Pulsed Electric Fields for vinification

Apertura-portaleThe medical community recommends drinking red wine, in moderation of course, because of the polyphenols it contains. So increasing polyphenolic content is one way to meet the needs of a market that is seeking both health related and sensory properties in wine.
Traditional red wine vinification doesn’t fully exploit the potential polyphenols contained in the grapes. These valuable compounds are found primarily in the vacuoles of cells and are difficult to extract. To enhance extraction yields, cell disintegration is necessary. The method that is mainly used is to treat the grapes with pectolythic enzymes which hydrolyze the polysaccharide compounds of the grape cell membrane, increasing the permeability so as to release the substances that are present in the cell. This all takes place at ambient temperatures and without the addition of anything foreign to the wine.
The research group of the Department of Industrial Engineering at the University of Salerno, under the direction of Professor Giovanna Ferrari, has taken another route, analyzing the polyphenolic content of wine after applying the technology of Pulsed Electric Fields ( PEF) to the grapes immediately after crushing and before the start of the maceration/fermentation phase.

The treatments

In the study, low strength pulsed electric fields (0.5 kV/cm to 1.5 kV/cm) were applied for intervals varying from 10 µs to 1 ms, producing two wines from two different bases (grapes from the Piedirosso and Aglianico varieties). The total content of polyphenols (in particular anthocyanins), colour intensity, various qualitative parameters and the level of permeability of the skins were measured. The antioxidant power of the polyphenolic content was also tested to verify that the PEF treatment did not have a negative impact. The same tests were conducted with the traditional vinification method and vinification with the addition of pectolythic enzymes in normal concentrations (20 g/t).
The grapes, harvested by hand and stored in a refrigerated area, were weighed, mechanically crushed and then manually destemmed. Then potassium metabisulfite (50 g/t) was added. Following this, the must and skins were manually separated with a sieve and weighed. The separation was carried out in order to subject only the grape skins to the PEF treatment of varying intensity and with a single pulse length of 10 µs. After the treatment, the skins were joined back with the must in the same proportions as before they were separated (56.6 % must, 43.4% skins).

The analysis

After the fermentation period (9 days), samples were drawn off with a traditional press. Every day during the maceration period, must samples were taken and put in a centrifuge at 5000 rotations for 5 minutes at a temperature of 10°C. The resulting supernatant liquid was subjected to various tests.
Obviously the most important parameter for determining the effectiveness of the treatments was the release of polyphenols. To determine this, the chromatic qualities of the extracts were analyzed by determining the colour intensity and measuring the concentrations, during all the maceration steps, of polyphenols and, in particular, the anthocyanins.
The permeability of the plant tissue (useful for understanding the impact on the extraction of polyphenols) was analysed by measuring the electrical properties (impedance and phase angle) of the skins. Finally, to verify the overall quality of the wine, tests were conducted to measure alcohol content, degrees Brix, pH, reducing and total sugars, total acidity and antioxidant potential.
All results were compared with those from the grape skins that were not treated and the skins that had undergone the enzyme treatment.

Results? Depends on the grape

In the Aglianico grape, all the samples treated with PEF showed increased concentrations of polyphenols (with higher levels depending upon the intensity of the treatment) compared to the untreated samples. Furthermore, the concentration of polyphenols in the samples that had undergone the most severe treatment (voltage at 1.5kV/cm and 1,000 to 2,500 pulses respectively) proved, after the fifth day of maceration, higher even than the samples treated with enzymes. On the last day of fermentation, the sample treated at 1.5kV/cm and 1,000 pulses and the sample at 1.5 kV/cm and 2,500 pulses showed higher concentrations of polyphenols compared to the control group, with 18% and 26% increase respectively.
On the other hand,  in the case of Piedirosso the release of polyphenols wasn’t any different when comparing the treated and the control group. Even the enzyme treatment didn’t reach a higher release of polyphenols. This information suggests that the structural differences of the Piedirosso base, compared to Aglianico, prohibit a more efficient release of polyphenols, regardless of the treatment applied.
Very similar results were found regarding anthocyanins. In the case of Aglianico, the samples that were subject to treatment with pulsed electric fields demonstrated greater colour intensity compared with the control group. Those treated with 1.5 kV/cm and 1,500 pulses and 1.5 k V/cm and 2,500 pulses showed better qualities compared with the sample treated with enzymes. For Piedirosso, again, there was no significant difference noted.

The effects on cell membranes

The effect of pulsed electric fields on the permeability of the cell membrane was confirmed, as well as the resulting improvement in the conductivity of plant tissue. However inconsistencies emerged between the data regarding the permeability of tissue and that relating to the release of polyphenols. The rate of permeability of samples treated with 1.5 kV/cm and 10,000 pulses was higher compared to the samples that were subjected to a treatment of 1.5 kV/cm and 1,000 pulses. The release kinetics of the samples undergoing the two treatments, instead, were inverted compared to the permeability results. In the case of Piedirosso, this difference was glaring. The treatment at 1.5 kV/cm and 1,000 pulses showed a significant degree of cell membrane permeabilisation, yet no increase in the release of polyphenols was noted.

Therefore…

The conclusion to be drawn is that the application of pulsed electric fields is utilizable as a technique for pre-treatment of Aglianico grape skins to increase and accelerate the release of polyphenols, reducing maceration times and increasing the health benefits of the final product.

Highlight: ANTIOXIDANT ACTIVITY AND SUGAR LEVEL

The results of the study oN the antioxidant activity of the wine are in line with the concentrations of polyphenols, proving the effectiveness of the two treatments applied at 1.5 kV/cm and the increased amount of polyphenols and anthocyanins in Aglianico showed an actual boost in antioxidant potential. Finally the treatment of the grape skins with pulsed electric fields did not alter the progression of sugar levels in the samples, or any qualitative parameter. 

Article by Elena Consonni

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