The unbearable lightness of hop chemistry

by Homebrewing

Usually homebrewers know hops are what makes your beer bitter, and provides the beer with antimicrobial properties.  Some might know the difference between alpha-acids and beta-acids, but like most, their eye’s glaze over if you go into too much more detail.  It’s important to know the details however, especially when some hops are difficult to find.  This article will help you understand the different compounds which make up your hops.

Bittering, Flavoring, and Aroma Hops

Hops are used for three purposes in your beer: flavoring, bittering, and hop aroma.  The antimicrobial aspects are great too, but most do not think about it.  The length of time you boil your hops determines the effect you are trying to achieve:bittering, flavoring or aroma.

Bittering hops are added near the beginning of the boil.  The longer you boil the hops (usually 60-90 minutes), the more alpha-acids are converted to their bitter form throughisomerization (the atoms in the molecule get rearranged).  Most of the isomerization happens in the first 45 minutes, but more can convert between 45 and 90 minutes.


Hops vary by use: bittering, flavoring and aroma

Aroma and flavoring hops are usually added later in the boil.  The boiling evaporates the hop flavor and aroma.  Flavoring hops are a compromise betweenbittering and aroma hops and are usually added halfway through the boil.  Aroma hops are added during the final minutes of the boil or after the boil.

The aroma in hops can diminish with processing or time.  For example, aroma whole hops are better than similar pellets, because the oil content decreases dramatically in pellets during the processing.  The oils oxidize at a rapid rate, even under ideal storage conditions.

Beers are dry hopped to add more aroma (and hop flavor since smell and taste are connected) by running the beer through a hop-back system or adding hops after the fermentation.  Dry hopping adds more of a grassy flavor, while adding hops late in the boil gives more of a floral or fragrant flavor.  The aroma is much stronger (not necessarily bad) when you dry hop.

If you want to dry hop your beer to add aroma, wait for at least 3 days of fermentation.  Hops, especially whole hops, can carry bacteria, molds, or wild yeast.  If you add the hops to your primary fermentation, you risk infecting your beer.  After three days your yeast will be able tooutcompete most anything, and your beer should be much safer from contamination.

Hop Acids

Hops used in your wort contain three main alpha-acids:humulone, cohumulone, adhumulone, and three beta-acids: lupulone, colupulone, adlupulone.  The easy way to distinguish between the two is the alpha-acids contain “humulone” in the name (from the genus name of hops, Humulus), and the beta-acids contain “lupulone” in the name (from the species name of hops, lupulus).

Hop alpha-acids

hops lupin

The yellow lupin contains the oils and acids to bitter and flavor your beer

Alpha-acids are the primary bittering acid for your beer.  Usually hops with high alpha-acid content are used as bittering hops.  Before boiling, hops are not bitter.  The alpha-acids isomerize during the boil to make the bitter flavors.  The isomerized versions of the acids (iso-alpha-acids) are what contributes the bitter flavor in your beer.  Cohumulone is the most easily extracted acid during brewing, and is considered the most important of the alpha-acids.

There is significant effort to quantitatively determine the bitterness contribution from hops in beer.  The current “gold standard” is International Bitterness Units (IBU).  One IBU is equal to 1 milligram of iso-alpha-acid per liter (0.0001335 of an ounce per gallon).

Another measurement common for homebrewers is the Alpha Acid Unit (AAU).  This unit was created by Dave Line (The Big Book of Brewing) and adopted by the American Homebrew Association (AHA).  One AAU/HBU equals one ounce of a one percent alpha-acid hop.  Two ounces of a 5 percent alpha-acid hop would give 10 HBUs.  Each HBU contributes 85 IBUs per liter (22.472 IBUs per gallon).

AAU / HBU = % alpha acid content X oz. of hops

To guestimate the IBU value for your beer, you can add up the AAUs to determine the total contribution of IBUs, and then divide by the size of your batch.  I say guestimate because this formula ignores length of boil and hop utilization.  The hop utilization will be different for worts with different gravities.  With AAU/HBU you are calculating the amount before you boil, not determining the amount after the brewing process is complete.  For a more accurate calculation ofIBUs, see Ray Daniels’ Designing Great Beers.

Hop beta-acids

Beta-acids also contribute a bitter flavor to your beer, but the bitter flavor is significantly more harsh and sometimes undesirable.  The beta-acids do notisomerize, they oxidize to produce bitter flavors.  The bitterness from beta-acids can come over time, long after the iso -alpha-acids have broken down.  This can give your beer a different bitter flavor after aging, which is sometimes desirable in aged beers.  The bitterness contribution from beta-acids is much less in your beer because the beta-acids are much less soluble.

