Ohmic heating and pulsed vacuum effect on

Innovative Food Science and Emerging Technologies 36 (2016) 112–119

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Ohmic heating and pulsed vacuum effect on dehydration processes and polyphenol component retention of osmodehydrated blueberries (cv. Tifblue) J. Moreno a,⁎, M. Gonzales a, P. Zúñiga a, G. Petzold a, K. Mella a, O. Muñoz b a b

Group of Emergent Technology and Bioactive Components of Food, Department of Food Engineering, Universidad del Bio-Bio, Casilla 447, Chillán, Chile Institute of Food Science and Technology, Universidad Austral de Chile, Independencia 641, Valdivia, Chile

a r t i c l e

i n f o

Article history: Received 30 March 2016 Received in revised form 10 June 2016 Accepted 12 June 2016 Available online 14 June 2016 Keywords: Ohmic heating Pulsed vacuum Osmotic dehydration Polyphenol components Blueberries

a b s t r a c t Blueberries are highly perishable fruits; therefore, emerging technologies focus on improving the bioactive compound retention and extending the shelf life. The aim of this study was to evaluate the effect of ohmic heating and vacuum pulses on the dehydration processes and polyphenol compound retention of osmodehydrated blueberries (cv. Tifblue). The treatments were performed using a 65% (w/w) sucrose solution, an electric field of 13 V/cm (100 V) at 30 °C, 40 °C or 50 °C for 300 min, and air drying at 50, 60 or 70 °C to obtain dried blueberries. The moisture content, soluble solids and phenolic compounds were analyzed. The combination of ohmic heating/ pulsed vacuum treatments intensifies mass transfer in osmodehydrated blueberries, especially at higher temperatures. Nevertheless, the polyphenol retention was greater at lower temperatures; hence, the application of an intermediate process temperature (40 °C) was selected as a pre-treatment prior to further drying. The treated samples improve the retention of polyphenols after drying compared with untreated samples. Therefore, the results of this research study suggest that the use of a pulsed vacuum and ohmic heating in the osmotic dehydration (PVOD/OH) treatment at 40 °C for 240 min and subsequent drying at 60°C could be the best process for dehydrating blueberries, considering that it improved mass transfer, achieved lower losses of phenolic components and reduced the drying time. Industrial relevance: Blueberries are an important fruit due their high bioactive compound content, especially polyphenols. Studies that involve emerging technologies application could add value to blueberries. Ohmic heating and pulsed vacuum as pre-treatments improve the efficiency of dehydration processes, focused toward bioactive compounds retention and achieving commercial viability. In this work have been applied PVOD/OH treatments at moderated temperatures and subsequently dried at 60 °C, obtaining promissory results. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Blueberries (Vaccinium corymbosum) have a rich content of phenolic compounds with a high antioxidant capacity against free radicals and reactive species (Giovanelli & Buratti, 2009), such that blueberry consumption may have a potentially beneficial effect on human health, leading these small fruits to be increasingly demanded by consumers (Petzold et al., 2016; De Souza et al., 2014; Namiesnik et al., 2014). Blueberries are high perishable fruits; therefore preservation and processing methods focus on extending the berry shelf life. Blueberry consumption has been associated with a protective effect against human diseases, such as cardiovascular disease and cancer (Navindra, 2010), due to the activity of their phenolic components, which are especially abundant in highly colored berries, including phenolic acids, flavonoids (flavonols, flavonoids) and anthocyanins (Skrovankova et al., 2016; Kalt et al., 2001). The combination of moderate-condition technologies is ⁎ Corresponding author. E-mail address: [email protected] (J. Moreno).

http://dx.doi.org/10.1016/j.ifset.2016.06.005 1466-8564/© 2016 Elsevier Ltd. All rights reserved.

necessary to protect their content of phenolic components and extend their shelf-life. Osmotic dehydration (OD) is a process founded on mass transfer, considered as a minimal-processing method that preserves the freshlike characteristics of fruits. It involves the following three simultaneous mass transfers: i) Removal of water from the sample, ii) intake of solids from the solution into the sample, and iii) elution of solute from the samples into the osmotic solution (Selvi et al., 2014). The application of a pulsed vacuum for a short period of time at the beginning of the osmotic treatment (PVOD) has beneficial effects on the kinetic process and quality in many fruits (Fito & Chiralt, 2000). PVOD technology is a mass transfer operation between a porous solid structure (fruit or vegetable) with a liquid phase that it is immersed in, where pressure gradients generated in the system cause a gas outlet and a liquid inlet into the interior of the porous structure (Fito, 1994), increasing the rate of water-related weight loss and solid gain (Moreno et al., 2004; Deng & Zhao, 2008). The cell permeability of their membrane network is also affected by electrical treatments (Moreno et al., 2011a, b; Moreno et al., 2012a, b; Simpson et al., 2015). Ohmic heating (OH) is a thermal process

