Effects of pulsed electric fields assisted osmotic dehydration on

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Journal of Food Engineering 183 (2016) 32e38

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Effects of pulsed electric fields assisted osmotic dehydration on freezing-thawing and texture of apple tissue Oleksii Parniakov a, *, Olivier Bals a, Nikolai Lebovka a, b, Eugene Vorobiev a Sorbonne Universit es, Universit e de Technologie de Compi egne, Laboratoire de Transformations Int egr ees de la Mati ereRenouvelable, EA 4297, Centre de Recherches de Royallieu, BP 20529, 60205, Compi egne Cedex, France b Institute of Biocolloidal Chemistry named after F. D. Ovcharenko, NAS of Ukraine, 42, blvr. Vernadskogo, Kyiv, 03142, Ukraine a

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 November 2015 Received in revised form 21 February 2016 Accepted 25 March 2016 Available online 30 March 2016

The effects of pulsed electric fields (PEF) assisted partial dehydration of apple tissue on freezing-thawing processes and texture characteristics were studied. The apple disks were treated to a high level of tissue disintegration (electrical conductivity disintegration index Z was z0.98) and then were subjected to osmotic dehydration (OD) in the apple juice-glycerol solutions using different concentrations of glycerol and time of dehydration. PEF-assisted OD resulted in noticeable acceleration of the freezing/thawing processes as compared with those for untreated samples. The main characteristics of freezing-thawing processes (freezing and thawing time and freezing temperature) and texture of samples in dependence on intrinsic moisture content in the sample were evaluated. The differences in these characteristics were explained by the differences in spatial distributions of soluble solids and water inside the samples. For some optimum parameters of osmotic treatments (e.g., at t ¼ 180 min, C z 20 wt. %), the texture after defrosting was comparable to that of fresh apples. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Osmotic dehydration Glycerol Apple Freezing-thawing Pulsed electric field

1. Introduction nchez et al., 2015) Freezing (Li and Sun, 2002), drying (Calín-Sa and osmotic dehydration (Garcia Loredo et al., 2013) are widely used as main food preservation techniques. However, the application of these techniques can cause undesirable changes of the qualitative characteristics of food products, colour, flavour, nutrient and textural properties (Stoecker, 1998). The aforementioned undesirable effects may be minimized or even eliminated by combination of different pretreatment techniques. For example, partial dehydration before freezing (dehydrofreezing) of fruits and vegetables allow obtaining the products with better quality (James et al., 2014; Li and Sun, 2002). The sugar alcohols (sorbitol, glycerol) and sugars (sucrose, trehalose) are widely used as osmotic agents. The glycerol is easily digested, non-toxic and is recognized as safe additive by regulation of the European Parliament (EC) N 1333/2008. The efficiency of above mentioned techniques can be noticeably improved with assistance of pulsed electric fields (PEF) (Donsi et al., 2010; Vorobiev and Lebovka, 2011). PEF-treatment allows significant acceleration of mass transfer properties without undesirable

* Corresponding author. E-mail address: [email protected] (O. Parniakov). http://dx.doi.org/10.1016/j.jfoodeng.2016.03.013 0260-8774/© 2016 Elsevier Ltd. All rights reserved.

changes in food tissues (Donsi et al., 2011; Jaeger et al., 2012; Odriozola-Serrano et al., 2013; Raso and Heinz, 2006). The positive effects of PEF-pretreatment on freezing and freezeedrying processes of different foods were already demonstrated (Ben Ammar et al., 2010; Jalte et al., 2009; Phoon et al., 2008; Shynkaryk et al., 2008; Wiktor et al., 2015). The PEF-assisted osmotic treatment by cryoprotectants allowed prevention of tissue softening after defrosting (Shayanfar et al., 2014, 2013) and strengthening the tissue texture after freezing-thawing (Parniakov et al., 2015). However, combined PEF and cryoprotectants-assisted dehydration at different osmotic conditions (concentration of osmotic agent, time and degree of dehydration) was never previously studied in details. This manuscript studies the effects of PEF-assisted partial dehydration of apple tissue on freezing-thawing processes and texture of apple after storage at freezing temperature and thawing. The PEF-assisted osmotic dehydration was done in apple juice glycerol solutions using different concentrations of glycerol and time of dehydration. The main characteristics of freezing-thawing processes (freezing and thawing duration and freezing temperature) and texture of samples in dependence on intrinsic moisture content in the sample were evaluated.

