The Mystery Of Skunky Beer: Aroma Triggers Explained

Beer drinkers are often familiar with the phenomenon of a beer developing a skunky aroma, but what causes this? Skunking is a chemical reaction that occurs when beer is exposed to light, specifically UV light, which is present in sunlight and fluorescent lighting. The reaction involves the compound iso-humulone, which is derived from hops, and creates a chemical called 3-methyl-2-butene-1-thiol, or 3-MBT, which is responsible for the skunk-like smell. This reaction can occur rapidly, even within minutes, and once it happens, the compound cannot be removed. While the skunky aroma is generally considered undesirable, some breweries are experimenting with intentionally skunked beers, challenging the notion that it is a flaw.

Exposure to UV light

The number one enemy of beer is light, specifically UV light. When hops are boiled down to make beer, they release chemical compounds called iso-alpha acids, which are bitter. When exposed to UV light, these compounds break down and interact with other molecules in the beer to produce a molecule that's almost identical to the one found in a skunk's smelly spray. This molecule is called 3-methyl-2-butene-1-thiol, or 3-MBT, and it has a strong skunky aroma. The process by which this happens is called "skunking" and it can occur in a matter of minutes, or even seconds, of direct UV exposure.

The only way to protect beer from skunking is to keep it away from UV light. This can be done by using LED lights in beer coolers, applying UV-blocking film to fluorescent lights, and keeping beer away from windows or using windows with UV-blocking film. The type of glass used for beer bottles also makes a difference. Clear and green glass offer little to no protection against UV rays, while brown glass blocks about 98% of harmful rays. Of course, cans and kegs offer 100% protection from UV light.

It's important to note that skunking is not caused by temperature changes or age. It is solely the result of a chemical reaction between UV light and the bitter compounds in beer. This reaction can be prevented by storing beer in dark glass containers, aluminium cans, or opaque bottles. However, once skunking has occurred, there is no way to reverse it. The human threshold for 3-MBT is very low, and even a tiny amount will be detectable.

To summarise, exposure to UV light causes a chemical reaction in beer that results in the formation of a compound with a strong skunky aroma. This can be prevented by shielding beer from UV light during storage and using appropriate packaging materials.

Hops and their compounds

Hops are the flowers (also called seed cones or strobiles) of the female hop plant, Humulus lupulus, a member of the Cannabaceae family of flowering plants. They are one of the four essential ingredients in beer, alongside barley, yeast, and water. Hops are added to the boil stage of brewing to unleash their "alpha" acids, which bitter and balance the sweetness of the malt. The hop flower gives beer bitterness when used early in the brewing process and aroma when added at the end.

Humulone, one of the compounds in hops, is the cause of the skunky aroma in beer when exposed to light. During the boil, humulone's shape is changed through isomerization, a chemistry term meaning that the bonds inside the molecule have been rearranged. After this isomerization has happened, the compound is called iso-humulone. If iso-humulone comes into contact with UV light, a reaction occurs that creates a chemical called 3-methyl-2-butene-1-thiol, or 3-MBT, which smells almost exactly like skunk spray. This reaction can happen in a matter of minutes, dramatically changing the aroma of the beer.

The hop plant is a vigorous, climbing, herbaceous perennial, usually trained to grow up strings in a field called a hopfield, hop garden, or hop yard when grown commercially. The first documented use of hops in beer is from the 9th century, though Hildegard of Bingen in the 12th century is often cited as the earliest documented source. Hops are used in brewing for their antibacterial effects and for their ability to balance the sweetness of the malt with bitterness and a variety of flavours and aromas.

Hops contain beta acids or lupulones, which are desirable for their aroma contributions to beer. The main components of hops essential oils are terpene hydrocarbons consisting of myrcene, humulene, and caryophyllene. Myrcene is responsible for the pungent smell of fresh hops, while humulene and its oxidative reaction products may give beer its prominent hop aroma. Together, myrcene, humulene, and caryophyllene represent 80 to 90% of the total hops essential oil.

Xanthohumol is the principal flavonoid in hops, with potential benefits under basic research for its potential properties. Hops are also used in herbal medicine in a way similar to valerian, as a treatment for anxiety, restlessness, and insomnia.

