NMR Spectroscopy: A Powerful Tool to Investigate the Role of Tannins in the Taste of Wine and their Health Protective Effect

Tannins in the skin and seeds of grapes used to make red wine are responsible for the two dominant sensory perceptions of astringency and bitterness. Astringency is a tactile sensation causing a dry, rough and puckering mouth-feel, while bitterness triggers an unpalatable harsh taste. Although these flavors are both associated with tannins, their mechanisms of action differ greatly. Astringency results from an interaction between the tannins and the saliva proteins, whereas bitterness is the result of an interaction between the tannins and the taste receptors located on the tongue. In the last decade, various studies using NMR spectroscopy have revealed new clues to the understanding of astringency perception at the molecular level. We now know the three-dimensional structure and the colloidal state of tannins are key factors in the mechanism of tannin-saliva protein interactions. Although the latter are undeniably related to astringency, it is only very recently we have learned that the lipids of oral cavity membranes and a fortiori provided by fat foods could also play a role in this complex sensory phenomenon. Indeed, strong interactions between tannins and membrane lipids have been highlighted in recent research supported by a fluidizing effect on membranes depending on the tannin structure. These findings show lipids interfere with tannin-saliva protein and tannin-taste receptor interactions involved in astringency and bitterness respectively. In addition to their role in taste, tannins as antioxidant molecules and in a larger extent polyphenolic compounds provided by foods are strongly suspected to have a positive role in many pathologies. Whereas their antioxidant properties have been widely demonstrated, their protective effect on membrane against lipid oxidation has been shown for the first time by NMR investigations. New insights into the location of tannins within the membrane have been proposed to explain their inhibitory effects on free radicals. Moreover, a synergistic effect has been evidenced proving the beneficial effect of food polyphenols as shown by epidemiological studies.

Keywords: Antioxidants, Astringency, Bitterness, Lipid oxidation, Membrane location, Molecular dynamics, NMR, Polyphenols, Synergistic effect, Tanninlipid interactions, Tannin-saliva protein interactions, Taste, Wine tannins.