As most beer drinkers already know, hops in beer are mainly responsible for its bitter flavor. More specifically, a group of acids commonly referred to as hop alpha acids provide beer’s bitter taste (I first touched on these acids in my article All About Hops).  While there are many different types of hop alpha acids, there are three in particular that are dominate in beer’s flavor profile.

These three major hop acids are called isocohumulone, isohumulone, and isoadhumulone. Additionly (to make it even more confusing), each one of these acids exists in two different molecular configurations notated as the “cis” and “trans” forms of the acids, giving us a total of six different major alpha acids in beer.

It’s interesting to note that these molecules start out as plain old cohumulone, humulone, and adhumulone; none of which are particularly bitter. It’s the brewing process that transforms these molecules into their “iso” forms and brings bitterness to the beer, as shown by the example in the diagram below:

Not a big difference in form, but a big difference in taste

The “cis” and “trans” forms of molecules can be a little tough to understand, even if you have a couple of semesters of organic chemistry under your belt. These forms are often described as the “handedness” of the molecule. In much the same way as your right and left hands are of the exact same form yet still mirror images of each other, many molecules can also exist in two distinct three dimensional form (although not always necessarily mirror images of each other).

Understanding the -cis and -trans forms of molecules is an important concept because these two forms can have very different chemical properties despite their similarity. We’ve all heard about the dangers of trans-fats in our diets. It’s only a very slight difference in the form of this fat in comparison to normal (cis) fats makes the trans-fat a danger to our health. Hop acids in their -cis and -trans forms also behave differently, with -trans acids degrading MUCH faster then the -cis acids.

These trans acids degrade so much faster that it’s been proposed that the ratio of -cis to -trans hop alpha acids provide a perfect marker for tracking the staling of beer over time. See the 2002 paper “A new parameter for determination of the extent of staling in beer” in the Journal of the American Society of Brewing Chemists for more detail on that.

Wouldn’t it be cool if we could figure out a way to only have the -cis form of alpha acids appear in our beer? Our IPA’s would stay hoppy for a lonnnnngg time then!

Jason

jason@brewbasement.com

www.brewbasement.com