Beer packaging

The number one enemy of beer is light, specifically UV light. When beer is exposed to UV light, it can develop a skunky aroma. This is caused by a chemical reaction between the UV light and the hops in the beer. Hops are an ingredient in all beers, and they contribute to the bitterness, preservation, flavour, and aroma of the beer. One of the compounds in hops that lends bitterness to beer is humulone. During the brewing process, humulone is isomerized, meaning its molecular bonds are rearranged, and it becomes iso-humulone. When iso-humulone is exposed to UV light, it creates a chemical called 3-methyl-2-butene-1-thiol, or 3-MBT, which smells almost exactly like skunk spray. This reaction can occur in a matter of minutes and only requires a tiny amount of 3-MBT to be detectable by humans.

To prevent beer from developing a skunky aroma, it is essential to protect it from UV light exposure. This can be achieved by using LED lights in beer coolers, applying UV-blocking film to fluorescent lights, and keeping beer displays in stores away from windows or using UV-blocking film on windows. The type of packaging that beer is sold in can also play a crucial role in preventing UV light exposure. Clear and green glass bottles offer little to no protection against UV rays, while brown glass bottles can block up to 98% of harmful rays. However, even brown glass bottles can let in enough light over time to cause skunking, so it is important to store beer in a cool, dark place. The best options for preventing UV light exposure are cans and kegs, which offer 100% protection.

Some beer brands, such as Heineken, have chosen to stick with iconic green glass bottles despite the risk of skunking. In these cases, it is important for consumers to be aware of the potential for skunking and to store the beer accordingly. It is also worth noting that not everyone has the same sensitivity to skunky aromas, and some people may even find the smell appealing. Additionally, while skunking can affect the taste of the beer, it is not harmful to health.

To summarise, preventing a skunky aroma in beer comes down to proper packaging and storage. By using the right materials, such as brown glass, aluminium cans, or kegs, and keeping beer away from direct UV light, the risk of skunking can be significantly reduced.

Human sensitivity to skunky odours

Humans are extremely sensitive to skunky odours, which are often described as sharp and musky. The human nose can detect skunk spray in tiny quantities, as low as one part per 10 billion.

The scent of skunk spray is designed to irritate the eyes and nose, causing redness, mucus production, and tears. It can also cause nausea and even vomiting. The scent is so strong that it can permeate practically everything it touches.

The human threshold for 3-MBT, the chemical compound in skunked beer, is very low, meaning that even a tiny amount will be detectable. The good news is that while humans are highly sensitive to 3-MBT, we also get used to it very quickly.

In terms of genetics, scientists have discovered a mutation in the scent receptor for body odour, making it easier for humans to pick up on nasty stenches. Conversely, the scent receptors for perfume may be waning in intensity.

Additionally, research suggests that chronic psychological distress may lead to functional changes in the olfactory system, causing heightened sensitivity to odours. For example, persons with anxiety are more perceptive of threatening stimuli than non-anxious individuals.

Overall, while humans are highly sensitive to skunky odours, there is variation among individuals, and not everyone shares the same sensitivity.

Beer temperature

Firstly, it is important to clarify that temperature fluctuations alone do not cause beer to become "skunky." Skunking is specifically caused by exposure to light, particularly UV light, which interacts with the hops in beer to create a compound that smells like skunk spray. This can occur within minutes of direct UV exposure, and clear and green glass bottles offer little to no protection against it.

However, temperature does affect beer in different ways. Similar to milk, beer is best preserved when kept cold. Storing beer at room temperature can drastically reduce its shelf life from nearly six months to only a few weeks. Exposing beer to very warm temperatures can affect its flavour in just a couple of days. The hop flavours and aromas will begin to fade, and malt flavours may transform into a generic "sweet" taste. In some cases, reminders of wet cardboard and paper may also develop.

While temperature changes between cold and warm do not directly cause skunking, they can still impact the beer's overall quality. Therefore, it is advisable to keep beer refrigerated to maintain its flavour and freshness for extended periods.

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