High voltage electric discharges assisted extraction of phenolic compounds from grape stems: Effect of processing parameters on flavan-3-ols,flavonols and stilbenes recovery

This work aimed at providing some novel aspects pertaining to flavan-3-ols, flavonols and stilbenes extraction from grape stems using high voltage electrical discharges (HVED). Treatment time, pH and ethanol concentration affecting the extractability of these compounds were optimized through response surface methodology. The results from the optimized extraction technique (pH = 2.5; Time = 4.0 ms: Ethanol = 50%) were compared to a conventional hydro-alcoholic extraction.HVED improved significantly the extraction of flavan-3-ols and flavonols but was less efficient on stilbenes. The efficiency of HVED is directly correlated with the other processing conditions (pH, ethanol concentration). Prolonged HVED treatment at low pH value was a positive combination for flavan-3-ols. The proposed procedure (pH = 2.5; Time = 4.0 ms; Ethanol = 50%) allowed the release of almost 35% of additional phenolic compounds, which can be attributed to a better extractability of flavan-3-ols (+ 21%) and of flavonols (+ 12%), compared to conventional hydro-alcoholic extraction.     

A comparative study of physical pretreatments for the extraction of polyphenols and proteins from vine shoots

This work examined the potential of valorization of vine shoots through their polyphenol and protein contents. However the choice of the experimental conditions targeted polyphenol extraction at the expense of proteins for further simplification of the purification process. The intensification of polyphenol and protein extraction by physical treatments (pulsed electric fields (PEF), high-voltage electrical discharges (HVED) and ultrasound (US)) was studied. A significant enhancement of polyphenol extraction was noticed with HVED, PEF and US. However, and for each treatment, the improvement of the extraction process started beyond a specific energetic threshold (HVED (10 kJ/kg), PEF (50 kJ/kg) and US (1010 kJ/kg)). HVED had the highest polyphenol and protein extraction yields with the lowest energetic prerequisite. Extracts of high polyphenol yield (34.5 mg of gallic acid equivalent (GAE) per g of dry matter (DM)) and high purity (89%) were obtained with HVED. Polyphenol and protein diffusion coefficients (m²/s) demonstrated HVED to better enhance the extraction process of those biomolecules. Similarly, the calculation of the electrical conductivity disintegration index, Z, showed the highest tissue damage for HVED and a rising cellular damage with the increased energetic requirement of each treatment.     

“Ice” juice from apples obtained by pressing at subzero temperatures of apples pretreated by pulsed electric fields

The impact of apple pretreatment by pulsed electric field (PEF) on juice extraction using the freezing-assisted pressing was studied. Apple discs were PEF pretreated at electric field strength of E = 800 V/cm and then air blast frozen inside the freezer (− 40 °C). Then, pressing experiments in a laboratory-pressing chamber (2–5 bars) were started at sub-zero temperature (− 5 °C). Time evolution of juice yield and its nutritional qualities were compared for PEF and untreated apple samples. High improvements of juice yield were obtained for freeze-thawed (FT) and PEF + FT samples. The combination of PEF + pressing (5 bar) at sub-zero temperature gave optimum results for juice extraction with high levels of carbohydrates, and antioxidant bioactive compounds. At fixed value of extraction yield, Y, PEF pretreatment improved nutritional parameters. E.g., at Y = 0.6, an increase in °Brix (by ≈ 1.27), carbohydrates (by ≈ 1.42), total phenolic compounds (by ≈ 1.16), flavonoids (by ≈ 1.09) and antioxidant capacity (by ≈ 1.29) was observed after PEF pretreatment.Industrial relevancePressing constitutes one of the most commonly used technologies at industrial scale to obtain fruit juices. However, during the pressing some undesirable chemical, physical and biological changes may occur in juices, thus reducing their nutritional and sensorial properties. For instance, the use of freezing-assisted pressing is a promising technique for the production of juice concentrates rich in sugars and other solids as the low temperature operation prevents undesirable modifications. But this method is rather expensive and requires strong control of the quality of “ice” juices, their sensory and compositional profiles. Thus, there is an increased search for obtaining new efficient methodologies for producing high quality juices. In this line, PEF-assisted pressing has been shown as a useful technology to increase juice yield. Therefore, the combination of PEF-assisted “ice” juice extraction by pressing of fruits at subzero temperatures may be a useful tool to improve the extraction yield of juices, thus improving their nutritional, physicochemical and sensorial properties.Keywords: “Ice” juice, Apple, Pulsed electric fields, Freezing-assisted pressing     

