Abstract

This article presents a review on the processing technique used in the production of wine. Pulsed Electric Field (PEF) is a non-thermal processing technique that is introduced as a pre-treatment of liquid and semi-solid foods (Ricci, Parpinello & Versari, 2018). The use of non-thermal processing techniques is due to its inactivation of harmful microorganisms by destroying the cell membranes of its targeted food groups. For exactly this reason, these methods are being used for the preservations of foods. In the recent years non-thermal methods especially Pulsed Electric Field (PEF) processes are being utilised in the food industry and in the production of wine, to extract the phenolic compounds as proficiently as possible while maintaining a high-quality wine product (Teusdea et al, 2017). The functioning principal of Pulsed Electric Field (PEF) is oriented on the applications of high voltage which is typically centred around 20kV/cm up to 70kV/cm, these pulses are presented to liquid foods and placed between two electrodes. The studies of these pulses are used in the winemaking process, to allow positive influences of PEF on vinification to eliminate most pathogenic microorganisms, reducing the maceration time, increasing the wines phenolic compound extractions, inactivating any oxidative enzymes and the acceleration of wine aging. The aim of this review is to expose the understating of Pulsed Electric Field process by summarizing it potential application in making wine and to also express the effects on the quality of wine (Ozturk & Anli), 2017).

Introduction

Pulsed Electric Field (PEF) is a new and emerging technological process for food processing and preservation based on the applications of the short and high-voltage pulses that penetrate food products. As mentioned earlier the typical voltage intensity that is used in the PEF treatment ranges from 20-70kV/cm with the pulse duration of micro to milliseconds. Pulsed Electric field technology has been presented to the food industry as the new emerging non-thermal treatment for the inactivation of microorganisms, while having the purpose of achieving improved preservation of food products and their textures, colours, flavours and nutritional value with respect to the traditional thermal processing methods (Gocławski, et al., 2017). When there is an external electric field applied to food products, the critical electric potential across the cell membrane is then induced, thus this potential causes the rapid electrical breakdown and mechanical changes in the cell membrane. Subsequently, the membrane permeability is drastically increased, and the pores are simultaneously formed in the membrane. Thus, the Pulsed electric field technology has been advanced and promoted in various food process such as sterilization, promoting extraction, reduction of pesticides residues, food dehydration and inactivation of enzymes. In addition to these applications, the Pulsed Electric Field treatments have been also used mainly in the use of liquid induced foods like fruit juice products, dairy products, liquid eggs and alcoholic beverages (Yang. et al, 2015). However, the use of electroporation has also been recommended as well for the extraction of bioactive compounds from plant materials, and for also increasing the extraction yield during the process of making fruit juices, therefore presenting new standpoints for the use of PEF technology in the food industry (Comuzzo et al., 2018).

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There are many thermal and non-thermal food process that can be used in the production of wine, some of these are High Pressure Processing, the use of ultrasonics, high hydrostatic pressures and high voltage electrical discharge. The use of new emerging technologies such as PEF and high voltage electrical discharge (HVED) have been applied to the microbial stabilization of liquid foods, and to the extraction of compounds in fruits and vegetables. The effects of microbial inactivation depend on the composition of electrical parameters and solutions being used (Delstart. et al, 2015). In contrary High-Pressure Processing (HPP) utilizes the application of even pressures between 100 and 600MPa to inactivate spoilage of yeasts, fungi and microorganisms in the production of wine, beer and other alcoholic beverages (Wyk, Farid & Silva, 2018). On the other hand, High Hydrostatic pressure (HHP) and ultrasonic are used to recycle the by-products of grapes by extracting their antiviral, antioxidants, antibacterial, anti-inflammatory, and anticarcinogenic properties. The extraction of these compounds is done with the use of HHP which enhances the mass transfer rates leading in the increase of cell permeability as well as increasing the secondary metabolite diffusion (Corrales. et al, 2007).

