Journal of Food Engineering 106 (2011) 253–261

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Journal of Food Engineering

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

Microstructure analysis on pre-treated apple slices and its effect on waterrelease during air drying

C. Ramírez ⇑, E. Troncoso 1, J. Muñoz 1, J.M. Aguilera 1

Department of Chemical and Bioprocess Engineering, Pontificia Universidad Católica de Chile, Avda. Vicuña Mackenna 4860, Macul, Santiago, Chile

a r t i c l e i n f o

Article history:Received 9 March 2011Received in revised form 12 May 2011Accepted 17 May 2011Available online 24 May 2011

Keywords:Apple slicesPre-treatmentsMicrostructureAir dryingWater lossModeling

0260-8774/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.jfoodeng.2011.05.020

⇑ Corresponding author. Present address: Departmand Environmental, Universidad Técnica Federico SanValparaíso, Chile. Tel.: +56 32 2654258.

E-mail address: cristian.ramirez@usm.cl (C. Ramír1 Tel.: +56 2 3544264.

a b s t r a c t

Microstructural studies have played an important role in the comprehensive understanding of some foodphenomena that occur during drying processes. The aim of this work was to study the effect of four pre-treatments (immersion in boiling water, vacuum impregnation, freezing/thawing, and uni-axial compres-sion) on apple microstructure characterized by cell cavities size parameter and how the changes inducedby these pre-treatments can affect the water loss rate during air drying. The quantitative structure anal-ysis showed that freezing/thawing and compression pre-treatments caused more damage on the applestructure, yielding larger cell cavities in comparison to vacuum impregnation and immersion in boilingwater and control. Apple slices that were frozen/thawed, compressed and immersed in boiling water dis-played the higher drying rates and diffusivity coefficients than the vacuum impregnated and control sam-ples. These results allow concluding that disruption of the cell walls can be considered as a factor thataffects the drying rate.

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1. Introduction

Over the past few decades, the microstructure of food has beenextensively studied, considering that many key phenomena thatcontrol the properties of food occur below 100 lm scale (Aguilera,2005). Therefore, food structure at the micro, meso or macro scalesplays an important role in mass transfer processes such as drying(Aguilera, 2005), which is defined as the evaporation of the major-ity of water normally present in a food into a vapor phase via appli-cation of heat under controlled conditions (Mujumdar andDevahastin, 2000). The drying process is affected by the distribu-tion of water inside a product and its variation over time, accordingto the prevailing moisture transport mechanism (i.e., moleculardiffusion, capillary flow, and hydrodynamic flow), physical struc-ture of the product and the external drying condition. In addition,drying is usually divided into an initial period of constant rate,where the main mass transfer mechanism is the capillary flow ofliquid water, and the subsequent periods of falling rates, where dif-fusion is the main mass transfer mechanism. The specific dryingmethod as well as the physicochemical changes that occur duringdrying seems to affect the quality of the dehydrated products(Aguilera and Stanley, 1999; Aguilera, 2003).

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ent of Chemical Engineeringta María, Avda. España 1680,

ez).

The effect of the drying conditions on the microstructure offruits and vegetables has been studied for many years, and somekey phenomena have been found at the microstructural level thatmay consequently affect food macrostructure. The most importantof these phenomena is cell shrinkage, which causes the majormodification in the global structure of the product and is directlyrelated to the loss of water during drying (Lewicki and Pawlak,2003; Ramos et al., 2004; Roca et al., 2008). However, it is alsoimportant to consider the effect of the sample structure prior todrying (e.g. structural changes induced by pre-treatments of theraw material) and how the structural changes could influence thedrying kinetics (Yong et al., 2006). For instance, it is known thatblanching of fruits and vegetables causes cell separation (Anders-son et al., 1994). Specifically, blanching produces structuralchanges at a cellular level that affect the mass transfer phenomenaduring drying (González-Fésler et al., 2008). Vacuum impregnationis a process that consists of the exchange of internal gas for anexternal liquid in the pores. This treatment generates importantchanges in the physicochemical and structural properties of foodsthat may affect their behavior in drying operations and the finalquality of the dried product, especially in highly porous foods suchas the apple (Fito and Chiralt, 2000).

