It’s understandable why bitterness is an acquired taste. Despite what surely must pain “iso-philes” (bitter beer lovers), the old “bitter beer face” commercials have some truth to them. It’s generally agreed that bitterness has been an evolutionary signal for a possible poison, so it’s no surprise if we are initially put off the taste of bitterness.
What’s not quite so understandable is how the sensation of bitterness differs for each of us. Much of the research I’ve been involved with has shown that various bitter compounds elicit different responses from each person, with few correlations able to be drawn. Each compound has different intensities and often different qualities as well, including harsh, medicinal, vegetative, lingering, etc.
The predominant source of bitterness in beer are the iso-alpha acids. These are derived from the alpha acids which are present in the flowers of the female hop plant, Humulus lupulus. These alpha acids are found in the lupulin glands of the flower, which look like tiny yellow pollen-like balls clustered together. Much of the other material in the hops which brewers value (such as aroma compounds) are also contained in the lupulin glands, but today we discuss the alpha acids.
There are actually 5 different alpha acids but the predominant versions which make up the majority are the three acids humulone, co-humulone, and ad-humulone. Each of these weak acids are similar in basic structure, but carry different variations on a hydrocarbon side chain (humulone: isovaleryl, co-humulone: isobutyryl, ad-humulone: 2-methylbutyryl).
Despite the fact that raw hops themselves are intensely bitter, alpha acids themselves have been demonstrated to have no bitterness.  In order to obtain the desired bitterness, the brewer’s must apply heat to the hops while in solution with the wort. This is done in the kettle, where a number of other important things are also going on. Once the alpha acids are in solution, the boiling temperatures in the kettle cause the isomerization reaction (where the number of atoms doesn’t change, but the structure does) which produces the iso-alpha acids. This reaction is shown below.
Commonly, these iso-alpha acids are found in beer at levels from a staggeringly low value of 1.6ppm (Michelob Ultra) to over 40ppm (Ruination IPA), values which I’ve personally obtained by HPLC. A common pale ale or IPA would land in the neighborhood of 25-35ppm.
While there are no practical limits to the amount of iso-alpha acids you could have in beer (aside from what the human palate can handle), the limitations arise instead when alpha acids are added to the kettle: the pH of the wort plays an important role in the solubility of the rather hydrophobic alpha acids (the iso’s, by contrast, are a bit more hydrophilic than their non-isomerized counterparts). The lower the pH, the fewer alphas you can get in solution. For this reason, I am often skeptical when I read labels which claim 100+ BU’s (Bitterness Units – a liquid extraction assay which provides a relatively decent approximation of the bitterness of a beer).
There are many more roles that alphas and isos play in brewing, but the spotlight is usually on bitterness. We’ll explore the other various ways they influence beer in later articles.
Fritsch, A., Shellhammer, T., Alpha-acids Do Not Contribute Bitterness To Lager Beer, J.ASBC., 65:1, 2007, p 26
edit: fixed language about alpha acid varieties