Alcoholic beverages typically contain 3% to 4% alcohol, they are consumed legally in over 100 countries. Therefore, the production and consumption of wine or any other alcoholic beverages are great for the economy. The production of wine involves several processes, these are crushing and pressing of the grapes, fermenting, aging and bottling and preserving. The crucial treatment in wine making is the control of the growth of microorganisms, controlling microbial growth results in poor quality wine and storage problems are likely to arise (Yang. et al, 2015). Furthermore, the Phenolic compounds are one of the most important quality parameters of red wine. The dark intense colours, and other attributes like wines astringency, aroma and bitterness are mainly ascribable to these substances in a large degree. The antioxidant and anti-inflammatory properties are also associated from the phenolic compounds in wine. There are two important phenolic compounds responsible in red wine and they are anthocyanins and tannins. Anthocyanins are located in the skin of grapes whereas tannins are located in the seeds and also in the skin of grapes. Anthocyanins holds an important role in the formation of the red colour in grapes through its pigments. The extraction of these phenolic compounds from the cells of the skin during the maceration step in making wine is controlled by the diffusion through the cell membranes. Usually in traditional wine making the extraction of phenolic compounds from the grape skin is a slow process that requires several days. In recent times there has been a lot of efforts to develop and improve the extraction of these phenolic compounds during winemaking process. Therefore, the application of pulsed electric field treatment to the grape pomace before maceration and fermentation step aids in the increase of extracting phenolic compounds during the vinification of different grapes (López. et al, 2009).

PEF Technology in winemaking

Pulsed Electric Field comprises of direct applications of short voltage pulses to materials that are placed between two electrodes. The classic system of PEF is the treatment of the pumpable fluids that are placed into a pulse generator that consists of high voltage, a treatment chamber and then a set of monitoring and controlling equipment. With the assistance of energy transfers to liquids that are subjected to PEF treatment can cause an increase in the temperature and electrical conductivity of the treatment medium. With there is an increase in temperature when processing this can present significant changes to the viscosity and stability of the cell membrane. The first process in Pulsed Electric Field treatment is electroporation which is the electrical breakdown of the cell membrane through externally applied electrical field of strong yields to raise the strength in cells plasma membrane. The role of this treatment is to inhibit the growth of microorganisms (Ozturk & Anli, 2017). A study was conducted by Comuzzo et al in 2018 to examine the effects on the wines composition and volatile compounds via using Pulsed Electric Field processing techniques, the study was examined on two hundred kilograms of grapes that was subjected to 8kV/30 in a PEF generator and a polymethyl cylindrical cell with two toroidal stainless steel electrodes. The mashed grapes were then transferred through a single-screw volumetric pump. The PEF treatments were then conducted in three repetitions with an electric field strength of 1.5kvcm with a single pulse duration of 0s of untreated mash, then 8s which matched with a total energy of 11kjkg and then followed with 16s with a matching energy output of 22kjkg. The PEF generator supplied square-wave pulses with frequencies of 600Hz for both the PEF treatments. These experiments were performed at approximate room temperature of 20C, after the PEF treatments were measured the temperature had a slight increase less than 5C for all samples. On the other hand, another study conducted by Abca & Evrendilek 2014, used a six treatment chambers that measures 0.29cm in diameter and 0.23cm in gap distance. The temperatures of post and pre-treatment at the outlet and inlet of the chambers were examined by the using K-type digital thermocouples. The treated samples were cooled in the chambers by cooling coils that were submerged in a water bath at 12C. Then a trigger generator was used to monitor and control the pulse duration time, pulse repetition rate and pulse delaying time. The applied voltage and current were 12,000 V max of output voltage and 60 A max of output current, and 16kjkg of specific energy was stored in the pulse generator. The preliminary tests were implemented for the wine samples and the results showed different electric field strengths of 0 (untreated) which was the control, 17kV/cm, 24kV/cm and 31kV/cm at levels of 10C, 20C and 30C were applied. All these treatments had a flow rate of 40mL/min, 3s off pulse duration and 500Hz of frequency were applied to the PEF treatments.