J. Moreno et al. / Innovative Food Science and Emerging Technologies 36 (2016) 112–119

in which heat is internally generated by the passage of an alternating electrical current (AC) through a body, such as a food system, that serves as an electrical resistance. The food components become parts of the electric circuit through which the alternating current flows, generating heat based on the intrinsic electrical resistance properties of the fruit (Salengke & Sastry, 2007). The main advantages of this technology are the uniformity of heating and improvements in quality with minimal structural, nutritional or organoleptic changes. The temperature can increase rapidly in OH, making the technology efficient in preserving product quality (Sarang, Sastry, & Knipe, 2008). The cell membrane electropermeabilization phenomena have been known for several decades and have received increasing attention because of their application in cells and tissues in food, as cell membrane electropermeabilization is based on measurements of electrical currents through planar bilayer membranes under the influence of strong electric fields and on the molecular transport of molecules into (or out of) cells subjected to electric field pulses (Kulshrestha & Sastry, 2006). Drying is commonly used to increase the stability and shelf life of food preparations and is one of the oldest forms of food preservation (Akbarian et al., 2014). However, the traditional air drying process damages the quality of fruits, causing oxidative damage, browning, and flavor loss (Aguilera & Karel, 1997). The application of concentrated sugar solutions or juices prior to the air-drying of fruits results in a better texture, color, and flavor than those of conventionally air-dried fruits (Torreggiani & Bertolo, 2004). Few papers have reported the effects of emerging technologies on the polyphenol components in blueberries. Sarkis et al. (2013) evaluated the effect of ohmic and conventional heating on the anthocyanin degradation. Brownmiller et al. (2008) analyzed thermally processed blueberry products (canned, juices, and purees) and observed a marked loss in total anthocyanins, suggesting the need for methods to retain the polyphenols in thermally processed blueberries. Lee et al. (2002) compared the impact of two pretreatments (heat and SO2) on the polyphenolics after juice processing of the blueberry, and their results showed no differences among the treatments and the control blueberry. Skrede et al. (2000) studied the loss of polyphenolic compounds in blueberry juice processed by pressing, pasteurization and concentration at 40 °C. There has been no report of the application of these emerging technologies for the retention of the bioactive compounds in blueberries. With respect to the combination of emerging technologies, Moreno et al. (2011a and 2011b) showed that the application of vacuum and ohmic heating on pears and apples accelerates the mass transfer at 50 °C. Additionally, the effect of ohmic heating and vacuum impregnation on the osmotic dehydration of strawberries has been studied (Moreno et al., 2012a; Moreno et al., 2012b). However, more studies are needed to identify the optimal pretreatment parameters and drying conditions to obtain a blueberry with a high retention of polyphenols and an extended shelf life. The aim of this study was to evaluate the effect of ohmic heating and vacuum pulses on the dehydration processes and polyphenol compound retention of osmodehydrated blueberries (cv. Tifblue). 2. Materials and methods 2.1. Sample preparation Blueberries (var. Tifblue) from Chile were obtained from commercial sources and stored at 4 °C until treatment. A 65°Brix sucrose solution was used as the osmotic solution, containing 2 g/L of potassium sorbate (C6H7KO2) and 1.13 g/L of calcium chloride (CaCl2) to help retain the firmness and increase conductivity.

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and ohmic heating (OH) at 30, 40, and 50 °C, controlled by a water bath with a thermostat. The ratio of solution:fruit was 3:1 (w/w) to avoid excessive dilution of the solution (Deng & Zhao, 2008). Osmotic treatments (OD, OD/OH, PVOD and PVOD/OH) were performed for 300 min to determine the mass transfer equilibrium, considering the loss of water and solid gain. In pulsed vacuum treatments, a 5 kPa vacuum was applied for 15 min at the beginning of the process, and then atmospheric pressure was restored (Moreno et al., 2004). In osmotic treatments combined with ohmic heating, the samples were immersed in a tank made of two concentric-cylindrical electrodes of stainless steel (3.7 and 19 cm of diameters) with a distance (d) of 7.65 cm between them, with a nonconducting plastic bottom (Moreno et al., 2011b), and orbital shaking at 100 rpm (Barnstead/Lab-Line MaxQ 2000, Iowa, USA) was employed (Moreno et al., 2012b) to maintain the electric field homogenous, verifying the temperature and voltage with Teflon-coated thermocouples (CPSS-116-24-PFA) connected to an OM-420 data-logger (Omega Engineering, Stamford, USA). The osmotic solution was subjected to an alternating current at 60 Hz and 100 V, which generated an electrical field (E) of 13 V/cm, as calculated by the following equation:

