The biggest reason that red wine is kept in dark bottles is to avoid damage from light. Light is a form of radiation that moves in waves. Short wavelengths, such as ultraviolet (UV) light, are very high energy, whereas longer wavelengths, such as infrared light, are much lower energy. Visible light falls right in the middle, with wavelengths between 400 and 750 nanometers.
Clear glass is transparent because it lets all wavelengths of visible light through. Thankfully, even clear glass can protect a bit from UV light. However, dark glass is needed to protect from visible light. Light-strike damage can occur under both natural sunlight and in artificial lighting. They are most commonly caused by wavelengths at 340, 380, and 440 nanometers, which includes both visible and ultraviolet light.
Light-strike damage, from either UV light or visible light at the blue end of the spectrum, causes a series of chemical reactions. Sulfur-containing amino acids will react with riboflavin or pantothenic acid, forming:
- Dimethyl sulphide (DMSP). Known as the “smell of the sea”, this is the metabolite given off by marine algae that gives ocean water that cabbage-y, fishy smell that is characteristic of marine environments.
- Dimethyl disulphide. This is the chemical emitted by the Dead Horse Arum Lily. The chemical attracts flies because of its similarity in smell to fetid meat.
- Hydrogen sulphide. This is the chemical that gives the aroma of rotton eggs.
These are certainly not things you want to see on wine tasting notes!
Amber bottles offer almost complete protection from ultraviolet light and significant protection from visible light. Green glass offers a little less UV protection, but, since it is usually used for wines that should be consumed more quickly, it is usually enough to prevent degradation of the wine. According to WRAP, amber glass blocks 90% of harmful rays, whereas green glass blocks closer to 50%.
There are also natural components of the wine that can help prevent light-strike damage. For example, phenols, such as tannins, can help protect wine. Because red wines have more tannins than white whites, red wines will be less likely to be damaged from light exposure that would ruin white wines.
However, since red wines tend to be cellared longer, they are more likely to be exposed to light before they are consumed, so they need to be kept in darker bottles. Thus, red wines are usually kept in amber bottles and whites are often kept in green ones.