Fault
Lightstrike
Light breaks down hop compounds into a thiol chemically close to skunk spray — the fastest fault in beer, and entirely preventable.
Also called skunked beer, skunky, sunstruck.
What it tastes and smells like
Responsible compound: 3-methyl-2-butene-1-thiol (MBT).
Ultraviolet and blue light break isohumulones — the hop compounds responsible for bitterness — into 3-methyl-2-butene-1-thiol, which is strikingly similar to a component of skunk defensive spray. The human threshold for it is among the lowest of any beer compound.
The reaction is fast. Beer in clear glass can develop a detectable skunk note within minutes of direct sunlight; green glass takes somewhat longer; brown glass blocks most of the responsible wavelengths and cans block all of them.
Styles this tends to affect
Not where it belongs — where it is most likely to appear when something has gone wrong.
- Blonde ale — A gentle, approachable pale ale with soft malt sweetness and restrained hopping — often a brewery’s entry-level beer.
- British golden ale — A modern British cask style: pale, crisp and citrus-hopped, developed to win drinkers back from lager.
- Cream ale — An American hybrid: ale-fermented then cold-conditioned, producing a smooth pale beer close to a lager in character.
- Dortmunder Export — A pale lager sitting between Helles and Pils: fuller and firmer than the first, softer and less bitter than the second.
- German Pils — Pale, dry and firmly bitter, with a crisp finish and a spicy-floral noble hop character — Germany’s answer to the Bohemian original.
- International pale lager — The global mainstream lager: pale, clean, lightly hopped and built for consistency and refreshment above flavour intensity.
- Kölsch — Cologne’s pale hybrid beer: ale-fermented then lagered, giving lager clarity and crispness with a whisper of fruit.
- Pre-Prohibition lager — A reconstruction of the American lager brewed before Prohibition: fuller, more bitter and more characterful than what replaced it.
If you are drinking it
A skunky aroma means the beer has been exposed to light, usually before you bought it. It is harmless. Some well-known imported brands in green glass are so consistently affected that drinkers have learned to expect the flavour as part of the brand — it is still a fault.
- Buy cans or brown glass where possible, avoid bottles displayed under bright light, and keep beer out of direct sun. (prevents)
- The reaction is irreversible. A skunked beer stays skunked. (cannot be reversed)
Not a safety issue. Beer’s alcohol, hop compounds and low pH make it inhospitable to organisms that cause illness.
What causes it
| Cause | Stage | How often | Who can act |
|---|---|---|---|
| Packaging in clear or green glass, which does not block the wavelengths that drive the reaction. This is a brand decision, not an accident. | packaging | most common | brewer |
| Display under strong shop lighting or in a sunlit window. Fluorescent and LED lighting can cause it given enough exposure. | retail storage | common | retailer |
| Beer left in sunlight — at a barbecue, in a car, or on a table outside. | consumer storage | common | consumer |
If you are brewing it
- Brewer: Package in cans or brown glass. Alternatively use reduced isomerised hop extracts, which are not light-sensitive. (prevents)
Where in the process this comes from
Steps that can produce this fault when they go wrong. Each links to what the step is for and how it is controlled.
- Packaging — Move finished beer into its final container with as little oxygen pickup as possible, because oxygen taken up here determines how the beer ages.
Often confused with
- Excess sulphur — rotten egg, struck match
- Yeast autolysis — rubber, burnt rubber
Questions
Why does some imported beer taste skunky?
Because it is sold in green or clear glass, which does not block the light wavelengths that break hop compounds down into a skunk-like thiol. The reaction takes minutes in sunlight. It is a packaging choice rather than anything about the recipe, and the same beer in a can would not have it.
Evidence
What BeerHQ knows about this, and how. Every statement carries the state of the evidence behind it rather than being presented flatly as fact.
Brown glass absorbs most of the wavelengths responsible for lightstrike, green and clear glass substantially do not, and aluminium cans exclude light entirely.
Backed by a source
Sources: Brewing: Science and Practice — print; Beer: Tap into the Art and Science of Brewing — print
Lightstrike occurs when ultraviolet and short-wavelength visible light photolyse hop-derived iso-alpha-acids, producing a radical that reacts with a sulphur source to form 3-methyl-2-butene-1-thiol.
Backed by a source
Sources: Beer: Tap into the Art and Science of Brewing — print; Journal of the Institute of Brewing
Chemically modified hop extracts — reduced iso-alpha-acids — cannot form the responsible thiol, which is how some widely-sold lagers are packaged in clear glass without skunking.
Backed by a source
Sources: Brewing: Science and Practice — print; For the Love of Hops — print
Lightstrike is a photochemical reaction independent of a beer’s age: a newly-packaged bottle left in sunlight will skunk within minutes, and a five-year-old can never will.
Backed by a source
Source: Beer: Tap into the Art and Science of Brewing — print
3-methyl-2-butene-1-thiol is structurally close to compounds in skunk defensive secretion and is perceptible at concentrations in the parts-per-trillion range, which is why lightstruck beer is recognised instantly.
Backed by a source
Source: Beer: Tap into the Art and Science of Brewing — print