O. Parniakov et al. / Journal of Food Engineering 183 (2016) 32e38

Nomenclature Brix C d D E h F F* n N t ti tf tm tPEF Dt Dtt

concentration of the total soluble solids in intrinsic juice. concentration of glycerol, wt.%. diameter of apple disk, mm. effective diffusion coefficient, m2/s. electric field strength, V/cm. thickness of apple disk, mm. force, N. texture index. number of pulses in the series. number of trains. time of dehydration, min. pulse duration, ms. effective freezing time, min. effective thawing time, min. time of PEF treatment, s. interval between pulses, ms. pause between trains, s.

2. Materials and methods 2.1. Materials Commercial apples (Jonagold) were purchased in the local supermarket (Compiegne, France) and were used in this work. The moisture content, measured by drying 25 g of the fresh apple tissue in one batch at 105  C to constant weight, was about 89 wt.% (that corresponds to 8.09 kg water/kg d.m.). The apple disk-shaped samples (d ¼ 29 mm in diameter and h ¼ 5 mm in thickness) were manually prepared immediately before experiments using the special cylindrical knife. 2.2. Methods The experimental setup is schematically shown in the Fig. 1. The apple samples were initially treated by PEF, osmotically dehydrated (OD) and then freezing-thawing experiments were done. Finally, the texture of the samples was analysed. 2.2.1. PEF treatment PEF treatment was applied using a monopolar PEF generator (5

PEF- treatment

33

temperature,  C. effective freezing temperature,  C. effective melting temperature,  C. intrinsic moisture content (humidity), wt. % (mass of water/total mass of the sample). electrical conductivity disintegration index.

T Tf Tm Wi Z

Greek symbols electrical conductivity, S/cm. characteristic diffusion time, min.

s t

Subscripts d damaged. i initial, intact. o optimal. Abbreviations OD osmotic dehydration. PEF pulsed electric field. U untreated sample.

kVe1 kA, Hazemeyer, Saint-Quentin, France). The PEF generator provided pulses of a near-rectangular shape, and N series of pulses were applied. Each separate series consisted of n pulses with pulse duration ti, time interval between pulses Dt and pause Dtt after each train. The total time of PEF treatment was regulated by variation of the number of series N and was calculated as tPEF ¼ Nnti. The current and voltage values were measured during the period between two consecutive series of pulses. The following protocol was used in PEF experiments: E ¼ 800 V/cm, n ¼ 10, ti ¼ 100 ms, Dt ¼ 100 ms, Dtt ¼ 10 s, and the value of N was varied in order to obtain the desirable tt, e.g., N ¼ 10 corresponded to t ¼ 0.1 s. The specific energy consumption for PEF treatment was 12 ± 0.6 kJ/kg. This data is in very good correspondence with previous works (Grimi et al., 2011). The chosen protocol of successive trains with long pause after each train allowed a good control of the plant tissue permeabilization without any significant temperature elevation (DT  3  C) during PEF treatment. Electrical treatment cell con de s sisted of a Teflon cylindrical tube (Atelier Genie des proce industriels, UTC, Compiegne, France) with z110-mm inner diameter and an electrode at the bottom. The seven apple disk-shaped samples were placed inside the cell on the bottom electrode and covered with fresh apple juice. After that the second electrode was

Osmotic dehydration

Freezing thawing

Treatment chamber Apple juice glycerol osmotic solution

Generator

T= Freezer

Air

Electrodes Apple juice

Disk-shaped sample Thermocouple

Fig. 1. The scheme of PEF- assisted osmotic dehydration and freezing experiments.

