Influence of the electrohydrodynamic process on the properties of dried button mushroom slices: A differential scanning calorimetry (DSC) study

food and bioproducts processing 9 5 ( 2 0 1 5 ) 83–95

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Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp

Influence of the electrohydrodynamic process on the properties of dried button mushroom slices: A differential scanning calorimetry (DSC) study Somayeh Taghian Dinani a,∗ , Nasser Hamdami b , Mohammad Shahedi b , Michel Havet c , Delphine Queveau c a

Department of Food Science and Technology, Shahreza Branch, Islamic Azad University, Shahreza, Iran Department of Food Science and Technology, College of Agriculture, Isfahan University of Technology, Isfahan 84156-83111, Iran c LUNAM, ONIRIS, GEPEA (CNRS UMR 6144), Rue de la Geraudière, BP 82225, 44322 Nantes, France b

a r t i c l e

i n f o

a b s t r a c t

Article history:

In this paper, drying of button mushroom (Agaricus bisporus) slices with an innovative drying

Received 1 September 2014

technique of hot air combined with Electrohydrodynamic (EHD) drying process was inves-

Received in revised form 31 March

tigated at three electrode gaps (5, 6, and 7 cm) and voltage levels (17, 19, and 21 kV). The

2015

effects of different hot air combined with EHD drying treatments on the temperature of

Accepted 8 April 2015

the mushroom slices, drying time, final color and protein denaturation features including

Available online 17 April 2015

enthalpy (H), onset temperature (To ), peak transition temperature (Tp ), conclusion tempera-

Keywords:

In addition, water state changes in DSC cooling thermograms of dried mushroom slices were

Electrohydrodynamic (EHD)

investigated. The results obtained by differential scanning calorimetry showed that the H

Mushroom slices

values in the DSC traces of the EHD-dried mushroom slices were reduced with a decrease

ture (Tc ), and temperature range (Tc –To ) of endothermic peaks were systematically evaluated.

DSC thermograms

in the electrode gap and an increase in the voltage. Specifically, among voltages of 21, 17,

Enthalpy changes

and 19 kV, a voltage of 21 kV resulted in the lowest H and Tc –To values and the highest Tp

Endothermic peaks

and To values. This result indicated that voltage had a significant effect on these responses.

Protein denaturation

Similarly, the DSC results showed a considerable effect of high electric field intensity on H, Tc –To and Tp responses related to protein denaturation in comparison to low electric field intensity. © 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1.

Introduction

Drying is one of the earliest methods of food preservation, representing a very important aspect of food processing (Bajgai and Hashinaga, 2001a). Conventional drying methods are convective, conductive, dielectric heating, and radiation as well as drying in superheated steam and lyophilization either alone or in different combinations (Bajgai et al., 2006). However, most of the commercial drying methods that have been developed are not very successful in terms of providing economical and

∗

high-quality products simultaneously (Taghian Dinani et al., 2014d). For instance, conventional hot air drying often causes heat damage and undesirably affects the color, texture, flavor, and nutritional value of dried foods. Although freeze–drying can be applied to produce products with excellent quality (Bajgai and Hashinaga, 2001a), it is slow and requires expensive equipment. Thus, it is rarely used for the preservation of inexpensive products such as cultivated mushrooms, and is limited to precious wild edible species and medicinal species (Argyropoulos et al., 2011). These problems have led to the

Corresponding author. Tel.: +98 3153292069; fax: +98 3153232701 2. E-mail address: [email protected] (S. Taghian Dinani). http://dx.doi.org/10.1016/j.fbp.2015.04.001 0960-3085/© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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application of novel techniques such as Electrohydrodynamic (EHD) drying, which desirably satisfies both issues, appropriate product quality and low energy consumption (Singh et al., 2012). The principal mechanism of EHD drying is a secondary flow which is known as ionic or corona wind (Ramachandran and Lai, 2010) obtained by applying a high voltage to a highly curved electrode (such as a pin, needle or wire). The air around the sharp electrode is ionized (Kamkari and Alemrajabi, 2010) and ions of the same polarity, including those of the corona or discharge electrode (sharp electrode), are propelled to the flat electrode (grounded electrode) at high velocities. Along their path, these ions collide with neutral air molecules and transfer their momentum to the neutral air molecules. The resulting flow of ions from the discharge electrode to the grounded electrode is called corona wind (Ould Ahmedou et al., 2009). The net effect of corona wind is destabilization of boundary layer, thus leading to a substantial increase in moisture removal rate (Ramachandran and Lai, 2010) due to the enhancement of heat and mass transfer coefficients (Lai and Wong, 2003). Additionally, the EHD drying technique has many pleasant features such as being simple (no moving parts involved), lightweight, non-mechanical and thus requiring little maintenance (Tansakul and Lumyong, 2008), having low acoustic noise, rapid control of performance by varying the applied electric field and by lowering power consumption (Feng and Seyed-Yagoobi, 2004). However, studies have shown that the EHD drying method is extremely effective at the first stage of drying i.e., constant-rate period. Moreover, similar to most drying techniques its effectiveness decreases as the drying process advances in time. It is speculated that the improvement in drying rate can be further enhanced by a supplementary heating element (Tansakul and Lumyong, 2008). Therefore, in this paper, an EHD drying system in combination with a supplementary heating element with a fixed high temperature (60 ◦ C) was applied to dry the mushroom slices as a novel drying technique. Then, the results obtained from this new approach were investigated. Mushroom was selected for drying as an important commercial edible fungus (Giri and Prasad, 2007) with a relatively short shelf life at ambient temperatures in comparison to other vegetables and fruits. As a result, it has to be processed by various methods in order to extend its shelf life. Among these methods, drying is a moderately inexpensive technique for mushroom preservation (Taghian Dinani et al., 2014c). Although EHD drying is a novel drying method with the mentioned advantages, the question of degradation of the product exposed to EHD drying still needs to be more deeply investigated. In the study of Hashinaga et al. (1999), HPLC analyses indicated no formation of foreign substances in EHDdried apple slices. Bajgai and Hashinaga (2001b) reported that the HPLC analysis of organic acids and sugars of spinach samples indicated no generation of new undesirable substances in the EHD-dried spinach samples compared to the convectivedried samples. Xue et al. (1999) analyzed EHD-dried whey proteins by the DSC technique. They reported that the ovendried whey proteins resulted in extensive changes in the number of electrophoretic bands and band intensity whereas no such changes were observed after EHD drying. The DSC analysis showed that the EHD-dried whey proteins and the native state of the whey proteins had similar thermograms. This review of the literatures reveals that the effect of electric field intensity on physicochemical changes and denaturation of dried foods has not been sufficiently explored and limited