Other processing techniques used in Winemaking

In relevance to winemaking there are other proposed thermal and non-thermal techniques used to make wine and in extracting of phenolic compounds. A study conducted by Corrales. et al, 2007 examined the extractions of anthocyanins from grape by-products by using ultrasonics and high hydrostatic pressure while comparing to pulsed electric field processing. The study examined the feasibility of the different emerging technologies as a potential extraction method for bioactive substances that can be used as natural antioxidants or colorants. The grape by-products were tested in three processing technologies, a pulsed electric treatment was applied using an electric field strength of 3kV/cm and the extraction was performed at 70C and held for 1 hour. Tests were conducted in a high hydrostatic pressure device which consists of a series of thermostatic micro autoclaves and samples were pressurised at 600MPa, at 70C and held for an hour as well. Ultrasonic extraction treatment was conceded in an ultrasonics bath with a heating frequency of 35KHz and held at 70C for an hour. The study resulted that the highest antioxidant extraction was obtained from the PEF (784.34 ± 150.41 μmol) treated samples and then followed by HHP (548.49 ± 47.97 μmol) and ultrasonics (308.13 ± 46.54 μmol). The use of 3kV/cm by the PEF treatment caused an irreversible effect on pores in the plant membrane which increases the extractability of polyphenols through the release of solutes into the solvent. Additionally, PEF treatment delivers an opportunity of inactivating some degrading enzymes which may lead to higher extractions in antioxidant activity compared to the other techniques. Another study by Delstart. et al 2015 examined the efficiency of pulsed electrical treatments for the inactivation of yeasts, with the applications of pulsed electric fields and high voltage electrical discharges (HVEDs) which was the replacements to the addition of sulphites, which is used in the inhibition of fermentation in sweet wine. These treatments were investigated by the influence of sulphite concentrations from 0mgL to 500mgL, the use of pulsed electrical fields of 4kV/cm for 0.25 to 6 milliseconds and HVED of 40kV/cm for 4 milliseconds, these treatments were done on the inactivation of total yeasts and non-Saccharomyces yeasts. The results of this study showed that sweet wine that consists of an alcohol content of 10% can have the probability to inhibit a high growth of non-Saccharomyces yeasts and Saccharomyces yeasts. Once the PEF treatments were examined it showed that there was a higher inactivation of non- Saccharomyces yeasts having 4 log cycles for the highest electrical filed strength and also has the longest treatment duration time. Compared to PEF treatment which had lower inactivation of non- Saccharomyces yeasts have only 3 log cycles, therefor these results demonstrate that non- Saccharomyces yeasts were more sensitive to high electrical treatments of Saccharomyces yeasts. Showing that HVED treatments are more effective in the inhibition of yeasts rather PEF treatments.

       2.1   Microbial growth controls in Wine

PEF technology has a great ability to control the microbial activity in wines. Alcoholic beverages possess the capability of resisting the spoilage caused by microorganisms due to the relatively high acidic and alcohol content. Although, there are particular microorganisms that can still survive in an alcoholic beverage and may pose a significant risk to the food safety (Yang. et al, 2015). These spoilage microorganisms are a part of the natural microbiota that is presented on the surface and in the grapes skin, they are responsible for the contamination of the grapes, then the wine and its contact surfaces such as their winery equipment and aging barrels. The growth of these natural microbiota restricts the development of the added fermentation starters to the wine and also impairs the sensory changes of the wine. Several approaches have been advancing in the wine making industry to control and avoid the development of these spoilage microorganisms in wine. As mentioned earlier the addition of Sulphur dioxide (SO2) to the process of wine play a significant role in decreasing the risk of microbial spoilage during the production of wine. Although, certain microbial sensitivity varies with the addition of sulphur dioxide between different microorganism strains. Studies have showed that new emerging non-thermal technologies that can be applied to microbial inactivation and these are usually high hydrostatic pressure and PEF. The use of these new technologies has proven that microbial activity can be controlled without modifying the sensorial properties of the wine (Puértolas, 2010). A study performed by Arenzana et al 2017 was done on reducing the microbial communities of wines after the fermentation process to improve the microbial activity and for developing the malolactic fermentation. This study used pulsed electric field technology for the microbial community reduction on four different wines. This resulted in the significant reductions in the yeast population, reductions of lactic acid bacteria, and acetic acid bacteria were completely eliminated after the PEF treatment. Therefore, this shows that PEF treatment was an essential tool for improving the malolactic fermentation performance and the inhibiting the growth of microorganisms while still preserving the wines sensorial properties. Moreover, a study done by Puligundla, Pyun and Mok 2018 investigated the decontamination of the spoilage microbes in low-alcohol red wine using electrode pulsed electric field treatment system. The study showed that electrical field intensities within ranges of 20-50kV/cm and with frequencies from 500 to 1500Hz used in the PEF treatment, detected the spoilage of microorganisms that included aerobic bacteria, yeast and lactic acid bacteria, and yeasts were found to be more susceptible to the PEF processing techniques, therefore PEF treatment considered to be more suitable to the control common spoilage microorganisms in wine production.