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O. Parniakov et al. / Journal of Food Engineering 183 (2016) 32e38

put on the top of the samples. The distance between the electrodes, 5 mm, was determined by the height of the sample. Freshly prepared apple juice (from the same apples Jonagold) was chosen as a natural medium in order to reduce the samples degradation and to improve electrical contact between the electrodes and the samples. The temperature inside the geometrical centre of the central sample was recorded in the online mode by a teflon-coated thermocouple Thermocoax type 2 (AB 25 NN). All data (electrical conductivity, voltage, current, temperature) were collected using a data logger and software adapted by Service Electronique (UTC, gne, France). Compie The degree of apple tissue permeabilization was evaluated using electrical conductivity disintegration index, Z (Lebovka et al., 2002):

Z ¼ ðs  si Þ=ðsd  si Þ

(1)

where s is the measured electrical conductivity and subscripts i and d refer to the conductivities of the intact and completely damaged tissue, respectively. All values of s were measured at the same temperature, T ¼ 20  C. The value of sd was estimated as the maximum attainable level of s for the given mode of treatment. It can be attained using long-lasting treatment (e.g., treatment during the time of 1 s at 800 V/cm for PEF treatment). The Z (tPEF) dependence for the same apple variety (Jonagold) was previously investigated in details (Parniakov et al., 2015). In this work the same protocol of PEF treatment was applied for obtaining the apple samples with high value of Z (Z z 0.95, tPEF ¼ 0.1 s). 2.2.2. Osmotic dehydration The osmotic dehydration (OD) of apple discs was done using apple juice-glycerol osmotic solutions with different glycerol concentrations, C ¼ 0e60 wt. %. Freshly prepared apple juice (from the same apples Jonagold) was used. The analytical grade anhydrous glycerol (BioUltra, Sigma-Aldrich) was used through out this study. PEF-treated samples were placed into a 600 ml beaker, for avoiding the noticeable changes in composition of the osmotic solution during diffusion experiments the solid-liquid ratio in the beaker was maintained at rather low level 1:20 (w/w), the temperature was held at T ¼ 20  C. For careful agitation, continuous mixing using a magnetic mixer (0e1, 250 min1, Bioblock Scientific, Heidolph, Germany) was applied and the beaker was closed in order to avoid any evaporation. The level of osmotic treatment was controlled by periodical measurements of the changes in  Brix of intrinsic juice expressed from the dehydrated apples. In order to study the effect of the glycerol concentration and time of OD the two different methods for sample preparation were applied:  Samples St: The concentration of glycerol was C ¼ 60 wt. % and the OD duration was varied (t ¼ 0e180 min).  Samples Sc: The OD was done at different glycerol concentrations (C ¼ 0, 20, 40, 60 wt. %) for a duration of t ¼ 180 min. The samples were taken from the osmotic solution in order to measure the concentration of the total soluble solids in intrinsic juice (juice expressed from the apples)  Brix and the intrinsic moisture content (humidity) Wi in the samples. The analysed samples were not used later in the diffusion experiments. Then juice was expressed from the sample and the value of  Brix was measured using a refractometer PR-201 (Atago, Japan). The humidity Wi was determined by drying of the apple disks at 105  C to constant weight. The values of Brix and Wi were used for characterisation of the effects of osmotic dewatering of samples.

2.2.3. Freezing-thawing experiments Apple samples Sc and St were frozen using a ventilated ultralow-temperature freezer MDF-U2086S (Sanyo, Gunma, Japan). It was supplied with a modular-type temperature controller SR Mini System (TC Ltd., Dardilly, France) and the software Spec-View Plus (SpecView Corporation, Gig Harbor, USA). The samples were placed inside the freezer at 40  C with an air flow rate of 2 m/s controlled by an electronic device VEAT 2.5 A (Air-technic, Firminy, France). Inner temperature of apple discs was measured with a T-type thermocouple of 0.5 mm of diameter (TC, Ltd., Dardilly, France) with an accuracy of ±0.1  C that was inserted into geometrical centre of the sample. Initial temperature of the sample before freezing was 20  C. The total freezing time, t, from the beginning of the cooling was z50 min and the final temperature of the samples was z35  C. The effective freezing temperature, Tf, was determined as a crosspoint of tangent lines of freezing and cooling part of T(t) curve. The effective freezing time, tf, was determined as the cooling time required to reach the temperature T ¼ 30  C. The thawing experiments were started immediately after freezing by placement of the sample into the similar ventilated chamber maintained at 20  C. 2.2.4. Textural test The textural properties were measured using a Texture Analyser TA-XT plus (Stable Micro Systems, Godalming, UK). The samples were placed on the flat platform of the texture analyser and stress relaxation tests were carried out. Pre-loading with 15% deformation compensated nonuniformity of the structure. In the force relaxation tests, the applied force was set at the level of 5 N, the speed of piston displacement was 1.0 mm/s and stress relaxation curves were recorded with 0.1 s resolution during 60 s. In order to quantify the difference in the sample texture at different protocols of apple tissue treatment, the texture index defined as (De Vito et al., 2008; Parniakov et al., 2015)

