Journal of Food Engineering 104 (2011) 621–627

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Journal of Food Engineering

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Influence of ohmic heating and vacuum impregnation on the osmoticdehydration kinetics and microstructure of pears (cv. Packham’s Triumph)

J. Moreno a,⇑, R. Simpson b, M. Sayas a, I. Segura a, O. Aldana a, S. Almonacid b

a Department of Food Engineering, Universidad del Bio-Bio, Casilla 447, Chillán, Chileb Department of Chemical Engineering and Environment, Universidad Técnica Federico Santa María, Casilla 110-V, Valparaíso, Chile

a r t i c l e i n f o

Article history:Received 16 November 2010Received in revised form 21 January 2011Accepted 25 January 2011Available online 1 February 2011

Keywords:Osmotic dehydrationOhmic heatingVacuum impregnationMicrostructurePear

0260-8774/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.jfoodeng.2011.01.029

⇑ Corresponding author. Tel.: +56 42 253173; fax: +E-mail address: jomoreno@ubiobio.cl (J. Moreno).

a b s t r a c t

The effect of ohmic heating (OH) and vacuum impregnation (VI) on the osmotic dehydration kinetics andmicrostructure of pears was studied. Three different dehydration levels (30, 40 and 50 �C) were used, byapplying VI or not (OD) and OH (100 V). Dehydrated samples showed that the application of OH duringthe osmotic treatments had significant effects on the kinetics of water loss and sugar gain as well as onthe microstructure of samples. The greatest water loss was observed with the OD–OH. The largestamount of solute gain and the smallest firmness loss were obtained in the VI–OH. In some treatments,the process time was reduced by as much as 40%. The SEM observations showed that cell deformationand cell rupture were significant in the OD–OH than on the VI–OH samples. The increases in the perme-ability of cell by OH explain the acceleration of mass transference and process time reduction.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Osmotic dehydration (OD) at mild temperatures, which is con-sidered minimal processing, preserves fresh-like characteristics offruits and can be used to obtain various pear products or ingredi-ents for many food products. OD preserves attributes such as color,texture and aroma, and it reduces water activity, giving high-moisture products extended shelf life.

The use of vacuum impregnation (VI) allows an increase in therate of water-related weight loss and solid gain, and it introducescontrolled quantities of a solution into the porous structure offruits and vegetables (Barat et al., 2002; Moreno et al., 2004; Dengand Zhao, 2008). Mass transfer during osmotic dehydration occursthrough the semipermeable cell membranes. The dominant resis-tance in the mass transfer in the biological materials changes frompartial to total permeability, leading to significant changes in tissuearchitecture (Aguilera and Lillford, 2000; Rastogi et al., 2000;Quiles et al., 2004).

Two resistances oppose mass transfer during the osmotic dehy-dration of fruits: internal and external. The fluid dynamics of thesolid–fluid interface governs the external resistance whereas theinternal, and much more complex, resistance is influenced by celltissue structure, cellular membrane permeability, deformation ofvegetable/fruit pieces and the interaction between the differentmass fluxes (Fito, 1994; Fito and Chiralt, 1997).

ll rights reserved.

56 42 253066.

Ohmic heating (OH) is a thermal process in which heat is inter-nally generated by the passage of an alternating electrical current(AC) through a body, such as a food system that serves as an elec-trical resistance. A food product, thanks to its numerous ionic com-pounds, is a conductor of electricity. In the ohmic heatingprocesses, the food components become the parts of the electriccircuit through which the alternating current flows, generatingheat in the foods based on their intrinsic properties of electricalresistance (Salengke and Sastry, 2007; Sarang et al., 2008; Zellet al., 2009).

The interest in OH technology is due the fact that products areof a superior quality to those processed by conventional technolo-gies (Castro et al., 2003). The main advantages claimed for thistechnology are uniformity of heating and improvements in qualitywith minimal structural, nutritional or organoleptic changes. Pos-sible applications include most of the heat treatments such asblanching, evaporation dehydration, and fermentation as well aspasteurization and sterilization.

