Words by Simon Difford
Light is as damaging to beer, wine and delicate liqueurs as oxygen and heat. Brown glass offers almost total protection, at least against short-term or low-intensity exposure. Green glass offers some protection but clear glass offers none. Aluminium cans or beer kegs give total protection.
The issue of light denigration is most obvious in beer where alpha acids (α acids) found in the resin glands of hop flowers give beer its refreshing hop bitterness. Light triggers the breakdown of hop alpha acid compounds to form a dimethyl allyl radical. This reacts with thiol sulphur-containing compounds to create 3-methyl-2-butene-1-thiol (MBT) which emits pungent 'skunky' or 'cat pee' aromas. Indeed the skunk's defensive spray consists mainly of thiol compounds. Hence, in beer tasting notes it is common to see the term 'skunked' or 'skunky' as part of the description of a beer's aroma and taste. MBT skunk notes are incredibly pungent and detectable at concentrations as low as 0.1 parts per billion.
The blue part of the visible light spectrum (350-550 nm) is the most damaging and in strong sunlight a beer can become light-struck in less than 20 seconds. The effects of fluorescent beer fridge lighting are much more gradual and my take days or even weeks before the beer's degradation is detectable.
People buy with their eyes so despite the risks some beer producers persist in using clear glass bottles and this has led to the development of 'light stable' hop extracts which can be used to bitter 'designer beers' such as Corona and Sol. Hence, if you are going to drink such a beer in a clear glass bottle, then perhaps do it in the dark - your beer won't be light-struck and you will not be seen drinking it!
Many liqueurs are also very sensitive to the effects of light, especially fruit liqueurs (crèmes) and dairy liqueurs - ever noticed that cream liqueurs tend to come in brown glass?
Store in a cool, dark place - really dark.