F * ¼ ðF  Fd Þ=ðFi  Fd Þ

(2)

was used. Here, F is the force measured at t ¼ 50 s, and subscripts I and d refer to the values of intact and completely damaged tissue, respectively. The previous force relaxation tests for PEF-treated and freeze-thawed apple tissues had shown that at t  50 s the changes in values of force are unessential. (De Vito et al., 2008). The previous studies evidenced that PEF-treated apple tissue with high level of disintegration (Z z 1) has stronger texture compared with the texture of maximally disintegrated freezethawed tissue (De Vito et al., 2008; Lebovka et al., 2004). That why the maximally disintegrated apple freeze-thawed tissue was used for estimation of reference value of Fd. The value of Fd was estimated as the minimum level of F attainable after long-lasting (z90 h) freezing and thawing treatment of apple tissue. Texture index is convenient for understanding of the viscoelastic properties of tissue. The value of F* is equal to 0 for completely damaged tissue, while for intact material F* is 1.0. 2.3. Statistical analysis All experiments and measurements of characteristics were repeated using, at least, five replicates. One-way analysis of variance was used for statistical analysis of the data using the Statgraphics plus (version 5.1, Statpoint Technologies Inc., Warrenton, VA). For each analysis, significance level of 5% was assumed. The error bars presented on the figures correspond to the standard deviations. Tukey's test was also performed on data for all pair wise comparisons of the mean responses to the different treatment

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groups. This test allows determination of treatments which are statistically different from the other at a probability level of P ¼ 0.05. 3. Results and discussions Fig. 2 presents kinetics of changes in concentration of the total soluble solids in intrinsic juice DBrix (DBrix ¼ Brix  Brix (0), where Brix (0) is an initial value before the osmotic treatment at t ¼ 0 min) and the intrinsic moisture content Wi in the sample during osmotic treatment of PEF-treated tissue at different values of C. DBrix and Wi reached their constant values (DBrix∞  and Wi∞ , respectively) after z3.5e5 h of OD. This time was rather small (t z 3.5 h) as compared to that observed for PEF untreated samples (t z 10 h). Similar acceleration of OD was reported earlier for different fruit and vegetable tissues subjected to electrical treatment (Vorobiev and Lebovka, 2011). The experimental data of DBrix (t) and Wi(t) were fitted using Fick's second law diffusion equation for an infinite disk of thickness h (Crank, 1975; Rastogi et al., 2002):

yb;w ¼ y∞ b;w 1  ,

nm X

i h ð2n þ 1Þ2 exp  ð2n þ 1Þ2 t=t

n¼0 nm X

!

(3)

2

ð2n þ 1Þ

n¼1 ∞ where y∞ (y∞ ¼ DBrix∞   Brix ð0Þ, ory∞ w ¼ Wi ð0Þ  Wi ) is a b;w b maximum (saturation) level of the variable in the limit of t/∞, tb,w ¼ (h/p)2/Db,w is a characteristic diffusion time, h is the thickness of the slab, and Db,w is an effective diffusion coefficients, the subscripts b and w refer to the time dependencies of DBrix and Wi, respectively. Here, nm is the maximum number of terms in the series. For satisfactory fitting the value nm ¼ 10 was used. The estimated values of Db and Dw (Table 1) were in correspondence with previously obtained values for effective diffusion coefficients (Amami et al., 2006; Jemai and Vorobiev, 2002). For example, the values Db ¼ 0.39  109 m2/s and Db ¼ 0.25  109 m2/s for PEF-treated (E ¼ 1000 V/cm) and untreated apple samples, respectively, were obtained for apples

Fig. 2. Examples of concentration of the total soluble matter in intrinsic juice DBrix (a) and the intrinsic moisture content Wi (b) time dependencies during OD of untreated (U) and PEF treated tissues (E ¼ 800 V/cm, tPEF ¼ 0.1 s). The symbols are the experimental data and the lines correspond to the fitting with Eq. (3).