information is available about this controversial issue. In thermal denaturation of foods, the three most important phase changes are protein denaturation, water (or ice) phase change and starch gelatinization (Zhang et al., 2002). In the current study, button mushrooms were analyzed for their water state and protein phase changes due to their high amount of water and protein, and their low amount of starch (Sharma and Vaidya, 2011). Thermo analytical methods such as DSC are considered to be convenient and reliable for monitoring changes (Colombo et al., 2010) in physico-chemical properties of materials as a function of temperature (Xue et al., 1999). The basic principle of DSC is to compare the rate of heat flow with that of the sample and the reference when heated or cooled at the same rate. Changes in the sample that are associated with absorption or evolution of the heat make a change in the differential heat flow, which is normally displayed as a peak on an experimental curve (Xue et al., 1999). A few DSC studies report on compounds having all of the above three phase changes (Zhang et al., 2002). To the best of our knowledge, this is the first well-explored DSC study on the effects of different levels of voltages and electrode gaps applied for drying of mushroom slices. In this study, in order to investigate protein changes of mushroom slices during drying, special attention was dedicated to determine protein denaturation features, including enthalpy (H), onset temperature (To ), peak transition temperature (Tp ), conclusion temperature (Tc ), and temperature range (Tc –To ) responses of endothermic peaks. Also, to investigate the water state in the dried mushroom slices, thermograms ranged from a temperature of −20 to 40 ◦ C were obtained. In addition, in this study, the influences of voltage and electrode gap factors on drying time, color of dried mushroom slices, and temperature of mushroom slices during drying were investigated.

2.

Materials and methods

2.1.

Experimental set-up

In the present study, mushroom slices were dried in hot air combined with an EHD drying system presented in Fig. 1. The corona wind was provided by a high-voltage DC power supply (Model Sefelec, Ottersweier, Germany) connected to six horizontal T1- thermocouple alloy (Ni90/Cr10) wires (Chromel® , Hoskins Manufacturing Company, Nebraska, United States) with a diameter of 0.254 mm, a length of 26 cm and an electrode space of 5 cm between the two neighboring wires. The wire electrodes were fixed on a Teflon framework (with 26 cm × 36 cm internal dimensions and 36.7 cm × 42.5 cm external dimensions) projected onto a fixed horizontal grounded metallic plate (23.1 cm × 32.8 cm rectangle stainless steel plate). The blanched mushroom slices were placed on a perforated plate (23.1 cm × 32.8 cm) and placed on the horizontal grounded metallic plate during drying. The electrode gap between the Teflon frame and grounded electrode was fixed by four poles of the desired height. The described EHD set-up was placed in a chamber (VC 7018 Vötsch Industrietechnik, Germany) at a controlled temperature (60 ◦ C) and humidity (10%) during all the drying experiments. During experiments, the sample weight was recorded using a data logger (Model AOIP, Evry, France) and the internal temperature of a blanched mushroom slice was recorded by a fiber optic temperature sensor (Model T1, Canada) connected to a

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Fig. 1 – Scheme of the experimental set-up. fiber optic temperature conditioner (Model ReflexTM , Québec, Canada) and to the described data logger. In the described hot air–EHD drying system, the onset voltage (start voltage of air ionization) was close to 16 kV. Furthermore, because spark and system interruption occurred at voltage levels higher than 21 kV and electrode gaps less than 5 cm, the applied voltage was set at 17, 19, and 21 kV for electrode gaps of 5, 6, and 7 cm in the following drying experiments, which generated an electric field strength range from 2.43 to 4.2 kV/cm.

2.2.

