Have you ever wondered whether light can affect wine or not? The answer is yes, yet it is necessary to understand its physics and chemistry to know about the so-called “light-strike reactions” that can impart off-odors to wine. The French call it goût de lumière.
Wine is sensitive to the damaging rays of light, particularly ultraviolet (UV) rays from sunlight and fluorescent lighting; that’s why wine (and beer) is packaged in dark bottles-the best example is Port in those familiar, dark, opaque bottles. Wines packaged in clear glass bottles are meant for quick consumption and should be stored in carton boxes until such a time.
All forms of radiation, including visible light ultraviolet (UV) rays and X-rays, have energy that is directly proportional to their frequencies or inversely proportional to their wavelengths-that is, high-frequency waves have short wavelengths, and vice versa. And the greater the energy, the greater the catalytic effect of chemical reactions in wine.
Of concern in wine are visible light and the sun’s UVA rays-yes, UVA rays because research has proven that even a short exposure of wine to sun (UVB and UVC are absorbed by the atmosphere), in the order of hours, can have detrimental effects. That’s because UVA rays lie in the 315-400 nanometer (nm) range of the electromagnetic spectrum and therefore has more energy than visible light, which lies in the 400-700 nm range.
Glass acts as a filter, but clear glass offers no protection against UVA and visible light. Green glass offers some protection, but only half as good as amber; dark-amber glass offers almost complete protection. So why is so much wine, particularly many of the ultra-premium wines of the world, packaged in green glass? This is likely for historical reasons when making green-colored glass was easiest and the effect of radiation on wine was unknown.
But glass is only half the story. We must now consider the optical properties of wine to then determine how radiation effects any chemical reaction.
Research has shown that wavelengths in the 375-440 nm range-that is, the high end of the UVA range and the blue (low) end of the visible light range-are most damaging to wine. The amount of radiation absorbed by wine in this range is directly proportional to opacity; therefore, red wine absorbs the most, and is most affected, while white wine absorbs the least-rosé wine is in the middle. As such, for marketing purpose where displaying color is important, white wine can be stored in clear glass, although not for extended periods of time; that’s why premium whites, such as age-worthy Chablis wines come in dead-leaf-colored glass, a golden-yellow color that provides better protection than green.
Opaque, deeply colored, rich reds absorb almost all radiation; however, these have the highest concentration of effect-inhibiting tannins, which provide protection. But if the concentration is low or the extent of exposure is long, unpleasant and off-putting aromas and flavors can develop. Specifically, red wine has sulfur-containing amino acids, namely, containing methionine and cysteine, which have a thiol (sulfur-hydrogen) side chain in their structures. These then react with naturally occurring vitamins, such as riboflavin (vitamin B2) and pantothenic acid (vitamin B5)-water-soluble vitamins that are easily destroyed by heat, oxygen and, that’s right, UV light. The products of this reaction are sulfur compounds such as hydrogen sulfide, dimethyl sulfide (DMS), and dimethyl disulfide (DMDS), responsible for imparting a range of foul smells from rotten eggs to cabbage and wet dog.
And now I’m left to wonder if my precious magnum of 1991 Château d’Yquem, in a clear-glass bottle, which I bought for my son born that same year, was affected by X-rays on its travel from the U.S. It was pre-9/11, however, the unyielding airport security guard simply couldn’t be convinced not to X-ray my precious bottle. I guess he was not a fine-wine aficionado.