35

Table 1 The fitting parameters for experimental data presented in Fig. 2 using the Eq. (3). Here. DBrixo∞ and W∞ i are the saturation levels of the variables in the limit of t/∞, tb,w is the characteristic diffusion time, Db,w is an effective diffusion coefficient and 2 r is a coefficient of determination. The subscripts b and w correspond to data extracted from measurements the values of Brix and Wi, respectively. C, % wt

DBrixo∞

20 60 C, % wt 20 60

13.4 44.6 W∞ i 74.9 40.0

± 0.9 ± 4.7 ± 1.0 ± 2.0

tb, min

Db, 109 m2/s

R2

61 ± 12 136 ± 33 tw, min 84 ± 10 122 ± 5

0.69 ± 0.14 0.31 ± 0.08 Dw, 1010 m2/s 0.50 ± 0.06 0.35 ± 0.03

0.990 0.997 R2 0.993 0.990

(Golden delicious variety) at T ¼ 20  C (Jemai and Vorobiev, 2002). The effective diffusion coefficients of solute obtained from OD experiments of apples (Golden delicious variety) in aqueous sucrose solution (C ¼ 44.5 wt.%) were 0.33  109 m2/s and 0.41  109 m2/ s for untreated and PEF treated (900 V/cm), respectively (Amami et al., 2006). The diffusion processes that control the changes in Brix and Wi, became slower with increase of glycerol concentration. It can reflect the impact of noticeably higher mixture viscosity and lower mutual diffusion coefficients Dm in the given watereglycerol mixtures (D'Errico et al., 2004; Ternstrom et al., 1996). The similar differences in mutual diffusion coefficients Dm were observed for the water þ glycerol mixtures at 25  C, e.g., Dm z 0.96  109 m2/s at C ¼ 20 wt.% and D12 z 0.60  109 m2/s at C ¼ 60 wt.% (Ternstrom et al., 1996). Fig. 3 shows changes in concentration of the total soluble solids in intrinsic juice DBrix versus the intrinsic moisture content Wi for the samples Sc and St. The relationship between Wi and concentration of glycerol C for the sample Sc is also shown. The good linear dependences were observed between DBrix and Wi, as well as for Wi and C:

Fig. 3. Changes in concentration of the total soluble matter in intrinsic juice DBrix versus the intrinsic moisture content Wi for the samples Sc and St (a) and value of Wi versus the concentration of glycerol C for the sample Sc (b). The symbols are the experimental data and the lines correspond to the linear dependencies according to Eqs. (4)e(5). Samples were pretreated by PEF (E ¼ 800 V/cm, tPEF ¼ 0.1 s).

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DBrix0 ¼ a  bWi

(4)

Wi ¼ c  dC

(5)

where a ¼ 76.3 ± 1.1, b ¼ 0.851 ± 0.015, R2 ¼ 0.996, and c ¼ 88.76 ± 0.58, d ¼ 0.702 ± 0.013, R2 ¼ 0.998. Fig. 4 demonstrates the typical examples of temperature changes during cooling (a,b) and heating (c,d) of the apple samples St (a,c) and Sc (b,d) at different values of intrinsic moisture content Wi. The correspondence between listed values of Wi and values of DBrix (samples St) or concentrations of glycerol C (samples Sc) can be evaluated from Fig. 3a or b, correspondingly. The temperature decrease initially was rather slow with the plateaus (Fig. 4a, b) that corresponds to the ice crystallisation zone (Chevalier et al., 2000). Such plateaus were observed in the temperature intervals between z3 to 30  C. The freezing-point depression can be explained by the presence of ionic and glycerol solutes inside apple samples (Franks, 1985). The second cooling stage is started when the most freezable water is converted to ice. At this stage the temperature began to decrease rapidly to the storage value (35  C) (Fig. 4a). During the heating process T(t) curves were initially stabilized at some temperature that corresponded to the melting temperature T z Tm (Fig. 4c, d). The estimated values of Tm on the base of the

