Light-struck taste in white wine: enological approach for its prevention

Abstract

Light-struck taste is a defect prevalent in white wines bottled in clear glass light-exposed for a considerable amount of time leading to a loss of color and appearance of sulfur-like odors. The reaction involves riboflavin (RF), a highly photosensitive compound that undergoes to intermolecular photoreduction by the uptake of two electron equivalents from an external donor, the methionine. The reaction includes different steps forming methional which is extremely unstable and decomposes to methane thiol and acrolein. The reaction of two molecules of methane thiol yields dimethyl disulfide. Methane thiol is highly volatile, has a low perception threshold (2 to 10 µg/L in wine) and confers aroma-like rotten eggs or cabbage. Dimethyl disulfide is less volatile, but the perception threshold is still low (30 µg/L) and has an aroma impression of cooked cabbage or onion. However, if light contact, at certain wavelengths, is avoided the reaction does not happen. The riboflavin is released by the yeast and its level up to 100 ppb is considered safe for the appearance and perception of this defect. In this study, fermentation trials of must were carried out by using 15 commercial yeast strains monitoring the fermentation trend, as well. The degradation kinetic was evaluated in both model solution and white wine exposed to light in the absorption wavelengths of RF (370 and 440 nm). Different clarifying agents and adjuvants were tested including different types of bentonite and carbon, and zeolite. Moreover, preliminary tests were performed on provoking the light-struck taste by illuminating a model solution added with gallic and ellagic tannins from oak, gall, grape seeds and skin, and glutathione, ascorbic acid and phenylalanine. The RF production by yeast was confirmed and it is a characteristic strain-dependent. Its concentration ranged 30-50 ppb, except for one strain which released 180 ppb. No correlation between the fermentation rate and the RF production was found. The selection of the yeast strain seemed to play a key role for the final concentration of RF in wine. RF disappeared after only 2 hours of illumination in both model solution and white wine. RF decay followed a 1st order reaction kinetic and the half-life time was doubled in case of white wine. Such a difference could be due to the matrix. Among the clarifying agents, all the bentonites tested (100 g/hL) led to a reduction of RF up to 60%. A lower decrease was found by using the zeolite (30%). The carbon showed the highest decrease of RF (90%). Lower RF reduction in terms of both concentration and decay rate was observed in white wine with all the clarifying agents investigated. However, in white wine, the treatment with carbon was the most effective and the “safe concentration” was reached after 2 hours adding 5 g/hL of carbon. The preliminary results on provoking the light-struck taste suggested the tannin extracts and glutathione could limit the appearance of this defect.