Changes in Cu fractions and riboflavin in white wines during short-term light exposure: impacts of oxygen and bottle colour

Copper in white wine can be associated with Cu(II) organic acids (Cu fraction I), Cu(I) thiol species (Cu fraction II), and Cu sulfides (Cu fraction III). The first two fractions are associated with the repression of reductive aromas in white wine, but these fractions gradually decrease in concentration during the normal bottle aging of wine. Although exposure of white wine to fluorescent light is known to induce the accumulation of volatile sulfur compounds, causing light-struck aroma, the influence on the loss of protective Cu fractions is uncertain. Riboflavin is known to be a critical initiator of photochemical reac-tions in wine, but the rate of its decay under short-term light exposure in different coloured bottles and for wine of different oxygen concentrations is not well understood. This study aims to gain insights into the rate of change in Cu fractions and riboflavin concentrations during the exposure of white wine with different oxygen concentrations and/or different bottle colours to light over a period of days to months.

A Chardonnay wine with an addition of 0.5 mg/L riboflavin, 0.3 mg/L Cu and different concentrations of oxygen (10 and 0.5 mg/L) was exposed to fluorescent light at 20°C in Flint coloured bottles. The wine was also investigated using other coloured bottles (Arctic blue, French green, Antique green and Amber) with the minimum oxygen concentration. The Cu fractions were quantified using colorimetry and riboflavin concentrations measured by ultrahigh-performance liquid chromatography. The results showed that for wine in Flint bottles with low oxygen, light exposure accelerated the decrease in Cu fraction I and II, with the change in Cu fraction I being most pronounced (i.e., a 10-fold decrease in 24 hours). In contrast, high oxygen concentrations resulted in no light-induced decrease in Cu fractions I or II. Riboflavin concen-trations became depleted after only 20 hours of irradiation under high oxygen concentrations, while 0.07 mg/L remained in the wine with low oxygen. The darker coloured wine bottles slowed the changes observed for Cu fractions and riboflavin from a minimum period of hours (i.e., for Flint bottles) to a maxi-mum period of months (i.e., for Amber bottles). Although light is known to induce light-struck aroma in wine, this study has demonstrated it can also accelerate the removal of protective fractions of Cu and this has implications for the general reductive development of wine.