analysis of chemical and structural changes in kiwifruit (actinidia deliciosa cv

8/10/2019 Analysis of Chemical and Structural Changes in Kiwifruit (Actinidia Deliciosa Cv

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Analysis of chemical and structural changes in kiwifruit (Actinidia deliciosacvHayward) through the osmotic dehydration

M. Castro-Girldez a, U. Tylewicz b, P.J. Fito a,, M. Dalla Rosa b, P. Fito a

a Instituto Universitario de Ingeniera de Alimentos para el Desarrollo, Universidad Politcnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spainb 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 25 August 2010

Received in revised form 22 March 2011

Accepted 26 March 2011

Available online 31 March 2011

Keywords:

Osmotic dehydration

Thermodynamics

Mass transfer

Enthalpyentropy compensation

Kiwifruit

a b s t r a c t

Osmotic dehydration experiments of kiwifruit (Actinidiadeliciosacv Hayward) were carried out in order to

apply a nonlinear irreversible thermodynamic model. Samples were immersed into 65% (w/w) sucrose

aqueous solution at 30 C during5, 10, 15, 20, 30, 45, 60, 90, 120, 180, 250, 320, 400, 720, 1440 min. Some

physicalchemical parameters were measured in fresh, treated and reposed (24 h at 30 C) samples. It

was possible to apply the enthalpyentropy compensation coupled to a nonlinear thermodynamic model,

obtaining the apparent bulk modulus and explaining the elastic answer of the tissue throughout the

osmotic process. The osmotic dehydration also produces losses in the native compounds of kiwifruit such

as citric acid and Calcium and Potassium.

2011 Elsevier Ltd. All rights reserved.

1. Introduction

Osmotic treatment is a dehydration technique usually used in

fruits and vegetables, which produces a reduction in food water

activity and therefore allows storing the foods for longer periods

improving the stability and quality of products. Osmotic

dehydration consists on the introduction of foods in a low water

activity solution in order to induce water outflows and inflows of

external solutes (Ferrando and Spiess, 2001). This treatment

depresses the food water activity and improves the biochemical

and microbiological stability. The structure of plant tissue can be

considered as one of the main factors for understanding the

osmotic dehydration process (Mavroudis et al., 1998). Osmotic

dehydration in a biological system involves complex mass transfer

phenomena simultaneously (chemical and also mechanical) pro-

ducing physical and structural changes. These changes weredescribed for isolated cells by Segu et al. (2010) and thermody-

namically quantified in apple tissue by Castro-Girldez et al.

(2011).

Kiwifruit is a biological system constituted by three distinct

tissue types, namely outer pericarp, inner pericarp and core. The

core is composed by spherical/ellipsoidal cells (0.1 and 0.2

mm diameter), whereas the outer pericarp is composed by large

cells (0.50.8 mm diameter) dispersed in a matrix of smaller cells

(0.10.2 mm diameter) (Hallett et al., 1992). The inner pericarp

consists on the locules, which are enclosed within locule walls

(Hallett et al., 1992). Each locule is composed by large radially

elongated thin-walled cells (0.20.4 mm >1 mm) and seeds,

while the locule wall is a narrow region composed by smaller

thicker walled cells (Hallett et al., 1992). Each of these tissues

has different chemical composition (Ferguson, 1980; MacRae

et al., 1989a), texture properties (MacRae et al., 1989b; Jackson

and Harker, 1997), cell wall composition (Redgwell et al., 1991,

1992) and cell characteristics (Hallett et al., 1992).

Gerschenson et al. (2001)reported that kiwifruit behaves as an

elastic solid with storage moduli dominating the viscoelastic re-

sponse when it is subjected to an osmotic dehydration process.