Comparison of the effect of pulsed electric field or high voltage electrical discharge for the control of sweet white must fermentation process with the conventional addition of sulfur dioxide

The present work discusses the efficiency of pulsed electrical treatments for the inactivation of yeasts. The application of pulsed electric fields (PEFs) and high voltage electrical discharges (HVEDs) as alternatives to sulfites, which are used as anti-microbial to stop the fermentation of sweet white wine, was investigated. The influence of sulfite concentration (from 0 mg·L^− 1 to 500 mg·L^− 1), PEF (from 4 kV·cm^− 1 to 20 kV·cm^− 1; from 0.25 ms to 6 ms) and HVED (40 kV/cm; 1 ms or 4 ms) treatments on the inactivation of total yeasts and non-Saccharomyces yeasts was determined. The addition of SO₂ (250 mg·L^− 1) resulted in 8 log total yeast reduction. The maximum yeast inactivation obtained with PEF and HVED was respectively 3 and 4 logs. The use of SO₂, HVED and PEF allows decreasing the non-Saccharomyces yeast level by 7, 5 and 4 logs respectively. However, the wine browning was less pronounced for the samples treated by PEF in comparison with HVED and SO₂ treatments. PEF seems to be the most suitable alternative technique to sulfite addition.     

Improving the pressing extraction of polyphenols of orange peel by pulsed electric fields

The influence of pulsed electric field (PEF) treatment on the extraction by pressing of total polyphenols and flavonoids (naringin and hesperin) from orange peel was investigated. A treatment time of 60 μs (20 pulses of 3 μs) achieved the highest cell disintegration index (Zp) at the different electric field strengths tested. After 30 min of pressurization at 5 bars, the total polyphenol extraction yield (TPEY) increased 20%, 129%, 153% and 159% for orange peel PEF treated at 1, 3, 5 and 7 kV/cm, respectively. A PEF treatment of 5 kV/cm to the orange peels increased the quantity of naringin and hesperidin in the extract of 100 g of orange peels from 1 to 3.1 mg/100 g of fresh weigh (fw) orange peel and from 1.3 to 4.6 mg100 g fw orange peel respectively. Compared to the untreated sample, PEF treatments of 1, 3, 5 and 7 kV/cm increased the antioxidant activity of the extract 51%, 94%, 148% and 192%, respectively.The results of this investigation demonstrate the potential of PEF as a gentle technology to improve the extraction by pressing of polyphenols from fresh orange peel. This procedure enhances the antioxidant capacity of the extracts, reduces extraction times and does not require using organic solvents.Industrial relevanceProcessing of orange fruits to obtain fresh juice or citrus-based drinks generates very large amounts of byproduct wastes, such as peels that are a rich source of polyphenols mainly flavonoids. Extraction of these compounds from orange peels is a crucial step for use of these compounds in the food and pharmaceutical industries as antioxidants. PEF-assisted extraction by pressing of polyphenols from fresh orange peels stands as an economical and environmentally friendly alternative to conventional extraction methods which require the product to be dried, use large amounts of organic solvents and need long extraction times.     

