As Scott continues his journey into the craft of home brewing for The Brew Club he will (unless he has a raging thirst) need to bottle his beer, which brings us quite nicely to the subject of ‘skunked beer‘.
Beer becomes ’skunked’ when it is unintentionally exposed to light; more specifically, to Ultra-Violet (UV) radiation which is most prevalent in sunlight and fluorescent light. Brown glass filters the most UV radiation, green less so, but the choice of which colour bottle to use depends on what type of beer you’re bottling.
The light sensitive chemicals are compounds derived from hops, which as you know are used in beer both as a preservative (traditionally in IPAs where the extra hop compounds allowed the ale to journey from England to India without spoiling) and as a flavouring – adding bitterness. It’s the bitterness compounds that are susceptible to skunking, rather than the aromatic oils. Which is why, despite the ‘hoppy’ nose of many fine lagers (such as Pilsner Urquell) there is less concentration of the bitterness compounds, which is why lagers tend to be bottled in green, or even clear glass bottles, and ales in brown.
Settle down… this is the science bit:
The actual chemicals, which could form up to 15% of the dry hop flowers are alpha-acids, these are transformed in the brewing process (by the addition of heat) into iso-alpha-acids and it is these that add the bitterness to a brew.
Subsequent addition of UV energy allows these acids to react with sulphur compounds in the ale (there are several hundred different chemical compounds inside a good ale) to form 3-methyl-2-butene-1-thiol (thiol indicates sulphur).
It is the sulphur in the 3-methyl-2-butene-1-thiol that shares the malodorous properties of stink bombs, well water and skunks – an offensive odour. Hence skunkiness, AKA ‘skunked beer’.
And time is not a factor in this; research has shown that a well-hopped beer can become ‘lightstruck’ in less than a minute in bright sunlight – perhaps that’s why real ale pubs in Britain tend to be dingy!
Other than selecting a bottle that filters the UV light, then brewers can modify their brews to reduce the iso-alpha-acids (but retaining alpha-acids) – such as by dry-hopping – this adds the aromatic nature of the hops, but less bitterness.
And that is the story of skunked beer. Do you take precautions to keep your beer from getting skunked? Have any skunked beer stories to share, or add anything to the above post? Please share!