Giving Grapes The Chair: Extraction of Polyphenols by Pulsed Electric Field Technology

Extraction of polyphenols, including anthocyanins and tannins, is a necessity in red winemaking, and has been a frequent focus of study in hopes to find a method that extracts these compounds at an optimal rate and concentration, while maintaining both affordability and sustainability.  These phenolic compounds are found in both the skins and the seeds of the grapes, and basically need to be released from these places and suspended into the fermenting wine.  Anthocyanins are located primarily in the upper layers of the epidermis (a.k.a. skin) of the grapes, and are mainly responsible for the

Photo By fr:Yelkrokoyade (author) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC-BY-SA-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons

Photo By fr:Yelkrokoyade (author) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC-BY-SA-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons

color of the wine.  Tannins, on the other hand, are located in both the skins and the seeds, and are responsible for many factors, including color stabilization as well as astringency, aging potential, sensory characteristics, and overall quality of the wine.

 

Several different winemaking techniques have been established in order to increase the extraction of these polyphenolic compounds, all of which attempt to increase the permeability of the grape skins (i.e. make the skins easier to pass through if you’re a polyphenol) often by significantly increasing (or decreasing) the temperature, or extending the maceration process for a much longer time period, which can result in increased energy consumption (higher carbon footprint) or sometimes a degradation in wine quality if the extraction is left going for too long.

 

One relatively new method for extracting phenolic compounds in grapes which is receiving more attention in the world of research is the use of pulsed electrified field treatments (PEF).  PEF treatments have been used to extract compounds from other products (i.e. juices), as well as for the inactivation of certain microorganisms.  Using PEF would be a “nonthermal” approach to grape polyphenol extraction, which has both winemakers and environmental scientists intrigued.   In a couple of experiments using white grapes, PEF was shown to significantly increase juice yield as well as a reducing in browning.  PEF has also been shown to be a very effective method for reducing spoilage organisms in wine, with one study showing a 99.9% reduction in spoilage organisms from must and wine after PEF treatments.

 

What about red grapes/wine?  PEF has shown to be effective in extraction of juice from white grapes as well as the protection of wine against spoilage organisms, but can it also increase the extraction of polyphenols from red grape skins and seeds?  The goal of the study presented today was to determine if PEF could enhance the extractability of phenolic compounds from Merlot grapes without separating the juice, at relatively low electric field strengths.  Since these effects are not yet known in red wine, the study also examined a few other electric field strengths as well as pulse times in order to find optimal conditions for extraction.

 

Methods

 

Merlot grapes from a vineyard in Bordeaux, France were harvested in 2008 and used for this study.

 

Standard winemaking techniques were carried out in 10L stainless steel tanks, and inoculation was performed using S. cerevisiae yeast FX10.  After fermentation, wines were racked and 20mg/L of SO2 was added.  The wines then were kept at 15oC and bottled at the completion of malolactic fermentation.

 

PEF treatments were created by a pulse generator from 5kV to 1kA.  5kg of grapes were placed in between two PEF electrodes and were “hit” with several series of pulses from the device (treatment times between 40 and 100ms).  The voltages and times were chosen such that the temperature of the system did not change throughout the treatment process.

The three PEF treatments were:

  • PEF1 = 700V/cm for 100ms
  • PEF2 = 500V/cm for 100ms
    Photo By Zuzu (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

    Photo By Zuzu (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

  • PEF3 = 700V/cm for 40ms

 

Alcohol content, pH, density, total acidity, sugar content, free SO2, and total SO2 levels were analyzed for each juice.  Phenolic compounds, tannins, and color intensity were measured for all musts and finished wines.

 

Sensory analyses were performed by 8 “experts” from the Laboratory of Flavors and Enology in the Department of Analytical Chemistry, at the University of Enology of Bordeaux.  Analyses occurred 7 months after bottling, and were assessed for basic taste and aromatic characteristics (including astringency, bitterness, intensity of aroma, and acidity.  Panelists were also asked to give an overall score for each wine, in addition to the individual sensory characteristic scores.

 

Results

 

  • Enological characteristics (alcohol content, pH, total acidity, etc) of the musts and wines were found to be in the normal range for Merlot grapes.
  • Alcoholic fermentation kinetics was not significantly different between PEF treatment wines and control wines.
  • Immediately after PEF treatment, color intensity increase in all PEF treatment wines compared with control wines.
    • After 7 months, color intensity was the same for PEF treatment wines and control wines.
    • Similar to color intensity, wine color tint also increased immediately after PEF treatment compared with control wines, while after 7 months there were no differences in color tint between PEF wines and control wines.
    • PEF-treated wines were consistently darker than control wines, and remained darker throughout the 7 month experimental period.
    • There were no significant color differences between the different PEF treatments after 4 days.
    • Anthocyanin and tannin levels were higher in PEF-treated wines than control wines, indicating the PEF treatment increased the permeability of the grape skins and seeds to allow for greater extraction of these compounds than the controls.
    • There were no differences in the state of polymerization of tannins in regards to the PEF1 and PEF3 treatments, however, there was an increase observed for PEF2.
    • PEF3 showed the greatest potential for decreasing astringency compared with control wines.
    • PEF1 showed an increase in the potential for greater astringency.