In this complex scenario, very little work has been carried out to

analyze the effect of osmotic dehydration on structure and compo-

sition of kiwifruit, and to understand the mechanisms that impulse

the mass transfer through this operation.On the other hand, thermodynamics is one of the approaches

used to understand the properties of foods and to calculate energy

requirements associated with transfer of heat and mass in biolog-

ical systems (Fasina, 2006). The enthalpyentropy compensation

theory has been reported in many processes (Ziga et al., 2008;

Liu and Guo, 2001). This theory pointed out that variations ofdH

are compensated by variations in dS. Thus, representing both ener-

gies, a linear relation is obtained (Eq. (1)).

dS adHb 1

wherea and b values are constants; a is inversely proportional to

the temperature (Ziga et al., 2008).

0260-8774/$ - see front matter 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.jfoodeng.2011.03.029

Corresponding author. Tel.: +34 96 387 9832; fax: +34 96 387 7369.

E-mail address: [email protected](P.J. Fito).

Journal of Food Engineering 105 (2011) 599608

Contents lists available at ScienceDirect

Journal of Food Engineering

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j f o o d e n g
http://dx.doi.org/10.1016/j.jfoodeng.2011.03.029mailto:[email protected]://dx.doi.org/10.1016/j.jfoodeng.2011.03.029http://www.sciencedirect.com/science/journal/02608774http://www.elsevier.com/locate/jfoodenghttp://www.elsevier.com/locate/jfoodenghttp://www.sciencedirect.com/science/journal/02608774http://dx.doi.org/10.1016/j.jfoodeng.2011.03.029mailto:[email protected]://dx.doi.org/10.1016/j.jfoodeng.2011.03.029


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The objectives of this work are to quantify the effect of the os-

motic dehydration treatment in the kiwifruit structure for short

treatment times in a system with a fast mechanical response,

and to develop a methodology for calculating the phenomenologi-

cal coefficient by applying the compensation theory.

2. Material and methods

Sucrose solution (65% w/w, 30 C), prepared with commercial

sugar and distilled water, was used as osmotic agent. Before the

experiment, kiwifruits (Actinidiadeliciosa 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 cali-meter in half slices (1 cm thickness) and the core was eliminated.

There were prepared 47 samples which were immersed in a vessel

containing the osmotic solution with continuous stirring. The rela-

tion between the fruit and the solution was of 1:20 (w/w) to avoid

changes in the solution during the process. The system was main-

tained at 30 C in a constant-temperature chamber. To prevent

evaporation the vessel was covered with a sheet of plastic wrap.

Preliminary kinetic studies were done at the same working con-

ditions in order to select the osmotic dehydration times. These re-

sults show that, in the first minutes of treatment, some structural

changes occur affecting the rest of treatment. Therefore, the se-

lected times for the present study were: 5, 10, 15, 20, 30, 45, 60,

90, 120, 180, 250, 320, 400, 720, 1440 min. Six samples were used

at each dehydration time.A flow-diagram of the experimental procedure is presented in

Fig. 1.

Three samples per osmotic treatment time were reposed at

30 C for 24 h, on Decagon containers, closed with parafilm, in or-

der to eliminate the concentration profiles in samples.

Mass, volume and surface water activity were analyzed for each

fresh, treated and reposed samples. Representative fresh samples

were used to determine the initial moisture, sugar content (Brix),

citric acid content, cation content and Cryo-SEM. Moisture and su-

gar content (Brix) were measured for each reposed sample. The

other three samples per time were used for the cation analysis, cit-

ric acid content and Cryo-SEM.

At each osmotic time, an aliquot of sucrose solution was also ta-

ken from the vessel. Water activity and Brix of the solution weremeasured at each time.

Mass was determined by using a Mettler Toledo Balance

(0.0001) (Mettler-Toledo, Inc., USA).

Volume was determined by analyzing the front and the section

pictures of samples using millimetres paper in the background by

using an imaging analysis with the software Adobe Photoshop

(Adobe Systems Inc., San Jose, CA, USA) in order to get the diameter

and the thickness of the samples.

Surface water activity was measured in the structured samples

with a dew point hygrometer Aqualab series 3 TE (Decagon De-

vices, 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). Su-

gar content was determined in a refractometer (ABBE, ATAGOModel 3-T, Japan). The titrable acidity (referred to citric acid)

was analyzed by usingAOAC Method 934.06 (2000).

