Characterization of tannins and prevention of light-struck taste: the enofotoshield project

AIM: Hydrolysable tannins resulted effective against the formation of light-struck taste (LST) in model wine [1]. The first activity of Enofotoshield project is to evaluate the effectiveness of tannins in limiting the LST in white wine. Therefore, tannins of different origin were characterized in terms of their chemical composition as well as their ability in preventing the appearance of LST that was firstly screened in model wine. METHODS: Fifteen commercial tannin-based formulas of different origin (grape skin and seeds, tea, oak, chestnut, cherry, acacia, quebracho, tara, nut gall, lemon) and extraction treatment (e.g. water, solvent) were considered. They were characterized in terms of total phenolics (Folin-Ciocalteau index and 280 nm; TPI), antioxidant capacity (DPPH assay), relative amounts of oxidized phenols, ellagitannins and proanthocyanidins content, the latter two for hydrolysable and condensed tannins, respectively. The oxygen consumption rate was assessed for all the formulas with and without sulfur dioxide. The impact on astringency and bitterness was also evaluated. The effectiveness against the appearance of LST was assessed in model wine solution added with the two actors of LST, riboflavin and methionine (Met), in both oxic and anoxic conditions. RESULTS: The tannin-based formulas showed a wide content of phenolics ranging from 462±28 to 1019±57 mg gallic acid/g powder for cherry tannins and gall nut tannins, respectively. Similarly, the antioxidant capacity strongly varied from 3.70±0.23 mM Trolox/g powder for grape skin tannins to 10.94±1.28 mM Trolox/g powder for nut gall tannins. Considering the ratio among the antioxidant capacity and TPI, tara tannins showed the greatest value. The oxygen consumption rate also differed and it was the lowest and the highest in the presence of and nut gall and chestnut tannins, respectively, when sulfur dioxide was not added. None of them affected both bitterness and astringency in white wine (up to 80 mg/L). Met decreased in all the conditions tested due to the light exposure and an increase of Met sulfoxide, the major compound deriving from Met oxidation [2], was observed. Sniffing trials showed the ability of most of these formulas in preventing the LST; some of the tested tannin preparations revealed only little differences in LST perception between oxic and anoxic conditions. CONCLUSIONS: Tannins can effectively prevent the appearance of LST. The tannin-based formulas with the best performances in terms of LST prevention and lowest impact on wine properties will be employed at bottling for the wine production at industrial scale.