Formation and prevention of light-struck taste white wine

Light-struck taste is a defect occurring white wines bottled in clear glass and exposed to light. It is manifested by a loss of color and aroma as a result of the presence of sulfur-like smells [1]. Its appearance is due to the reduction of riboflavin (RF), a high photosensitive compound, and the oxidation of methionine (Met) to give methional which is unstable under light and decomposes to acrolein and methanethiol. Two molecules of the latter compound can yield to dimethyl disulfide [1]. Methanethiol is highly volatile, has a low perception threshold (0.3 to 3 µg/L in wine) and confers rotten eggs-like or cabbage-like aromas. Dimethyl disulfide is less volatile, but the olfactory perception threshold is still low (30 µg/L) and it has an aroma impression of cooked cabbage or onion. Concentrations of RF lower than 80-100 µg/L can decrease the risk of light-struck taste appearance [2]. The wine treatment with charcoal can lead to a depletion of RF in white wine [3], but it can have a detrimental effect on sensory properties of white wine. In order to limit the appearance of the light-struck taste, certain antioxidants (sulfur dioxide and glutathione) and wood tannins (oak, chestnut and galla) were tested in model wine containing RF and Met and exposed to light. The concentrations of these two compounds were monitored as well as the content of volatiles. The RF was completely degraded under light independently to the presence of Met. On the contrary, this amino acid underwent to photodegradation only in presence of RF. The sulfur dioxide limited the appearance of the defect maybe due to the formation of a complex riboflavin-sulfur dioxide making the vitamin less susceptible to the photo-degradation. The molar ratio degraded RF:degraded Met ranged from 1:8 to 1:20, much higher than one previously indicated in literature [1]. As expected, the increasing concentrations of RF led to major levels of sulfur compounds. Similarly, higher amounts of Met strongly affected the formation of volatiles which content increased as Met increased. The wood tannins could exert a protective effect, the galla tannins in particular. In fact, the lowest levels of volatiles, namely methanethiol and dimethyl disulfide, were found when the gallotannins were added. Nevertheless, the volatiles were lower in presence of both chestnut and oak tannins in comparison to the model solution. The content of oxidized phenols could be the main actor against the formation of the light-struck taste since it was the highest into the galla tannins. The oxidized phenols could bind the sulfur compounds and, consequently, they could be reduced back to phenols. The protection of white wine against the appearance of the light-struck taste can be achieved by adding the wood tannins, gallotannins in particular, before bottling. As this defect can appear for higher levels not only of RF, but also of Met, low concentrations of these compounds can also play a protective effect allowing the maintenance of the wine quality during the shelf-life .