Analytical determinations described above were obtained by

triplicate.

Cation quantification was carried out by means of an ion chro-

matograph (Methrom Ion Analysis, Herisau, Switzerland), using an

universal standard column (Metrosep C2150, 4.0 150 mm)

along with an eluent composed of tartaric acid (4.0 mmol/L) and

dipicolinic acid (0.75 mmol/L), equipped with electronic detectors.

In every case, the fruit samples were previously homogenized at

9000 rpm in an ULTRATURRAX T25 for 5 min and centrifuged (J.P.

Selecta S.A., Medifriger-BL, Barcelona, Spain) at 4000 rpm for

20 min. Afterwards, 1 mL of supernatant was diluted with

Milli

-Q water in a 50 mL Erlenmeyer flask. The clarified extractwas filtered through a 0.45 lm Millipore filter; 15 mL was used

to analyze the cation content. Measurements were taken in

duplicate.

2.1. Low temperature scanning electron microscopy (cryo-SEM)

A Cryostage CT-1500C unit (Oxford Instruments, Witney, UK),

coupled to a Jeol JSM-5410 scanning electron microscope (Jeol,

Tokyo, Japan), was used. The sample was immersed in slush N 2(210 C) and then quickly transferred to the Cryostage at 1 kPa,

where sample fracture took place. The sublimation (etching) was

carried out at 95 C; the final point was determined by direct

observation in the microscope, working at 5 kV. Then, once againin the Cryostage unit, the sample was coated with gold in vacuum

Nomenclature

aj activity of the chemical speciej ()G Gibbs free energy (J)Gj partial Gibbs molar free energy of the speciej (J/mol)Jj molar flux of the speciej (mol/m

2s)Lj phenomenological coefficient of the specie j (mol

2/

J s m2)Mr molecular weight (g/mol)M mass (g)n number of moles (mol)P absolute pressure (atm)R ideal gases universal constant (J/mol K)A surface area (m2)S entropy (J/mol K)H enthalpy (J/mol)T temperature (K)t time (min)V volume (m3)x mass or molar fraction ()z mass or molar fraction of the liquid fraction ()

F force (N)l displaced distance (m)F molecular force (N/mol)

Greek alphabetlj chemical potential of the speciej (J/mol)l0j chemical potential of reference of the speciej (J/mol)m specific volume (L/mol)D variation of a variableg apparent bulk modulus

Subscript and superscript0 initial time24 time 24 h after the treatmentLP liquid phasemax maximums sucrosew watert time (min)

600 M. Castro-Girldez et al. / Journal of Food Engineering 105 (2011) 599608


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(0.2 kPa), applied for 3 min, with an ionizationcurrent of 2 mA. The

observation in the scanning electron microscope was carried out at

15 kV, at a workingdistance of 15 mmanda temperature6130 C.

3. Results and discussion

The osmotic dehydration operation with 65Brix solution pro-

duces compression and relaxation phenomena (Fito and Chiralt,1997). These mechanisms could be observed by analyzing the mass

and volume variations. Fig. 2 shows the overall mass variation,

water loss and sucrose gain through the osmotic treatment. In

the figure, the decrease in total mass is explained by the high water

losses while the sucrose mass increases during the osmotic treat-

ment. InFig. 3, the volume variation is observed; it is possible to

appreciate the overall decrease in volume through the osmotic

treatment. The half slice of kiwifruit can be divided in two zones

(see Fig. 4c), zone A represents the external zone composed by

homogeneous reservoir cells explained in the introduction; and

zone B represents the internal part of the tissue, where the reser-voir and vascular cells are combined in radial way. These two zones

show different directions in the mechanical tension, during the

contraction and expansion.

Fig. 1. Diagram of the experimental procedure.Cryo-SEM was made in fresh and treated samples until 60 min of treatment.