E¼

V d

The temperature was controlled by a refrigeration system because ohmic heating is a high-temperature, short-time process (Leizerson & Shimoni, 2005; Zareifard et al., 2003). Blueberry drying was conducted in circulating air in a convective dryer (Memmert 750, Frankfurt, Germany) with the parameters of a flow rate of 1.5 m/s, hot air temperatures of 50, 60, or 70 °C, and a duration until the humidity was 20% of the weight of the samples. Each batch of technological repetitions was dried separately. 2.3. Analysis of samples composition The moisture content (Xw t ) was determined by drying the samples until constant weight was achieved at 60 °C in a vacuum oven at 10 kPa, according to the method defined by the Association of Official Analytical Chemists (AOAC, 2000); the soluble solid content (Xss t ) was determined by grinding 3 g of the sample with 25 mL of distilled water by an Ultra-Turrax (Ika-Werke, model T25 basic, USA); and the homogenized solution was analyzed using a digital refractometer (Leyca Mark II, Buffalo, NY, USA). The water and soluble solid contents were determined for both fresh and treated samples to evaluate the compositional changes induced by osmotic treatments, applying the equations

Xw t ¼

W 0 −W t ss ; Xt ¼ W0

W 0 Xw t þ W A W 0 ð100−AÞ

where Wt and W0 represent the sample weights at times t and 0, respectively; W represents the weight of distilled water added; and A corresponds to the sample °Brix. ss Changes in the soluble solid and water contents (ΔMw t and ΔMt , respectively) were calculated by the equations " ΔM w t ¼

0 w M0t X w t − M0 X 0

M00

# ; ΔM ss t ¼

" # 0 ss M0t X ss t −M 0 X 0 M 00

2.2. Ohmic heating and pulsed vacuum osmodehydration treatments Osmotic dehydration at atmospheric pressure (OD) and pulsed vacuum osmotic dehydration (PVOD) were conducted with conventional

where M0t and M00 represent the sample weights at times t and 0, respecss w ss tively, and Xw t , Xt , X0 and X0 are the water (w) and soluble solid (ss) mass fractions of the sample at times t and 0, respectively.

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2.4. Total polyphenolic content (TPC)

3. Results and discussion

The total phenolic content (mg GAE/g 100 g d.m.) was determined spectrophotometrically by the Folin–Ciocalteu reagent method (Silveira et al., 2007). Blueberry extracts (250 μL) were mixed with 1250 μL Folin–Ciocalteu reagent and 2500 μL sodium carbonate, and the absorbance at 760 nm was measured on a T70 UV/vis spectrophotometer after 30 min of incubation at room temperature. Gallic acid was used to generate a standard curve, and the results were expressed as gallic acid equivalents (GAE) on a dry weight basis. All of the measurements were performed in triplicate.

3.1. Compositional changes

2.5. Flavonoid and non-flavonoid content by high-performance liquid chromatography (HPLC) The quantification of phenolic compounds (flavonoid and nonflavonoid) in blueberry extract was performed by HPLC (Perkin Elmer, series 200, USA), consisting of an autosampler, a binary pump, a column compartment, a diode array detector, and a Purospher STAR® 100 RP18e column (125 × 4 mm, 5 μm particle size). The extract was obtained using a method previously described (Arranz, Silva, & Saura-Calixto, 2010), where 1.5 mL homogenate extract was filtered through Teflon filters (PTFE). The reversed-phase HPLC method used to analyze flavonols (myricetin) and flavan-3-ol monomers (catechin and epicatechin) has been previously described (Ritchey & Waterhouse, 1999). The column temperature was maintained at 45 °C. Phenolic compounds were identified by comparing the retention times and spectral data with those of the standards. The detection wavelength was 520 nm for anthocyanins and 280 nm for non-flavonoid and flavonols, and the results were expressed as mg/100 g d.m.

2.6. Statistical analysis The results were subjected to an analysis of variance (ANOVA) and an LSD test using Statgraphics Plus 5.0 software with 95% confidence levels (with significance determined by p ≤ 0.05). As the main sources of variance, the osmotic dehydration treatments and the process temperatures were considered with a factorial randomized design (4 × 3). In the same way, the drying process was performed with a 4 × 3 factorial design that considers the osmotic dehydration treatments and drying temperatures. Fig. 1 shows a schematic presentation of the experimental procedure that was used to obtain the dried blueberries. All of the measurements were made in triplicate, and the mean values were reported.

Fig. 1. Schematic presentation of the experimental procedure used to obtain dried blueberries. OD: osmotic dehydration with conventional heating; OH: osmotic dehydration with ohmic heating; PVOD: pulsed vacuum osmodehydration with conventional heating; PVOD/OH: pulsed vacuum osmodehydration with ohmic heating.