Sample preparation

Fresh Agaricus bisporus mushrooms were supplied from a shop in Nantes, France. In order to perform all the experiments, mushrooms having a homogeneous size and color were selected and washed with tap water so that dust and impurities adhered to them would be eliminated. Afterwards, the white parts of mushroom fruiting bodies were cut into slices with an average thickness of 5 mm using an electric slicer (model SOFRACA® , Morangis, France). The mushroom slices were blanched in boiling water for 2 min and were immediately cooled down by tap water. The surface of the blanched mushroom slices was smoothly wiped with tissues. After removing excess water, around 90 g of blanched mushroom slices were uniformly spread in a single layer on a perforated plate, and placed on the grounded electrode for exposure to the EHD field. The average thickness of the blanched mushroom slices was 3.1 mm and the averaged diameter of the blanched mushroom slices was 33.1 mm. During all the experiments, the weights and temperatures of mushroom slices were recorded at intervals of 10 s. The drying process was continued until the weight of the mushroom slices remained constant. Then, at the end of each drying

experiment, the quality aspects and DSC thermograms of the dried samples were measured. The drying experiments for each combined hot air–EHD drying treatment were performed at least twice.

2.3.

Procedures

2.3.1.

Moisture content and drying rate

The moisture content of mushroom slices was measured at least in triplicate through the hot air oven method. Representative weighed, blanched, and dried mushroom slices were kept in the oven (Memmert, Schwabach, Germany) at 70 ◦ C temperature until a constant weight was obtained. An electronic balance (Model Radwag, AS 220/C02, Radom, Poland) was used for weighing the mushroom slices, and then moisture content (wet basis (%) and dry basis (kg water/kg dry matter)) was calculated (Taghian Dinani et al., 2014d). The drying rate (DR) of mushroom slices was calculated using Eq. (1) and expressed as kg water/kg dry matter. min: DR =

M1 − M2 t2 − t1

(1)

where t1 and t2 are two different times (min) during drying; M1 and M2 are the moisture content (kg water/kg dry matter) of the mushroom slices at time t1 and t2 , respectively (PereaFlores et al., 2012).

2.3.2.

Color

The colors of both the fresh and dried mushroom slices were measured using a chromameter spectrophotometer (Model CM 3500d, Minolta, Japan), with a measuring area of 8mm-diameter. For maximum accuracy, at least seven color measurements of the blanched and dried mushroom slices

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were taken for each treatment. In this investigation, Hunter L, a, and b chromatic scales were used to calculate the color difference (E) using formula (2):



2

2

E = (L0 − L) +(a0 − a) +(b0 − b)



2 0.5

2.3.3.2. DSC program at a temperature range of 10 to 120 ◦ C. (2)

where E indicates the degree of total color change of the dried mushroom slices in comparison to the color values of the blanched mushroom slices having color values of L0 , a0 , and b0 (Giri and Prasad, 2009). Briefly, L, which represents the degree of whiteness and blackness of the mushroom slices, ranges from L = 0 (for black color) to L = 100 (for white color), a, which denotes the degree of redness and greenness of the samples, ranges from negative values (−a) for green color to positive values (+a) for red color, and b, which indicates yellowness and blueness, ranges from negative values (−b) for blue color to positive values (+b) for yellow color (Tarhan et al. 2010).

2.3.3.

Differential scanning calorimetry

In this investigation, the thermal behavior of samples was assessed using a ␮DSC 7 (Setaram, Caluire, France) device connected to a cooling bath (JULABO, Labortechnik GmbH, 77960 seelbach, Germany). The instrument was calibrated using Naphthalene. In order to avoid any water condensation at low temperatures, Nitrogen was used as a sweep fluid during the experiments. The programming and the elaboration of the spectra were performed with Data Acquisition (version 4.1D—TA Instruments) and Calisto Processing (version 1.065—Setaram, Caluire, France) software packages. At first, mushroom slices (143 mg) were accurately weighed to 0.01 mg precision by an electronic balance (Denver Instrument, Arvada, Colorado, United States) into a 1 ml pre-weighed ␮DSC cell, and then were sealed with a crimper. Afterwards, the cells were placed in the ␮DSC and the two programs were run for each sample.

2.3.3.1. DSC program at a temperature range of −40 to 20 ◦ C. The first program was run to determine the amount of freezable water (FW) (if there was any in the dried samples) and unfreezable water (UFW) of the dried mushroom slices between the temperature of −40 and 20 ◦ C. For this program, the initial temperature in the cell was set to −40 ◦ C, and a scan at 1.2 ◦ C min−1 was programmed up to the final temperature of 20 ◦ C. 15 and 10 min dwell isotherms were programmed at the initial and final temperatures. For this program, the reference cell was an empty cell, similar to the one containing the sample. The base line was acquired with two empty cells. The endothermic transition could be attributed to the melting of ice to water. The phase change enthalpies were obtained by comparison of the reference scan base line with the sample scan. If there was an endothermic transition in the thermograms it could be used to calculate the FW and UFW (%) values at 0 ◦ C by Eqs. (3) and (4), respectively (Xanthakis et al., 2013): FW% =

Hsample × 100 Hwater × ms

UFW% = Wwater % − FW%

70 ◦ C. At least two runs per sample were carried out for each treatment.

