Upgrading of grape skins: Significance of plant cell-wall structural components and extraction techniques for phenol release

In grape skins, phenols may be classified as (1) cell-wall phenols, which are bound to polysaccharides by hydrophobic interactions and hydrogen bonds, and (2) non-cell-wall phenols, encompassing phenols confined in the vacuoles of plant cells and phenols associated with the cell nucleus. The phenolic composition of wines determines the colour quality, the sensory, and the potential health promoting properties of wines, and the extraction of phenols from the grapes into the must and wine is to a large extent governed by how the phenols are bound and entangled in the grape skins. Degradation of cell-wall polysaccharides is a fundamental step to improve the release of phenols from grape skin whether this is in wine-making or in upgrading of wine pomace. Cellulases, hemicellulases, pectinases, and other enzymes able to catalyze the hydrolysis of bonds in plant cell-wall polysaccharides can be employed to decompose the cell-wall structure. In addition, novel extraction principles and optimization of extraction conditions such as temperature, solvent-to-solid ratio, use of supercritical fluids and new extraction cell designs have shown promise for optimizing the release of phenols from grape skins for valorization of wine pomace. An in-depth knowledge of how phenols are bound in grape skins will allow us to employ the most suitable techniques to release phenols in order to optimize the phenol-related properties of wine and maximize the phenol recovery from grape byproducts.