The hops used in brewing are the female flower or cone (called the strobile) of the humulus lupulus plant. The flower contains a number of leaves called bracts that are arranged around a central stem called a strig. At the base of each of the bracts are the lupulin glands that contain the resins and essential oils that provide the flavor and aroma compounds that are of interest to brewers. In a fresh cone you can see the lupulin glands along the central stem which will appear bright yellow in color.
Hops are usually sold as either whole cones or pellets. Whole hops are simply dried hop cones, while pellets have been ground up and squeezed into a small pellet. Pellets takes up less space and makes them easier to ship and store, usually resulting in a lower price. Furthermore, hop pellets are more stable as the pellet has less surface area for oxidation to occur. One of the potential drawbacks of pellets, is that heat caused during production can drive off some the volatile oils that contribute to flavor and aroma This is less of a problem than it used to be, as the better hop providers using modern equipment appear to have accounted for this problem.
The bitterness in beer is primarily attributable to Alpha acids, although other compounds can contribute to the overall perception of bitterness. The amount of alpha acids contained in the hop are represented as a percentage that can usually be found on the packaging. Alpha acids are not soluble in water (or wort) and need to be isomerized before they will go into solution. The isomerization of alpha acids is a chemical reaction that occurs when the alpha acids are subjected to heat during the boil.
The bitterness of beer is most often measured in International Bitterness Units, or IBU. In order to measure IBU a sample of beer is run through a spectrometer. As this level of analysis is not available to most homebrewers, or even craft brewers, the IBU level is often estimated by means of calculation. There are a number of different calculations such as Tinseth, Garetz and Rager. Each varies in the utilization estimate and will come up with different results. These formula are pre-built into most homebrewing software. As a hombrewer it really doesn’t matter which one you use, as long as you always use the same formula for consistency.
In addition to alpha acids, there are a number of other compounds that contribute to the flavor and aroma profile. While alpha acids are the primary driver of hop bitterness, beta acids can also contribute to bitterness albeit via different pathways. Beta acids also have some responsibility for the antibacterial qualities of hops.
Essential oils contribute to flavor and aroma, and can be found in the lupulin glands that are located along the central stem of the hope cone. There are four primary essential oils found in hops; humulene, myrcene, caryophyllene, and farnesene. The two oils that are most often discussed are humulene and myrcene. Humulene is responsible for earthy and spicy flavors found in traditional noble hops. Myrcene is responsible for the citrus and fruit like aromas and flavors that are characteristic of american style hops. Caryophyllene and farnesene are thought to be responsible for the characteristic hoppy aroma.
The essential oils are highly volatile, and are easily driven off during boiling. Myrcene has a boiling point of just 160°F and humulene has a boiling point of 210°F. Temperatures above their boiling point will cause the oils to vaporize and be driven off with time. These oils are also prone to oxidation and is one of the reasons why old hops that have not been stored properly not only lose their desired flavor and aroma, but can take on negative qualities as well.
When discussing hops and recipe formulation, hops are often referred to as “additions” and the additions are measured in reference to the amount of time left on the boil. Generally bittering additions are added to the wort and boiled for 60 minutes allowing time for isomerization. Flavor and aroma additions come at the end of the boil, with a traditional flavor addition happening with 15 minutes left on the boil and an aroma addition happening with 5 minutes left or at “flame out”.
“Dry hopping” can be used to extract further hop flavors and aromas. The hops are most often added after the beer has been moved to a secondary fermenter, but they can be added to the primary as long as fermentation is complete. Fairly recent research has indicated that most if not all of the beneficial compounds are extracted within two to three days of contact with the beer.
“First wort hopping”, is where hops are added to the wort before the wort is brought to a boil. I have yet to see any systematic studies on the process, but the thought is that the lower temperatures allow the essential oils to form stable complexes. Anecdotal evidence suggests that first wort hopping provides a softer bitterness and allows for increased hop utilization. In a tasting I conducted at NHC, a beer brewed with only first wort hops had a surprising amount of flavor and aroma.
There are a few off flavors associated with hops. Excessive use of hops, or hops that are left in contact with the beer for too long can result in vegetable and grass flavors. Hops can oxidize, resulting in off flavors and aromas described as cheesy. Finally, UV light can cause iso-alpha-acids to turn into 3-methyl-2-butene-1-thiol, which produces a “sulfur” or “skunk like” aroma. There are “advanced hop” products which allow large brewers to add bitterness directly to the beer, and many of these products have been stabilized to prevent skunking.
(C) 2014-2015 by Matthew E. Schaefer all rights reserved -Used With Permission; except as to the image upon which no copyright is claimed.