh, treated samples and those of the osmotic solution can be appreciated for the initial times (from 0 to 180 min of treatment). The osmotic treatment decreases the relaxation frequency and the spectra are decreasing in the dipolar losses zone as treatment progresses. In Fig. 3, the spectra of the kiwifruit samples from 180 to 1440 min of treatment can be appreciated. It is possible to observe how the spectra of samples are being flattened with treatment progression, reaching the osmotic solution spectrum. Moreover, the ionic losses of the spectra decrease drastically when increasing treatment times; this effect might be due to the fact that most of the native ionic compounds are lost during the osmotic

Fig. 4. Loss factor variation at relaxation frequency throughout the process time in treated samples.

treatment (Talens, Escriche, Martínez-Navarrete, & Chiralt, 2003). Fig. 4 shows the evolution of loss factor at relaxation frequency throughout the process time where it is possible to observe the decrease in the loss factor with treatment progression. In Fig. 5, the direct relation between the loss factor at relaxation frequency and the surface water activity is demonstrated. The loss factor at relaxation frequency is proportional to the water molecular motion, the most important dipole in biological systems (at this frequency, the induction effect needs a water molecule with rotation and motion capacity); furthermore the surface water activity represents the fugacity of water molecules in the area measured by the electric field. Thus, both variables must be related because both represent the capacity motion of water molecules. So the loss factor at relaxation frequency could be a good variable to follow the water lost through a dehydration process. The osmotic dehydration operation can be well described by using a thermodynamic analysis of the Gibbs Free Energy. In order to understand and to estimate the water transport through the interface (surface in contact with the osmotic solution), it is necessary to determine the free energy variation across the interface per mol of water. Gibbs free energy variation could be estimated in a biological tissue as can be appreciated in Eq. (3) (Demirel, 2002). dG = −SdT + VdP + Fdl + ψde + ∑ μ i dni

ð3Þ

i

35

ε''

Treatment time

30

40

25

ε'' (fR)

y = 146,68x-111,68 R² = 0,9725

35

20 30 15 25 10 Osmotic solution

5 0 0.1

1

10

20

100

15

f (GHz) Fig. 3. Dielectric loss factor spectra from fresh (―) and treated samples at different times of osmotic dehydration: a) 180 min ( ), 250 min (....), 320 min (− − −), 400 min ( ), 720 min ( ), 1440 min ( ), osmotic solution at 0 min ( ), osmotic solution at 1440 min ( ).

aws 10 0,84

0,86

0,88

0,9

0,92

0,94

0,96

0,98

1

Fig. 5. Loss factor variation at relaxation frequency with regard to surface water activity through the treatment.

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0,8

-ln (ε''OS/ε''e)

0,1

y = 4,8112x + 0,1987 R² = 0,9568

ΔMw

0,7 0

0,6 -0,1

0,5 0,4

y = 0,7319x-0,6009 R² = 0,933

-0,2

0,3 -0,3

0,2 0,1 0

-0,4

ln (awOS/awe) 0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

    00 ε OS aOS Fig. 6. Relationship between − ln 00 e and ln we throughout the treatment. ε aw

-0,5

-ln (ε''OS/ε''e) -0,6

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8



 00 ε OS Fig. 8. Relation of − ln 00 e with the overall mass variation throughout the treatment. ε

Following the model published by Castro-Giráldez, Fito, and Fito (2011) for fruit osmotic dehydration in sucrose solution, the chemical potential, engine of the water transport, can be defined as:

ext

Δμw = νw ΔP + Fw Δl + RT

OS a J ln se s as Jw

+

OS a RTLn we aw

ð4Þ

Loss factor at relaxation frequency has been related with the surface water activity in Fig. 5; then the water activity term of Eq. (4) can be obtained by using loss factor at relaxation frequency as follows: 00

Where νw represent the molar partial volume, Fw the mechanical storage force expressed by mol, l represents the elongation, as represents the sucrose activity, J represents the molar fluxes for sucrose and water; superscript e represents the external liquid phase (apoplastic way) close to the interface, and superscript OS represents the sucrose solution. It was demonstrated that the mechanical terms of Eq. (6) are mainly affected by the volume variation (Castro-Giráldez, Tylewicz, Fito, Dalla Rosa, & Fito, 2011). Moreover, in this process, the high positive mechanical terms induce outflows of internal liquid, increasing the water loss and reducing the sucrose gain. Thus, the main cause of samples weight and volume lost is the mass water variation.

0

ΔMW

-0,1

y = 0,9153x-0,7907 R² = 0,9481

-0,2

− ln

ε OS =k· 00 εe

ln

aOS w aew

! ð5Þ

This new expression can be used to control the changes induced by the water transport. In Fig. 6, the relationship between both terms of Eq. (5) is observed, showing a good correlation. Therefore, the water loss, which is the driving force in the structural changes, can be related with the new term defined in Eq. (5), obtaining a control function (see Eq. 6). This relation is shown in Fig. 7. 00

ΔMW = f

OS

ε ln 00 e ε

! ð6Þ

As was mentioned above, the amount of overall mass variation is caused mainly by water loss because the high mechanical forces reduce the sucrose transport. Fig. 8 shows the relation between the overall mass variation and the new dielectric term of Eq. (5), showing the possibility to control the mass variation of samples through the

ΔMW

0

-0,1 -0,3 -0,2 -0,4

y = 1,3587x-1,2446 R² = 0,955

-0,3 -0,4

-0,5

-0,5 -0,6

-ln (ε''OS/ε''e)

-0,6

0,7

-0,7

V/V0

-0,7 0

0,1

0,2

0,3

0,4

0,5

0,6

0,8

  00 ε OS Fig. 7. Relation of − ln 00 e with the water mass variation throughout the treatment. ε

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

Fig. 9. Relation of water loss with samples deformation.

0,9

1

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0,8

627

has been defined based in the water chemical potential in order to obtain relations between the dielectric measurements and the structural changes. A non-destructive method to analyze the physical, chemical and structural changes of kiwifruit through the osmotic treatment has been developed.

-ln (ε''OS/ε''e) 0,7 0,6 0,5

References 0,4 0,3

y = 1,422x-0,4526 R² = 0,9244

0,2 0,1

V/V0 0 0

0,2

0,4

0,6

0,8

1



 00 ε OS Fig. 10. Relation of − ln 00 e with samples deformation throughout the treatment. ε

osmotic treatment by using the dielectric properties in a rapid and non-destructive way. In an osmotic dehydration treatment, the volume variation of kiwifruit is induced by the water loss (Castro-Giráldez et al., 2011). Fig. 9 shows the direct relation between the deformation and the water loss. As was explained before, the new dielectric term of Eq. (5) can be used to control the changes induced by the water transport; thus, the deformation of kiwifruit through the osmotic treatment can also be controlled by measuring the dielectric properties as can be observed in Fig. 10. 4. Conclusions The direct effect of the surface water activity with the loss factor at relaxation frequency has been demonstrated. A new dielectric relation

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