The most common odor in commercial beer and craft beer is the unpleasant “Light-struck” flavor and aroma. It can cause your beer to emit a strong smell (similar to a skunk) and make it unpleasant to drink.
What Is Light-Struck?
When light comes into contact with beer, the ultraviolet rays interact with hop compounds and produce a chemical called MBT. This chemical is the same one that skunks spray, so you can imagine the smell it emits. Humans can detect it at levels as low as one part per billion. Beer with this peculiar smell is referred to as “skunked”. The light-struck process can occur within a few seconds, so if you’re lying on the beach enjoying beer, you might experience this smell.
Of course, if your beer is stored in opaque containers like kegs, cans, ceramic bottles, or stainless steel bottles, the taste won’t change much. However, if your beer is stored in a green or transparent glass bottle, you need to be careful as it may have been light-struck. Transparent glass bottles cannot block any ultraviolet rays, while green glass bottles can block about 20% of the ultraviolet rays, but still allow 80% of them to enter the beer.
Why Does Beer Get Skunked?
Although most beer drinkers are aware of the “skunked” phenomenon, they don’t know how it occurs. Hops used in brewing beer contain light-sensitive compounds. When these compounds are exposed to strong light, a photo-oxidation reaction takes place, producing a strong flavor-active compound called 3-methyl-2-butene-1-thiol (MBT). MBT is one of the most potent flavor substances known and is often associated with being “skunked”. This pungent odor compound is similar to the notorious defensive spray used by skunks.
In 1875, German chemist Dr. Carl Lintner first reported that beer exposed to light would develop an unpleasant taste and smell. In 1960, Yoshiro Kuroiwa announced that the main component of the odor was MBT, which originated from the photolysis of isohumulones. Furthermore, Kuroiwa’s team discovered that the skunked taste would be more pronounced under blue light in the visible spectrum (350-500nm). Under strong sunlight, the reaction was almost instantaneous, and the presence of MBT could be detected in beer exposed for less than 10 seconds. Under less intense conditions, these reactions would still occur, but it could take days or even weeks for the off-flavor to become apparent.
Light-Struck and Beer Bottle Types
In the real world, beer is affected by light with a wavelength between 400-500nm (the blue end of the spectrum) and ultraviolet rays with a wavelength less than 400nm. Brown beer bottles can reduce the impact of light exposure and block light below 500nm. In contrast, green beer bottles can only block ultraviolet rays below 400nm, while transparent bottles provide almost no protection against light-struck.
What Impact Does Light-Struck Have on Beer Bottle Choice?
The odor-producing reaction involves the cleavage of the isopentenyl side chain of the iso-alpha acid in hops. This cleavage is photocatalytic and can lead to the formation of dimethylallyl radicals. Free radicals react with sulfur-containing compounds (thiols) to form MBT. While green and clear glass provide little protection against this reaction, brown glass is very effective in blocking it (at least for short-term or low-intensity exposure). To provide the best protection from light, it is recommended to transfer beer to opaque kegs or cans. Despite the lack of protection from light, some breweries still insist on using transparent and green glass bottles. This is because the color of beer bottles has become an important part of the brewery’s brand strategy.
When beer bottles do not provide protection, modern hop science offers an alternative to prevent the formation of MBT from hops. “Tetra” and “Hexa” hops are modified by reducing side-chain double bonds, which inhibits the photodegradation reaction. These advanced hop products are based on liquid or supercritical carbon dioxide extracts of hops, known as humulone. These resins are then alkaline isomerized to humulone in a solution, and isohumulone can be further reduced to produce bitter compounds that won’t degrade into MBT. These specialized bitter hops are referred to as “light-stable” products. They are used in the brewing process to make beer less susceptible to the harmful effects of sunlight. However, sunlight may still cause other flavor changes unrelated to hops.
How to Avoid Skunked Beer?
Avoiding skunked beer is simple: keep the beer away from light exposure. The best way is to package beer in an opaque container, such as a keg. Commercial breweries can also use cans for packaging. If glass bottles are the only option, it is recommended to use brown ones as they are more effective at blocking harmful ultraviolet rays compared to green or clear glass. However, even brown glass bottles can’t provide 100% insulation from light, so they still need to be stored in a dark place. For homebrewers, using glass bottles or light-transmitting jars during fermentation or storage may lead to the formation of off-flavors. It’s best to cover them with a towel or black garbage bag and store them in a dark area.
Of all the off-flavors that affect beer, skunking is the easiest to avoid. Proper packaging and storage are all it takes to prevent these unpleasant flavors.