(3) (4)

where H sample is the heat of melting expressed in J/g, Hwater is the heat of melting of pure water, which is equal to 333.50 J/g, ms is the mass of the sample and Wwater is the total water content of the sample (%), obtained by the oven method at

The second program was run to determine the enthalpy or H (the peak area of the DSC transition curve), onset temperature (To ), peak transition temperature (Tp ), conclusion temperature (Tc ), and temperature range (Tc –To ) of the dried mushroom slices between temperatures of 10 and 120 ◦ C to investigate protein changes during the drying processes. For this program, the initial temperature in the cell was set to 10 ◦ C and a scan at 1.2 ◦ C min−1 was programmed up to the final temperature of 120 ◦ C. 15 and 10 min dwell isotherms were programmed at the initial and the final temperatures. Also, for this program, at least two runs per sample were carried out using a sealed empty cell as a reference.

2.4.

Statistical analysis

In this paper, Complete Randomized Design (CRD) was used to compare nine EHD treatments (5 cm—21 kV, 5 cm—19 kV, 5 cm—17 kV, 6 cm—21 kV, 6 cm—19 kV, 6 cm—17 kV, 7 cm—21 kV, 7 cm—19 kV, and 7 cm—17 kV) with each other. Additionally, a Factorial Experiment in a Randomized Complete Block Design with two variables of voltage (at three levels of 17, 19, and 21 kV) and electrode gap (at three levels of 5, 6, and 7 cm) was used to study the effects of voltage and electrode gap on the investigated responses. An analysis of variance (ANOVA) was performed by the General Linear Model procedure (GLM) of the SPSS software, version 20 (IBM Corporation, New York, USA) at a 95% confidence level (p ≤ 0.05). For both statistical methods, the Duncan’s New Multiple Range Test was utilized to determine differences between the mean values. In addition, the reported results are presented as mean ± standard deviation (SD) for both statistical methods. Multiple regression equations of different responses were obtained using the Statgraphics Centurion XVI software (StatPoint Technologies, Inc., Warrenton, VA, USA) and their contour analyses were provided using Sigmaplot version 11 build 11.0.0.77 software (WPCubed GmbH, Germany).

3.

Results and discussion

3.1.

Temperature profiles

Fig. 2 depicts the variation of internal temperature of the mushroom slices during 100 min of hot air combined with EHD drying treatments at different levels of voltages (17, 19, and 21 kV) and electrode gaps (5, 6, and 7 cm) at 60 ◦ C. The rise in the temperature of the mushroom slices during drying increased as the electric field intensity heightened (Taghian Dinani et al., 2014a). This increase was more significant at 21 kV voltage than at 17 and 19 kV voltages. Chaktranond and Rattanadecho (2010) reported that applying electric fields above a sample at a high air temperature (60 ◦ C) led to a faster increase of temperature in the sample. Furthermore, they demonstrated that a higher applied voltage caused a higher temperature; this is in agreement with our observations. These researchers proposed that corona wind enhanced the convective heat transfer coefficient, and the degree of heat and mass transfer enhancement was dependent on the level of the applied voltage (Chaktranond and Rattanadecho, 2010). In fact, when the EHD drying was performed at a high temperature such as 60 ◦ C, there was enough heat supplied to the

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44 41

Temperature (°C)

38 35 32

5cm-21kV 6cm-21kV 7cm-21kV 5cm-19kV 5cm-17kV 6cm-19kV 7cm-19kV 6cm-17kV 7cm-17kV

29 26 23 20 0

20

40

60

80

100

Time (min) Fig. 2 – Temperature profiles of mushroom slices in the combined hot air–EHD drying system at different voltages and electrode gaps.

Fig. 3 – Variation of drying rate with moisture content of mushroom slices at different voltages and electrode gaps. sample. Subsequently, the temperature of the sample was not decreased by the enhanced water evaporation of EHD drying (Cao et al., 2004).

3.2.

Drying rate curves

Fig. 3 illustrates the drying rate vs. moisture content (%) obtained from hot air combined with an EHD drying system at different voltages (17, 19, and 21 kV) and electrode gaps (5, 6, and 7 cm) at 60 ◦ C. This figure reveals that 7 cm—17 kV and 5 cm—21 kV treatments had the lowest and the highest drying rates during drying and consequently the highest and the lowest drying time required to obtain constant moisture content, respectively. The objective of Fig. 4 is to show the relationship between drying time, voltage, and electrode gap. In addition, the regression equation and the associated R2 value are shown under this figure. In this equation and other equations in the following sections, voltage is in kV and electrode gap is in cm. Drying time can be calculated at any voltage and electrode gap in the investigated ranges using the equation given under Fig. 4. Therefore, using these kinds of equations reduces the requirement for laboratory tests at different voltages and electrode gaps. Fig. 4 also shows that time values steadily decreased with increasing voltage and decreasing electrode gap. This figure reveals that

Fig. 4 – Response surface plot for time response as a function of voltage (V) and electrode gap (G). Response surface equation: Drying time = −944.24 + 47.9792V + 269.335G − 0.33125V2 − 8.18125VG − 7.225G2 , (R2 = 0.861).

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Fig. 5 – Effects of different combined hot air–EHD drying treatments on (a) L, (b) a and (c) E responses of dried mushroom slices. Data are shown as the mean ± SD. For each response, means with different lower case letters are significantly different (p ≤ 0.05).