Recovery of colorants from red prickly pear peels and pulps enhanced by pulsed electric field and ultrasound

The aim of this work was to evaluate the potential of the non-conventional pre-treatments; pulsed electric fields (PEFs) and ultrasounds (USNs), to enhance the extraction of red colorants from red prickly pear (Opuntia stricta Haw.) peels and pulps. The overall goal was to valorize the thick part of the fruit, being discarded for fruit's consumption. PEF and USN treatments were first optimized using fruit slices, followed by a supplementary aqueous extraction (+ SAE) up to 1 h. The optimal conditions were then applied for peels and pulps, separately. Results showed that PEF + SAE and USN + SAE enhanced significantly the extraction of red colorants (betanin/isobetanin), compared to untreated tissues. Promising results for colorants recovery were obtained from fruit peels, using both pre-treatments, compared to that obtained from pulps, and without pre-treatment. Scanning electron microscopy revealed cell denaturation after PEF and USN pre-treatments, which can provide better recovery of the intracellular compounds with less impurity.Industrial relevanceWastes and by-products generated during fruit processing constitute a great source of high-added value compounds, which have the potential to be used as food additives and/or as nutraceuticals. Opuntia fruits constitute a great source of bioactive compounds. In particular, Opuntia fruit processing by-products are interesting as they contain a great amount of potential food additives, including food colorants (i.e. betanin/isobetanin), thus constituting an important alternative to replace synthetic colorants that have been restricted by governmental organizations. In this line, pulsed electric field and ultrasounds are proposed in this work as promising technologies for the enhancement of the extraction of colorants from red prickly pear peels and pulps.     

Effect of pulsed electric field (PEF)-treated kombucha analogues from Quercus obtusata infusions on bioactives and microorganisms

Pulsed electric field (PEF) is a promising non-thermal food preservation technology. The objective was to study inactivation of yeasts in PEF-treated kombucha analogues prepared from Quercus obtusata infusions. Fermentation conditions of infusions from Q. obtusata were time (7 days), sugar (10%), starting culture (10%), and inoculum (2.5%, at 25 °C). The PEF treatment considered using square waves, an electric field strength (37.3–53.4 kV/cm), PEF processing time (445.3–1979.2 μs), an output temperature (18.31 ± 0.98 °C), an input energy (21.2 - 136.5 KJ/L), and two feed flow rates (51.42 and 102.85 L/h). pH, °Brix, color determinations, microbiological testing, total phenolic, flavonoid content, DPPH test, and UPLC/ESI/MS/MS analysis were done. No changes at different PEF conditions were observed for pH and °Brix. Higher color changes were observed at higher specific energies. Acid-acetic bacteria were more sensitive to PEF than yeasts. Lower specific energies render products with higher polyphenolic content and antioxidant capacity.Industrial relevancePulse electric field is an interesting alternative to preserve kombucha analogues from oak leave infusions with minimal changes in physicochemical characteristics, antioxidant activity and bioactive compounds. The present work describes the effect of feed flow and specific energy on the several characteristics of fermented beverages, determining conditions for best processing.     

Impact of Electric Pulse Treatment on Selective Extraction of Intracellular Compounds from Saccharomyces cerevisiae Yeasts

Treatments by high-voltage electrical discharges (HVED, needle-plate electrode geometry, U?=?40 kV, t _p?≈?0.5 μs) and pulsed electric field (PEF, plate–plate electrode geometry, E?=?5–40 kV/cm, t _p?≈?8.3 μs) were evaluated as tools for selective extraction of different intracellular components from the wine Saccharomyces cerevisiae (bayanus) yeasts in a 0.5% (w/w) aqueous suspension. The pulses in the form of damped oscillations and exponential decay were applied in HVED and PEF modes of treatment, respectively. The extraction efficiency results obtained using HVED and PEF techniques were compared with those for high-pressure homogenization technique. The HVED and PEF treatments always resulted in incomplete damage of yeast cells, though efficiency of HVED was higher than that of PEF. The high selectivity of extraction of ionic substances, proteins, and nucleic acids was demonstrated; e.g., electric pulse treatments at E?=?40 kV/cm and N?=?500 allowed extraction of ≈80% and ≈70% of ionic substances, ≈4% and ≈1% of proteins and ≈30% and ≈16% of nucleic acids in cases of HVED and PEF modes, respectively.     