For a food item consisting of a liquid–particulate mixture, heatcan be generated using OH at rates that are the same or compara-ble in both the liquid and particulate phases if the electrical con-ductivities of the two phases are equal. OH thus provides atechnology for processing particulate foods at the rate of an hightemperature short time (HTST) process but without the limitationsof heat transfer to particulates found in conventional HTST pro-cesses (Sarang et al., 2008). Temperature can increase rapidly inOH, making the technology efficient HTST treatment that preserveproduct quality. Homogeneity of the process depends on the

622 J. Moreno et al. / Journal of Food Engineering 104 (2011) 621–627

homogeneity of electrical conductivity in the chamber, electrodegeometry and the resistance time of solid–liquid mixture in theheather (Eliot-Godéreaux et al., 2001).

Wang and Sastry (1997) evaluated the effects of an ohmic pre-treatment to vegetables and found no significant changes in themoisture content of the final product. Eliot et al. (1999) studiedthe influence of precooking by OH on the firmness of cauliflowerand concluded that OH combined with low-temperature precoo-king in saline solutions offers viable solution to HTST sterilizationof cauliflower florets. In a study by Castro et al. (2003), the suitabil-ity of several strawberry-based products to be processed by OHwas tested.

The phenomenon of cell membrane electropermeabilization hasbeen known for several decades and has recently received increas-ing attention because of its applicability to the manipulation ofcells and tissues (Weaver and Chizmadzhev, 1996). Strictly speak-ing, our knowledge is phenomenological, as it is based on measure-ments of electrical currents through planar bilayer membranes(BLM) under the influence of strong electric fields and on themolecular transportation of molecules into (or out of) cells sub-jected to electric field pulses (Kulshrestha and Sastry, 2006).

In a previous study osmotically dehydrated raspberries (Rubusidaeus) treated with ohmic heating at a variable voltage (to main-tain a temperature between 40 and 50 �C) and an electric fieldintensity <100 V/cm showed that processing time was significantlyreduced by the ohmic heating. In some cases, this reduction in timeeven reached 50% (Simpson et al., 2007). Vacuum impregnation(VI) and ohmic heating (OH) pre-treatment with and without addi-tion of citric acid were used to enhance the mass transfer kineticsduring osmotic dehydration (OD) of apple cubes. The water lossand sugar gain during osmotic dehydration were significantly in-creased when the apple tissue was pre-treated with addition of cit-ric acid. The pre-treatments led profound changes in apple fruitstructure, evaluated measurements of fruit firmness and electricalconductivity (Allali et al., 2009).

The aim of this work was to evaluate the effect of ohmic heatingand vacuum impregnation on the osmotic dehydration kinetics andmicrostructure of pears (cv. Packham’s Triumph).

2. Materials and methods

2.1. Sample preparation

Fresh pears (cv. Packham’s Triumph) from Chillan (Chile) wereobtained from commercial sources and stored in a refrigerator at4 �C. The fruits were selected according to their appearance (ripe-ness, size and color). The pears were peeled and cut into 1-cm3

cubes. The samples were dipped in a 1.5% ascorbic acid and 3%citric acid solution for 3 min to prevent enzymatic browning. A su-crose solution of 65 �Brix was used as an osmotic solution. Thesolution contained 2 ppm of potassium sorbate (C6H7KO2) as a pre-servative and calcium chloride (CaCl2) 1.13 [g/L] to retain firmnessand increase conductivity.

2.2. Osmotic treatments

The osmotic dehydration was carried out at atmospheric pres-sure (OD), and the vacuum impregnation (VI) was conducted withconventional and ohmic heating (OH) at 30, 40 and 50 �C. The pro-cessing time was 300 min in the osmotic treatments (OD, VI, OD–OH and VI–OH). A vacuum pulse was applied for 5 min at 50 mb atthe beginning of the process. After this step, the atmospheric pres-sure was restored to complete the process time (Moreno et al.,2004). For the OH treatments, the samples were immersed in aconcentric cylinder tank (3.7 and 19 cm in diameter) made of

stainless steel with a plastic bottom connected to the generatorby two electrodes. The osmotic solution was subject to an alternat-ing current at 50/60 Hz and 100 V, generating an electrical fieldintensity of 13 V/cm, for 5 h. The temperature was measured withCPSS-116-24-PFA Teflon-coated thermocouples. A data-loggerOM-320 (Omega Engineering, Stamford, USA) was employedto simultaneously measure the temperature, voltage and current.To standardize the temperature, orbital shaking at 100 rpm(Barnstead/Lab-Line MaxQ 2000, Iowa, USA) was employed.

