Beer Science: Essential Oils

As the last of a three-part write-up on important compounds found in hops, this installment deals with essential oils.  The last two science write-ups were regarding alpha acids and beta acids, and can be found by clicking the links.  But the focus of this article is essential oils.  The flavors in beer imparted by the hop plant come as a result of many different chemicals acting in unison.  Therefore, the combination of alpha acids, beta acids, and a myriad of compounds known collectively as essential oils all make up the aroma/flavor profile and perceived "hoppiness" of a beer.  There is little debate that the essential oils composition in hops is just as important as the composition of the alpha and beta acids in hops.

There are over 300 identified compounds that are believed to contribute to hop profiles.  Some of these are perceived as pleasant, while others are less desirable.  Perceived bitterness is predominantly determined by the presence of alpha acids and beta acids.  However, other aromas and flavors are found in the vast array of chemicals comprising the essential oils of hops.


Essential oils make up only about 0.5-3% of the total hop weight.  Essential oils are comprised of 80-90% hydrocarbons and an oxygenated fraction.  Most of the hydrocarbons are very volatile and evaporate during the boil, leaving the oxygenated fraction behind.  In fact, all three of the essential oils listed below- myrcene, humulene, and caryophyllene - all boil off when added to the beginning of the brewing process.  There are methods to maintain the aroma presence of these chemicals, and that will be discussed after each of these essential oils is discussed in more detail.


Important Essential Oils

Three noteworthy essential oils are listed here along with their perceived aroma/flavor.

(1) Myrcene - Floral, Geranium leaf

Myrcene is the most prevalent of all the essential oils in mature hop plants.  Myrcene comprises anywhere from 50 and upwards or 70% of the total amount of essential oils.  As the hop cone ripens, the amount of myrcene present in hop cone rises.  Therefore, the percentage of myrcene can be a useful indicator of hop maturity.

The following hops possess a large percentage of myrcene in their total hop oil:
Cascade (45-60%)
Centennial (45-60%)
Amarillo (68-70%)
Simcoe (60-65%)
Citra (60-65%)

Myrcene is the most potent of the essential oils, and the human nose can detect as small as 13 parts per billion (ppb) of this compound.  However, myrcene is a present in the hydrocarbon fraction and therefore most of the myrcene will boil off during the brewing process.

(2) Humulene - Woody, Balsamic

Humulene is most pronounced in hop varietals such as Hallertau Mittelfrüh (55%), Saaz (35-40%) and UK Kent Golding (~36% of total oil).  Like myrcene, humulene is volatile and therefore most humulene is boiled off during the brewing process. 

(3) Caryophyllene - Floral, Black pepper

Caryophyllene is found as an essential oil in clove, hemp, rosemary, and hops.  This compound imparts a black pepper, spicy, floral aroma.  And like myrcene and humulene, caryophyllene also does not survive the brewing process due to the volatility of the chemical.


Late Hop Additions

Although myrcene , humulene, and caryophyllene do not survive a typical brewing process, there are methods to insure that the aroma presence of these essential oils in beer is maintained.  When added late (or dry-hopped) in the brewing process, these volatile hydrocarbons do not have time to completely boil off.  Thus, these interesting aromas can be preserved and flavor profiles can be enhanced during the brewing process.

An interesting note: the essential oils mentioned here are used in the perfume industry.  Apparently the woody, floral aromas of these hops render not only beers irresistible, but also humans!

References:
Oxford Companion to Beer
Beer Sensory Science: myrcene
Beer Legends Humulene Oil Table
Craft Beer Academy Essential Oils