The water (ΔMwt) and soluble solid (ΔMsst) content changes of the treated blueberries are presented in Figs. 2, 3 and 4 at 30 °C, 40 °C, and 50 °C, respectively. The proportion of ΔMwt was greater than that of ΔMsst in all of the treatments, and both parameters were enhanced when the combined treatments of pulsed vacuum and ohmic heating (PVOD/OH) were applied at higher temperatures, due to their synergistic effect. Significant differences between pulsed vacuum (PVOD, PVOD/ OH) and atmospheric processes (OD, OD/OH) can be observed for ΔMwt, but only slight differences were detected for ΔMsst, noting that pulsed vacuum treatment improved the mass transfer of the osmotic dehydration, obtaining a greater water loss and solid gain over the other samples. It was identified that the mass transfer reached equilibrium at 240 min into the process, according to the process time used by Kucner et al. (2013). Once the equilibrium was determined, all of the subsequent studies of the polyphenols were performed for 240 min. The OH treatment reduced the process time because the mass transfer is favored by the presence of the following two effects: thermal denaturation and electroporation (Simpson et al., 2007). With respect to electropermeabilization, Moreno et al. (2011a) concluded, following the observation of the microstructure of osmodehydrated pears by scanning electron microscopy (SEM), that electric field (E = 13 V/cm) produced changes in cell structure including reduction of the middle lamella thickness, plasmatic membrane rupturing and cell size decreasing caused by the electroporation and thermal effects. Similar results were obtained by Moreno et al. (2011b) on apples that OD/OH provoked changes in the shape and thickness of the lamellae, reduced cell size, and especially increased cellular breaking. The increase in temperature and the pulsed vacuum application facilitated water loss and promoted the uptake of osmotic solution into the tissue pores, so that the sample reached equilibrium faster than by the osmotic treatment (OD) alone. Data found in the literature confirms that low-pressure pretreatment makes it possible to remove the gas present in the pores of the raw material, which leads to an increased surface of mass transfer and facilitates further dehydration under atmospheric pressure (Rastogi et al., 2002), thereby decreasing the process time and reducing energy consumption (Deng & Zhao, 2008). The water mass fraction (Xw), solute mass fraction (Xss), water content (ΔMwt) and soluble solid (ΔMsst) changes of fresh and processed

Fig. 2. Water content (ΔMwt) and soluble solid (ΔMsst) changes (mean values) of osmotic treatments with 65°Brix osmotic solution. Osmotic dehydration at atmospheric pressure (OD), atmospheric pressure/ohmic heating (OD/OH), pulsed-vacuum (PVOD), and pulsed-vacuum/ohmic heating (PVOD/OH) during 300 min at 30 °C.



blueberries after different treatments (300 min) are shown in Table 1. The treatments combined with ohmic heating (OD/OH and PVOD/OH) increased the concentration levels compared with the sample treated by conventional heating (OD) at 30 °C. The application of electric fields causes increased diffusion through foods, which favors water loss and solid gain (Kemp & Fryer, 2007) and promotes changes in the structural and compositional profiles of the treated samples (Chiralt & Talens, 2005). PVOD treatments promoted slightly higher concentration levels of the samples compared with OD treatments. These results agree with the action of the hydrodynamic mechanism, which accelerates mass transfer (Fito & Chiralt, 1997; Moreno et al., 2000). Thus, the application of pulsed vacuum for a short time has beneficial effects on the osmotic dehydration kinetics and quality in several fruits (Fito & Chiralt, 2000). 3.2. Polyphenol and monomer contents after osmotic treatments Blueberries are rich sources of bioactive compounds, including polyphenols, such as flavonoids, phenolic acids, tannins, and anthocyanins,