2.3. Analysis of physicochemical properties

The water content (Xw) and soluble solids (Xss) were measuredin the fresh and treated samples to determine the compositionalchanges promoted by osmotic dehydration. The moisture contentwas determined by the technique defined by the Association ofOfficial Analytical Chemists (AOAC, 2000). The soluble solids weredetermined by a digital refractometer (Leyca Mark II, Buffalo, NY,USA). All measurements were made in triplicate, and the averagevalues were reported.

Mass transfer parameters and changes in the water and solublesolids (DMw

t and DMsst, respectively) were calculated using Eqs.(1) and (2); Mot and Mo0 represent the sample weight at time tand 0, respectively, and Xw

t, Xsst, Xw

0 and Xss0 are the water (w)

and soluble solid (ss) mass fractions in a sample at time t and 0,respectively.

DMwt ¼

Mot � X

wt �Mo

0 � Xw0

Mo0

� �ð1Þ

DMsst ¼

Mot � X

sst �Mo

0 � Xss0

Mo0

� �ð2Þ

The color of the pears was evaluated using a Minolta ChromaMeter CR-200 (Minolta Corp., Osaka, Japan). The instrument wascalibrated with a standard white plate (L = 97.59, a = �0.07,b = 1.59). The CIE Lab coordinates used D65 illuminant, with a 2�observer as reference system. A glass Petri dish containing a sam-ple was placed above a white plate, and the L, a and b values wererecorded. The measurements were made in triplicate and in threedifferent places on each sample, and the mean values were re-ported. The hue angle (h�abÞ and chroma ðC�abÞ were also calculatedfor Eqs. (3) and (4), respectively. The firmness of the sampleswas determined using a Texture Analyser TA-XT2 (Stable Micro-systems, Haslemere, UK) that had a Kramer shear cell with fiveblades. The press crosshead speed was set at 2 mm/s. Rectangularpieces of 4 � 3 � 0.3 cm were measured in each mechanical test.The mechanical parameter considered was the maximum peakforce (Fmax), reported as N. Ten replicates were performed for eachtreatment.

H�ab ¼ arctgba

ð3Þ

C�ab ¼ ða2 þ b2Þ12 ð4Þ

2.4. Structural analyses

Structural analyses were carried out using a scanning electronmicroscope (SEM) and a transmission electron microscope (TEM),that is, a Jeol JSM–6380LV and a Jeol JEM 12000EX-II, respectively,both of OXFORD Instruments, UK. The fresh and treated sampleswere analyzed from the surface to the center. Sample fixationwas done by immersion in glutaraldehyde (2–4%) for 4–24 h in a0.1-M phosphate buffer (pH 7.2–7.4) at 4 �C. A second fixationwas made with a 1% osmium tetraoxide solution for 1–2 h in a

J. Moreno et al. / Journal of Food Engineering 104 (2011) 621–627 623

0.1-M phosphate buffer at 4 �C. The samples were dehydrated byimmersion in ethanol solutions (30%, 50%, 70%, 85% and 95%) for15 min each. After that, the samples were immersed in pure etha-nol for 20 min. The samples were then critical point dried in adryer (BALZERS-UNION, Balzers, Liechtenstein), covered with athin silver layer, vacuum evaporated and covered with a gold coat.The samples were examined at an acceleration voltage of 20 kV.

2.5. Statistical analysis

A statistical analysis of the data was performed through ananalysis of variance (ANOVA) using Statgraphics Plus 5.0 software(Statgraphics, 2000). The effects of the osmotic dehydration treat-ments (OD, VI, OD–OH and VI–OH) and temperature (30, 40 and

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 30 60 90 120

Tim

ΔMw

tΔM

sst

Fig. 1. Water content (DMwt) and soluble solid (DMss

t) changes of osmotic treatments w(OD), vacuum impregnation (VI) at 50 mb for 5 min, ohmic heating (OD–OH) at 100 V a

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 30 60 90 120

Tim

ΔMw

tΔM

sst

Fig. 2. Water content (DMwt) and soluble solid (DMss

t) changes of osmotic treatments w(OD), vacuum impregnation (VI) at 50 mb for 5 min, ohmic heating (OD–OH) at 100 V a

50 �C) on parameters such as moisture, soluble solids, color, andfirmness were analyzed as factorials in a completely randomizeddesign (factorial design 4 � 3). The differences among mean valueswere established by the least significant difference (LSD) at 5%.