heating curves were approximately the same as previously estimated from data of differential scanning calorimetry (see insert to the Fig. 4d (Parniakov et al., 2016)). After the finishing of the ice thawing the temperature began to increase again. The effective thawing time, tm, was defined as the heating time required for exceeding the sub-zero level (Fig. 4d). Fig. 5 compares the effective freezing tf, (a); thawing times, tm, (b) and freezing temperature, Tf(b) for the samples St and Sc. The obtained data evidenced that PEF-assisted osmotic treatment results in noticeable acceleration of the freezing/thawing processes as compared with those for untreated samples. These results are in good correspondence with the previously reported data (Ben Ammar et al., 2010; Parniakov et al., 2015). The differences in kinetics and freezing temperature were also observed for the samples St and Sc. The most accelerated kinetics was observed for the sample St and the pronounced differences in the values tf and tm was observed at intermediate values of Wi (z60e70% wt). The noticeably smaller freezing temperature for the sample Sc as compared with sample St was observed. These differences for the samples St and Sc can reflect the differences in spatial distributions of water and solute molecules in the samples. The more homogeneous distribution of different matter was expected in the sample Sc that was obtained with application of long term dehydration at different glycerol concentrations. From the

Fig. 4. Evolution of temperature, T, inside the geometrical centre of the sample during cooling (a,b) and heating (c,d) at different values of the intrinsic moisture content Wi for the St (a,c), Sc (b,d) and untreated (U) samples. Here, tf and Tf are the effective freezing time and effective freezing temperature, respectively (a); tm is an effective thawing time and Tm is an effective melting temperature (c). Inset to (d) shows effective melting temperature, Tm, versus intrinsic moisture content Wi (Parniakov et al., 2015). Samples were pretreated by PEF (E ¼ 800 V/cm, tPEF ¼ 0.1 s).

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37

Fig. 6. Texture index, F*, versus intrinsic moisture content, Wi, for the samples Sc and St. Here, arrows show values of F* for rehydrated samples.

Fig. 5. The effective freezing, tf, (a) and thawing, tm, (b) times, and effective freezing temperature, Tf, (c) versus the intrinsic moisture content Wi for the samples St and Sc. u , T u for untreated samples are also presented. Samples were pretreated The data Tfu , Tm f by PEF (E ¼ 800 V/cm, tPEF ¼ 0.1 s).

other hand, the distributions of the values of Wi, and Brix inside the sample St may be essential. Diffusion theory evidences the presence of spatial distributions of solutes inside the sample during the diffusion (Crank, 1975). These distributions can be evaluated using the Fick's diffusion equation. The presence of highly inhomogeneous distribution of solutes inside apples discs was also confirmed in recent experiments (Parniakov et al., 2015). The profiles of intrinsic moisture and glycerol can be schematically presented as shown in Fig. 7. Note that the experimentally determined values of Wi, and Brix are spatially averaged over the volume of the sample. The increase of OD duration results in a deeper penetration of the glycerol inside and in the increase of water diffusion outside the apple tissue. That's why the values of tf, tm, Tf for sample St reflect the crystallization and thawing processes in sample with highly inhomogeneous spatial distributions of moisture and glycerol. Overall the concentration of glycerol in the centre of the sample for the given value of Wi was higher for the sample Sc and the smaller freezing temperature was observed for this sample as compare with sample St (Fig. 5c). Finally, the texture relaxation tests were done for freeze-thawed St and Sc samples. Fig. 6 shows the texture index, F*, versus intrinsic moisture content, Wi. The values F* gone through the maximum at Wi¼Wi0 as values of Wi increase for both St (Wi0 z63 wt. %) and Sc (Wi0 z75 wt. %) samples. The maximum texture indices were rather higher (z0.9e0.95) and comparable with texture index of the fresh untreated apple (F* ¼ 1.0). The optimum parameters of OD were t z 90 min, C ¼ 60 wt. % (Wi0 z63 wt. %) and t ¼ 180 min, C z 20 wt.