In turn, the freezing treatment prior to drying may also be usedto improve mass transfer during the dehydration process. Freezingcan induce changes in the tissues by destroying cells (i.e., ice crys-tals push the cells apart or rupture cell walls, producing large voidswithin the tissue) or enlarging intercellular air spaces. For example,Funebo et al. (2002) found that the drying rate of apple tissue

Nomenclature

b parameter in Eqs. (3) and (4) (dimensionless)Deff effective moisture diffusion coefficient (m2/s)Do effective diffusivity at t = 0 (m2/s)F0 Fourier number (dimensionless)L half thickness of the slice (m)MR moisture ratio (dimensionless)N number of data pointsR2 coefficient of correlationRMS root mean squaret drying time (s)

V valueX moisture content (g water/g dry basis)

Subscriptse value at equilibrium timeexp experimentalfitted fitted0 value at initial timet value at any time

254 C. Ramírez et al. / Journal of Food Engineering 106 (2011) 253–261

increased when freezing was applied before dehydration. Addi-tionally, the thawing induces further damage to the material. Thus,the combination of both processes can affect the physical proper-ties and diffusivity during dehydration. On the other hand,mechanical damage induced by compression of the raw materialaffects the microstructural properties. It is known that tissue fail-ure under compression or tension (i.e., by using extreme stresses)is due to cell wall rupture (De Belie et al., 2000). Deformation andrupture of cells using compression or tension can modify the phys-ical properties of the final product.

Microstructural changes induced by different processing condi-tions of the raw material can be measured using specializedmicroscopy techniques. Quantitative structure analysis has beenused to establish relationships between drying conditions andmicrostructure. It has been demonstrated that the shrinkage ofcells and intercellular spaces during the drying of apple slices is di-rectly related to the size parameters and moisture content (Mayoret al., 2005; Ramos et al., 2004). Thus, several geometrical featuresof raw material can be related to its drying behavior, which can befitted by different models.

To analyze the drying process, mass transfer can be studied bythe application of Fick’s law of diffusion (Atarés et al., 2009;Bialobrzewski, 2006; González-Fésler et al., 2008; Roca et al.,2008; Sacilik and Elicin, 2006; Velic et al., 2007), which has beenfrequently used to predict the internal moisture distribution dur-ing the first falling rate period of food dehydration. Likewise,empirical models have been used to describe the kinetics of dryingof different foods (Kaya et al., 2007; Reyes et al., 2007; Sacilik andElicin, 2006; Simal et al., 2005).

The aim of this work was to study the effect of four pre-treat-ments (immersion in boiling water, vacuum impregnation, freez-ing/thawing, and uni-axial compression) on the size quantitativeparameters (area, perimeter and diameter) of cell cavities of appleslices (as model food) and how these changes affect the water re-lease rate during air drying. To model the water loss of the appleslices during drying, Fick’s law of diffusion was used with constantand variable effective diffusive coefficients.

2. Materials and methods

2.1. Materials

Apples (cv. Granny Smith), with an initial water content of6.6 ± 0.1 g/100 g dry basis and total soluble solids of 11.9 ± 0.8 Brix,were purchased from a local grocery store in Santiago, Chile, andstored in a refrigerator at 4 �C prior to the experimental runs. Pre-viously to sample preparation, the apples were equilibrated atroom temperature before each pre-treatment and subsequent dry-ing. Three apple slices (diameter: 38 mm; thickness: 3 mm) werecut from a cylinder obtained from the parenchymatous region ofthe apples in a perpendicular direction to the apple’s axe (equato-

rial zone), using a stainless steel tubular cork borer and a knife. Thefirst slice was taken at a depth of 2 mm under the skin. The thick-ness-diameter ratio of the apples slices was smaller than 0.1(3 mm/38 mm = 0.079), whereby the geometrical shape of thesamples was considered as seminfinite plates, in which fluxes ofwater and soluble solids are unidirectional (Azuara-Nietoet al., 2003). Distilled water was used for the preparation of allsolutions.