3. Results and discussion

Variations in water content and soluble solids due to osmoticdehydration at atmospheric pressure (OD) with vacuum impregna-tion (VI) and ohmic heating (OD–OH and VI–OH) at temperaturesof 30, 40 and 50 �C were compared (Figs. 1–3). Fig. 1 shows thewater loss and sugar gain of processed pears cubes at 30 �C, itcan be observed that water loss was greater in processed sampleswhen VI and OD–OH treatments were applied. Likewise, the solid

150 180 210 240 270 300

e (min)

ΔMsst OD 30°C ΔMsst VI 30°C

ΔMsst OD-OH 30°C ΔMsst VI-OH 30°C

ΔMwt OD 30°C ΔMwt VI 30°C

ΔMwt OD-OH 30°C ΔMwt VI-OH 30°C

ith 65 �Brix osmotic solution for 5 h. Osmotic dehydration at atmospheric pressurend vacuum impregnation/ohmic heating (VI–OH) were carried out at 30 �C.

150 180 210 240 270 300

e (min)

ΔMsst OD 40°C ΔMsst VI 40°C

ΔMsst OD-OH 40°C ΔMsst VI-OH 40°C

ΔMwt OD 40°C ΔMwt VI 40°C

ΔMwt OD-OH 40°C ΔMwt VI-OH 40°C

ith 65 �Brix osmotic solution for 5 h. Osmotic dehydration at atmospheric pressurend vacuum impregnation/ohmic heating (VI–OH) were carried out at 40 �C.

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 30 60 90 120 150 180 210 240 270 300

Time (min)

ΔM

wt

ΔM

sst

ΔMsst OD 50°C ΔMsst VI 50°C

ΔMsst OD-OH 50°C ΔMsst VI-OH 50°C

ΔMwt OD 50°C ΔMwt VI 50°C

ΔMwt OD-OH 50°C ΔMwt VI-OH 50°C

Fig. 3. Water content (DMwt) and soluble solid (DMss

t) changes of osmotic treatments with 65 �Brix osmotic solution for 5 h. Osmotic dehydration at atmospheric pressure(OD), vacuum impregnation (VI) at 50 mb for 5 min, ohmic heating (OD–OH) at 100 V and vacuum impregnation/ohmic heating (VI–OH) were carried out at 50 �C.

Table 1Composition parameters reached by the fresh and processed samples (processingtime = 5 h).

Treatments Xw Xss

Fresh 0.874 ± 0.015a 0.098 ± 0.017a

OD 30 �C 0.485 ± 0.023b,c 0.482 ± 0.029c

VI 30 �C 0.479 ± 0.003b,c 0.454 ± 0.009b,c

OD–OH 30 �C 0.500 ± 0.005b 0.453 ± 0.001b,c

VI–OH 30 �C 0.482 ± 0.006b,c 0.469 ± 0.012b,c

OD 40 �C 0.489 ± 0.009b,c 0.465 ± 0.022b,c

VI 40 �C 0.488 ± 0.003b,c 0.448 ± 0.023b

OD–OH 40 �C 0.470 ± 0.019c 0.527 ± 0.012d

VI–OH 40 �C 0.430 ± 0.011d 0.482 ± 0.018c

OD 50 �C 0.419 ± 0.005d,e 0.533 ± 0.026d

VI 50 �C 0.405 ± 0.004e 0.579 ± 0.029e

OD–OH 50 �C 0.354 ± 0.005f 0.568 ± 0.001e

VI–OH 50 �C 0.377 ± 0.004f 0.562 ± 0.017e

When there are no significant differences at 5.0%, homogeneous groups in eachvariable, according to a LSD test, they are identified by the same superscript letter.

