How Beer Fermentation Enhances Hop Aroma

Hops are the flowers of the hop plant (Humulus lupulus) and are a key ingredient in beer, providing balance, bitterness, and aroma. They have been used in brewing beer for centuries, originally for their preservative properties. Today, hops are still used to extend the shelf life of beer, but they also play a crucial role in defining its flavour and aroma profiles. The hop plant has separate female and male plants, and only female plants are used for commercial production. The flowers contain essential oils that contribute to specific aromas, ranging from floral and herbal to citrusy and piney. The specific combination of terpenes in a beer can vary depending on the hop variety, the timing of the hop addition, and the brewing technique. For example, dry hopping – the process of adding hops to the beer after fermentation – can enhance hop aroma without adding additional bitterness.

Hop oils contain compounds that give beer its distinct aromas

Hop oils are derived from the hop flower or strobile, and they account for about 1-3% of the weight of the cone. They are made up primarily of a hydrocarbon fraction (containing only hydrogen and carbon) and an oxygenated fraction (containing hydrogen, carbon, and oxygen). Hop oils also contain small amounts of sulfur-containing compounds. The hydrocarbons typically make up 80-90% of the total oil content; the terpenes myrcene and β-pinene and the sesquiterpenes β-caryophyllene and α-humulene are the most prevalent constituents.

Hop oils are responsible for many of the flavor and aroma characteristics in beer. If you drink a pale ale or an IPA with a citrus or pine aroma, you can thank the hop oils. The hop oils are delicate and evaporate if the hops are boiled for too long, so hops for flavor and aroma are typically added towards the end of the boil.

There are four primary hop oils found in hops: myrcene, humulene, caryophyllene, and farnesene. Myrcene is the most prevalent hop oil, often comprising 50% or more of the total oils in the hop cone. It is commonly associated with floral or citrus aromas in beer. Humulene is the second most common hop oil, though sometimes it may be in greater quantity than myrcene. It contributes woody, spicy, and herbal characteristics and tends to withstand high temperatures better than myrcene. Caryophyllene has a distinctive woody and herbal aroma and often contributes an herbal character to beer. Farnesene usually represents less than 1% of the oils in the hop, though it may be as high as 10% or more of the total oil content. It contributes a woody or herbal character to the beer.

Hop oils can be added at any stage of the brewing process – kettle, fermenter, or post-fermentation. Techniques such as dry hopping and wet hopping add aroma without extra bitterness.

Hop aroma is enhanced by the process of dry hopping

Hops are added to beer during the brewing process to impart unique flavours and aromas. The hop aroma is enhanced by the process of dry hopping, which involves adding hops to the fermenter or keg after fermentation. This technique adds aroma without the bitterness that is usually associated with hops.

During the boil, hops release alpha acids that provide the bitter flavour needed to offset the sweetness of the barley malt in beer. However, the delicate aromatic oils that give hops their distinctive aroma are often lost during the boiling process. Dry hopping helps to preserve these aromatic oils, resulting in a burst of hoppy aroma that is characteristic of many modern beer styles, especially American beers.

The process of dry hopping involves adding hops to the beer after primary fermentation has completed. This allows the hops to soak in the finished beer for several days to several weeks. The longer the hops are left in contact with the beer, the more pronounced the hop aroma will be. However, it is important to note that overexposure to hops can result in a "grassy" or "oily" flavour, so brewers must carefully control the duration of dry hopping.

The type of hops used for dry hopping is also important. Aromatic hops with low alpha-acid levels (6% or less) are generally preferred for dry hopping as they have a higher percentage of the fragile aromatic oils needed for this technique. Examples of suitable hops include Saaz, Tettnanger, Hallertauer, Goldings, Fuggles, Cascade, and Willamette. Brewers should select hops that match the origin and style of the beer, such as using English Goldings hops for an English Ale or India Pale Ale.

In addition to the type and amount of hops, the form of hops used for dry hopping can also impact the final product. Plug or pellet hops are generally preferred, especially when using a glass carboy with a narrow neck. Whole hops can be difficult to get in and out of the fermenter and may require a mesh bag to contain and separate them from the beer. Pellet hops can cause foaming when added to the fermenter due to their large surface area, while plug hops are easier to handle and fit through the carboy neck.

Dry hopping is a popular technique used by both homebrewers and commercial craft brewers to enhance the hop aroma in their beers. It is particularly well-suited for brewing styles with high hop rates, such as IPAs, Pale Ales, some Stouts, and California Common (Steam) beers. By carefully selecting the type and amount of hops and controlling the duration of dry hopping, brewers can create beers with a strong hop aroma that appeals to drinkers who appreciate the distinctive flavour and fragrance of hops.