2.2. Pre-treatments

To generate visible and quantifiable changes at a microstruc-tural level in apples slices prior to air drying, four pre-treatmentswere applied to the raw apple slices (control):

Immersion in boiling water: raw apple slices were immersed in1 l of distilled water at 100 �C for 2 min. Then to drain the samples,they were put on a mesh grid (5 mm) in a vertical position at roomtemperature (25 �C) for 5 min. The final moisture content of theimmersed apple slices was 8.8 ± 0.1 (g water/g dry basis).

2.3. Vacuum impregnation

Raw apple slices were immersed in a CaCl2 solution (�5.3 gwere dissolved in one liter of distilled water) contained in a vessel.A vacuum pressure of 53.33 kPa was applied using a vacuum pump(Gast Manufacturing Inc., model DOA-P104-BN, MI, USA) for15 min at room temperature (25 �C), and then atmospheric pres-sure was restored. The samples were allowed to drain on a stain-less steel mesh grid (5 mm) in a vertical position at roomtemperature (25 �C) for 5 min. The final moisture content of thevacuum-impregnated apple slices was 8.0 ± 0.1 (g water/g drybasis).

2.4. Freezing/thawing

Raw apple slices were subjected to freezing at �30 ± 1 �C for16 h into a freezer (Kenmore, model 253.9284210, USA) and weresubsequently thawed in a Petri plate (with lid to avoid furthermoisture loss) at 1 �C for 4.5 h. The frozen/thawed apple slices finalmoisture was 5.5 ± 0.2 (g water/g dry basis).

2.5. Uni-axial compression

Apple cylinders (diameter 38 mm; thickness 30 mm) were uni-axially compressed by a texturometer TA-XT2 (Texture Technolo-gies Corp., USA), using a flattened tip (diameter 70 mm). Cylinderswere compressed to 5 mm with a speed test of 0.5 mm/s. Thediameter changes after compression (±1 mm) were not consideredfor the posterior analysis. Then, from each compressed cylinder,one slice was obtained (diameter: 38 mm; thickness: 3 mm), cutfrom the most exposed side of the compression test. The final

C. Ramírez et al. / Journal of Food Engineering 106 (2011) 253–261 255

moisture content of the compressed apple slices was 7.1 ± 0.1 (gwater/g dry basis).

2.6. Air-drying process

Air drying of the control and pre-treated apple slices was per-formed in a pilot dryer (Fig. 1). Two electrical resistances (1 KW)and a fan were mounted on the inside of the stainless steel tube(diameter: 0.11 m) thermally insulated with glass wool. A bankof tubes of diameter 10 mm and 100 mm of length was placedinside of the stainless steel tube to induce a laminar flow of air.One apple slice was dried each time. The dryer condition was setto 65 ± 1 �C, with 15% of relative humidity and air velocity of1.2 m/s. This air velocity was used to minimize the external resis-tance to mass transfer from the sample surface to the air steam(Saravacos and Maroulis, 2001). Weight changes of the controland pre-treated apple slices during the drying process were mon-itored automatically at intervals of 2 min using a digital balance(0.001 g). Samples were dried for 90 min, period selected fromexploratory runs. The drying experiments were carried out in trip-licate with slices obtained from different samples (apples) in orderto include the variability between samples.

2.7. Mathematical modeling of drying curves

A dimensionless moisture ratio (MRt) was calculated for thedried apple slices from the drying curves as:

MRt ¼Xt � Xe

X0 � Xeð1Þ

where Xt is the moisture content at any time t (g water/g dry basis),Xe is the moisture content at the equilibrium time (g water/g dry ba-sis), and X0 is the initial moisture content (g water/g dry basis). Val-ues of Xe are considered relatively small compared to Xt or X0. Thus,(Xt � Xe)/(X0 � Xe) can be simplified to (Xt/X0) (Doymaz and Pala,2002).