7 cm—17 kV and 5 cm—21 kV treatments had the highest and the lowest drying times (330.9 ± 4.8 min and 229.2 ± 17.7 min, respectively) required to obtain constant moisture content. Hence, it represents the predominant effect of electric field intensity on the drying rate and time, where a higher electric field intensity (i.e. a lower electrode gap or a higher voltage) led to a shorter drying time. When the electric field intensity increases, the corona wind becomes stronger resulting in a rise in the heat transfer and consequently a drop in drying

time (Taghian Dinani et al., 2014a). Furthermore, a high electric field intensity led to an internal temperature increase in the mushroom slices (Fig. 2) resulting in faster removal of moisture from the sample. Cao et al. (2004) reported that the drying rate increased when the electrode gap decreased and the voltage increased. Also, Bai et al. (2008) reported that the higher the voltage between the two electrodes, the faster the drying rate of kelp. These reports are in accordance with the current study.

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Fig. 6 – Response surface plots for (a) L, (b) a and (c) E of dried mushroom slices as functions of voltage and electrode gap. Response surface equations: (a) L = 121.141 − 12.1446V − 3.66313G + 0.393125V2 − 0.100625VG + 0.19G2 , (R2 = 0.783), (b) a = −33.995 + 1.61083V + 6.22G − 0.0325V2 − 0.1475VG − 0.2725G2 , (R2 = 0.566), (c) E = 179.099 − 13.409V − 14.9232G + 0.415201V2 − 0.23145VG + 1.47172G2 , (R2 = 0.616).

3.3.

Color

Fig. 5(a) illustrates a comparison of the mean values of L response (lightness reduction of the dried mushroom slices in comparison to the blanched ones). This figure shows that the L values ranged from 0.70 ± 0.26 for 7 cm—17 kV treatment to 16.04 ± 6.91 for 5 cm—21 kV treatment. The results indicate that the L value for the 5 cm—21 kV treatment was not significantly greater than that for the 6 cm—21 kV treatment; furthermore, the L value for the 7 cm—17 kV treatment was significantly different from those of all the other treatments except for the 6 cm—17 kV, 7 cm—19 kV, and 5 cm—17 kV treatments (p ≤ 0.01). In fact, increasing voltage and decreasing electrode gap elevated the L response; this corresponds to a reduction in the lightness of the dried mushroom slices. The results obtained by CRD statistical analysis presented in Fig. 5(b) show that the a response was not significantly affected by different combined hot air- EHD drying treatments (p ≥ 0.05). However, the results obtained by the Factorial statistical analysis method demonstrated that the a values were significantly influenced by the voltage (p ≤ 0.05). The a values and the a absolute values of the dried mushroom slices were

significantly increased by increasing voltage; this corresponds to a high degree of red color in the dried mushroom slices. Decreasing L parameter and increasing a parameter could be related to the higher electric field intensity and higher internal temperature of the mushroom slices (Fig. 2) at higher voltages and at lower electrode gaps. However, results show that b was not significantly affected by different combined hot airEHD drying treatments, voltages, and electrode gaps (p ≥ 0.05). Therefore, the parameters L and a were proven to be the main components that play significant role in the color difference (E) changes of the mushroom slices dried by different hot air–EHD drying treatments. Fig. 5 (c) shows that the E response was not significantly affected by the treatments (p ≥ 0.05). However, the results obtained using the Factorial statistical analysis show that the E values were significantly influenced by the voltage (p ≤ 0.05). Decreasing voltage reduced the E value of the dried mushroom slices; moreover, the E value observed at 17 and 19 kV was significantly (p ≤ 0.05) less than that observed at 21 kV. A 2.42% and 32.69% increase in the E value at 19 and 21 kV (12.25 ± 0.74 and 15.87 ± 3.50, respectively) in comparison to that at 17 kV (11.96 ± 2.90) was calculated, respectively (Taghian Dinani

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Temperature (ºC) 0

10

20

30

40

50

60

70

80

90

100

110

120

Endotermic Heat Flow (mW)

7cm-17kV 7cm-19kV 7cm-21kV 6cm-17kV 6cm-19kV 6cm-21kV 5cm-17kV 5cm-19kV

5 mW

5cm-21kV

Fig. 7 – DSC thermograms of the mushroom slices dried by a combined hot air–EHD drying system at different voltages and electrode gaps. et al., 2014b). Li et al. (2006) reported that the color of EHD dried okara cake became distinctly browner than that of the control; this observation is in agreement with the present study. Figs. 6a–c depict the results of multiple regression analysis, through which equations have been established between L, a and E responses, and the independent variables of voltage and electrode gap of the combined system. Using the relevant equations under this figure, L, a, and E responses can be calculated in terms of the two independent variables, voltage and electrode gap.

3.4. Characteristics of DSC thermograms of the dried mushroom slices over a temperature range of 10 to 120 ◦ C Fig. 7 gathers the DSC thermograms of mushroom slices being dried by hot air combined with an EHD drying method at three levels of voltages (17, 19, and 21 kV) and three levels of electrode gaps (5, 6, and 7 cm). These were continued until constant moisture content was obtained. This figure demonstrates that the DSC curves of the dried mushroom slices had one apparent major endothermic peak. According to Fig. 7, it seems that treatments at high voltages or low electrode gaps decreased the size of endothermic peak. Because measurements of the changes in these peaks throughout the treatments allowed a precise comparison of different combined hot air–EHD drying treatments on the dried mushroom slices; H and endothermic characteristics of endothermic peaks containing To , Tp , Tc , and Tc –To were determined from the obtained DSC thermograms and analyzed in the two following sections.

