Beer Science: Alpha Acids

Alpha acids are an essential chemical component to any good beer (note: beta acids is another discussion).  In fact, yours truly has mentioned alpha acids in previous posts including The Chemistry of the Skunk.  This science installment sought to answer the following two questions about alpha acids:

(1) Where do alpha acids come from?
(2) How do hops differ in alpha acid composition?


(1) Where do alpha acids come from?

Alpha acids are contained in hop plants, called Humulus lupulus by scientists.  Specifically, alpha acids are produced in glands present in mature female hops called lupulin glands.  Lupulin glands contain a yellow powdery substance that looks like pollen when extracted from cones (technically called strobiles by botanists) of the hop plant.  Lupulin is packed with resins and oils that, when brewed, impart hop bitterness flavors to beer.

One interesting trivia factoid is that lupulin is believed to possess medicinal properties.  Lupulin acts as a sedative.  I found one company in the Netherlands called Azarius (link in the references section) where you can actually purchase lupulin powder to help cure anxiety and sleeplessness.  Not surprisingly lupulin tastes bitter, so they recommend mixing with honey to drink as a tea-like concoction.  I also found some "nervous system calming" lupulin available from Amazon.com.  Personally, I don't feel I need any additional hop extracts in my diet, but maybe you do?

(2) How do hops differ in alpha acid composition?

Different hop strains contain different amount of lupulin, and thus, different amounts of alpha acids.  When beer is brewed, a certain percentage of the lupulin alpha acids are converted into iso-alpha acids.  These iso-alpha acids are one factor responsible for perceived bitterness by beer drinkers.  In the absence of brewing, the alpha acids themselves are not very soluble in water and thus do not actually impart any substantial bitterness.  The brewing process converts these insoluble alpha acids into their more soluble bitter chemical counterparts, iso-alpha acids.

As a homebrewer, I always wondered the difference was between bittering and flavoring hops.  Why did I add some hops early in the brewing process, with 60 minutes to go?  And why did I add some hops late in the brewing process, with less than 5 minutes remaining?  The answer lies in the alpha to iso-alpha conversion chemistry.  The more time the hops are brewed for, the more time the alpha acids have to convert to iso-alpha acids.  The majority of late addition hops do not have time to convert alpha acids to iso-alpha acids.  Therefore, when added early to brewing hops are called bittering hops and when added late to brewing they are called flavoring hops.

The amount of iso-alpha acids in the final beer are more commonly referred to as International Bitterness Units, or IBUs.  In a laboratory, the iso-alpha acids can be quantified using a spectrophotometer and solvent extraction,

There are also other chemicals present in the oils found in hops that are equally important to brewers as alpha acids.  These chemicals, myrcene, caryophylellene, farnesene, and others collectively impart a flavor profile to beer.  The brewer must consider not just the alpha acid composition, but also the concentration of these other chemicals when deciding which hop to choose for a recipe.

Hops have undergone rigorous dissection by chemists - literally.  There are some amazing charts available that show which hop plant contains how much alpha acid and total oils.  Brewers make use these charts when deciding which hop will best suit their flavor profile.  Here are just a couple of charts I found.

But what actually causes some hops plants to produce more lupulin (and therefore alpha acids) than other plants?   That's a bit of a tougher question to answer, at least for me.  And this question is better tackled by hop growers than myself.  But part of the answer is: different strains of hops inherently possess different concentrations of lupulin.  I chose not to dig deeper into the rabbit hole of bioagriculture, but I assume that farmers and geneticists alike are currently breeding hops with a high concentration of lupulin.  In fact, I was invited to an experimental hop tasting in a fortnight at Pearl St. Brewery.  It will be interesting to see what novel hop strains evolve on the market as the demand for hops continues to grow.


References:
2 Noble Dogs Brewing: When Should You Harvest Hops?
Azarius in the Netherlands. Can buy lupulin here.
Hops Chart
Alpha Analytics Hop Chart (Hop Union)