The Chemistry Of Hops: Aromatic Compounds In Brewing

Hops are the flowers of the hop plant, Humulus lupulus, and have been used in brewing beer for centuries. They are added to the brewing process at different stages to achieve different effects, with the initial bittering addition balancing the sweetness of the malt and providing bitterness to the beer. The second aroma addition is added later to give the beer a unique aroma and flavour.

The essential oils in hops are considered volatile and will evaporate quickly if left in the boil too long. Most aromatic hop oils are best extracted at medium whirlpool temperatures of 160–170°F (71–76°C). The four main essential oils that contribute to the aroma of hops are myrcene, humulene, caryophyllene, and farnesene. Humulene, in particular, is responsible for the characteristic hoppy aroma of beer.

Hops contain essential oils that give beer its aroma

Hops are the flowers of the hop plant (*Humulus lupulus*), which has separate female and male plants. Only the female plants are used for commercial production. Hops have been used in brewing beer for centuries, originally for their preservative properties. Today, hops are used primarily as a bittering, flavouring, and stability agent in beer. They impart bitterness, floral, fruity, or citrus flavours, and aromas to beer.

• Myrcene (β-myrcene) is the most prominent oil and adds resinous pine, citrus, and fruity flavours to beer.
• Humulene (α-caryophyllene) is the second most abundant hop oil and adds woody, hoppy, and spicy notes to beer.
• Caryophyllene (β-caryophyllene) adds black pepper, spicy, and herbal aromas to beers.
• Farnesene (β-farnesene) is found in trace amounts or not at all in many hops. When present, this oil adds fresh, green, and woody aromas to beer.

In addition to these four, there are over ten other possible oils within hops, though most are at undetectable levels or do not contribute much to the overall flavour of the finished beer. Sulphur-containing aroma compounds in hop oil, known as thiols, are responsible for the tropical aromas in many recent hop varieties.

The four main essential oils in hops are myrcene, humulene, caryophyllene, and farnesene

Myrcene is the most abundant and potent of all the oils in hops. It is responsible for the pungent smell of fresh hops and is associated with citrusy and resinous-pine aromas. Humulene is a natural terpene most associated with woody and pine aromas. Hops with higher amounts of humulene tend to be more floral, herbal, and black peppery in character. Caryophyllene adds flavours of black pepper, spiciness, and herbs. Farnesene is associated with floral aromas and slight notes of woods and citrus.

Hops also contain thiols, which are sulphur-containing aroma compounds

Hops contain many different compounds that contribute to the aroma and flavour of beer. The female flowers of the hop plant, Humulus lupulus, are used to add balance to the finished product through bitterness, flavour, and aroma.

Three thiols that have been identified in hops are:

• 4-mercapto-4-methylpentant-2-one (4MMP) or 4-methyl-4-sulfanylpentan-2-one (4MSP), which smells and tastes of box tree, black currant, and ribes
• 3-mercaptohexan-1-ol (3MH), which is often described as exotic, smelling of rhubarb and citrus
• 3-mercaptohexyl acetate (3MHA), which is reminiscent of passion fruit and guava

These compounds are also prominent in wines such as Sauvignon blanc and Riesling, although they do not occur naturally in grapes. They are formed during fermentation from precursors present in grape must.

The amount of hops added at different stages of the brewing process affects the aroma

Hops are the flowers of the hop plant, Humulus lupulus, and are added to the brewing process at different stages to achieve different effects. The initial addition, known as the bittering addition, is added early in the brewing process to balance the sweetness of the malt and provide bitterness to the beer. The second addition, known as the aroma addition, is added later in the process to provide the beer with a unique aroma and flavour. Finally, dry hopping is the addition of hops to the fermented beer, enhancing the hop aroma and creating a hops-forward flavour.

Aroma hops, also known as finishing hops, are typically added towards the end of the boil to preserve their delicate flavours and aromas. Aroma hops contain volatile oils that evaporate easily and contribute to the floral, herbal, or spicy notes of the beer. By adding the hops later in the process, the evaporation of these essential oils is prevented, thus imparting a "hop taste" or "hop aroma" to the beer.

Dry hopping is another technique used to enhance the hop aroma without adding additional bitterness. This involves adding hops to the fermented beer and allowing them to steep in the liquid for several days or weeks. The technique of dry hopping creates a hops-forward aroma and flavour, with the compounds extracted by cool wort or beer being quite different from those found in the wort after the boil.

The timing of hop additions can vary depending on the desired aroma and flavour profile of the beer. For a more bitter beer, bittering hops are boiled for a longer period, typically 60-90 minutes. For a more delicate aroma and flavour, aroma hops are added towards the end of the boil or during the whirlpool stage. Dry hopping can be done for a few days to a few weeks, depending on the desired intensity of the hop flavour and aroma.

By carefully controlling the timing and techniques of hop additions, brewers can create a wide range of beers with unique and complex flavour and aroma profiles.

Hops are added at the bittering, aroma, and dry hop stages

Hops are added to the brewing process at different stages to impart different characteristics to the beer. The three main stages are bittering, flavour/aroma, and dry hopping.

Bittering

Hops added at the beginning of the boil are primarily used for bittering. The hops are boiled for 60 minutes or more, and this process isomerises alpha acids in the hops, creating iso-alpha acids which are responsible for the bitter taste of beer. All beers will have at least one addition of bittering hops, and this is essential to balance the sweetness of the malt.

Flavour/Aroma

Hops added during the boil, but with 15-30 minutes remaining, are primarily used for adding flavour. The shorter boiling time means that less of the bitterness will be extracted, but the crisp, hoppy flavour will be present in the final product.

Hops added during the last 10 minutes or less of the boil are primarily used for adding aroma. The hop oils responsible for the aroma are extremely volatile and will be driven off in the steam of the boil very quickly, so a short boiling time is essential.

Dry Hopping

Dry hopping is the process of adding hops to the fermenter after the yeast has been added. This is a recent phenomenon, and it adds aroma to the beer without boiling off the volatile oils.

Other Factors

The type of hop used will also affect the flavour and aroma of the beer. Aroma hops are low in alpha acids, with a percentage of less than 10%, and a 1:1 ratio of alpha to beta acids. This makes it difficult to add significant bitterness to the beer. However, as alpha acids isomerise at temperatures above 175°F (79°C), some bittering can still occur.

Dual-purpose hops have high amounts of both alpha and beta acids, as well as essential oils, and can be added at any stage of the brewing process.

The time and temperature of the boil will also affect the flavour and aroma of the beer. The longer the hops are boiled, and the higher the temperature, the more likely it is that bitterness will be extracted and flavour lost.

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