3.4.1. Enthalpy changes (H) of DSC thermograms over a temperature range of 10 to 120 ◦ C Fig. 8 depicts a comparison of the mean values of the H response of the mushroom slices dried by different hot air combined with EHD drying treatments. The results indicate that H was significantly affected by the treatments (p ≤ 0.01). This figure shows that the H values ranged from 11.31 ± 1.69 J/g for 7 cm—21 kV treatment to 27.47 ± 7.12 J/g for 7 cm—17 kV treatment. In addition, the results indicate

that the H value for 7 cm—21 kV treatment was not significantly less than that for 5 cm—17 kV, 5 cm—19 kV, 5 cm—21 kV, 6 cm—21 kV, and 6 cm—19 kV treatments. Furthermore, the H value for 7 cm—17 kV treatment was significantly different from all of the treatments except for the 7 cm—19 kV, 6 cm— 17 kV and 6 cm—19 kV treatments (p ≤ 0.01). Results of the Factorial statistical analysis indicate that the H values in the DSC traces of the EHD-dried mushroom slices were reduced with the voltage increase and electrode gap decrease. Results show that the H value of the dried mushroom slices at 21 kV (14.52 ± 3.27 J/g) was significantly less than that at 17 and 19 kV (20.54 ± 8.24 and 20.23 ± 6.59 J/g, respectively) (p ≤ 0.05). However, there was no significant difference (p ≥ 0.05) in the H mean value of 17 and 19 kV voltages. 41.46% and 39.32% increases in the H of 17 and 19 kV voltages in comparison to that of 21 kV were calculated, respectively. Also, the results indicate that the H value of the dried mushroom slices at a 5 cm electrode gap was significantly (p ≤ 0.01) less than those at 6 and 7 cm electrode gaps, but there was no significant difference (p ≥ 0.05) in the mean value of H for 6 and 7 cm electrode gaps. 42.73% and 59.16% increases in H values at 6 and 7 cm electrode gaps (19.64 ± 3.82 J/g and 21.90 ± 9.30 J/g, respectively) in comparison to a 5 cm electrode gap (13.76 ± 2.44 J/g) were calculated, respectively. Fig. 9 depicts the results of multiple regression analysis, through which an equation has been established relating the H response and the independent variables of voltage and electrode gap in the combined drying system. Using the relevant equation under Fig. 9, the H values can be calculated in terms of voltage and electrode gap. So, although a decreasing electrode gap and an increasing voltage (electric field intensity increasing) reduced the drying time necessary for obtaining constant moisture content for the mushroom slices processed at 60 ◦ C (Fig. 4), it decreased the H values due to the considerable effect of high electric field intensity in comparison to the low electric field intensity on mushroom slices. Specifically, the low H value of the mushroom slices dried at 21 kV could be correlated with the decrease in lightness and increase in redness (increase in a absolute values) of these samples (Figs. 5(a and b) and 6(a and b)) due to the high intensity of the

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Fig. 8 – Effects of different combined hot air–EHD drying treatments on H of dried mushroom slices. Data are shown as the mean ± SD. Means with different lower case letters are significantly different (p ≤ 0.05). electric field. Two possible explanations for the H reduction in the mushroom slices dried at high electric field intensity (higher voltage or lower electrode gap) in comparison to those dried at low electric field intensity can be offered: The first explanation can be attributed to protein denaturation due to the high electric field intensity. In fact, proteins are in equilibrium between the native (folded) conformation and denatured (unfolded) state (Dima et al., 2011). Changes in the secondary, tertiary or quaternary structure of a protein molecule are usually referred to as denaturation (Arntfield and Murray, 1981). Because DSC data can reflect the thermal denaturation of the proteins that have remained native-like in a sample, the higher the level of protein denaturation, the fewer the native-like proteins remaining and the lower H in a DSC trace (Kazemi et al., 2011). Xiang et al. (2011) reported that applying pulsed electric fields of intensity 22 and 25 kV/cm induced changes in the microenvironment of the tryptophan residues of soy protein from a less polar to a more polar environment. They indicated that these changes in the polarity were caused by partial denaturation and modification of the protein structure under the pulsed electric field

Fig. 9 – Response surface plot for H response as a function of voltage (V) and electrode gap (G). Response surface equation: H = −580.485 + 39.7907V + 75.3341G −0.674625V2 − 2.61019VG − 1.80575G2 , (R2 = 0.759).

