Photodegradation of riboflavin in white wine: impact of contrasting light sources and ethanol contents

Riboflavin is a photoinitiator that is known to promote the accumulation of off-aroma compounds in white wine. However, the combined impact of different non-ionising radiation wavelengths and wine ethanol concentrations on this process is not well understood. An aqueous tartrate-buffer solution and a de-alcoholised Chardonnay (0.4 % (v/v)) wine, both supplemented with 0.5 mg/L riboflavin and adjusted to different ethanol concentrations (0.4 or 12.4 % (v/v)), were bottled with low oxygen concentrations. Samples were stored in darkness or exposed to different commercially available light sources (cool white fluorescent, cool white light-emitting diode (LED), and yellow LED). Concentrations of riboflavin were monitored over 7 and 14 days for the tartrate-buffer solutions and Chardonnay wines, respectively, while the Chardonnay wines also had measurements of volatile sulphur compounds and sensory analysis conducted at the end of the storage. Riboflavin degradation rates were highest when exposed to cool white LED, with progressively lower decay rates evident when exposed to cool white fluorescent or yellow LED, and for control samples stored in darkness. This order corresponded to increased photon energy within the wavelength range of 350–520 nm. After one day of exposure to cool white LED or fluorescent light, concentrations of hydrogen sulfide and methanethiol in the Chardonnay exceeded aroma threshold levels by 12-fold and 4-fold, respectively. The Chardonnay exposed to cool white LED was perceived to have ‘sulphur/rotten egg’ aroma, whereas exposure to cool white fluorescent light was associated with a ‘cabbage’ aroma. Ethanol only accelerated the loss of riboflavin in the Chardonnay exposed to cool white LED, but had little impact on volatile sulphur compound concentrations or the outcome of sensory evaluation. Exposure of the Chardonnay to yellow LED resulted in no reductive aromas being perceived by the sensory panel, which was consistent with the lower accumulation of volatile sulphur compounds and a decreased rate of riboflavin loss. As a consequence, a yellow LED source may offer considerable benefit when displaying wines in retail and consumer settings, where lighting is necessary.