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Beer KnowledgeHops, from A to Z

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25 Oct 2023
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What are hops?

Hops are in fact a type of climbing plant that can be grown for a long period of time which also flower periodically. The hops that are used to make beer are not the fruits but the female flowers of the plant. When the female flowers that do not pollinate, the lupulin glands bloom in yellow color, which is where the resins and essential oils for adding flavors and aromas to the beer are held. Meanwhile, the male flowers do not consist of the lupulin gland, and thus, are not used for making beer.

Hops are diecious, which means that male and female flowers grow as separate plants. Once pollination between the male and female flowers results in viable seeds, then the hops are no longer usable to make beer. Therefore, only the female flowers are grown at hop farms.



 

Components of hops

The female flowers of hops (hereinafter ‘hops’) are made up of cellulose (which makes up the cell wall of plant cells) by more than half, 15% protein, 10% moisture, 8% minerals, and small amounts of polyphenol, lipids and fatty acids, and pectin. Depending on the type of hop, resins make up about 15-25% and essential oils make up about 0.5-3%. Resins mainly consist of α-acids and β-acids, among which α-acids isomerize to iso-α-acids when heated. It is the iso-α-acids of hops that give beer the bitterness. Meanwhile, essential oils are the substances that adds hop aroma to the beer. Up to this point, these information may seem familiar if you had taken a course in homebrewing orhave interest in beer (or read the Beer Post).

 

 

Essential oils

Let’s take a step further into learning about hops. First, essential oils. There are three parts to essential oil: hydrocarbons, oxygenate compounds and sulfur containing compounds. Further classification are shown in the table.

The reason for laying out the components of essential oil is to show the composition ratio and because the flavor of the beer is influenced by the combination of these components. For example, the grassiness of fresh IPA comes from hexenal and aldehyde, while the familiar citrus aromas of hops like orange and grapefruit come from various esters, nerol and linalool. The floral and fruity aromas come from linalool, geraniol, citronellol, 4MMP and 3MH. For herbal or resin (like pine) aromas, monoterpene like myrcene and sesquiterpene alcohols like humulenol I and II play a critical role. Depending on the composition, hops give different flavors and aromas, so it would be a good idea to go through them to get the desired characteristics. 

 

Don’t worry about where to get such information - websites that provide information on hops also have detailed composition lists.

 

 

Resins

Now, let’s get into resins. Like essential oils, resins are mainly found in the lupulin gland and consist of α-acids and β-acids, as well as soft and hard resins. The one to take note of is definitely the α-acids, which refers to various substances like humulone, cohumulone, adhumulone, prehumulone, and posthumulone. As mentioned previously, α-acids are isomerized to iso-α-acids (in case of humulone, isomerized to isohumulone), adding the bitterness to beer. Additionally, the preservative role of hops comes from the iso-α-acids. However, with the modern refrigeration technology and advancement of packaging, the preservative role is no longer taken into much consideration. Rather, the focus of measuring α-acids is for the bitterness. The bitterness measurement of the α-acids is referred to as IBU (International Bitterness Unit).


Other components like β-acids and soft resins also influence the bitterness of beer but the contribution level is very low compared to α-acids. On the other hand, hard resins do not contribute to the bitterness but act as surfactants or affect the aroma of the beer. Also, resins oxidize or polymerize with time, gradually losing its role in giving bitterness. Beers like lambic that are not characterized by the bitterness and hop aromas use hops that have been stored for some time for the aforementioned reason.

These days, hop extracts are often used to increase the yield rate of resins. When brewing, hops are added as they are harvested as wet hops or in pellet forms (ground and compressed) for more convenient distribution.


Although the pellet form is convenient, the use of hop extracts have grown recently because of the disadvantages of the pellet form like the low yield rate of α-acids, beer loss from the moisture absorption of hop flowers (the green leaf part), and development of undesired off-flavors when stored improperly. In terms of the yield rate of iso-α-acids, wet hops give about 25-30% yield rate, while pellets give about 35-40%. However, when extracts are used, the yield rate goes up to 40-45%. Additionally, hop extracts lead to better consistency in terms of bitterness.


Various methods exist for extracting hops, but the most representative methods include using organic solvents like ethanol or hexane and utilizing liquid carbon dioxide or supercritical carbon dioxide. Generally, the hop extracts are prepared by grinding the hops using macerator, extracting the hop components like resins, filtering out the hop residues, and evaporating to concentrate the product. From these steps, the loss of essential oils is immense, so hop extracts are not used to for adding aromas. Instead, hop extracts are generally used for bittering.


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