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which individually or synergistically help protect against cardiovascular disease, cancer, inflammation, obesity, diabetes and other chronic diseases (Wu et al., 2010). The effect of ohmic heating and pulsed vacuum in the osmodehydratration of blueberries on the polyphenol, anthocyanin, flavan-3-ol and phenolic acid contents are shown in Table 2. All treatments reduced the contents of these bioactive compounds after the osmotic processes, showing significant differences (p b 0.05) with respect to the fresh sample. It has been reported that running the osmotic dehydration process for 24 h at room temperature in a sucrose solution at 60°Brix causes important losses in the bioactive compounds of blueberries, such as total phenolics and total and individual anthocyanins (Giovanelli et al., 2013). However, comparing the treatments, the samples processed at lower temperatures (30 °C or 40 °C) were less damaged with respect to the phenolic compound contents than those treated at 50 °C, especially for the atmospheric pressure treatments (OD and OD/OH), demonstrating a clear temperature effect on the phenolic compounds. This temperature dependence was especially evident in the PVOD/OH treatment at 50 °C, reaching a loss of approximately 50% of the total polyphenol content compared with the fresh sample and a loss of 70–80% of the flavonoids and non-flavonoids. This loss could be explained by the migration of phenolic compounds to the osmotic solution as a result of the temperature increase, leading to a rise in the diffusion flow rate, and high temperature also hampers the selectivity of the cell membranes (Kucner et al., 2013). Anthocyanins are actually localized in the skin and can be removed by dissolution in the osmotic solution because they are water-soluble molecules (Osorio et al., 2007). Additionally, these pigments readily degrade during thermal processing (over 50 °C), which could affect the color and nutritional properties of foods (Patras et al., 2010). 3.3. Drying curve Before drying, the best osmotic process temperature was selected. On the one hand, the mass transfer of water loss and solid gain was greater at higher temperatures, but without significant differences between 40 °C and 50 °C; on the other hand, there was a higher retention of polyphenols in osmotically treated samples at 40 °C compared with at 50 °C (see Section 3.2). Hence, the temperature of the osmotic treatments (OD, OD/OH, PVOD and PVOD/OH) was selected to be 40 °C. The processed samples were dried at different temperatures (50 °C, 60 °C and 70 °C) until reaching a humidity less than or equal to 20%. To achieve this parameter, drying times were applied varying according to the air temperature and osmotic treatment applied. Untreated samples were also dried at the different temperatures as controls. The drying curves for the osmotic treatments and control samples at 50 °C, 60 °C and 70 °C are shown in Fig. 5. It can be observed that the time required for decreasing the free moisture of the sample was shorter at higher drying temperatures for all of the samples, reducing the time of the untreated sample from 57.5 h at 50 °C (Fig. 5a) to 14.17 h at 70 °C (Fig. 5c). This temperature effect was expected because higher temperatures facilitate the removal of water (Price, Sabarez, Storey, & Back, 2000). In addition, it could be observed that the application of osmotic treatments reduced the drying times compared with the control. Giovanelli et al. (2013) found that the untreated blueberry required a longer drying time (420 min) at 70 °C because the initial moisture was much higher in these samples compared with the osmotically treated samples, which needed 300–360 min to reach a constant weight. In Fig. 5a, it can be observed that the time of dehydration in the samples treated by ohmic heating was shorter than those treated by conventional heating. OD/OH had the fastest dehydration process, reaching a final moisture content in 22.5 h, while the OD and pulsed vacuum treatments required 45–54 h. The temperature of 60 °C decreased the time of sample dehydration to half (Fig. 5b), where the drying

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Table 1 Mean values and standard deviations of water mass fraction (Xw), soluble solid mass fraction (Xss), water content (ΔMwt) and soluble solid (ΔMsst) of fresh and processed samples by osmotic dehydration at atmospheric pressure (OD), atmospheric pressure/ohmic heating (OD/OH), pulsed-vacuum (PVOD) and pulsed-vacuum/ohmic heating (PVOD/OH) treatments during 300 min. Treatments – 30 °C 30 °C 30 °C 30 °C 40 °C 40 °C 40 °C 40 °C 50 °C 50 °C 50 °C 50 °C

Fresh OD OD/OH PVOD PVOD/OH OD OD/OH PVOD PVOD/OH OD OD/OH PVOD PVOD/OH a, b… h

Xss

Xw a

a

0.833 ± 0.025 0.785 ± 0.008b 0.718 ± 0.024cd 0.675 ± 0.004efg 0.673 ± 0.033efg 0.729 ± 0.033c 0.693 ± 0.009cde 0.665 ± 0.004efgh 0.638 ± 0.014gh 0.700 ± 0.080cde 0.685 ± 0.015def 0.646 ± 0.003fh 0.632 ± 0.029h

0.150 ± 0.012 0.213 ± 0.020bc 0.220 ± 0.005bc 0.240 ± 0.011cd 0.270 ± 0.009def 0.220 ± 0.016bc 0.214 ± 0.013b 0.252 ± 0.019def 0.270 ± 0.004ef 0.242 ± 0.002cde 0.240 ± 0.012cd 0.261 ± 0.046def 0.280 ± 0.007f

ΔMwt

ΔMsst

– 0.071 ± 0.003a 0.078 ± 0.003a 0.134 ± 0.021bc 0.160 ± 0.023cd 0.110 ± 0.008ab 0.160 ± 0.009cd 0.184 ± 0.009de 0.228 ± 0.017ef 0.151 ± 0.009bcd 0.158 ± 0.010cd 0.267 ± 0.004f 0.270 ± 0.004f

– 0.048 ± 0.003a 0.055 ± 0.010ab 0.051 ± 0.002ab 0.080 ± 0.007bc 0.060 ± 0.008abc 0.070 ± 0.005abc 0.057 ± 0.009ab 0.087 ± 0.002c 0.077 ± 0.005bc 0.070 ± 0.008abc 0.047 ± 0.005a 0.087 ± 0.008c

when there are no significant differences at 5%, homogeneous groups in each column are identified by the same superscript letter, according to a LSD test.

