Hops Essentials – Oils, Resins and Tannins

Hop cones. In whole leave, pellet or plug form, hops add aroma, flavor and bitterness to beer. Photo by Scott Hall.

Photograph by Scott Hall

Hops, the beloved herb of brewers everywhere. With so many varieties creating such a wide range of flavor, aroma and bittering qualities, hops is both an intriguing and mysterious aspect of the brewing process. But it wasn’t always that way.

Before hops came into widespread use, the magic and mystery of brewing involved the use of various herbs and spices for bittering and flavor to counter the sweet maltiness of fermented brews. Some common herbs and spices used were sweet gale, ground ivy, mugwort, heather, ginger, caraway seed, nutmeg and cinnamon. Collectively, the various combinations of herbs and spices used in producing these beverages was called gruit.

You may have heard of grut ales, brewed by some microbreweries as a tribute to the ancient ways of brewing.

Across Europe, a slow shift to the widespread use of hops in brewing occurred between the 11th and late 16th centuries, though the earliest recorded use of hops was in A.D. 736, maybe used in combination with other herbs in a gruit.

You can get as technical as you want exploring the history of hops and hop chemistry, or you can skim the surface and get a general idea to understand how they work. Either way, learning about hops on any level will enable you to use them more creatively in your brewing and may even help you gain more control over your beer.

It’s Not quite so simple
When I started brewing I naively assumed that hop leaves were more or less like tea leaves, and that the bitterness in beer was caused by hop tannins. Like steeping a cup of tea, I figured the longer you left the hops in the boil the more bitter the beer would be. That rudimentary assumption proved incorrect.

Tannins are present in hops but at a lower percentage than in tea, so the bittering effect of hop tannins on beer is negligible, and certainly not the main player in beer bitterness.

Though hop tannins can affect a beer if too much hops are used in the boil, or left in the fermenter for too long, problem tannins are more likely to come from grain husks during the mash process. Excessive mash temperatures, high mash water pH, and prolonged mash time can draw more tannins out of the husks.

Aside from a bitter and astringent edge to the beer flavor, grain husk tannins, or even hop tannins, can contribute to protein haze often referred to as “chill haze.” You would notice this cloudiness after putting your home brew in the refrigerator.

Hop Flavors and Aromas – Essential Oils
The key components responsible for hop flavor, hop aroma and hop bitterness in beer are the essential oils and resins found in and on the hop leaves themselves. Incidentally, most tannins are found in the stem of the hop cone, and in a very low percentage in the hop leaves; in the area of about 4%, compared to, say, 15% for total hop resins.

Essential oils found in hops, myrcene, humulene, caryophyllene and farnesene are considered by researchers to be the main compounds responsible for aroma. Aroma hops are typically added during the last 10 minutes of the boil, or dry hopped. When added late in the boil they will also contribute to flavor.

Essential oils released into the wort during the latter stage of the boil will react with oxygen. This will also occur during storage and aging. The reaction of these oils with oxygen creates secondary oils called oxygenated hydrocarbons, which are responsible for hop flavor in a finished beer.

Any hop can be used as an aroma hop. Whether they have a low or high alpha acid percentage is relatively insignificant, since they will be added very late in the boil, infused or dry hopped. By very late, I mean during the last 5 to 10 minutes of the boil.

The decision on what hop to use for aroma in your home brew depends on whether a particular hop variety is appropriate for a specific style of beer, or has an aroma that you find appealing and would like to incorporate into your recipe.

Brewers need to take a little more care with the hops they choose for flavor, because these hops are placed a little sooner in the boil than for aroma, typically after the last 20 minute mark in a 60 minute boil. Adding very high alpha acid hops at this point can still affect bitterness and may overpower your beer, so some adjustment of the bittering hops will be necessary.

Residual Flavor Compounds
While high alpha hops can be used for aroma and flavor, lower alpha acid hops have no substantial bittering effect, and the oxygenated hydrocarbons of the essential oils will not survive into the finished beer.