The impact of pulsed electric field treatment on selected bioactive compound content and color of plant tissue

The aim of this study was to analyze the impact of pulsed electric field treatment on selected bioactive compound content and color of plant tissue. For this purpose apple and carrot tissues were treated by pulsed electric field (PEF) at 0, 1.85, 3, 5 kV/cm and 0, 10, 50 and 100 pulses which corresponded to the specific energy input of 0–80 kJ/kg. The electroporation efficiency was assessed by the means of electrical conductivity (EC) measurement. Immediately and 60 min after the PEF application the optical properties of both materials were measured and compared. Moreover, the total carotenoid content was determined in the case of carrot, whereas in the case of apple the free radical scavenging activity and the total polyphenol content was examined. The highest EC values were observed when 30 and 40 kJ/kg were delivered to the carrot and apple samples, respectively. The total color change (ΔE) of carrot tissue as a result of PEF treatment was smaller (ΔE_0min = 1.64–5.51; ΔE_60min = 1.33–3.91) than in the case of apple samples (ΔE₀ = 0.48–7.20; ΔE₆₀ = 1.25–21.87) which could be linked to the different chemical compositions of these raw materials. The application of PEF at 1.85 kV/cm regardless of the applied pulse number increased the total carotenoid content (TCC) up to 11.34%. In turn, the maximal increase of total polyphenolic content (TPC) and antioxidant activity (EC50) in the case of apple tissue was observed for samples treated by 10 pulses at 1.85 kV/cm. The utilization of 3 kV/cm decreased the TCC up to 25.33% whereas the application of PEF at 5 kV/cm and 100 pulses decreased the TPC and EC50 up to 35.93 and 32.95%, respectively. The research indicated that PEF can be used to enhance the extractability of bioactive compounds from plant tissue and to modify its color.Industrial relevanceThe paper discusses the impact of PEF treatment on bioactive compound concentration in apple and carrot tissues. Moreover, it deals with the color changes after the PEF treatment. Due to the fact, that pulsed electric field could be applied to the processes which depend on the heat and/or mass transfer such data should be considered of great importance. Firstly, the bioactive compounds (e.g., polyphenols, carotenoids) are very important regarding both their nutritional value and technological functionality (antioxidant properties, coloring agent production, etc.). Moreover, the PEF treatment efficiency depends on many different factors linked to the raw material properties and to the treatment protocol and conditions. This paper demonstrates that it is possible to improve the extractability of the carotenoids or polyphenols by PEF application. However, incorrectly selected PEF treatment parameters can result in the decrease of the bioactive compounds content in the solid material, as described and discussed more deeply in the paper. Color is one of the most important quality parameters. Especially that it is directly perceived by the consumers. As it can be read in the paper pulsed electric field treatment could be used to modify the color of plant tissue before any further processing. Such data is also important considering the minimal process food, which PEF could be involved in (e.g., special minimal process food for elderly people). In the presented experiment the color has been measured directly after the PEF application and after 60 min of it. Such information could be beneficial when considering the proper time interval between particular technological steps (when PEF is applied). i.e. the phenomena that the color change after PEF is accelerating (in the case of apple tissue) should be considered before other processing. The vast majority of publications present this kind of data regarding juices. In our study more complicated matrix is discussed. Thus, the paper, according to the best knowledge of authors, is the first which discusses the impact of PEF treatment on bioactive compound concentration of solid-like, plant material — such often used as a raw material in many different food applications (e.g., yogurt or smoothie ingredient) or further processed (e.g., dried or frizzed).     

Comparison of aqueous extraction efficiency and biological activities of polyphenols from pomegranate peels assisted by infrared,ultrasound, pulsed electric fields and high-voltage electrical discharges