treatment; this subsequently led to an increase in the surface hydrophobicity. Arntfield and Murray (1981) explained that the reduction in H values was because of the structure weakening due to an increase in the number of positive charges or an excess of repulsive negative charges. In fact, when the net surface charge of the proteins was effectively neutralized, the maximum values for H were obtained. Considering the mentioned conclusions and the nature of the high electric field between the grounded and discharge electrodes, a hypothesis is proposed in the present study. This hypothesis represents that the H reduction of the mushroom slices dried at high electric field intensity can be attributed to the weakening of the sample protein structure due to an increase in the number of positive or negative charges which arise from the high electric field. In addition, protein molecules are dielectric and the strong electric field is even expected to orient the protein molecules in the direction of the field. Thus, an order is created as the protein dries and the entropy of the protein molecules reduces (Xue et al., 1999). Singh et al. (2013b) reported that under the effect of the nominal electric field strength of 0.002 V/nm and 0.004 V/nm, soybean hydrophobic protein reoriented itself in the direction of the electric field; additionally no considerable effect was observed on the secondary structure and surface properties of the protein. However, under an electric field strength of 3 V/nm, the protein unfolded and nearly all the helical structures were damaged. As a result, the electric field strength of 3 V/nm considerably affected the protein conformation. Furthermore, it was reported in another study that static electric field strengths of 0.001 V/nm and 0.002 V/nm induced conformational changes in the gliadin protein and that could alter the structure of the protein (Singh et al., 2013a). These studies can confirm our hypothesis. The second explanation can be attributed to exothermic reactions, especially aggregation of denatured proteins during DSC processing. In fact, the residual enthalpy is a net value consisting of a combination of endothermic reactions such as the disruption of hydrogen bonds and exothermic processes such as protein aggregation. As a result, an enhanced aggregation of denatured proteins might cause a reduction in the denaturation enthalpy (Ibanoglu and Erc¸elebi, 2007) of the mushroom slices dried at high electric field intensity (probably with more denatured proteins). In fact, aggregation of the denatured molecules is involved in the formation

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Fig. 10 – Effects of different hot air combined with EHD drying treatments on onset temperature (To ), peak transition temperature (Tp ), conclusion temperature (Tc ), and temperature range (Tc –To ) of endothermic peaks. Data are shown as the mean ± SD. For each response, means with different lower case letters are significantly different (p ≤ 0.05).

of new intermolecular bonds, and thus would be expected to reduce the net intensity of the denaturation endotherm (Fitzsimons et al., 2007). Anyway, the endothermic nature of all of the thermograms in Fig. 7 is an indication of the very large contribution of hydrogen bond disruption (Arntfield and Murray, 1981) and protein denaturation during the DSC process; this means a lack of complete denaturation of proteins during different combined convective-EHD drying treatments, in spite of high electric field effects during drying treatments. The reduction of the H values of the mushroom slices dried at high electric field intensity in comparison to those dried at low electric field intensity can be attributed to one or a combination of both mentioned explanations. Nevertheless, determination of the most important reason needs more investigations.

3.4.2. Temperature characteristics of endothermic peaks over a temperature range of 10 to 120 ◦ C The results obtained by CRD statistical analysis presented in Fig. 10 show that the To response was significantly affected by treatments (p ≤ 0.01) and that ranged from 32.48 ± 4.78 ◦ C for a 5 cm—19 kV treatment to 49.30 ± 4.74 ◦ C for a 6 cm—21 kV treatment. The To value for the 6 cm—21 kV treatment was not significantly greater than that for the 5 cm—21 kV, 7 cm—21 kV, 5 cm—17 kV, and 6 cm—19 kV treatments. Also, the To value for the 5 cm—19 kV treatment was not significantly lower than that for the 7 cm—17 kV, 6 cm—17 kV, and 7 cm—19 kV treatments (Fig. 10). The results obtained by the Factorial statistical analysis method show that the To of 21 kV (46.09 ± 3.70 ◦ C) was significantly (p < 0.01) greater than those of 17 and 19 kV (38.61 ± 3.61 ◦ C and 37.57 ± 5.20 ◦ C, respectively), but there was no significant difference between the To of 17 and 19 kV voltages. There was a 16.23% and 18.48% decrease in To of 17 and 19 kV in comparison to the To of 21 kV, respectively. In addition, the results obtained by the Factorial

statistical analysis method show that the To values were not significantly influenced by the electrode gap factor (p ≥ 0.05). Fig. 10 shows that the Tp response of the dried mushroom slices was not significantly affected by the treatments (p ≥ 0.05). The results obtained by the Factorial statistical analysis method show that the Tp of 21 kV (97.64 ± 5.84 ◦ C) was significantly (p ≤ 0.01) greater than those of 17 and 19 kV (91.09 ± 5.09 ◦ C and 87.92 ± 3.00 ◦ C, respectively), but there was no significant difference between those of 17 and 19 kV. There was a 6.71% and 9.95% decrease in the TP of 17 and 19 kV in comparison to that of 21 kV, respectively. Results show that the Tp values were not significantly influenced by the electrode gap factor (p ≥ 0.05). The increase in the Tp value of 21 kV is probably a reflection of the denatured proteins removed at low temperatures, resulting in a higher average value of Tp for the remaining proteins (Arntfield and Murray, 1981). Badea et al. (2012) reported the simultaneous decline in enthalpy and increase in denaturation temperature; this is in accordance with the present study. The decrease of the enthalpy with increasing Tp response can be attributed to the existence of exothermic reactions (Colombo et al., 2010). Liu et al. (2011) reported that increases in peak temperatures and decreases in enthalpy changes in the DSC thermograms for soy protein components were observed when the pulsed electric field treatment intensity was increased; this observation suggested that pulsed electric field treatment could reduce the internal interactions. They suggested that enthalpy changes of soy protein fractions arose from the denaturation of proteins and decline in the ordered structure inferring the decrease in the strength of hydrogen bonding and other interactions within protein molecules. These results are in accordance with this study. Fig. 10 indicates that the Tc response was significantly affected by the treatments (p ≤ 0.01) and the Tc for the dried mushroom slices ranged from 109.50 ± 3.34 ◦ C for 5 cm—17 kV to 115.84 ± 0.09 ◦ C for 6 cm—19 kV drying treatments. The

