Alpha Acids - The Bitter Truth

(Thanks to Mike Jablonsky for the title).  A lot of brewers know that alpha acids cause bitterness somehow.  In this article I will discuss just a little bit of their structure, names, and use.

First of all, why are these called acids?  There are no carboxylic acid groups in the molecule (see the structures below and note the absence of a carboxylic acid group).  The reason they are called acids is that alpha acids are more soluble in basic pH.

Humulone is just one of the three common examples of the class of molecules called humulones. One step in the biosynthesis by the hop plant (Humulus lupus) is the transfer of the side chain from an amino acid to the humulone precursor.  I have aligned the structures of the amino acid above the humulone, so that you can see where the side chain went onto the humulone molecule.  The other two humulone structures are cohumulone and adhumulone:

Looking at the bottom of the molecular structure, we see a hydroxyl group, and an isoprenyl group. Isoprenylation (commonly called prenylation) is a common building process in plant biochemistry.  If you replace the bottom hydroxyl of humulones with an isoprenyl group, you get the beta acids (lupulones). Lupulones are even less soluble in water, due to the switching out of a -OH group which can hydrogen bond with water, for a "hydrophobic chain" which cannot favorably interact with water.

Neither the alpha acids nor beta acids dissolve at a high enough concentration to impart any perceivable bitterness.  You must change these structures through isomerization.  That will be a separate topic for another day.