The effects of aqueous extraction of bioactive compounds from pomegranate peels using conventional extraction (CE) and extraction assisted by infrared irradiation (IR), ultrasound (US), pulsed electric fields (PEF), and high-voltage electrical discharges (HVED) have been compared. For the extractions assisted by US, PEF and HVED, the saturation in extraction was observed approximately at the same specific energy input in the order of W ≈ 90–100 kJ/kg. HVED assisted extraction enhanced the recovery of polyphenols by ≈3 and ≈1.3 times as compared to US and PEF assisted extractions, respectively. Scanning electron microscopy (SEM) data evidenced that the highest yield of total polyphenols after the HVED treatment can reflect the presence of a strong damage of the microstructure of pomegranate skins. The obtained data on inhibition of growth of A. flavus and biosynthesis of aflatoxin B1 were explained accounting for the presence of different synergetic effects of phenolic compounds on inhibition of different bioactivities. All the studied extracts (0.2 mg/mL) demonstrated the higher inhibition efficiency for S. aureus (up to ≈80%) as compared to E. coli (up to ≈33%). PEF selectively extracted and enhanced the recovery of ellagic acid (≈740 μg/g DM), whereas HVED (≈345 μg/g DM) intensified gallic acid extraction compared to US, IR, HVED and WB.     

Extraction of anthocyanins from grape by-products assisted by ultrasonics,high hydrostatic pressure or pulsed electric fields: A comparison

Extracts from grape by-products contain bioactive substances such as anthocyanins which could be used as natural antioxidants or colourants. The effect of heat treatment at 70 °C combined with the effect of different emerging novel technologies such as ultrasonics (35 KHz), high hydrostatic pressure (600 MPa) (HHP) and pulsed electric fields (3 kV cm^− 1) (PEF) showed a great feasibility and selectivity for extraction purposes. After 1 h extraction, the total phenolic content of samples subjected to novel technologies was 50% higher than in the control samples. Therefore, the application of novel technologies increased the antioxidant activity of the extracts being the extractions carried out with PEF four-fold, with HHP three-fold and with ultrasonics two-fold higher than the control extraction. In addition, the extraction of individual anthocyanins was studied showing a selective extraction based on the glucose moieties linked to the anthocyanidins; anthocyanin monoglucosides were better extracted by PEF, whereas the acylated ones were extracted by HHP.Industrial relevanceThis study examines the feasibility of different emerging technologies such as high hydrostatic pressure, pulsed electric fields and ultrasonics as potential extraction methods for bioactive substances from grape by-products. Grape by-products represent a low-cost source of valuable bioactive compounds such as anthocyanins, with great industrial applications as colourants or nutraceuticals. The higher yields obtained in extractions carried out by high hydrostatic pressure and pulsed electric fields are of major interest from an industrial point of view, since solvent amounts were reduced and extraction times shortened. Thus, the combination of emerging technologies for extraction purposes and low-cost raw materials is an economical alternative to traditional extraction methods according to industry demands and a sustainable development.     

Pulsed-electric-field-assisted extraction of anthocyanins from purple-fleshed potato

The influence of pulsed electric field (PEF) treatment on the anthocyanin extraction yield (AEY) from purple-fleshed potato (PFP) at different extraction times (60–480 min) and temperatures (10–40 °C) using water and ethanol (48% and 96%) as solvents has been investigated. Response surface methodology was used to determine optimal PEF treatment and optimise anthocyanin extraction. A PEF treatment of 3.4 kV/cm and 105 μs (35 pulses of 3 μs) resulted in the highest cell disintegration index (Z_p = 1) at the lowest specific energy requirements (8.92 kJ/kg). This PEF treatment increased the AEY, the effect being higher at lower extraction temperature with water as solvent. After 480 min at 40 °C, the AEY obtained for the untreated sample using 96% ethanol as the solvent (63.9 mg/100 g fw) was similar to that obtained in the PEF-treated sample using water (65.8 mg/100 g fw). Therefore, PEF was possible with water, a more environmental-friendly solvent than ethanol, without decreasing the AEY from PFP.     