Sensory Analysis

  • The control wine received higher scores for sulfur, slight sweetness, and mineral taste (average score = 12 out of 20).
  • PEF1 was noted for harsh astringency and had a high score for reduction characteristics (average score = 10 out of 20).
  • PEF2 was noted for being softly aromatic, metallic, with a taste of paper (average score = 13 out of 20).
  • PEF3 was noted for having a more complex structure, with aromas and flavors of leather and meat.  Additionally, the wine was perceived as fruitier with a longer finish and “less aggressive” than other treatment wines (average score = 15 out of 20).
  • Overall wine preference by the panel was in the following order: PEF3 > PEF2 > Control > PEF1.

 

Conclusions

Overall, the results of this study showed that PEF treatments with low electric pulses (between 500 and 700V/cm) and relatively short pulse times (between 40 and 100ms) were enough to cause an increase in permeability of Merlot grape skin and seed cells.  The sensory analysis showed clear differences between the different treatments, even though to you and me, the differences between 500 and 700V/cm or 40 and 100ms aren’t really distinguishable.  Specifically, for those treatments with higher energy (i.e. 700V/cm), the tannins extracted were perceived as more bitter and astringent compared with those treatments exposed to pulses of lower energy (i.e. 500V/cm), though the “intermediate” treatment of the higher pulse energy but shorter pulse time resulted in the most favorable wine in the sensory analysis.

 

In general, at the lower electric field levels, the extra polyphenolic compounds extracted into the wine resulted in positive sensory changes, while some of the higher electric field levels resulted in the over-extraction of phenolics, resulting in greater bitterness and astringency.  The authors suggested that this technique using short low energy electric pulses could lead to reduced maceration times in Merlot grapes, though this wasn’t explicitly studied.

 

The use of the electric field pulses in general increased extraction of polyphenols from Merlot grapes, however, higher

Photo By Ryan O'Connell (originally posted to Flickr as BARREL PIGEAGE) [CC-BY-SA-2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons

Photo By Ryan O’Connell (originally posted to Flickr as BARREL PIGEAGE) [CC-BY-SA-2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons

energy and longer pulse times seemed to over-extract some of the bitter tannins and resulted in a wine with higher astringency that preferred by the sensory panel.  The results of this study indicate that while these results are certainly promising, more work needs to be done to find the optimum electric field and pulse time necessary to extract the desired level of polyphenols from the wine in order to decrease the time needed for maceration yet still increase the positive sensory characteristics of a well-extracted wine.

 

I would be interested in seeing more combinations of electric field strength and pulse times tested, as well as the carbon footprint created by this method compared with current methods.  This method does seem to shorten the length of time needed for maceration, and does not require an increase in energy expenditure in terms of increasing temperatures, however, I would still like to see some data on the energy expenditure and carbon footprint of the electric pulse system, as electricity sadly doesn’t come for free.

 

Overall, I think this study was a really nifty pilot study on the topic of using electric pulse energy to decrease maceration time and increase polyphenol extraction of red wines, and I am hoping to see more detailed analysis in the future related to the topic.

 

What do you all think of this study?  Please feel free to comment (I believe the commenting system is now functional again ;) ).

 

Source: Delsart, C., Ghidossi, R., Poupot, C., Cholet, C., Grimi, N., Vorobiev, E., Milisic, V., and Peuchot, M.M. 2012. Enhanced extraction of phenolic compounds from Merlot grapes by Pulsed Electric Field treatment. American Journal of Enology and Viticulture 63(2):205-211.

5 comments for “Giving Grapes The Chair: Extraction of Polyphenols by Pulsed Electric Field Technology

  1. Robert
    July 26, 2013 at 12:34 pm

    Electrifying study.There I said it. But very interesting study and it would be interesting to do this in a large fermenter set-up. I hope you continue to follow this one.

    • Becca
      July 26, 2013 at 1:30 pm

      HAR HAR HAR! ;) Seriously though, I love it ;) I’ll keep my eyes peeled, so hopefully someone will continue to work on this research! Thanks for reading and commenting! Cheers!

  2. peter
    August 6, 2013 at 4:53 am

    it would be really good to try and differentiate the extraction between the skin and seed tannins. You could suspect that the skin cells would be easier to rupture and getting these polyphenols out is a priority over the seed PPs.

    • Becca
      August 6, 2013 at 6:16 pm

      That’s a great idea, Peter! It would be interesting to examine this difference! Thanks for your comments and insights!

  3. March 26, 2014 at 5:25 am

    Good work but 10 ms was probably too much.
    We have been working with it :) and you are right. A mild treatment, 500V/cm to 1000V/cm in a bunch of 50 us pulses can shorten the time for phenolic extraction. We had industrial tests last summer (to be published) and will go for real operation this summer in Europe.

What do you think about this topic?