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Temperature (°C) 0

-40

-30

-20

-10

0

10

20

-2.2

7cm-17kV 7cm-19kV 7cm-21kV

Heat Flow (mW)

-1.1

-3.3

6cm-17kV 6cm-19kV 6cm-21kV

-4.4

5cm-17kV 5cm-19kV 5cm-21kV

-5.5

Fig. 11 – DSC cooling thermograms of the mushroom slices dried by a combined hot-air–EHD drying system at different voltages and electrode gaps.

results indicate that the Tc value for the 5 cm—17 kV treatment was significantly different from those for all the other treatments except for the 7 cm—21 kV and 6 cm—21 kV treatments (p ≤ 0.01). In addition, the Tc value for the 6 cm—19 kV drying treatment was not significantly different from those for all the other treatments except for the mentioned treatments (6 cm—21 kV, 7 cm—21 kV and 5 cm—17 kV) (Fig. 10). The results obtained by the Factorial statistical analysis represent that the Tc values were not significantly influenced by the voltage and electrode gap variables (p ≥ 0.05). The results obtained by CRD statistical analysis presented in Fig. 10 indicate the Tc –To response, which was computed to understand the temperature differences of endothermic peaks, was significantly affected by the treatments (p ≤ 0.01). The Tc –To response for the dried mushroom slices ranged from 62.88 ± 5.35 ◦ C for the 6 cm—21 kV treatment to 80.79 ± 4.68 ◦ C for the 5 cm—19 kV treatment. The results indicate that the Tc –To value for the 6 cm—21 kV treatment was not significantly less than those for the 5 cm—17 kV, 7 cm—21 kV, and 5 cm—21 kV treatments. The Tc –To value for the 5 cm—19 kV treatment was not significantly different those for all the other treatments except for the mentioned treatments (p ≤ 0.01). Results show that the Tc –To value of 21 kV (66.94 ± 4.72 ◦ C) was significantly (p ≤ 0.01) less than those of the 17 and 19 kV (74.52 ± 6.60 ◦ C and 77.29 ± 4.22 ◦ C, respectively), but there was no significant difference (p ≥ 0.05) in the Tc - To values of 17 and 19 kV. Compared to the Tc –To value of 21 kV, an 11.32% and 15.46% increase in the Tc –To values of the 17 and 19 kV was calculated, respectively. Although the To and Tp values at 21 kV were greater than those at 17 and 19 kV, the value of the Tc –To response was less. The reason for the broadness of the typical single DSC peak (for instance, for 17 and 19 kV) can be attributed to the increasing number of denatured proteins (Chandrapala et al., 2011) during the DSC process; this means a fewer number of denatured proteins during drying processes. Xue et al. (1999) reported that a peak broadening might also indicate the existence of intermediate chemical forms of proteins.

3.5. DSC thermograms of the dried mushroom slices over a temperature range of −40 to 20 ◦ C Food properties are much related to the dynamic molecular mobility state of water in foods. In terms of molecular sorption, monolayer and multilayer water have been attributed to bound water, referred to as unfreezable water. Moreover, the term “freezing” of water refers to the crystallization of water (ice formation) on cooling to subzero temperatures (Li et al., 1998). Fig. 11 shows the DSC cooling thermograms of samples do not represent a detectable endotherm peak. Because an endothermic peak can be attributed to the melting point of crystallized water, based on these diagrams, it is possible to conclude that after using hot air combined with EHD drying treatments at different electric field intensity, free water in the dried mushroom slices was eliminated (Bchir et al., 2012). Therefore, the final dried mushroom slices using different hot air- EHD drying treatments presented only unfreezable water; this observation confirms that no special differences exist between the water state of mushroom slices dried by the mentioned drying treatments (Bchir et al., 2009).

4.

Conclusions

In this investigation, a hot air combined with an EHD drying system was designed and applied for drying mushroom slices through a multiple wire to plate configuration. The effects of three levels of voltages and three levels of electrode gaps on the drying temperature, drying time and DSC thermograms of the dried mushroom slices were experimentally verified. The main conclusions are listed below: • Decreasing the voltage and increasing the electrode gap elevated the internal temperature of mushroom slices and reduced the time required for obtaining a constant moisture content. • The mushroom slices dehydrated at high electric field intensity levels (higher voltages), represented a relatively low L

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value and high a and E values. Thus, the product color was adversely affected by the combined hot air–EHD drying processes realized by high voltages. • DSC curves of the dried mushroom slices illustrated one apparent major endothermic peak. The H value in the DSC traces of the EHD-dried mushroom slices was reduced as the voltage increased and the electrode gap decreased, respectively. In addition, the mushroom slices dried by a voltage of 21 kV showed the lowest H and Tc –To values and the highest Tp and To values in comparison to those dried by the voltages of 17 and 19 kV; this indicated that voltage had a significant effect on these responses. • DSC cooling thermograms of the dried mushroom slices lacked an endothermic peak indicating no existence of freezable water in the mushroom slices dried by all the combined hot air–EHD drying treatments.

Acknowledgement Special thanks is addressed to Luc Guihard (ONIRIS) for developing the experimental set-up and his valuable technical contributions.

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