Because very little of the essential oils, which contribute to flavor and aroma, will come through in the final beer after a long time in the boil, it is better to use small amounts of a high alpha acid hop for bittering, than to use more of a lower alpha acid hop. Overuse of low alpha acid hops early in the boil can result in astringent off flavors, or haze. If there is any chance that hop tannins are going to affect your beer, through flavor or by contributing to protein haze, it is in that scenario.

That said, the essential oils myrcene and humulene do oxidize and degrade to other flavor compounds which do survive the boil and contribute to flavor. These residual flavors vary among hop varieties and can lend floral, spicy, citrus or piney notes to a finished beer. But it is best to add hops for flavor a little later in the boil to minimize any of the adverse affects mentioned above.

Alpha Acids and IBUs
The alpha acids present in hop resins that contribute to the hop bitterness in a finished beer include humulone, cohumulone, adhumulone, posthumulone and prehumulone.

When bittering hops are added to the boil these alpha acids undergo a chemical change called isomerization. Isomerization causes the atoms of these molecules to change their structure, but the molecule is the same, just in a different form. They become isomers, one for each of the original alpha acids in the hops, isohumulone being the most common. These isomers of the alpha acids, known as iso-alpha acids, are responsible for hop bitterness in beer.

The concentration of these iso-alpha acids in the finished beer is measured in International Bittering Units or IBUs, represented in parts per million. The higher the number, the greater the bitterness.

For example, Magic Hat’s #9 has 20 IBU, a fairly mild bitterness. Sierra Nevada Pale Ale has 37 IBU, just a touch more bitter. Compare those to a Sierra Nevada Torpedo IPA at 65 IBU, or 75 IBU for something like Flying Dog’s Wildeman Farmhouse IPA and you get the idea. While impressive, the latter are a still bit tame compared to some, Founders Devil Dancer Triple IPA for instance, which boasts 200 IBUs.

I refer you to BeerTutor.com. According to their list of 100 Most Bitter Beers, Flying Monkey’s Alpha Fornication tops the IBU chart at 2500, which is off the scale bitter. A close second is Mikkeller X Hop Juice, at 2007 IBU. Regarding the excessively high IBUs, BeerTutor goes on to say, “…keep in mind that the higher end of this list is basically gimmicky as the human mouth can only taste up to about 120 IBUs.”

How do we calculate IBUs?
So, how would a home brewer determine the IBU of a beer? In his book Designing Great Beers, Ray Daniels explains that the basic formula for calculating IBU is rather simple, but there is a little snag: the utilization factor, which needs to be calculated and included in the formula.

Daniels includes a utilization chart in his book, but luckily someone else has figured it out for us and posted their chart online. For hop utilization (U%) I refer you to Glenn’s Hop Utilization Numbers at RealBeer.com.

The basic idea is to take the weight of hops, in ounces (W) added to the beer, multiply that by the alpha acid percentage (A%), multiply that result by the utilization factor (U%), then multiply by a correction factor to convert to parts per million (7,489), and finally divide the result by the final fermenter volume (V) multiplied by a gravity correction (GC) factor.

The equation would look like this:

W x U% x A% x 7,489

(V x GC)

To calculate your Gravity Correction (GC) factor — the bottom number in the IBU equation — do this: 1 + [(BG – 1.050) ÷ 0.2 ] where BG is your Boil Gravity. If you’re boil gravity is less than 1.050 the GC would be 1.0.

How Important is Control?
Most of us start out guestemating the amount of hops to use in our homebrews. We toss in 2 ounces for bittering and then an ounce or so for aroma. But at some point, as we develop as brewers, we try to get more control over what’s happening, and what we taste in our beers. We get the bug, crave more hops and start piling it in, but without some idea of what’s happening and how to properly leverage the essential oils and alpha acids, we’re flying blind.

Hopefully this post will help you get a better handle on hops and how it really affects your beer. For further reading and information, please refer to the references found at the bottom of this post.

Happy hopping your homebrew!


References:
Ray Daniels, Designing Great Beers: The Ultimate Guide to Brewing Classic Beer Styles
Hops Chemistry (PDF)
Wikipedia, Hops
Wikipedia, Tannin
Wikipedia, Gruit
Wikipedia, Isomerization
Wikipedia, Humulone
BeerTutor.com, 100 Most Bitter Beers (IBUs)