Fig. 7. The schema of OD of electroporated apple tissue. Here, the profiles of intrinsic moisture and glycerol are schematically presented.

% (Wi0 z75 wt. %) for the samples St and Sc, respectively. Note, that these results are in full correspondence with recently reported data on strengthening of texture for the osmotically dehydrated PEF-treated tissues after defrosting (Parniakov et al., 2015; Shayanfar et al., 2014, 2013). The observed texture index can reflect the water and glycerol content inside the treated samples. In order to check the effect of glycerol on strengthening of texture the additional diffusion experiments using the immersion of samples in apple juice for 3 h were done. The final texture indexes decreased for both samples down to F*z0.2. It means that the effects of strong texture of samples are mainly related with presence of the osmotic agent (glycerol) inside the tissues.

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O. Parniakov et al. / Journal of Food Engineering 183 (2016) 32e38

4. Conclusion Pre-treatment by pulsed electric fields allows more effective OD of apple tissue in apple juice-glycerol solutions. OD was applied for different durations (t ¼ 0e180 min) at the fixed concentration of glycerol (C ¼ 60 wt. %) (sample St) and for different values of C (0e60 wt. %) at the fixed value of t (¼180 min). Freezing-thawing processes and textures of the samples St and Sc were tested. PEFassisted OD results in noticeable acceleration of the freezing/ thawing processes as compared with those for untreated samples. For the fixed Wi the most accelerated kinetics was observed for the sample St. The pronounced differences in the values tf and tm were observed at intermediate values of Wi (z60e70 wt. %). The noticeably smaller freezing temperature for the sample Sc as compared with sample St was explained by higher concentration of glycerol in the centre of the sample. After defrosting the strong texture with texture index of F*z0.9e0.95, comparable with the texture of the fresh apples (F* ¼ 1.0), was observed for the samples at some optimum value of intrinsic water content Wi0 z63 wt. % (for the sample St) and Wi0 z75 wt. % (for the sample Sc). This strong texture reflected the presence of glycerol inside the tissue. The subsequent rehydration in apple juice resulted in noticeable decrease of texture index down to F*z0.2. Acknowledgements The authors appreciate the support from the COST Action TD1104 (EP4Bio2Med - European network for development of electroporation-based technologies and treatments). Parniakov O. thanks thesis grant provided by the French Ministry of Education. References Amami, E., Vorobiev, E., Kechaou, N., 2006. Modelling of mass transfer during osmotic dehydration of apple tissue pre-treated by pulsed electric field. LWT Food Sci. Technol. 39, 1014e1021. Ben Ammar, J., Lanoiselle, J.-L., Lebovka, N.I., Van Hecke, E., Vorobiev, E., 2010. Effect of a pulsed electric field and osmotic treatment on freezing of potato tissue. Food Biophys. 5 (3), 247e254.  Kharaghani, A., Lech, K., Figiel, A., Carbonell-Barrachina, A.,  nchez, A., Calín-Sa Tsotsas, E., 2015. Drying kinetics and microstructural and sensory properties of black chokeberry (Aronia melanocarpa) as affected by drying method. Food Bioprocess Technol. 8, 63e74. Chevalier, D., Bail, A., Le Ghoul, M., 2000. Freezing and ice crystals formed in a cylindrical food model: part I. Freezing at atmospheric pressure. J. Food Eng. 46 (4), 277e285. Crank, J., 1975. The Mathematics of Diffusion. Clarendon Press, Oxford. D'Errico, G., Ortona, O., Capuano, F., Vitagliano, V., 2004. Diffusion coefficients for the binary system glycerolþ water at 25 C. A velocity correlation study. J. Chem. Eng. Data 49, 1665e1670. De Vito, F., Ferrari, G., Lebovka, N., Shynkaryk, N., Vorobiev, E., 2008. Pulse duration and efficiency of soft cellular tissue disintegration by pulsed electric fields. Food Bioprocess Technol. 1, 307e313. Donsi, F., Ferrari, G., Maresca, P., Pataro, G., 2011. Food quality: control, analysis and consumer concerns. In: Medina, D.A., Laine, A.M. (Eds.), Food Quality: Control, Analysis and Consumer Concerns. Nova Science Publishers, Inc., pp. 505e554

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