Riboflavin (RF) is a well-known photosensitizer,
responsible for
the light-induced oxidation of methionine (Met) leading to the spoilage
of wine. An NMR approach was used to investigate the role of gallic
acid (GA) and sulfur dioxide (SO
2
) in the RF-mediated photo-oxidation
of Met. Water solutions of RF and Met, with and without GA or SO
2
, were exposed to visible light for increasing time in both
air and nitrogen atmospheres. Upon light exposure, a new signal appeared
at 2.64 ppm that was assigned to the S(O)CH
3
moiety of
methionine sulfoxide. Its formation rate was lower in a nitrogen atmosphere
and even lower in the presence of GA, supporting the ability of this
compound in quenching the singlet oxygen. In contrast, SO
2
caused relevant oxidation of Met, moderately observed even in the
dark, making Met less available in donating electrons to RF. The competition
of GA versus Met photo-oxidation was revealed, indicating effectiveness
of this antioxidant against the light-dependent spoilage of wine.
A pro-oxidant effect of SO
2
toward Met was found as a possible
consequence of radical pathways involving oxygen.
A Box–Behnken
experimental design was implemented
in model
wine (MW) to clarify the impact of copper, iron, and oxygen in the
photo-degradation of riboflavin (RF) and methionine (Met) by means
of response surface methodology (RSM). Analogous experiments were
undertaken in MW containing caffeic acid or catechin. The results
evidenced the impact of copper, iron, and oxygen in the photo-induced
reaction between RF and Met. In particular, considering a number of
volatile sulfur compounds (VSCs) that act as markers of light-struck
taste (LST), both transition metals can favor VSC formation, which
was shown for the first time for iron. Oxygen in combination can also
affect the concentration of VSCs, and a lower content of VSCs was
revealed in the presence of phenols, especially caffeic acid. The
perception of “cabbage” sensory character indicative
of LST can be related to the transition metals as well as to the different
phenols, with potentially strong prevention by phenolic acids.
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