A study of mechanisms involved during the extraction of polyphenols from grape seeds by pulsed electrical discharges

This paper presents an experimental study of the influence of pulsed electrical discharges of low energy (3 to 10 J) on the extraction of polyphenols from grape seeds. To obtain a better understanding, three basic phenomena involved in the whole discharge process are studied separately: pulsed electric field (PEF), pre-breakdown phase (streamer), and breakdown phase (arc). The polyphenol extraction is much more efficient with arcs, compared to streamers and PEF. Therefore, during the discharge process, the enhancement of polyphenol extraction can be mainly ascribed to the final arc phase. The total energy per unit mass CWp required to extract 5000 mg GAE/100 g DM with the arc (CWp = 16 kJ/kg) is 27 times lower compared to streamers alone, and 47 times lower compared to PEF. An optical study shows that the mechanical effects of arcs (shock waves, expanding cavity, and strong turbulence) are much more pronounced compared to streamers. These mechanical effects are responsible for the fragmentation of grape seeds, and strongly promote the release of polyphenols. Other parameters such as the test cell shape and the inter-electrode distance also show that the polyphenol extraction is further enhanced when grape seeds are in close proximity with the breakdown arc. In optimized conditions, the extraction efficiency with low energy discharges can be significantly higher than previous measurements with high energy discharges. Industrial relevance This paper presents relevant information for the design of generating electrical discharges treatment. The study also addresses a specific case of use of by-products and shows the effectiveness of such technology.