2.7.1. Diffusional modelsTwo models based on the Fick’s law of diffusion were used to

describe the water loss during the air drying of the apple slices. Be-cause the diameter of the apple slices was considerably larger thanthe thickness, it was assumed that mass transfer occurred only inthe axial direction. In the first model, the solution of Fick’s lawwas used for one-dimensional transport, with the assumptions

Fig. 1. Schematic represen

that moisture migrates only by diffusion, that negligible shrinkageoccurs and that the diffusion coefficients and temperature are con-stant, and is given by (Crank, 1975):

MRt ¼8p2

X1i¼1

1

ð2iþ 1Þ2exp

�ð2iþ 1Þ2p2Deff t

4L2

!ð2Þ

where Deff is the effective moisture diffusion coefficient (m2/s), t isthe drying time (s) and L is the half thickness of the slice (m).

The second model considers that the physical properties of thematerials vary with changes in the microstructure of the productduring drying (Moyano and Berna, 2002). This model results in:

MRt ¼8p2

X1i¼1

1

ð2iþ 1Þ2exp

� 2iþ 1ð Þ2p2

4ð1þ bÞ 1þ D0t

L2

� �1þb ! !

ð3Þ

where D0 is the effective diffusivity at t = 0 (m2/s) and b is a dimen-sionless parameter. In Eq. (3), Deff was postulated as a function oftime as (Alvarez and Legues, 1986):

Deff ¼ D0ð1þ F0Þb ð4Þ

with F0 the Fourier number defined by F0 = D0t/L2. The parametersin the models described by Eqs. (2) and (3) were estimated by non-linear regression procedures performed by the Solver tool of Micro-soft Excel (Microsoft Corporation, Redmond, Washington, USA),using the minimization of the root mean square (RMS) as the objec-tive function, defined as follows:

RMS ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1N

XN

i¼1

Vexp � V fitted

Vexp

� �2vuut ð5Þ

where N is the number of data points, Vexp is the experimental va-lue, and Vfitted is the fitted value by the two models analyzed. Themodels based on Fick’s law of diffusion (Eqs. (2) and (3)) were fittedto the experimental data using first 500 terms of the series.

2.8. Microstructure and image analysis

The samples were prepared for light microscopy analysis. Thecontrol and pre-treated apple slices were fixed in a solution (5%formaldehyde, 5% acetic acid, and 90% ethanol 70%) for 48 h atroom temperature. Then, the samples were dehydrated in a gradedseries of water/ethanol at 50%, 70%, 95%, and 100% to complete thedehydratation and then to be embedded in paraffin. Three to six18 lm-thick microslices were obtained with a Jung K manual

tation of hot air drier.

256 C. Ramírez et al. / Journal of Food Engineering 106 (2011) 253–261

microtome (Jung Co., Heidelberg, Germany), collected on a cleanglass slide and treated with an albumin/glycerin solution to aidthe adhesion. Slides were placed on a warming tray (40 �C) for3 h to allow the microslices to flatten and the water to evaporate,so that the microslices can adhere to the slide. Then, the microslic-es were deparaffinized using xylene (3 baths, 10 min each bath)and hydrated using an ethanol series (100%, 95%, 70%, and 50%,5 min each step, ending with water for 1 min). The microsliceswere stained with a safranin O solution for 30 min (0.5 g safranin,50 mL ethanol at 95% and 50 mL distilled water) and then washedin water to remove all excess stain from the tissue. Subsequently,another dehydration was performed in ethanol (50%, 70%, 95%,and 100%, 5 min each step). Finally, tissues of the microslices werestained for 3 min with Fast green (0.1 g Fast green, 10 mL absoluteethanol, and 10 mL eugenol) to stain the cell walls in the samples.To improve the color contrast, the tissue sections were washedtwice (10 min each) with a eugenol solution (50 mL eugenol,25 mL absolute ethanol, and 25 mL xylene). The sections were fi-nally cleared after three changes of 100% xylene (10 min each)and were subsequently air dried. A coverslip was mounted withPermount to make a permanent preparation. This stain methodwas performed for the examination of the tissue morphology, inparticular, the cell wall.