Fig. 2. Evolution of overall mass (), water mass (j) and sucrose mass (N) variation through the osmotic treatment.

M. Castro-Girldez et al. / Journal of Food Engineering 105 (2011) 599608 601


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Kiwifruit behaves as a viscoelastic solid and it is possible toappreciate, at the beginning of the treatment, the important

expansions and contractions of the tissue (Fig. 4a and b). Fig. 5a

shows a Cryo-SEM micrograph of fresh kiwi where the intracellular

and extracellular spaces and membrane/wall system are identified,

showing the extracellular spaces filled of air. In the first 5 min of

treatment, the fruit suffers an important lost of turgid, and a 25%

of contraction can be appreciated in the overall volume; Fig. 5b

shows the tissue with the extracellular space filled with liquid.

Afterwards, until 15 min of treatment, the fruit suffers expansions,

recovering 93% of the initial volume, recovering the zone A; Fig. 5c

shows the membrane/wall system intact, thus the expansion must

be a response of the tissue to the first fast contraction. At 20 min of

treatment, the fruit again is contracted strongly (32% of the initial

volume), disappearing again the zone A and contracting the zone B.

After this time, the expansion/contraction phenomena are less

marked and, at long osmotic times, the fruit is completely retracted

(Fig. 4a) and zone B is absolutely contracted. It is important to

highlight that although the fruit suffers important expansions at

the beginning of the treatment, it is dehydrating and losing overall

weight (Fig. 2).

Second expansion occurs at 30 min, as shows Fig. 4b, recovering

no more than 5% of volume;Fig. 6a and b, show Cryo-SEM pictures

of samples treated 30 min and figures c and d show Cryo-SEM pic-

tures of samples treated 45 min. All pictures show an initial plas-

molysis so incipient. Therefore the second expansion must be

also an elastic response to the second fast contraction.

The water and solute fluxes promoted in the osmotic treatment

can be estimate with the mass variation, the process time and the

variation of the samples surface (Eqs. (2)and (3)):

Jw DMwMoDtAMrw

2

Js DMsMoDtAMrs

3

whereJw is the water flux (molw/s m2),Js is the sucrose flux (mols/

s m2) during the treatment,tis the process time, A is the measured

Fig. 3. Evolution of the volume variation through the osmotic treatment.

Fig. 4. Kiwifruit deformation through the osmotic dehydration; (a) pictures of fresh and treated kiwifruit at different treatment times; (b) volume variation curve at shorttreatment times and (c) detail of the zones of the kiwi tissue with the sense of the deformations.



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surface area of the sample (m2), Mrw is the molecular weight of

water (18 g/mol) and Mrs is the molecular weight of sucrose

(342 g/mol). It is possible to observe in Fig. 7 that both fluxes are re-

duced with treatment time promoted with the decreasing in driving

forces.

Fig. 8 shows a scheme of surface water activity measured in the

fresh, treated and reposed samples; these measurements represent

the water activity of the surface of each sample. After the repose

time, the moisture was analyzed in whole sample. The moisture

of treated samples was estimated by balances. Fig. 8 shows also

the relationship between the moisture in dry basis and the surface

water activity of the treated, reposed samples and data from pure

solutions of water and sucrose (Starzak and Mathlouthi, 2006). In

this figure it is possible to observe that the reposed samples main-

tain the same relationship surface water activity/moisture as the

pure sucrose solution at different concentrations. Therefore, the

water and sucrose concentrations are homogeneous in the reposed

samples because moisture is an average value of the whole sample

and the water activity is just the surface value. Thus, the concen-

tration profiles through the tissue are negligible. The surface water

Fig. 5. Cryo-SEM pictures at different treatment times; (a) fresh kiwi at 350; (b) treated sample at 5 min, at 1000and (c) treated sample at 15 min, at 200.

Fig. 6. Cryo-SEM pictures at different treatment times; (a) treated sample at 30 min, at 1000; (b) treated sample at 30 min, at 500; (c) treated sample at 45 min, at 500and (d) treated sample at 45 min, at 500.



