The researchers indicate that squeezing the grapes has to be sufficiently intense to be able to extract the phenols from the internal layer of the skin.

The research group 'Food Colour and Quality', which is part of the Faculty of Pharmacy at the University of Seville, has published two scientific articles in which the reason that different quantities of phenols can be extracted from the solid parts of the grape during winemaking. The aim of this project is to improve the extraction of compounds from the skin and the seeds of the grape as it has been shown that phenols have health benefits and add quality to wine.

"In the case of wine, phenols add organoleptic qualities of great importance, and on these the quality of the wine directly depends. The appearance of these compounds in the wine mainly comes from the solid parts of the grape (skin and seeds) during winemaking," explains the University of Seville researcher Julio Nogales, who also says that "we already knew this, but now we can relate the structural composition of the skin and the seeds with the ease with which we can obtain phenols and understand the differences in phenol extraction according to whether we use the external or internal surface of the skin."

This study was carried out using vibrational spectroscopy, which includes infrared and Raman spectroscopy. These techniques study the vibrations that take place in the molecular links from two complementary points of view, allowing the identification of what the most important families of compounds are in the residue that remains after the extraction of phenols.

"As the level of ripening of the grape increases, the cellular wall of the material of the berry starts to degrade and the phenols are released more easily. The external layer, called the cuticle, does not contain polyphenols and is mainly made up of hydroxyl fatty acids, while the layer closest to the pulp is where the majority of phenols are founds in the skin. Therefore, during the process of winemaking, the crushing stage has a great influence on the extractability of the phenols from the skin, as inefficient crushing does not generate enough contact between the grape-juice and the internal layer of the skin," states Nogales.

In the case of the seeds, on the other hand, the diffusion of phenols decreases as the grape ripens because lignification occurs (sealing of the plant walls through the deposit of lignin) in much of the material, impeding the diffusion of phenols.

In the laboratory

For the realisation of this study, samples of red grapes suitable for winemaking were collected from two plots from the Denominación de Origen Condado de Huelva from the 2014 and 2015 harvests. The skins and the seeds were separated in the samples to try to determine the ease of extraction of phenols from these two sources.

After discovering the ease of extraction of phenols from the samples, they were characterised spectrally (FTIR and Raman) to understand their structural composition. Then, different statistical analyses were carried out to show the possible relationships between the structural composition of the non-extractable material and the ease of release of phenols as observed in these samples. As a result of this study, the researchers have confirmed that there exist clear structural differences between the external and internal parts of the skin and that these differences are related to the quantity of phenols that can be extracted from each part.

The greatest differences could be attributed to spectral areas related to pectins, monosaccharides, polysaccharide, lipids and waxes, all of which are components of the skin and it was determined that the quantity of polysaccharides and the degree of esterification of the pectins significantly influences the extractability of phenols. Similar results were found in the case of seeds.