Photomicrographs of the cell structure were taken with a digitalcamera (CoolSnap-Pro, Media Cybernetics, Inc., USA), which wasmounted on a light microscope (Olympus BX50, Optical Co., Ltd.,Tokyo, Japan). The magnification used was 4�, with an image res-olution of 865 pixels per mm. Two microslices (3 � 15 mm) perpre-treatment were analyzed to perform a scan through all slices,obtaining 30 images per slice with an image size of 1040 � 1392pixels (1.20 � 1.61 mm). Thereby, 60 images were generated persample. To quantify the changes in the apple structure, cell cavities(i.e., space enclosed by a cell wall) were characterized in terms oftheir cell area, cell perimeter and cell diameter by using an auto-matic image processing (from gray level to binarized images)method based on the gray level difference between adjacent pixels,which was performed using Matlab code (Mathworks, Inc., version7.0.1 Release 14, USA). Then measurements of cell cavities (ob-tained from binarized images) were carried out by the ImagePro-Plus software (Media Cybernetics, Inc., Version 4.0, USA).

To establish a representative structural analysis, a successivelyincreasing number of cells cavities were considered in this evalua-tion and the mean values of the morphological features were quan-tified. A cell cavities count was considered representative when aconstant value of their morphological characteristics of size wasobtained (Graham and Yang, 2003). Histograms of the morpholog-ical features for size were constructed using Microsoft Excel(Microsoft Corporation, Version 2003, USA).

2.9. Statistical analysis

Significance tests of the results were conducted using an analy-sis of variance (ANOVA), and the differences between the groupmeans were analyzed by a multiple-range test. Statistical signifi-cance was set at a probability level of 0.05 (p < 0.05) and was deter-mined using the Statgraphics Plus software (Statistical GraphicsCorporation, Version 5.1, Rockville, USA).

3. Results and discussion

3.1. Microstructure of control and pre-treated apple slices

The effect of sample size on morphological parameters such asthe mean cell area and mean cell perimeter, according to the num-ber of cells quantified, is shown as an example in Fig. 2 for the

control and immersed in boiling water samples (data related tothe other pre-treatments is not shown, because they presentedsimilar tendencies). There was no correlation between the cellnumber and the variability of the size parameters obtained fromthe samples analyzed. In general, the cell number required to ob-tain constant values was dependent on the type of pre-treatmentapplied to the sample. For example, the cell number required tocharacterize samples that were immersed in boiling water and vac-uum impregnated were 3138 and 10580, respectively. In the caseof the apple slices that were immersed in boiling water, the cellcavities number required for determination of constant values ofthe mean area and mean perimeter was lower than the total quan-tified amount (Fig. 2B). The same behavior was found for thosesamples subjected to vacuum impregnation, freezing/thawingand compression. However, the control samples did not followthe same behavior (Fig. 2A), and the entire sample size (9309 cells)had to be considered to obtain constant values for the mean cellarea and mean cell perimeter.

To study the microstructure of the control and pre-treated ap-ple slices, photomicrographs of these samples were obtained(Fig. 3), and the results of the quantitative analysis for their sizefeatures are summarized in Table 1 and Fig. 4. Control apple slicespresented a mean cell cavity area of 0.0036 mm2, mean cell cavityperimeter of 0.2598 mm, and mean cell diameter of 0.0601 mm.However, these values were lower than those reported by Lewickiand Pawlak (2003) for Idared raw apples. Thus, in comparison to re-sults obtained with other works, it is possible to find differences inthe quantitative analysis of morphological size characteristics, dueto different factors that affect the structural morphology, such asthe variety of apple, degree of ripeness, and handling during prep-aration of sample for microscopy.

The size parameters for cells of apple slices that were immersedin boiling water showed similar values to those found in the con-trol apple slices (Table 1). However, the most frequent area, perim-eter and diameter values are larger than the control, which impliesthat a higher number of large cell cavities are present in the struc-ture (Fig. 4). These results are in agreement with the values re-ported by González-Fésler et al. (2008) for blanched apples,where it was found that cellular membranes were broken and cellwalls were damaged, appearing interrupted in many places andthat the cells became slightly more irregular in shape.