time for OD/OH was 13.3 h, compared with 24.1 h for the control. The samples dehydrated at 70 °C (Fig. 5c) showed a further reduction in drying time compared with the other temperatures, with the OD/OH drying time reduced to 10 h and those of PVOD/OH and PVOD to 10.7 and 10.8 h, respectively. Hence, the application of osmodehydration prior to air drying decreases the moisture content without causing a phase change, preserving the quality of the blueberries and reducing the air-drying time of the sample as a result of the synergistic effect of pulsed vacuum and ohmic heating. 3.4. Polyphenols after drying The total polyphenol (TPC), flavonoid and non-flavonoid contents of fresh blueberry, untreated (control) and samples processed at 40 °C by osmotic treatments (OD, OD/OH, PVOD, PVOD/OH) after drying at 50 °C, 60 °C and 70 °C are shown in Table 3. It can be observed that the samples subjected to drying at different temperatures showed reduced contents of these bioactive compounds compared with the fresh samples, but in general, the application of osmotic treatments improves the retention of polyphenols compared with the control sample. The behaviors of the monomeric flavonoids or non-flavonoids were diverse with respect to the different drying temperatures and treatments. Catechin and epicatechin showed higher contents, expressed as mg/100 g of dry matter, and have been studied with regard to their benefits, such as antioxidant,

UV-radiation protection, proapoptotic and antiangiogenic activities (Folmer et al., 2014). The retention percentages of anthocyanins and flavan-3-ols of the dried samples compared with fresh blueberries are provided in Table 4. At 60 °C, the delphinidin retention was higher by OD treatment, followed by PVOD/OH, which was greater than that of the equivalent treatment at 70 °C. The best retention percentage of cyanidin was obtained by PVOD/OH at 60 °C and that of malvidin was very similar for all treatments, fluctuating in a retention range of 24–33%. The anthocyanin stability is not only affected by the process or temperature, but there are also other factors, such as pH, light, oxygen, enzymes, ascorbic acid, sugars, sulfur dioxide or sulfite salts, metal ions, and copigments (Pereira et al., 2010), which could affect the monomeric anthocyanin retention. The flavan-3-ol retention also differs between them, showing that in general, catechin had an elevated retention for all treatments (57–82%), while the epicatechin retentions were considerably reduced by osmotic processes (8–14%). Nevertheless, the majority of monomeric polyphenols are less damaged in osmodehydrated samples with combined pulsed vacuum and ohmic heating treatments compared with the untreated sample, especially when the drying temperature was 60 °C. These results could be explained by the protection given by the pulsed vacuum treatment due to solute impregnation into the pores protecting the natural tissue structure, improving the texture and quality and lowering the drip loss in the subsequent drying process by limiting

Table 2 Mean values and standard deviations of total polyphenol (TPC), flavonoids and non-flavonoids content of fresh and osmodehydrated blueberry with sucrose solution of 65°Brix at moderate temperatures by 240 min. (OD) osmotic dehydration at atmospheric pressure, (PVOD) osmotic dehydration with pulsed-vacuum and (OD/OH and PVOD/OH) osmotic treatments combined with ohmic heating. Osmotic treatment

Fresh OD OD/OH PVOD PVOD/OH OD OD/OH PVOD PVOD/OH OD OD/OH PVOD PVOD/OH a, b… k

TPC (mg GAE/100 g d.m.) – 30 °C 30 °C 30 °C 30 °C 40 °C 40 °C 40 °C 40 °C 50 °C 50 °C 50 °C 50 °C

845.0 ± 89.0a 626.8 ± 32.7b 534.6 ± 65.1c 475.4 ± 49.4def 468.1 ± 19.8def 613.8 ± 25.1b 481.1 ± 29.3de 455.8 ± 10.6ef 441.6 ± 26.6f 500.7 ± 62.4cd 495.1 ± 13.4d 454.2 ± 34.1ef 402.7 ± 6.7g

Flavonoids (mg/100 g d.m.)

Non-flavonoids (mg/100 g d.m.)

