Understanding Hops: Alpha and Beta Acids

2013 Homegrown Chinook

Hops, or Humulus Lupulus, are one of the key ingredients in brewing beer. Hops used for beer come from the female plant and the cones consist of water, total resins, essential oils, tannins, monosaccharides, pectins, amino acids,lipids, and wax.  Total resin (which makes up roughly 15% of the total cone weight) and essential oils (which make up .5% of the total cone weight), together hold the most value to brewers. While the essential oils contribute to the aroma and flavor of the beer, the total resin contains the alpha and beta acids.

Alpha Acids:
Alpha acids are the main source of bittering for beer and are measured by the percentage of total weight of the hop. But, alpha acids are not soluble in wort and impart no bitterness. During the boil, alpha acids are isomerized by heat, into iso-alpha acids. This conversion is slow and almost never completed. After a one hour boil, only 25-30% of the alpha acids have been converted into iso-alpha acids. Therefore, the longer the hops are boiled, the more iso-alpha acids are in the final beer, creating more bitterness. We calculate this by IBUs (International Bitterness Units). The gravity of the wort also has an affect on iso-alpha acid conversion. Higher gravity worts have a slower conversion of alpha acids to iso-alpha acids. This is why extract brewers that only boil 3-4 gallons of their wort and top off with water to reach 5 gallons, need more hops to reach the same IBUs as brewers that use full boils.



There are three main types of alpha acids found in hops: cohumulone, humulone, and adhumulone. When looking at bittering hops it is important for brewers to look at the cohumulone percentages, usually posted as a percentage of the alpha acids. Cohumulone imparts a very harsh characteristic to the wort. Furthermore, cohululone is isomerized into the boiling wort faster than the other alpha acids. The following table shows some popular, American bittering hops and their alpha acid break down:

taken from Hop Union

If you are looking for a smooth bitterness, select a hop with a low cohumulone percentage like Simcoe, Magnum, Northern Brewer, or Horizon. If you are looking for a harsher bitterness select a bittering hop like Columbus, Galena, or Chinook.

Beta Acids:
There are three types of beta acids : lupulone, colupulone, and adlupulone. Beta acids are measured by percentage of the total weight of the hop, like alpha acids. Conversely, beta acids are not soluble in wort and beer and contribute no bitterness during the boil. Instead, beta acids can affect the bitterness of the a beer during and after fermentation. Beta acids  oxide in the finished beer to produce a harsh bitterness. Furthermore, beta acids can develop during dry hopping, long fermentation, and aging beer. When aging a beer it is important to look at the alpha acid : beta acid ratio. As beer ages alpha acids loose their bittering power while oxidized beta acids increase their bittering power, resulting in the preservation of bitterness as the beer ages. It is simply a preference in brewing. American styles like pale ales, IPAs, and double IPAs are meant to be drank fresh and will rarely see the effects of beta acid bitterness, beyond dry hopping. Conversely, German brewers prefer oxidized beta acid bitterness in many of their aged lagers. The chart below shows the ratio of alpha and beta acids in popular hops:

taken from Hop Union

Understanding alpha and beta acids is important because not all bitterness and IBUs are the same. Like in many aspects of brewing, experiemtnation is key. Finding what percentage of cohumulones when bittering an IPA or alpha acid : beta acid ratio when when aging a barley wine is key in designing great beers. While bitterness is the main purpose of hops essential oils also play an important role in brewing.

Resources:
Hop Union
Hop Character - Chris Colby, BYO, Hop Lover's Guide
The Alpha and Beta of Brewing -  Kay Witkiewicz, www.lovetobrew.com