In contrast, vacuum impregnation was the pre-treatment thatproduced the lowest values of microstructural features for appletissue in terms of cell cavity size (see Table 1). The mean area ofcells cavities was 0.0036 mm2, with a mean perimeter of0.2529 mm and a diameter of 0.0592 mm. It has been shown thatthe presence of calcium in the matrix helps maintain the integrityof the cell wall due to cross-linking of pectic polymers and increasethe degree of cell-to-cell contact. Additionally, the cell walls ap-peared clearer and better defined, but similar to fresh fruit (Aninoet al., 2006; González-Fésler et al., 2008). The frozen/thawed appleslices presented the highest damage in apple microstructure. Themean and most frequent values of size measurements in the cellcavities of frozen/thawed samples were higher than those of thecontrol and the other pre-treatments studied (Fig. 4). The resultspresented thus far indicate that freezing/thawing and immersionin boiling water are the most aggressive pre-treatments. Duringan inappropriate freezing condition in qualitative terms, it is com-mon to find cellular damage due to ice crystals formation whichbreak cell membranes and walls. Hence, the cells appeared irregu-lar in shape and tissue distortion was observed due to the growingof ice crystals with cell separation (Roy et al., 2001; Delgado andRubiolo, 2005), which it is in accordance with our analysis.

Finally, for compressed apple tissue, the mean cell size was lar-ger than control, immersed in boiling water and vacuum impreg-nated samples, but closer than the ones subjected to freezing/

Fig. 2. Effect of the sample size on the morphological features: (d) mean cell area, and (s) mean cell perimeter for (A) control, and (B) immersed in boiling water samples.

C. Ramírez et al. / Journal of Food Engineering 106 (2011) 253–261 257

thawing pre-treatment. This finding suggests an important damageon the structure mainly due to the collapse induced in the vegetaltissue; therefore, this pre-treatment modifies the tissue micro-structure through cell wall rupture generated by extreme stressduring testing (Alamar et al., 2008). Donald et al. (2003) directlyobserved the effect of compression on vegetal tissue by using envi-ronment scanning electron microscopy (ESEM), finding that cellslose turgor and rupture as well as the release of cell content occursin specific zones when the compression was applied.

3.2. Modeling of drying curves

A continuous decrease in the moisture content of pre-treatedapple slices was observed during the drying process (Fig. 5), whichwas significantly affected (p < 0.05) by the pre-treatments. Thedrying rates of immersed in boiling water, freezed/thawed andcompressed samples were higher than vacuum-impregnated andcontrol samples (Fig. 6), which induced a more rapid decrease intheir moisture contents. In general, the decrease in moisture con-tent was predominantly given by a falling-rate period. Hence,internal diffusion may be assumed to be the mechanism responsi-ble for water loss during the drying process. In addition during thefirst 4 min, it was observed an increase in the drying rate for con-trol, vacuum impregnated, freezed/thawed and compressed sam-ples, period in which the final surface temperature was reached.

Only the samples subjected to immersion in boiling water entirelypresented a continuous falling-rate period, probably due to thesuperficial starch gelatinization when the apple slices were pre-treated and, subsequently, a further barrier to the water removalwas imposed for the constant-rate drying. Similar results werefound by Chen et al. (2006), who visualized under ESEM that theboiled sample dried faster compared with the fresh sample underthe same drying condition, especially at the later stage of drying(after the initial free water evaporation). This result is most likelydue to the fact that the internal resistance to mass transfer wouldbe less than that in the raw material.

Water diffusivity can be calculated from the experimental dry-ing data using Fick’s second law. However, several models havebeen used to fit the kinetics of water loss during drying of differentproducts (Funebo et al., 2002; Andrés et al., 2004; Simal et al.,2005; Menges and Ertekin, 2006; Kaya et al., 2007). In this work,two models based on the Fick’s second law were studied. Thekinetics of water loss was better fit by the variable diffusivity mod-el than the constant diffusivity model (Fig. 5). Parameters and RMSvalues obtained for the variable diffusity model are shown in Table2. The usual analytical solution of the Fick’s law (Eq. (2)) did not fitthe experimental measurements of water loss during drying, prob-ably because this equation does not consider possible variations(e.g. microstructural changes, shrinking, sample temperature vari-ation) induced by the drying conditions. Thus, when biological

Fig. 3. Photomicrographs of representative samples for: (A) control apple slices; and apple slices subjected to the following pre-treatments: (B) immersion in boiling water;(C) vacuum impregnation; (D) freezing/thawing; and (E) compression. Bars represent 0.25 mm (see Section 2 for details).