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activity of treated samples is lower than that of the reposed sam-

ples; it denotes concentration profiles in the kiwifruit tissue and

an important internal transport is driven; only at the end of the

treatment, the treated and the reposed values concur with the pure

solution values; this fact denotes the disappearance of concentra-

tion profiles in the tissue.

Fig. 9 shows the water flux and the volume variation during the

24 h of repose time. In the figure, two steps can be observed: the

first one, composed by the samples treated less than 180 min,

shows fluctuant fluxes promoted by the shrinkage and swelling

of samples during the repose time (mechanical behaviours); the

second step, composed by the samples with treatment times high-

er than 180 min, shows a continuous decrease of fluxes without

volume variation, suggesting that the engine of the transport dur-

ing the repose time must be the gravity.

In order to determine the chemical and mechanical changes in

the kiwifruit tissue through the osmotic treatment, it is possible

to apply a thermodynamic analysis in terms of the Gibbs free

Fig. 7. Variation of water (d) and sucrose (s) fluxes versus time through the treatment.

Fig. 8. Relationship between the moisture in dry basis and the water activity for samples dehydrated (d), reposed 24 h (s), and sucrose solutions () obtained from Starzakand Mathlouthi (2006).



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energy, satisfying the enthalpyentropy compensation. For this

purpose, an interface was defined as the fruit surface in contact

with the osmotic solution (Fig. 10).

Using the relations of Maxwell (Demirel, 2002) (Eqs. (4) and

(5)), Eq.(6)is obtained for nonelectrolytic and isothermal system.

dG dHTdSSdT dUPdV VdPTdSSdT 4

dG VdPSdTFdl udeX

ldni 5

PdV TdS FdlX

ldni 6

Obtaining the entropy variation as follows:

dS1

T PdV Fdl

Xldni

7

Castro-Girldez et al. (2011)developed a non-equilibrium ther-

modynamic model for describing the long time osmotic dehydra-

tion through the apoplastic ways in apple tissue. The authors

defined the water chemical potential for a non ionic added solute

and an isotherm system, as follows:

dG

dnw

dlextw mwdPFwdlRTln

aOSsaes

JsJw

RTLnaOSwaew

8

where asis obtained from the bibliography (Lide, 2004), superscript

erepresents the external liquid phase (apoplastic way) close to the

interface, and superscriptOSrepresents the sucrose solution.

Therefore, it is possible to define the entropy variation by water

mol using Eqs.(7)and (8) as follows:

T dS

dnw RTln

aOSsaes

Js

JwRTln

aOSwaew

FwdlpwdV 9

Water and sucrose transport with the deformation tissue must

satisfy the entropyenthalpy compensation, therefore the relation-

ship between the entropy and the enthalpy must follow the Eq. (1).

In order to estimate those parameters, it is necessary to develop an

iteration with the free energy, entropy and enthalpy as it is

explained inFig. 11.

In Fig. 11, the phenomenological coefficient (Lw) is the supposed

variable to iterate and the compensation equation works to esti-

mate the error of the iteration. In the estimation of entropy and en-

thalpy, the term of expansions (pwdV) is present in both

developments; thus, in the compensation equation the term is can-

celled, being the entropy and the enthalpy without this term with

asterisk superscripts (dS

and dH

, respectively). On the other hand,the mechanical terms are packed in pressure term (vwdP) and the

elongation term (Fwdl), but the elongation term is important only

at the beginning of the dehydration process, when the turgid loss

mechanisms occur.

After the iteration, the phenomenological coefficient obtained

was Lw= 2.46 105 mol2/ J s m2. In fruits, different values of

phenomenological coefficients of water transport through the

protoplast membrane have been published; Ferrando and Spiess

(2002, 2003) showed in osmotic dehydration treatments with

sucrose solution at 30 C, 1 106 mol2/ J s m2 for onions, 5.2

106 mol2/ J s m2 for carrots and 6.8 106 mol2/ J s m2 for pota-

toes. Segu et al. (2006) reported the value of 4.5 105 mol2/

J s m2 for apple (var Fuji). Phenomenological coefficient for apo-

plastic transport in osmotic dehydration treatment with sucrosesolution at 30 C was published by Castro-Girldez et al. (2011)

Fig. 9. Water flux and volume variation during the repose time.