      2.2    Phenolic compounds presented in Wine

The phenolic substances presented in wine play an important role in wines quality parameters. These compounds consist of mainly anthocyanins, flavan-3-ols, flavanols, stilbenes and other wine acids. These compounds are responsible for the contribution to the wine’s organoleptic attributes, specifically to the colour, bitterness, astringency and mouthfeel. These phenolic compounds in their monomeric form, are the pigments that are responsible for the red colour in red wines, they are also responsible for the stabilization of red wine colour in the aging process, with the developments of red polymeric pigments with other polyphenols and specifically flavan-3-ols. Studies have revealed that some phenolic compounds can exhibit antioxidant and free radical properties, that can play an important role in an individual’s health by reducing the risk of various degenerative diseases, like osteoporosis, cancer and cardiovascular diseases. The phenolic content in wine relies on the phenolic content of different varieties of grapes that are used in vinification. The extraction of polyphenols under vinification process depends solely on winemaking processes, due to this reason different techniques like the use of maceration enzymes, thermovinification, grape freezing and increasing the fermentation temperatures are used (Puértolas, 2009). The extraction of these phenolic compounds is greatly assisted by using PEF processing techniques. A study conducted by Corrales et al. 2007 showed that with the application of PEF treatment with 3kV/cm, 30 pulses and 10kJ/kg was applied to the grape skins and had an increased in the total phenolic content and the anthocyanins concentrations in the extraction medium. Therefor the use of PEF technologies increased the antioxidant activity by being extracted by PEF four-fold, and the anthocyanins were better extracted using PEF techniques. In contrary experiments performed by Saldaña et al, 2016 depicted that the influence of electric field intensities of 3 and 5kV/cm and pulse width of 5, 20, 50 and 100s were applied to vinification of wine, the results implied that effective extractions of polyphenols from grapes that were treated with pulses longer that 5s. Delsart et al 2014 presented that the concentrations of anthocyanins were superior in the PEF treated wines compared to other novel technologies, concentrations of tannins were low in wines with HVED treatments while PEF treatments obtained a slight increase. Anthocyanins and tannins are directly connected to the characteristics of the colour in red wine, thus the increase or decrease of these compounds will have a significantly impact on the colour of the wine. The ability of PEF processing techniques to inactivate an oxidative enzyme is a correlation to a more colourful wine. All these tests indicated that electric pulsed treatment can have a significant implication on the wine’s organoleptic properties. 

Conclusions

On the basis of the studies presented in this review we can conclude that PEF processing techniques offer an interesting perspective in the production of wine. It has been summarised that PEF technologies demonstrated the enhance of extracting the phenolic compounds while controlling the microbial activity in wineries, thus the reduced maceration time. For certain compounds, the conclusions of previous experiments show that PEF treatment conditions, PEF systems and equipment’s and different kinds of wine are not entirely the same in production. The demonstrations of other advanced processing technologies such as ultrasonics, high hydrostatic pressure was mentioned and how they enhance the extraction of phenolic compounds compared to Pulsed electric field processes. Spoilage of microorganisms can be controlled with ambient temperatures via the use of PEF techniques, PEF also has the ability to inhibit the growth of various yeasts and bacteria under the suitable treatment conditions that can be used in the production of standard wine making. Even with these findings the effects of PEF treatment on wine production remains unclear and therefore future studies should be conducted to address the issues in PEF processing techniques. Due to the increasing attention of wine making in companies using innovative and new emerging technologies, the results obtained from PEF technologies in wine making should be monitored and controlled to improve their process applications without affecting the organoleptic compounds in wine.

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