Resistance of Campylobacter jejuni to heat and to pulsed electric fields

The resistance of Campylobacter jejuni NCTC 11351 to heat and to pulsed electric fields (PEF) was studied at different treatment intensities (temperatures between 52 and 60 °C, and electric field strengths between 15 and 35 kV/cm, respectively). The influence of the growth phase, the pH of the treatment medium and the presence of sodium pyruvate in the recovery medium was also examined. A model based on the Weibull distribution was used to describe the inactivation curves, and times for the first decimal reduction were calculated (δ values). C. jejuni cells did not increase their resistance to heat nor to PEF upon entrance into stationary phase. The acidification of the treatment medium from 7.0 to 4.5 caused a sensitization of C. jejuni to heat (δ value at 55 °C × 1/4); on the contrary, resistance to PEF was increased (δ value at 25 kV/cm × 2.5). The absence of pyruvate in the recovery medium prevented recovery of a high percentage of heat-treated cells, but did not affect PEF survival. Whereas C. jejuni can be considered a heat sensitive organism (δ value at 55 °C and buffer of pH 7.0 of 2 min, z value 4.40 °C), it showed a relatively high resistance to PEF as compared to other vegetative cells (δ value at 25 kV/cm and buffer of pH 7.0 of 7 pulses, z_PEF value 8.20 kV/cm). Results obtained in this investigation indicate that Campylobacter spp. should be taken into account for the design of PEF treatments for food hygienization.Industrial relevanceBefore PEF can be commercially implemented it is necessary to determine its efficacy on pathogenic microorganisms of interest in order to ensure safety of food. There is no data available about the resistance of C. jejuni to pulsed electric fields, although it is now recognised as the leading cause of bacterial food-borne gastroenteritis throughout the world. In this research we characterize the resistance to heat and to PEF of C. jejuni NCTC 11351. Physiological factors affecting its survival to both agents are also explored.     

Application of electrical treatments in alcoholic solvent for polyphenols extraction from grape seeds

The polyphenols extraction from grape seeds was studied. In order to increase the polyphenols yield and the extraction kinetics, three different pre-treatments were applied on grape seeds: pulsed electric fields (PEF) (8–20 kV/cm, 0–20 ms), high-voltage electrical discharges (HVED) (10 kA/40 kV, 1 ms) and grinding (180 W, 40 s). These electrical treatments can damage cell membranes (PEF/HVED) and cell walls (HVED) thus accelerating the compounds extraction but have different effects on the product fragmentation. The diffusion was then performed after pre-treatment with a mixture of water and ethanol. The PEF efficiency was improved when the treatment was performed at 50 °C in the presence of ethanol. PEF-assisted diffusion was then compared to HVED-assisted diffusion and to diffusion of ground grape seeds. These two pre-treatments increased both the extraction kinetics and the maximum polyphenols yield. For both cases, the final polyphenols content was 9 g GAE/100 g DM after 15 min of extraction with grinding and after 60 min with HVED. When PEF were applied in a suspension containing ethanol, the maximum polyphenols yield was also reached after 60 min. The subsequent solid–liquid separation was faster with PEF as compared to ground and HVED-treatments.     

Microbiological and physicochemical stability of raw,pasteurised or pulsed electric field-treated milk

Pulsed electric field (PEF) processing was investigated as an alternative dairy preservation technology that would not compromise quality yet maintain safety. PEF processing of raw whole milk (4% fat) was conducted at two processing conditions (30 kV/cm, 22 μs, at either 53 or 63 °C outlet temperature) and compared with two thermal treatments (15 s, at either 63 or 72 °C) and a raw milk control and replicated twice. Milk bottles (2 L) from each treatment were incubated for two weeks, at 4 and 8 °C, and assessed for total plate count, pH, colour, rennetability, plasmin activity and lipid oxidation. The microbial quality of the thermal (72 °C/15 s) and PEF (63 °C) were similar. A drop in pH occurred after a change in counts was observed. Rennetability was not different between the treatments. Short chain acids dominated the volatile profile of the milk samples. Concentration of volatiles derived from microbial activity, namely 2,3-butanedione, acetic acid and other milk lipid derived short chain free fatty acids (e.g. butanoic and hexanoic acids), followed the trend of microbial activity in milk samples.Industrial relevanceResearch on the application of PEF to control spoilage and pathogenic microorganisms and enzyme systems in milk spans a wide array of processing equipment and reaction conditions. While industrial scale PEF processing of liquid milk for preservation and improved quality seems generally possible, substantiation of lower thermal damage under safe and scalable PEF conditions is required to enable economic feasibility.     