624 J. Moreno et al. / Journal of Food Engineering 104 (2011) 621–627

gain was greater in those treatments that VI, VI–OH and OD–OHwere applied. In this case the samples reached the equilibrium ofwater loss to 180 min and solid gain to 150 min of beginning theprocess (Fig. 1). Vacuum impregnation and ohmic heating havean effect on the moisture content and levels of solutes reached inthe samples and, subsequently, affect the water and solute massvariation. Fig. 2 shows water loss and solid gain values of samplestreated at 40 �C, in this case, the water loss was lower in OD treat-ment and few differences were observed between the other treat-ments (VI, OD–OH and VI–OH). The solid gain was greater insamples treated with VI–OH and OD treatments at 40 �C. Fig. 3shows the result of mass transfer kinetics at 50 �C, the water losswas greater with OD–OH treatment, and solid gain was greaterin treated samples with VI–OH treatment. In both temperatures(40 and 50 �C) the samples reached the equilibrium of water lossto 150 min and solid gain to 90 min of beginning the process (Figs.2 and 3). The most temperature dependent variable is DMw, thehigher temperature induce the higher water loss. Solid gain showsa slight increase with temperature in terms of net mass changes at50 �C (Lombard et al., 2008). When comparing the results from theexperiments, it was evident that the processing time was signifi-cantly reduced when ohmic heating was used. In some cases, thistime reduction even reached 40%, due to the coupled action of dif-ferent mass transport mechanisms to different extents (Morenoet al., 2004)). The increases in the temperature, vacuum and electr-opermeabilization effects promoted the gain of osmotic solutioninto the tissue pores, thus reaching equilibrium in the sample withless water loss (Simpson et al., 2007. The treatments at 30 �C sug-gest lower process driving forces and subsequently longer treat-ment times. This effect may result in a greater number of celllayers affected by the osmotic treatment at a determined overallconcentration of the sample, which is consistent with a flatterwater concentration profile (Fito et al., 2001; Lombard et al., 2008).

Table 1 shows the mass fraction of water (Xw) and the solublesolids (Xss) for the fresh and treated samples at the end of theprocess (5 h). The vacuum impregnation (VI) and ohmic heating(OD–OH and VI–OH) treatments at 50 �C had increased concentra-tion levels compared with the OD treated samples. This result isconsistent with the action of hydrodynamic mechanisms coupledwith diffusional osmotic phenomena, which accelerate mass

transfer. This decrease is due to the exchange of native liquid inthe internal content of the plant tissue with external solution sol-ids. This behavior can also be explained in terms of different struc-tural effects induced by vacuum impregnation (Martínez-Monzóet al., 2001). However, though an increase in Xss values wasobserved, they were not significant (p < 0.05) between treatmentsVI, VI–OH and OD–OH at 50 �C (Table 1). Previous studies showan increase of diffusion through foods using alternating electricfields, this making solid gain and water loss favorable (Kemp andFryer, 2007; Allali et al., 2009).

Table 2 shows the means and standard deviations of the colorCIE parameters L⁄, a⁄, b⁄, h�ab (Hue angle) and C�ab (Chroma) of thefresh and processed pears. Differences can be observed among L⁄

of VI treatment and others treatments, a decrease in lightness(L⁄) of 40% was observed in the vacuum (VI or VI–OH) samplescompared with the fresh samples but there is no significant differ-ence from each other. Whereas, the samples treated with OD andOD–OH showed an increase in L⁄ values of 5% Chrome was the col-or attributes that subject to the greater changes; VI and VI–OH (30and 40 �C) treatments provoke loss of color chrome, especially VItreatment at 50 �C. Nevertheless, OD (40 and 50 �C), OD–OH and

Table 2Firmness and color evaluation in the fresh samples and at the end of each treatment (processing time = 5 h).