Table 1Values of morphological features of cells for control and pre-treated apple slices.

Treatment Cell number Area (mm2) Perimeter (mm) Diameter (mm)

Mean ± SD* Most frequent Mean ± SD Most frequent Mean ± SD Most frequent

Control 9309 0.0036 ± 0.0047 0.0019 0.2598 ± 0.2221 0.1548 0.0601 ± 0.0412 0.0435Immersion in boiling water 3138 0.0034 ± 0.0051 0.0037 0.2508 ± 0.2305 0.1751 0.0575 ± 0.0376 0.0414Vaccum impregnation 10580 0.0036 ± 0.0041 0.0016 0.2529 ± 0.2136 0.1215 0.0592 ± 0.0383 0.0345Freezing/thawing 4891 0.0058 ± 0.0068 0.0040 0.3275 ± 0.2472 0.1541 0.0679 ± 0.0397 0.0380Compression 6104 0.0050 ± 0.0055 0.0015 0.2817 ± 0.2130 0.1409 0.0675 ± 0.0442 0.0346

* Standard deviation.

258 C. Ramírez et al. / Journal of Food Engineering 106 (2011) 253–261

products are subjected to drying processes, the particular geomet-ric shape, solid structure or unpredictable shrinkage promoted bythe process can be considered using a variable diffusivity coeffi-cient. For all pre-treatments tested, the Deff increased with the dry-ing time (Fig. 7). Freezed/thawed, compressed and immersed inboiling water apple slices presented the higher Deff values, whichindicates that these pre-treatments largely promote water diffu-sion during the drying process due to the considerable microstruc-tural changes induced by them in the raw material.

3.3. Relationship between microstructure and the drying rate of appleslices

Results obtained from the quantitative structure and dryingcurve analysis suggest that the changes in the apple microstructurecould be a factor that determines the drying rates of apple tissue.For example, the freezing/thawing and compression pre-treat-ments generated the most damage and disruption at the cellularlevel, increasing the size in terms of the mean cell area, perimeterand diameter (Table 1). Above results were supported by the high-er drying rates found for these pre-treatments, which implies thatwater was easily removed due to the structural damage induced bythese pre-treatments. The high degree of damage on the cell mem-branes and cell walls can facilitate the release of water from thematrix to the exterior because of the increase in intra-cellularspaces, inter-cellular channels and formation of new pathwaysthrough the structure, facilitating the water movement between

cells to the exterior. The results presented in this investigationare concordant with those from other works. For instance, Eshtia-ghi et al. (1994) found that using freezing as a pre-treatment to thedrying of green beans, carrots and potatoes resulted in the highestdrying rate by enhancing water transfer. Freezing disorders the tis-sue structure and results in better diffusion, therefore decreasingdrying time (Lewicki, 2006).

The analysis of the pre-treatment of immersion in boiling waterwas complex because the high diffusivity coefficient values wereclose to those obtained by freezing/thawing or compression pre-treatment. In addition, the mean area, mean perimeter and meandiameter for samples that were immersed in boiling water werelower and similar to the control and vacuum impregnated samples.Nevertheless, the size distribution curve (Fig. 4) shows that the val-ues for the cell area are closer to the freezing/thawing pre-treat-ment than to the less invasive pre-treatments. Therefore, largercell sizes are predominant in these samples, which could explainthe high diffusion values. Alvares et al. (1995) studied the effectof blanching as a pre-treatment in strawberries, showing that theDeff of water in strawberries was strongly affected by blanchingdue to the complete disruption of cells, the middle lamella practi-cally disappeared and distortion of microfibrils occurred, indicat-ing the degradation of the hemicellulosic polysaccharides presentin the cell wall.

On the other hand, the vacuum impregnation pre-treatment didnot induce changes in the drying rate of the samples with respectto the control. At a microstructural level, this pre-treatment did not

Fig. 4. Frequency distribution curves of: (A) cell area, (B) cell perimeter, and (C) cell diameter for (d) control; (s) immersion in boiling water; (.) vacuum impregnation; (4)freezing/thawing; and (j) compression pre-treatments.