Inactivation of Salmonella Typhimurium and Staphylococcus aureus by pulsed electric fields in liquid whole egg

In this study, the lethal effectiveness of pulsed electric fields (PEF) on the inactivation of Salmonella enterica subs. enterica ser. Typhimurium and Staphylococcus aureus in liquid whole egg (LWE) has been investigated. Maximum inactivation levels of 4 and 3 Log₁₀ cycles of the population of Salmonella Typhimurium and S. aureus were achieved with treatments of 45 kV/cm, 30 μs and 419 kJ/kg, and 40 kV/cm for 15 μs and 166 kJ/kg, respectively. The non-linear kinetics of inactivation observed for both microorganisms at all the investigated electric field strengths were described by mathematical equations based on the Weibull distribution. The developed equations enabled to compare the microbial resistance to PEF and to establish the most suitable treatment conditions to achieve a determined level of microbial inactivation. PEF treatments varying from 30 kV/cm, 67 µs and 393 kJ/kg to 45 kV/cm, 19 µs and 285 kJ/kg allow to reduce 3 Log₁₀ cycles the population of the microorganism of concern in PEF food processing of LWE, Salmonella Typhimurium.Industrial relevanceThe data presented in this investigation in terms of electric field strength, specific energy and treatment time result of relevance to evaluate the possibilities of PEF technology to pasteurize LWE with this technology. The models developed in this study can be applied to engineering design, and for the evaluation and optimization of the PEF technology as a new technique to obtain Salmonella free LWE.Based on our results it is not recommended to apply treatments of energy levels higher than 250 kJ/kg, since PEF lethality hardly increased but markedly augmented the energetic costs. For these energy values, PEF technology by itself is not sufficient (3 Log10 cycles in the best case scenario) to assure the safe security of LWE. Therefore, intelligent combinations of PEF with other preservation technologies have to be developed in order to use pulsed electric fields as an alternative to heat pasteurization of LWE.     

Application of the pulsed electric field to release bound phenolics in sorghum flour and apple pomace

The mild intensity pulsed electric field (PEF) was studied to release bound phenolics in sorghum flour (SF) and apple pomace (AP). In this study, SF and AP, naturally fermented at optimized conditions, were treated at different flour to water ratios (FWRs) of 10, 27.5, 45% (w/v) and 5, 8.75, 12.5% (w/v), respectively. In addition, three levels of PEF electric field intensity (EFI) as 1, 2 and 3 kV/cm were used to treat SF and AP for the treatment time of 500, 875 and 1250 μs. Both the treated samples were analyzed for total phenolic content (TPC), antioxidant activity (AA) and phenolic characterization. For SF, optimized conditions were determined as 45% (w/v) FWR, 2 kV/cm EFI and 875 μs treatment time, while these values were 12.5% (w/v) FWR, 2 kV/cm EFI and 500 μs for AP. At these conditions, TPC and AA of SF were 24.8% and 33.9%, respectively higher than the control SF, while for AP, these numbers were observed as 37.4% and 86%, respectively. The study suggests that PEF treated SF and AP may be very useful for the preparation of processed foods with the increased levels of phenolic antioxidants.Industrial relevanceSorghum, millets and fruit or vegetable by-products contain high amounts of flavonoids and phenolic acids among other bioactive compounds. Results explicate that the application of a PEF treatment following the fermentation could improve the phenolic antioxidants in the sorghum flour and apple pomace. Consequently, this technology could be used in the food industry to enhance the healthy properties of millets and other by-products prior to further processing like mild extrusion or baking. Also the use of the mild intensity pulsed electric field has the benefit of minimizing any damage to the other nutrients of any product.     