Treatments Firmness** (N) Color***

L⁄ a⁄ b⁄ h⁄ab C⁄ab DE⁄

Fresh 975.3 ± 92.4c 76.4 ± 2.6c �3.9 ± 0.9b 14.4 ± 0.9b 105.2 ± 2.4a,b 15.6 ± 0.8b,c 0OD 30 �C 872.0 ± 70.2a,b 76.2 ± 2.2c �3.2 ± 0.4a,b 13.6 ± 1.1a,b 103.4 ± 1.5a 14.0 ± 1.1b 2.6 ± 2.2a

VI 30 �C 894.3 ± 39.8a,b 45.7 ± 2.4a �2.7 ± 0.4a 10.9 ± 1.4a 109.5 ± 4.7c 11.3 ± 1.3a,b 25.8 ± 6.5d

OD–OH 30 �C 883.2 ± 54.6a,b 81.9 ± 1.9d,e �4.9 ± 0.4d 16.6 ± 1.4b,c 106.6 ± 1.0b,c 17.3 ± 1.4d 3.9 ± 2.1a

VI–OH 30 �C 928.5 ± 59.6b,c 45.7 ± 2.4a �3.4 ± 0.4a,b 12.8 ± 1.6ª,b 105.2 ± 1.5a,b 13.3 ± 1.6b 33.1 ± 3.4e

OD 40 �C 844.4 ± 28.4a,b 83.9 ± 2.5e �4.2 ± 0.7b,c 16.6 ± 2.5b,c 104.4 ± 0.8a,b 17.1 ± 2.6c,d 6.3 ± 3.7b

VI 40 �C 870.4 ± 65.2a,b 48.7 ± 1.9a,b �3.6 ± 0.4b 11.9 ± 0.9a,b 110.4 ± 3.4c 12.4 ± 0.8b,c 29.9 ± 4.4c

OD–OH 40 �C 854.9 ± 41.6a,b 80.6 ± 2.0d �4.4 ± 0.1c,d 17.6 ± 1.4c 104.3 ± 0.8a,b 18.1 ± 1.4d 7.2 ± 2.3b

VI–OH 40 �C 893.9 ± 63.6a,b 43.3 ± 1.8a �2.6 ± 0.3a 11.8 ± 0.8b 102.6 ± 0.7a 12.1 ± 0.9a,b 33.3 ± 2.5d

OD 50 �C 825.8 ± 46.5a 78.6 ± 1.5c,d �3.6 ± 0.3b 16.1 ± 1.9b,c 102.9 ± 1.0a 16.5 ± 1.9c 5.8 ± 3.0a

VI 50 �C 879.9 ± 68.2a,b 42.6 ± 1.7a �2.5 ± 0.3a 10.0 ± 1.0a 104.3 ± 1.1a,b 10.3 ± 1.0a 35.8 ± 3.4f

OD–OH 50 �C 851.2 ± 39.0a,b 80.8 ± 2.5d,e �4.3 ± 0.3c 16.4 ± 2.3b,c 105.0 ± 1.6a,b 16.9 ± 2.3c,d 10.5 ± 4.3b

VI–OH 50 �C 843.5 ± 21.9a,b 52.3 ± 1.9b �3.5 ± 0.4a,b 17.3 ± 2.6c 101.8 ± 1.3a 17.6 ± 2.6d 24.4 ± 2.6c

When there are no significant differences at 5.0%, homogeneous groups in each variable, according to a LSD test, they are identified by same superscript.** Values represent the mean and standard deviation of 10 analyses.

*** Values represent the mean and standard deviation of 9 analyses.

J. Moreno et al. / Journal of Food Engineering 104 (2011) 621–627 625

VI–OH (50 �C) treatments increases the chrome value as comparedwith fresh fruit. The greater total color difference (DE) in respect tofresh fruit was obtained for VI and VI–OH treatments, principallydue to loss of clarity in accordance with the transparency gain.However, small changes were associated with the OD and OD–OH treatments due to chrome increase. The vacuum-impregnationtreatments led to lower L⁄ values. These lower values were associ-ated with transparency gained due to air loss, an effect that is pro-duced by the total or partial substitution of the air present in thepores by the impregnation solution (Moreno et al., 2000).