Delphinidin

Cyanidin

Malvidin

Catechin

Epicatechin

Gallic acid

p-Coumaric acid

356.1 ± 11.7a 164.8 ± 2.2c 122.3 ± 0.3f 105.4 ± 1.1h 116.2 ± 0.4g 159.9 ± 0.2d 212.6 ± 1.9b 137.9 ± 1.1e 113.6 ± 1.2g 134.6 ± 2.4e 135.4 ± 2.7e 103.4 ± 1.9h 72.7 ± 0.7i

118.7 ± 4.1a 73.3 ± 0.3bc 66.8 ± 0.2d 51.3 ± 1.3e 47.3 ± 0.1f 72.9 ± 0.3c 76.5 ± 0.9b 52.3 ± 0.0e 52.9 ± 1.5e 53.5 ± 1.6e 53.9 ± 1.3e 41.5 ± 0.2g 33.9 ± 0.1h

194.9 ± 0.5a 109.3 ± 0.5b 79.8 ± 0.4d 66.7 ± 0.2g 72.8 ± 0.3e 105.6 ± 2.8bc 104.9 ± 0.4c 72.7 ± 0.5e 69.0 ± 0.5f 70.6 ± 1.2e 71.3 ± 0.6e 55.9 ± 0.3h 44.3 ± 0.4i

596.5 ± 32.5a 283.3 ± 0.9b 329.8 ± 7.3e 201.8 ± 11.6df 178.7 ± 17.4d 263.3 ± 11.9bc 297.6 ± 17.8eb 225.2 ± 13.1fc 195.7 ± 7.4df 177.3 ± 11.6d 213.3 ± 5.8df 125.7 ± 4.3g 122.2 ± 3.4g

4017.6 ± 3.2a 1150.8 ± 3.8e 1782.7 ± 9.4b 596.1 ± 4.3j 800.7 ± 0.2i 1513.1 ± 5.6c 1329.1 ± 9.4d 930.2 ± 9.6g 919.1 ± 4.2g 839.1 ± 4.8h 968.3 ± 2.9f 667.3 ± 9.3j 427.9 ± 0.5k

168.7 ± 0.5a 128.7 ± 0.1c 126.0 ± 0.8c 85.2 ± 1.5g 86.1 ± 1.4g 115.8 ± 0.2d 131.9 ± 0.8b 101.5 ± 0.4e 89.5 ± 1.6g 86.5 ± 1.2g 91.4 ± 0.2f 78.2 ± 0.3h 60.0 ± 0.7i

210.2 ± 1.5a 123.4 ± 1.3b 109.9 ± 0.1d 87.0 ± 1.8e 70.6 ± 2.5h 113.5 ± 1.1cd 116.7 ± 1.9c 81.6 ± 0.5f 93.6 ± 1.6e 73.9 ± 1.2g 74.5 ± 1.4g 56.7 ± 0.9i 51.8 ± 0.7j

when there are no significant differences at 5%, homogeneous groups in each column are identified by the same superscript letter, according to a LSD test. d.m.: dry matter.


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Fig. 5. Drying curves of untreated and processed samples at 40 °C by osmotic dehydration at atmospheric pressure (OD), atmospheric pressure/ohmic heating (OD/OH), pulsed-vacuum (PVOD) and pulsed-vacuum/ohmic heating (PVOD/OH) treatments, under different drying temperatures a) 50 °C, b) 60 °C and c) 70 °C.

collapse and cellular disruption. Additionally, the removal of oxygen from the pores by pulsed vacuum reduces the oxygen availability for the polyphenoloxidase activity (Zhao & Xie, 2004).

On the other hand, the OH application shortens the drying time, thus favoring bioactive component retention and producing higher-quality products than conventional heating, which can decrease the nutritional

Table 3 Mean values and standard deviations of total polyphenol (TPC), flavonoids and non-flavonoids content of fresh, untreated and osmotically dehydrated blueberry at 40 °C after drying convective at moderate temperatures. (OD) osmotic dehydration at atmospheric pressure, (PVOD) osmotic dehydration with pulsed-vacuum and (OD/OH and PVOD/OH) osmotic treatments combined with ohmic heating. Osmotic treatment

Fresh Untreated OD OD/OH PVOD PVOD/OH Untreated OD OD/OH PVOD PVOD/OH Untreated OD OD/OH PVOD PVOD/OH a, b… m

Dried temperature (°C)

– 50 50 50 50 50 60 60 60 60 60 70 70 70 70 70

TPC (mg GAE/100 g d.m.)

845.0 ± 89.0a 59.2 ± 3.0m 331.3 ± 5.4b 160.6 ± 3.8i 130.6 ± 4.1j 114.7 ± 3.3k 106.7 ± 5.8l 293.2 ± 1.8c 229.1 ± 2.4f 212.5 ± 5.4h 278.2 ± 2.9d 114.3 ± 4.3k 336.2 ± 7.3b 239.1 ± 2.4e 227.1 ± 4.3f 222.2 ± 1.6g

Flavonoids (mg/100 g d.m.)

Non-flavonoids (mg/100 g d.m.)

