Understanding and preventing the photocatalytic cycle responsible for the occurrence of light-struck flavor in white wines and Champagnes – DeLight

The formation of the light-struck fault is a complex process leading to the formation of volatile sulfur species whose olfactory perception threshold is very low. Previous studies have led to the hypothesis that the absorption of visible light by riboflavin leads to a photo-oxidation reaction of sulfur amino acids (methionine, cysteine) naturally present in wines. As part of the De-Light project, we conducted extensive photochemical studies to fully elucidate the photoinduced mechanism responsible for the formation of volatile sulfur species. These studies highlighted important gaps in previous studies that led to the formulation of mechanistic steps that are in contradiction with our experimental observations. In parallel, studies by a panel of sensory experts made it possible to make the link between the perception of the light-struck fault and the concentration of volatile sulfur species in order to develop a technique that can limit the appearance of the light-struck fault by trapping methionine, one of the precursor compounds.

We were able to show that only methionine and riboflavin participate in the photocatalytic cycle responsible for the formation of the taste of light, and that the mechanism proposed in the literature is incorrect. The importance of the oxygen content, the pH of the solution, and the limited protective action of sulfites was quantified. Furthermore, we observed that the formation of volatile sulfur species continues even in the dark after exposure to light. Thanks to these results, we developed a porous resin capable of selectively trapping methionine and thus lowering its concentration below the critical threshold identified by the panel of sensory experts. This resin is inert, easy to synthesize, recyclable, and likely suitable for food contact. It is very effective: only 150 mg are enough to permanently protect a bottle of Champagne. It can also be used to protect dairy products against the appearance of the analogous organoleptic defect. A patent application has been filed and discussions are underway with an industrial partner for its industrialization. Other wine sectors have contacted us with a view to extending the scope of protection to other organoleptic defects such as cork taste or smoke taste.

The project led to the discovery of a technical solution to prevent the formation of the light-struck fault through the use of a resin to trap methionine during the winemaking process. This discovery will be developed by an industrial partner as part of a maturation project.

The project benefited from strong cooperation between the partners, which enabled the consortium to generate excellent results both from a fundamental and an applied perspective. A first article explaining the photoinduced mechanism responsible for the light-struck fault appeared in the journal Food Chemistry (IF = 7.5), which is the most cited experimental journal in food science. These results were also the subject of two invited conferences (including an international invited lecture) and two oral communications. In parallel, the development of a resin capable of selectively trapping methionine in hydroalcoholic solutions is a real feat and has been the subject of a patent application. An industrial group has come forward to support its development through a maturation project in order to achieve a commercial product. The filing of the patent (in December 2021) makes it now possible to communicate these results more widely through publications in high-impact peer-reviewed scientific journals. This project also enabled the training of a thesis student and a postdoctoral researcher.

White wines and Champagnes are very sensitive to visible light, which causes a "light-struck" fault resulting from the photoinduced oxidation of certain amino acids by riboflavin. The phenomenon is well documented but there is as yet no complete understanding of the complex mechanistic pathways leading to the formation of numerous sulfur-containing compounds. The DeLight project will undertake a complete mechanistic investigation of the photocatalytic cycle by combining sensory analyses by expert panels with studies performed on model solutions and authentic samples. The results will be used to deliver a demonstrator for the protection of white wines and Champagnes resulting from the combination of natural photoprotectors with advanced winemaking/fermentation techniques and packaging solutions. The project will be a joint effort between the Pernod Centre de Recherche (CPR) and G.H. Mumm, and the Institute of Molecular Sciences (ISM, UMR 5255) at the University of Bordeaux. Together, we bring expertise in the areas of packaging, fermentation, and the chemistry of wines (CPR), winemaking (G.H. Mumm) as well as photochemistry and molecular recognition (ISM). The two partners are perfectly complementary and have collaborated previously on fundamental projects involving the photochemistry of anethol. During project DeLight, the two partners will work closely together and be jointly responsible for the success of two key tasks: the development of protecting strategies and the prototyping of a proof-of-principle demonstrator.
By undertaking a complete mechanistic investigation based on product studies, transient absorption spectroscopy and spectroelectrochemistry, we will contribute to advancing the scientific knowledge of the formation of the light-struck flavor and foster the emergence of new solutions to this wide-reaching problem. There is no solution to this problem to date, with the exception of using opaque containers to exclude light. Therefore, a combination of factors needs to be leveraged to improve the light-fastness of white wines and champagnes. To this end, we will elucidate the fundamental processes leading to the appearance of the light-struck flavor and propose a multi-pronged approach to solving the problem. This represents the first comprehensive mechanistic study of the photochemical processes and can lead to a long-term industrial solution to the problem of light-struck fault.