Combined densification and pulsed electric field treatment for selective polyphenols recovery from fermented grape pomace

The aim of this study is to assess a new process for the valorization of fermented grape pomace using pulsed electric fields (PEF). The combination of densification and PEF treatment was applied on grape pomace of low relative humidity, without any addition of conductive liquid. The kinetics of extraction and the composition of polyphenols were evaluated throughout the subsequent hydro-alcoholic extraction at different temperatures.Optimal parameters of PEF treatment (field strength E = 1.2 kV·cm^− 1; energy input W = 18 kJ·kg^− 1; density ρ = 1.0 g·cm^− 3) increased the content of total polyphenols regardless of the temperature of extraction. The ratio of total anthocyanins to total flavan-3-ols at 20 °C was equal to 7.1 and 9.0 for control and PEF treated modalities, respectively. These results demonstrate the selective nature of PEF treatment in anthocyanin extraction, and thus reveal new possibilities to produce extracts with different biochemical compositions.Industrial relevanceThis study examines the feasibility of densification combined with PEF pre-treatment of relatively low humidity grape pomace for the enhancement of bioactive compounds extraction. The concentration of total phenolic compounds obtained after PEF treatment showed that the use of this technique is relevant for an industrial use, since solvent amount and extraction time can be reduced. Moreover, the selective nature of PEF opens the opportunity to produce extracts of different biochemical compositions. This process is an alternative to conventional pre-treatments of raw material (e.g. dehydration and grinding), which have impacts on product quality and are more energy consuming.     

Influence of treatment temperature on the inactivation of Listeria innocua by pulsed electric fields

The effect of treatment temperature on the bactericidal effectiveness of pulsed electric fields (PEF) applied on Listeria innocua suspended in McIllvaine buffer was investigated. Electric field intensity and number of applied pulses were applied in the ranges of 31–40 kV/cm and 5–35 pulses, respectively. Studied treatment temperatures were sustained for 10 s, and ranged between 19°C and 59°C depending on the amount of energy delivered by the PEF treatment. The application of PEF at higher temperatures proved to be more effective than either PEF at low temperatures or the applied thermal treatments by themselves. A maximum bacterial inactivation of 6-log cycles was obtained by applying either: 20 pulses of 40 kV/cm at 65°C, 25 pulses of 36 kV/cm at 61°C, or 31 pulses of 31 kV/cm at 56°C. On the other hand, a thermal treatment of 66°C sustained for 30 s reduced the bacterial population on its own by only 5-log cycles, and the application of 60 pulses of 31 kV/cm at 30°C caused only 3-log cycles of bacterial inactivation. The findings in this study suggest that PEF technology may be effectively used as an enhanced mild thermal preservation method.     

Effect of pulsed electric fields on the structure and frying quality of “kumara” sweet potato tubers

The aim of this research was to study the effect of pulsed electric fields (PEF) on the microstructure of “kumara” sweet potato (Ipomoea batatas cv. Owairaka) and its quality after frying. Whole sweet potato tubers were treated at different electric field strengths ranging from 0.3 to 1.2 kV/cm with specific energy levels between 0.5 and 22 kJ/kg. Cell viability was determined using tetrazolium staining to investigate the uniformity of the PEF effect across tubers. Based on the patterns of viable cells it was observed that the effect of PEF was not homogeneous across the tuber. This result was also supported by the pattern of enzymatic browning due to PEF facilitating the reaction of polyphenoloxidase and phenols. PEF treatment resulted in significant softening of the ground tissues, but not on the dermal tissues, as determined by texture analysis. With respect to frying quality, tubers pre-treated with PEF at electric field strength of 1.2 kV/cm and fried at 190 °C had an 18% lower oil content than non-PEF treated samples. The kinetics of browning as a function of frying time could be described by a fractional conversion model. The activation energy (Ea) of the browning rate during frying increased (more temperature sensitive) due to PEF pretreatment at 0.5 kV/cm and 1.2 kV/cm. It implies that PEF pretreatment allows frying the potato chips at lower temperature in order to achieve the same brown colour intensity as the non-PEF treated tubers. This study shows clearly that PEF could reduce the energy required for cutting and frying of kumara.Industrial relevanceThis study provides evidence that the effect of PEF processing on whole kumara tubers is not uniform, demonstrating heterogenous distribution. These findings provide important information for food industry to design appropriate PEF processing conditions for solid materials. More importantly, PEF treatment reduced the energy required for cutting and frying of kumara, and reduced the oil content in the fried kumara chips.