Delphinidin

Cyanidin

Malvidin

Catechin

Epicatechin

Gallic acid

p-Coumaric acid

356.1 ± 11.7a 138.1 ± 0.6g 124.9 ± 0.5i 128.0 ± 0.8h 146.9 ± 0.3f 153.8 ± 1.4e 124.1 ± 0.9i 203.3 ± 0.8b 141.1 ± 1.6g 125.4 ± 0.4i 164.1 ± 0.5d 123.6 ± 0.7i 153.2 ± 0.5e 190.1 ± 0.3c 193.3 ± 0.8c 139.3 ± 0.5g

118.7 ± 4.1a 66.8 ± 0.3d 69.4 ± 0.4c 56.1 ± 0.6g 64.4 ± 0.6e 53.7 ± 0.5h 60.1 ± 0.5f 63.1 ± 0.5e 66.4 ± 0.6d 75.6 ± 0.9b 78.6 ± 1.3b 61.9 ± 0.2ef 45.8 ± 0.8i 62.6 ± 0.4ef 60.3 ± 0.5f 76.1 ± 0.4b

194.9 ± 0.4a 63.6 ± 0.7c 63.0 ± 0.5c 64.6 ± 0.6bc 64.3 ± 0.5bc 55.6 ± 0.4f 57.5 ± 0.2e 51.5 ± 1.4g 62.3 ± 0.8c 47.6 ± 0.2h 51.4 ± 1.4g 59.4 ± 0.3d 56.3 ± 0.6ef 58.8 ± 0.5de 56.4 ± 0.3e 66.7 ± 0.2b

596.5 ± 12.5a 486.8 ± 2.1b 430.7 ± 2.3d 359.1 ± 1.4h 440.2 ± 1.6c 430.2 ± 1.1d 405.1 ± 1.6f 395.2 ± 1.8g 342.3 ± 1.0i 408.2 ± 1.1f 418.2 ± 1.5e 398.6 ± 2.2g 335.6 ± 1.5j 356.8 ± 2.3h 396.1 ± 1.6g 435.8 ± 2.1d

4017.6 ± 13.2a 356.4 ± 2.6i 318.1 ± 2.7j 358.1 ± 2.9i 519.0 ± 2.2c 544.1 ± 3.6b 315.2 ± 2.9j 446.2 ± 2.3f 373.0 ± 3.2h 511.8 ± 2.7d 500.1 ± 3.1e 303.5 ± 2.9k 320.9 ± 2.4j 427.2 ± 2.9g 424.6 ± 2.2g 444.6 ± 2.6f

168.7 ± 2.5a 74.8 ± 0.9c 77.2 ± 0.5b 67.1 ± 0.3d 67.5 ± 1.1d 67.8 ± 0.9d 73.4 ± 0.2c 67.7 ± 0.3d 61.2 ± 0.5e 66.2 ± 0.8d 59.6 ± 0.6ef 74.9 ± 0.1b 59.3 ± 0.1ef 57.0 ± 0.5f 58.0 ± 0.6f 67.4 ± 0.5c

210.2 ± 1.5a 117.9 ± 0.5c 122.6 ± 0.6b 90.9 ± 0.6e 89.9 ± 0.6f 93.9 ± 0.7e 111.8 ± 0.3d 90.1 ± 0.4ef 81.2 ± 0.6h 72.1 ± 0.5i 84.7 ± 1.2g 112.8 ± 0.4d 86.9 ± 0.5g 72.1 ± 0.3h 79.8 ± 0.8h 82.1 ± 0.7h

when there are no significant differences at 5%, homogeneous groups in each column are identified by the same superscript letter, according to a LSD test. d.m.: dry matter.

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Table 4 Flavonoid polyphenols retention (%) of untreated and osmodehydrated samples at 40 °C after drying convective at moderated temperatures, compared to fresh sample (control). Treatment osmotic

Untreated OD OD/OH PVOD PVOD/OH Untreated OD OD/OH PVOD PVOD/OH Untreated OD OD/OH PVOD PVOD/OH

Dried temperature

50 °C 50 °C 50 °C 50 °C 50 °C 60 °C 60 °C 60 °C 60 °C 60 °C 70 °C 70 °C 70 °C 70 °C 70 °C

Anthocyanins

Flavan-3-ols

Delphinidin (%)

Cyanidin (%)

Malvidin (%)

Catechin (%)

Epicatechin (%)

39 35 36 41 43 35 57 40 35 46 35 43 53 54 39

56 59 47 54 45 51 53 56 64 66 52 39 53 51 64

33 32 33 33 29 29 26 32 24 26 30 29 30 29 34

82 72 60 74 72 68 66 57 68 70 67 56 60 66 73

9 8 9 13 14 8 11 9 13 12 8 8 11 11 11

a, b… k when there are no significant differences at 5%, homogeneous groups in each column are identified by the same superscript letter, according to a LSD test. d.m.: dry matter.

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