On a side note for hop growers, beta-acids, specifically colupulone, the main component of the beta-acid fraction of hop extracts, was shown to repel the two-spotted spider mite.  In Repellent and oviposition-deterring effects of hop beta-acids on the two-spotted spider mite (1996), higher beta-acid amounts were effective in repelling the mites and reducing the number of eggs laid.  This suggests hops grown with higher amounts of beta-acids would require less pesticides protecting against spider mites.

Alpha-beta ratio

The alpha-beta ratio of hops is simply the ratio of alpha-acid content over beta-acid content.  Some brewers prefer a ratio close to one because most noble hops are close to a ratio of 1.  Other brewers prefer higher ratios so thebittering stays consistent as the beer ages.  As alpha-acids degrade, beta-acids oxidize keeping the bitterness consistent over time.

Chemical Analysis of hops


The oils in beer contribute more aroma and flavor.  Hops are added late in the boil, or sometimes after the primary fermentation (dry-hopping) to give your beer more hop flavor.  Add your hops late in the boil because boiling breaks down or drives off the oils which contribute aroma and flavor.  Hops will usually list the total oil content, as well as each type of oil.


Humulene is a hydrocarbon (contains only hydrogen and carbon atoms) which gives the beer an elegant taste and aroma.  Most noble hops (and other aroma hops) have a high amount ofhumulene.  Because humulene degrades quickly, it is very unlikely your beer will contain humulene unless the aroma hops are added very late in the boil or if you are dry hopping.  The products from humulene degradation (i.e. – humulene epoxides) usually give the beer a spicy characteristic.


Another common hydrocarbon myrcene, is characterized with a pungent and sometimes offensive aroma.  Noble hops have very low amounts of mrycene.  Like the humulene, it is very unlikely to remain in your beer.  It degrades quickly as well.

Myrcene does contribute to the final beer flavor however.  There are several compounds like linalool and geraniol which give the beer floral flavors, and many believe this is from the myrcene since the resulting compounds are closely related.


Caryophyllene produces a similar spicy flavor to humulene when boiled or oxidized.  It usually accounts for 5 to 15 percent of the total oil, but it is in higher amounts in the aroma and noble hops.


Farnesene is found in very small amounts in most hops.  Some hops list only trace amounts.  Again this compound is found in higher amounts in noble, sometimes as much as 20%.  There is little known about the flavor contribution however, so here’s another possible research study for graduate students!

Remain %

Since all hops are an organic substance, they will degrade over time.  The acids available when you pick the hop will break down over time.  This is why it is important to use fresh hops.  Not all hops degrade at the same rate.  Some will last a long time in storage, while others will spoil quickly.  The remain percentage of a hop is the amount of alpha and beta acids remaining after 6 months of storage at ambient temperature.

This doesn’t mean all hops go bad with aging, in fact research has shown some hops actually improve with age.  Lambic recipes sometimes use Saaz hops which are aged over 2 years.  Usually it is the aroma hops which improve with age.

Choosing alternative hops

hops harvest

Harvesting my Spalt hops

With the hop shortages in past years, many homebrewers are forced to choose alternate hops when crafting their beers.  There are substitution charts for the most common hops, but not all hops are listed.  If you are looking to replace a hop in your recipe not listed in a substitution chart, the best course of action is to look at the chemical properties of your hop and find another hop with similar properties.  There might still be slight differences in flavor, but more often than not, the flavors will be similar.

Using hop extracts

There are not too many homebrewers using hop extracts in their beer, which is due more to romanticism rather than reason.  Many large commercial brewers use hop extract instead of hops.  Hop extracts have significantly less variability than fresh hops.  This allows the brewer to predict the hop effect on the beer with much higher precision.  This is very important to produce a consistent flavor in your product.

If you are a “green” brewer, you might also want to consider using hop extract.  If you use whole hops, you will get about 1/3 of the potential aroma and bitterness while boiling.  This means you are wasting about 2/3 of the hop potential.  More fresh hops need to be planted, harvested, and transported to flavor your beer.  Personally I’m not that green, so I use the real hops.

Don’t miss anything

New articles are out regularly and new videos come out every week. Make sure you subscribe!

Credits and Links

  • none



DJ Spiess

DJ Spiess

Beer buddy

I live in Denver, Colorado. This blog is everything about beer, wine, cider, mead and other spirits.
I am a avid homebrewer and winemaker. I’ve been making my own beer and wine for many years. I started making beer when I was in college (mostly because the drinking age in the United States is 21). My first few beers were horrible. The beers are much better now, and I often supply my neighborhood with free beer! It is a great hobby!