Hops Aroma: How Long Does It Last?

Hops are an essential ingredient in beer, adding balance to the finished product through bitterness, flavour, and aroma. The female flower of the hop plant, Humulus Lupulus, is used to make beer. The hop oils that give beer its aroma are extremely volatile and will be driven off in the steam of the boil almost immediately. Therefore, aroma hops must not be boiled for long. They are typically added during the last 5 minutes of the boil or at flameout. However, hops can lose their aroma over time due to oxidation and other factors. Proper storage and packaging are crucial to preserving the aroma of hops. Additionally, factors such as carbonation, serving temperature, and yeast activity can also impact the aroma profile of the final beer product.

Hops aroma is lost due to oxidation during storage

Hops are added to beer to balance it with bitterness, flavor, and aroma. The essential oils in hops are responsible for the aroma of the beer. These oils are very volatile and most are lost during the kettle boil. However, some oils survive, especially if the hops are added late in the boil.

The hops aroma compounds are lumped into the category of essential oils. These oils make up 1 to 1.5% of the hop's dry weight. There are more than 200 essential oils found in hops, and their distribution can be used to "fingerprint" a variety. Only four essential oils are commonly referred to: myrcene, humulene, caryophyllene, and farnesene. These compounds are known as terpenes. Terpenes are responsible for not only the aroma of hops but also the aroma of fruits, flowers, herbs, and the like.

Terpenes also change during storage. These compounds change chemically (mainly due to oxidation) and become less volatile. This means that beers made from hops with a high percentage of terpenes (essential oils) have better aroma retention. That's one of the contradictions that keep home brewers on their toes; oxidation may improve aroma retention but will damage other hop flavors.

Hop oils, like hop bitter resins, are easily lost during storage due to their susceptibility to oxidation. In storage trials at the U.S.D.A. Hop Research Lab, oil losses were found to range from 28-90% in six months at room temperature, depending on the variety. These losses can be slowed by storing hops in the freezer, preferably in a package that allows no air or water exchange (i.e., "barrier"). The best advice to follow in this area is to buy the best hops you can find and take good care of them, especially if they are aroma hops.

The beta acids in hops also degrade due to oxidation during storage. Unlike alpha acids, however, their oxidation products are bitter. This means that oxidized beta acids make up some of the bittering lost by alpha-acid oxidation during storage.

Oxidized beta acids also have an aroma: cheese. This cheesy aroma is due to volatile fatty acids released from oxidized beta acids. These volatile fatty acids are identical to compounds found in aged cheeses, such as parmesan and romano.

Hop merchants are skilled at the sensory evaluation of hops and are always on the lookout for cheesy hops. If you smell your hops and start thinking about a big plate of pasta, snap out of it! Hops should smell like hops. If they’re cheesy, don’t use them.

Hop oils are volatile and get driven off during the boil

Hop oils are highly volatile and prone to oxidation, which means they are easily driven off during the boil. This is why traditional flavour additions are added with 15 minutes left on the boil, and aroma additions are added at the very end of the boil or at "flame out".

The four main hop oils that add flavour late in the brewing process are myrcene, humulene, caryophyllene, and farnesene, and each has a different boiling point. Myrcene, the largest of the hop oils, makes up 40-60% of the hop oil content in many American hop varieties, though most noble hops are low in myrcene. It has a boiling point of 147°F/63.9°C, and can even volatilise in a high-temperature steep, so it will be lost if boiled. Humulene, the traditional noble hop oil, has a boiling point of 210°F/99°C, just below the boiling point of water, so it is usually added as a late boil or post-boil addition. Caryophyllene, which is a major aroma component in many traditional English hops, has a boiling point of 262°F/129°C. Farnesene, found in the coating of apples and other fruits, has a boiling point of 203-257°F/95-125°C, and is the smallest of the hop oils.

Because three of the four main hop oils have a boiling point below 212°F/100°C, brewers often add their aroma/flavouring hops after the boil has ended to preserve these oils. This can be done by steeping hops after the boil, during the cooldown, using a hop-back, whirlpool additions, or dry hopping after fermentation. All of these methods have the advantage of not boiling off the oils and a much lower risk of oxidation as the boil or fermentation removes most of the oxygen from the wort/beer.

Dry hopping at high krausen can lead to diminished flavours and aroma

Dry hopping is a common practice among homebrewers and professional brewers. It involves adding hops to the beer after the wort has been cooled, usually in the primary or secondary fermenters, or directly in a keg. This process adds aroma and flavour to the beer without imparting much bitterness. The hops being added to the cold wort release their aroma compounds, which are usually fruity, dank, or herbal.

However, dry hopping at high krausen, or the peak of fermentation, can lead to diminished flavours and aromas. This is because the hops are exposed to heat, which can cause the volatile oils that contribute to flavour and aroma to boil off. Additionally, the longer the hops are exposed to heat, the more likely it is that bitterness will be extracted instead of flavour. This is a delicate balance, as the active yeast during high krausen can also process any oxygen that is accidentally added to the wort, reducing the risk of oxidation, which can also negatively impact the flavour and aroma of the beer.

To avoid diminished flavours and aromas when dry hopping at high krausen, brewers can take several precautions. One method is to use a hop bag, which can help prevent hop matter from clogging the keg or racking cane. Another technique is to cold crash the beer before bottling or kegging, which causes the hop matter to drop to the bottom of the fermenter. Sanitization is also important, as good sanitization techniques can reduce the risk of infection when dry hopping.

Overall, while dry hopping at high krausen can be a successful technique for adding flavour and aroma to beer, it requires careful consideration and attention to avoid negative impacts on the final product.

Exposure to oxygen can cause the loss of hop aroma compounds

The hop aroma compounds are susceptible to oxidation, which leads to their loss during storage. The essential oils in hops, which are responsible for the aroma, can be lost due to oxidation and absorption by packaging materials.

Hop oils are composed of hydrocarbons, oxygenated compounds, and sulfur-containing compounds. The hydrocarbons, such as myrcene and b-pinene, typically make up 80-90% of the total oil content. The oxygenated fraction, including linalool and geraniol, contributes to the floral and fruity notes in hops. The sulfur-containing compounds, although present in small amounts, are important for the aroma as they have very low aroma detection thresholds.

To preserve the hop aroma, it is crucial to minimise oxygen exposure during the packaging process. Additionally, using non-absorptive packaging materials can help prevent the absorption of hop oil compounds. Storing hops in the freezer, in barrier packaging, can also slow down the oxidation process and preserve the aroma.

Hop aroma is closely tied to carbonation

However, one source states that carbonation does not have a huge impact on hop aroma. The same source suggests that if you want to increase hop aroma, you should use more aroma hops.

Another source suggests that hop flavours are closely tied to hop aroma, which is significantly impacted by carbonation. The same source recommends being patient if tasting flat beer, as the hop flavours may be lacking until the beer is properly carbonated.

The impact of carbonation on hop aroma is a complex and evolving area of research. One experiment found that carbonating with hop-infused CO2 did not have a dramatic effect on hop aroma, as tasters were unable to reliably distinguish a beer carbonated in this way from one carbonated normally. However, the experimenter himself felt that the beer carbonated with hop-infused CO2 had more pine and dankness. The experimenter also suggested that using this method on a more nuanced style of beer or packing more hops into the AromaCan (the device used to infuse the CO2 with hop aromatics) might result in a more noticeable effect.

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