The maximum force values obtained from the mechanical testindicated that the osmotic treatments resulted in decreased firm-ness (Table 2). All the treatments differ significantly from the freshsample but very few differ from each other, although a small

Fig. 4. Scanning electron microscopy (SEM) micrographs of parenchyma tissue from fresOD, (c) OD–OH and (d) VI–OH. IS, intercellular space; CR, cell rupture; CW, cell wall.

decrease of firmness values are observed due at increase of tem-perature and OD treatments. The smaller firmness differences inrespect to fresh fruit were obtained for VI and VI–OH treatments.The application of the vacuum induced a greater firmness byreplacing the osmotic solution in pores due to air loss, thus obtain-ing a more compact and less deformed tissue than that produced atatmospheric pressure (Moreno et al., 2004; Castelló et al., 2010).Moreover, it has been reported that when a treatment for aw reduc-tion is applied, changes of texture in vegetal tissue are moredependent on physical and chemical changes due to the transfor-mation of protopectin to soluble pectin and to sugar diffusion inintercellular spaces, which causes a loss of turgor and the move-ment of ions from the cell wall to the media (Glen and Poovaiah1990; Fito et al., 2001; Quiles et al., 2004).

h and treated pears with a 65% (w/w) sucrose solution at 50 �C. (a) Fresh control, (b)

Fig. 5. Transmission electron microscopy (TEM) micrographs of parenchyma tissue from fresh and treated pears with a 65% (w/w) sucrose solution at 50 �C. (a) Fresh control,(b) OD, (c) OD–OH and (d) VI–OH. ML, middle lamella; PL, plasmalemma; TN, tonoplast; PT, protoplast.

626 J. Moreno et al. / Journal of Food Engineering 104 (2011) 621–627

To better understand the effects of osmotic dehydration andohmic heating at the cell level on texture characteristics, electronmicroscopy was used. The tissue of fresh ‘‘Packham’s Triumph’’pear is composed of numerous cells, small intercellular space anda high degree of cell compartmentalization. In the fresh samples,cells are swollen and closely bonded to each other (Fig. 4a) andmeans of a well-limited medium lamella (Fig. 5a). Inside the cells,a vacuole occupies most of the protoplast, and both the plasma-lemma and tonoplast are kept close to the cellular wall (Fig. 5a).OD treatment produces plasmolysis on the cell, which become de-formed, contracted and collapsed (Fig. 4b); the intercellular spacesare contracted and deformed. The protoplast and cellular contentappear in plasmolysis and retracted to the center of the cell (Fig5b–d). During the treatments, the cells changed their shape and re-duced their size, results that may be explained by the native liquidloss. The effects of the OD–OH and VI–OH treatments are shown inFigs. 4c, 4d, 5c and 5d, these show a cell deformation and cellularrupture, being greater in OD–OH treatments. Figs. 4d and 5d showsthat the thickness of the middle lamellae between cells was greaterin the VI–OH than the OD–OH treatments, as was cellular breakagedue to the electro-thermal effect. These results explain the differ-ences in firmness values found between the two treatments. Thegreater firmness observed in the VI–OH samples may be explainedby the presence of a polymeric material. This material could be apolymeric compound or concentrated sugars, which may havebeen formed by interactions of the middle lamella pectin withthe osmotic solution solutes. Moreno et al. (2000, 2004) havereported a similar phenomenon in strawberries and papayas.

4. Conclusions

The application of vacuum impregnation and ohmic heatingduring osmotic treatment had significant effects on the kinetics

of water loss and solid gain in pear samples. The combination of os-motic dehydration at atmospheric pressure and the ohmic heating(OD–OH) treatments favored mass transfer with the increase of theworking temperature. The greatest water loss was observed in theOD–OH treatment at 50 �C. The largest amount of solute gain andthe lowest firmness loss were obtained with VI at 50 �C. Color dif-ferences induced by osmotic dehydration were related to changesin lightness (L⁄) and loss of clarity consistent with the transparencygain, especially in the VI treatments. However, a great loss of firm-ness throughout the osmotic treatments at atmospheric pressure(OD) was obtained. The pears’ microstructure showed that the os-motic dehydration and ohmic heating provoked changes of shape,a reduction in cell size, a change in thickness of the middle lamel-lae and increased cellular breaking by the electro-thermal effect.Under the studied conditions, VI and VI–OH at 50 �C representthe best processing conditions for dehydrating pear pieces in a su-crose solution at 65 �Brix. In some treatments, the reduction inprocess time reached 40%.

Acknowledgements

Authors Jorge Moreno and Ricardo Simpson are grateful for thefinancial support provided by CONICYT through FONDECYT ProjectNo. 1070946.

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