Hop aroma is influenced by the timing of additions during the brewing process

The longer hops are boiled, the more bitter the beer becomes. Therefore, bittering hops are added early in the process and boiled for up to 60 minutes. This releases more alpha acids, which contribute to bitterness.

Aroma hops, on the other hand, are added towards the end of the boil, typically during the last 5–10 minutes, or at flameout. This is because hop oils responsible for aroma are extremely volatile and will be driven off in the steam if boiled for too long. Adding aroma hops late in the process emphasises their aromatic qualities and minimises the bitterness they impart.

Flavouring hops are generally added with 15–30 minutes remaining in the boil. During this timeframe, very little bitterness will be extracted, but a crisp hoppy flavour will be achieved.

Techniques such as dry hopping and wet hopping are also used to add aroma without extra bitterness. Dry hopping involves fermenting beer with hops in it, while wet hopping involves brewing with fresh, undried hops within 24 hours of being harvested.

Hop aroma is influenced by the type of hops used

The type of hops used can be categorised into three types: bittering, finishing, and dual-purpose. Bittering hops have high alpha-acid levels and are responsible for the bitter flavour of a beer. Finishing hops, on the other hand, have high aroma levels and are added at the end of the brewing process to contribute to the aroma. Dual-purpose hops are a combination of both bittering and finishing hops.

The timing of hop additions during the brewing process also plays a crucial role in the final aroma of the beer. Aroma hops are typically added later in the process to prevent the evaporation of essential oils, which contribute to the hop aroma. Techniques such as dry hopping, where hops are added during fermentation, and wet hopping, using fresh, un-dried hops, are popular ways to enhance the hop aroma without adding extra bitterness.

The specific variety of hops chosen will also influence the aroma. Traditional European "noble" hops, such as Hallertau, Tettnanger, Spalt, and Saaz, are known for their low bitterness and high aroma. These hops are commonly used in European-style lagers, such as Pilsener, Dunkel, and Oktoberfest/Märzen. On the other hand, American hops, like Cascade, Columbus, and Centennial, tend to have higher alpha-acid levels and contribute to the bitter flavour of beers like IPAs.

In addition to the type of hops and the timing of additions, the quality and freshness of the hops are also important factors in achieving the desired hop aroma. Storing hops in cold temperatures and using vacuum-sealed packaging can help preserve the essential oils and aromatic qualities of the hops.

Hop aroma is influenced by the technique used to add hops

The technique used to add hops plays a significant role in the development of hop aroma in beer. The timing and method of hop addition can enhance or diminish the aromatic qualities of the final product. Here are some ways in which the technique of adding hops influences hop aroma:

• Dry Hopping: This technique involves adding hops during fermentation or after the beer has finished fermenting. Dry hopping is widely used to enhance the hop aroma without contributing additional bitterness. The hops are typically left in the beer for a few days to a few weeks, depending on the desired intensity of the hop aroma and flavour.
• Aroma Addition: Aroma hops, also known as finishing hops, are added towards the end of the boil. This technique emphasises the aromatic qualities of the hops while reducing the bitterness they impart. The later the hops are added, the more their aromatics are highlighted. Aroma hops contain volatile oils that evaporate easily, contributing to the floral, herbal, or spicy notes in the beer.
• Bittering Addition: The initial hop addition, known as bittering hops, occurs early in the brewing process and is primarily responsible for balancing the sweetness of the malt and providing bitterness. The longer the hops are boiled, the more bitterness they contribute. However, a longer boil time can also lead to the evaporation of aromatic compounds, making bittering hops less suitable for imparting aroma.
• Hop Amounts: The amount of hops used can also influence the aroma. Using a larger amount of hops, especially when added late in the boil or during dry hopping, can intensify the hop aroma. However, there is a risk of excessive bitterness and the development of a grassy taste if too many hops are used.
• Hop Quality and Storage: The quality and storage conditions of hops can impact their aromatic contribution. Properly stored hops that are less than a year old and kept in nitrogen-flushed packaging or continuously refrigerated tend to retain their aromatic qualities better. Improperly stored hops may develop an off-flavour, such as a grassy or vegetal note.
• Hop Timing and Steep Times: The timing of hop additions and the length of steeping times can affect hop aroma. Adding hops close to the end of the boil, such as during the whirlpool stage or with a short steep time, can help preserve delicate hop flavours and aromas. Covering the kettle after late-hop additions may also reduce the loss of aromatic compounds through evaporation.

Frequently asked questions