0.0

0.2

0.4

0.6

0.8

1.0

0 1100 2200 3300 4400 5500

Drying time (s)

MR

t

Fig. 5. Experimental and fitted moisture ratio during drying of apple slicessubjected to different pre-treatments: (�) control; (4) immersion in boiling water;(j) vacuum impregnation; (N) freezing/thawing; and (h) compression, for variablediffusivity model.

0.0003

0.0007

0.0011

0.0015

0.0019

0.0023

0 2 4 6 8

Moisture content (g water/g dry basis)

Dry

ing

rate

(g w

ater

/g d

ry b

asis

· s)

Fig. 6. Influence of pre-teatments on drying rate of: (�) control; (4) immersed inboiling water; (j) vacuum impregnated; (N) freezed/thawed; and (h) compressedapple slices.

C. Ramírez et al. / Journal of Food Engineering 106 (2011) 253–261 259

affect the structure of apple slices; the cell damage was not appar-ent and the quantitative analysis yielded similar results comparedto the control. González-Fésler et al. (2008) reported that the coef-ficient of diffusion of the vacuum impregnated apples was not sig-nificantly affected by the pre-treatment, and its value was similarto the control apples. This finding was most likely due to the fact

that calcium binds to the plant cell walls and forms cross-links,especially with pectins of the middle lamella, hence affecting tex-ture and shrinkage of the material that is being dried (Lewicki,2006).

As it has been demonstrated that the structure of raw materialscan control the drying processes, different pre-treatments could beused to largely modify the food matrix previous to its processing.Compact structures, such as those found in the control or vacuum

Table 2Parameters and RMS values for the variable diffusivity model.

Treatment Variable diffusivity model

D0 � 1010 (m2/s) b % RMS

Control 1.04 ± 0.06 6.18 ± 0.88 4.69Immersion in boiling water 1.07 ± 0.04 6.87 ± 0.45 3.92Vacuum impregnation 0.97 ± 0.16 7.35 ± 1.17 4.02Freezing/thawing 1.35 ± 0.30 6.41 ± 2.04 4.90Compression 1.28 ± 0.16 5.42 ± 1.17 4.47

1.0

2.3

3.6

4.9

6.2

7.5

0 1100 2200 3300 4400 5500

Drying time (s)

Def

f x

1010

(m

2 /s)

Fig. 7. Effective moisture diffusivity during drying of pre-treated apple slices; (�)control; (4) immersed in boiling water; (j) vacuum impregnated; (N) freezed/thawed; and (h) compressed samples.

260 C. Ramírez et al. / Journal of Food Engineering 106 (2011) 253–261

impregnated samples, decrease the water release from them due tothe limited pathways as a result of reduced space, while the waterrelease is faster in a more open structure, with damaged cell walls.Hence, food matrices containing higher cell cavities facilitate waterdiffusion during the drying process.

4. Conclusions

Different pre-treatments were applied to apple slices with thepurpose of inducing changes at the microstructural level in the tis-sue, and the effect on the drying rate was evaluated. The quantifi-cation of these microstructural changes was based on themeasurement of cell cavities size. Vacuum impregnation pre-treat-ment did not induce important changes on the structure with re-spect to the control samples. Nevertheless, freezing/thawing,immersion in boiling water and compression were pre-treatmentsthat seriously modified the structure by increasing the cell cavitiessize, mainly by disruption of cell walls. The drying rate was af-fected by the pre-treatment, especially by the processes of freez-ing/thawing, immersion in boiling water and compression,decreasing the moisture content of the samples more quickly thanin the control and vacuum impregnated samples. These resultssuggest that cell cavities size can be considered as a factor that af-fects the water release from the tissue. Considering that watermoves in vegetal tissue through cell membranes, plasmodesmataand cell walls, we can conclude that the microstructure character-ized by cell size may play an important role in the transport phe-nomena associated with the drying process. Additional studiesare requested with the aim of including the microstructural effecton the water release behavior during the drying process.

Acknowledgments

The authors acknowledge the discussions with Ph.D. studentsWenceslao Medina and Rommy Zúñiga, and the Chilean National

Commission for Science and Technological Research-CONICYT forfinancial support to Cristian Ramírez.

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