Transition metals and light-dependent reactions: application of a response surface methodology approach

Abstract

Light-induced reactions can be responsible for detrimental changes of white and rosé wines. This is associated to the photo-degradation of riboflavin (RF) and of methionine (Met) causing the appearance of light-struck taste (LST). The fault is associated to the formation of volatile sulfur compounds (VSCs), namely methanethiol (MeSH) and dimethyl disulfide (DMDS), leading to cooked cabbage, onion and garlic odours-like [1]. Aside these pathways, other oxidative reactions can occur involving iron and copper that can generate quinones. Moreover, the ability of copper in binding the compounds containing a free thiol group is well known. 
This study aimed to evaluate for the first time the combined effect of iron, copper, and oxygen on LST formation in model wine.
A Surface Response Methodology approach was used considering 3 variables, as iron, copper and oxygen. Based on the experimental design, 15 runs (light-exposed and kept in the dark) were performed in model wine. Furthermore, to better understand the influence of phenolics, the same experimental design was applied in the presence of catechin and caffeic acid, used as model phenols. RF, Met, VSCs, and sensory were determined.
No RF was found in any light-exposed sample analysed. The major decrease of Met was revealed in model solution in which MeSH and DMDS were the highest. The presence of phenolics limited the degradation of Met and, consequently, the formation of MeSH and DMDS. In particular, in most of the runs where caffeic acid was added, VSCs were lower than in the runs in model wine and in the presence of catechin. The presence of iron (10 mg/L and 5 mg/L with oxygen 3 mg/L) led to a higher content of mercaptans in model wine as well as in the presence of catechin and caffeic acid. Our findings suggest that besides RF and Met, the susceptibility of a wine in developing LST appeared to be related to the presence of transition metals as well as to the different phenols that would ordinarily be present in wine. 
This study represents a further step for the deeper comprehension of the photo-induced reactions allowing to pursue the LST prevention in white wine.