Fig. 10. Kiwifruit tissue scheme with the simplified phases used in the thermody-namic model (adapted fromCastro-Girldez et al., 2010).

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forappletissueobtainingthevalue: Lw= 1.095 105 mol2/ J s m2. In

animal tissue, phenomenological coefficient wascalculated for pork

meat salting process, Lw= 2.7 105 mol2/ J s m2 (Castro-Girldez

et al., 2010).

Fig. 12shows the relationship between the entropy and enthal-

py, resulted from the iteration, where the linear adjust of values

between 10 min to the end of treatment shows a good correlation

coefficient (R2 = 0.9941). It is possible to observe that, throughout

the first 5 min of treatment, the non consideration of the elonga-

tion term in the entropy estimation is not negligible. Using Eq.

(1) and the elongation in the entropy estimation (Eq.(9)), the elon-

gation term can be estimated by using next equation:

TdS

dnwRTln

aOSsaes

JsJw

RTlnaOSwaew

1 a Fwdl 10

Fig. 13 shows the elongation term through the dehydration pro-

cess, where it is possible to observe the high values of this term

during the lost of turgid pressure throughout the first minutes oftreatment.

Fig. 14 shows the volume variation as a function of pressure

variation throughout the interface estimated as was explained in

Fig. 11. The linear relation obtained allows calculating the apparent

bulk modulus following Eq.(11)(Bourne, 2002).

g DP

DVVi

11

The apparent bulk modulus obtained (3.9 MPa) reflects the elas-

ticity of wall-membrane system of kiwifruit. This value is similar to

the values of different elastic modulus obtained in other fruits:

1.84 MPa for Golden Delicious apples (De Baerdemaeker et al.,

1982), 39 MPa for different varieties of apple (Rosenfeld et al.,

1992), 5 MPa for an unripe green apples to 0.5 MPa for overripe

fruit (De Baerdemaeker et al., 1982; Duprat et al., 1997), 2.5 MPa

for pineapples (Chen and De Baerdemaeker, 1993), 1.9 MPa for

pears (Chen and De Baerdemaeker, 1993).

Osmotic solution only contains water and sucrose, then theactivity gradient of other native substances present in kiwifruit is

Fig. 11. Scheme of the iteration process developed in order to estimate the phenomenological coefficient.

Fig. 12. Relationship between the entropy and the enthalpy.



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Fig. 13. Evolution of the elongation term through the dehydration process.

Fig. 14. Volume variation as a function of pressure variation throughout the interface.

Fig. 15. Variation of citric acid during the treatment.



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so high and promotes high losses of this native substances. These

activity terms are coupled to contractions/expansions of the tissue

which provokes changes in kiwifruit composition; Figs. 15 and 16

show the changes in citric acid and in the most important kiwifruit

cations respectively during the osmodehydration treatment. It is

possible to appreciate how these components are reduced duringthe treatment.

4. Conclusions

It is possible to apply the enthalpyentropy compensation cou-

pled to a nonlinear thermodynamic model for determining the

water transport through the apoplastic ways in kiwifruit and to de-

scribe the behaviours involved in the osmotic dehydration. The

apparition of concentration profiles and the structural changes

was also determined. It is also possible to obtain the apparent bulk

modulus by the mechanical terms estimated in the thermody-

namic model, explaining the elastic answer of the tissue through-

out the osmotic process. The osmotic dehydration also produces

losses in the native compounds of kiwifruit such as citric acid

and Calcium and Potassium.

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Fig. 16. Variation of ions K+ and Ca+2 during the treatment.


analysis of chemical and structural changes in kiwifruit (actinidia deliciosa cv

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