The Alpha and Beta of Brewing By: Kay Witkiewicz
No matter how experienced of a homebrewer you are, you have probably already noticed that hops are incredibly complex. Their vast variety of aromas, flavors, and bittering powers are all due to differences in acid and essential oil contents, thus giving each hop its distinguishing characteristics. Alpha and beta acids are the two main organic compounds that determine bitterness, flavor, and shelf life of your beer, while essential oils with obscure names, such as myrcene, farsene, and humulene, impart unmistakable hop aromas we know and love.
Alpha acids, expressed as a percentage of the total weight of the hop, are the main source of bittering in our beautiful brewing flowers. There are three main alpha acids and each contributes its bittering characteristics in a different way: humulone, adhumulone, and cohumulone. In order to fulfill their bittering potential, hops need to be boiled so that the alpha acids can undergo a chemical change in their make-up called isomerization and dissolve in your wort. The biochemical make-up of hops is as fascinating as it is difficult to understand, so here’s what you need to know: when these alpha acids oxidize, either during storage of the hop itself or as your beer ages, they lose their bitterness and begin to emanate rancid, cheesy aromas and accompanying tastes. Side effects from oxidation are one of the reasons why IPAs typically don’t age well and why you should store your hops as airtight and as cold as possible. Moreover, hops high in cohumulone (the typical range is between 20 and 50% of alpha acids) tend to impart bitterness that many perceive as unpleasantly harsh. Even though your Hopunion hop packets don’t provide you with figures beyond alpha and beta acid content, the company’s website offers a nifty guide that allows you to dive deep into the molecular world of Humulus lupulus, but be careful, lupulin glands are sticky and once you’re stuck you might want to learn everything there is to know about hops.
Also expressed as a percentage of total hop weight, beta acids, on the other hand, are insoluble in wort or beer and have no bittering powers until they become oxidized. Similar to alpha acids, beta acid constituents include lupulone, adlupulone, and colupulone. However, slight differences in their molecular structures cause beta acids to not isomerize during the boil. So, if they’re practically inert, why should you care about beta acids? The ratio of alpha to beta acids determines to what extent bitterness will diminish in your hop as it oxidizes and in your beer as it ages. Ratios close to 1:1 (alpha:beta) are prized in the hop trade and can often be found in aroma varieties, such as Liberty and numerous noble hops, while ratios of 3:1 are common with many revered American super-alpha cultivars, such as Simcoe, Summit, and Chinook, and ratios of 1:2 or beyond seem rare, except I do have a packet of German Hersbrucker with a beta acid content four times higher than its alpha acid content. As your hop or beer oxidizes, alpha acids lose their bittering power, but beta acids just then unleash their potential, in theory preserving at least some of the bitterness. Despite this preservation, there are qualitative differences in the bitterness derived from alpha and beta acids, the latter generally being more harsh and lingering than the former. Unless you’re brewing Belgian beers the traditional way (with aged hops that have lost their bittering powers almost entirely), hop oxidation is neither tasty nor desirable, so the fresher, the better.
Although they constitute only 0.5 to 3% of the total hop weight, essential oils are responsible for the complex bouquets of floral, herbal, spicy, and all imaginable citrus notes. Even though over 300 different molecular compounds have been associated with different aromas, scientists have only scratched the surface of what makes hops so enticing. Of the panoply of essential oils, a few are worth getting to know closer: myrcene, farsene, and humulene. When we think of nonspecific, general hop aroma, chances are we’re thinking of myrcene—green, fresh, floral, and citrusy. Centennial, Cascade, and pretty much any American variety contain noticeable amounts of myrcene, but due to its extreme volatility, common with all essential oils, not much of this bewitching aroma tends to remain in your beer unless you carefully late- and dry-hop your brews. Farnesene is myrcene’s European counterpart, creating the distinctive spicy, herbal, and floral notes of noble hops such as Saaz and Tettnanger. Lastly, humulene creates a similar bouquet, but we more commonly associate it with finely spiced British hops, such as UK Kent Golding. Each hop aroma profile is distinct, and while these three compounds play a large part in that, the combinations of aroma molecules is seemingly endless.
Alpha and beta acids are the be all and end all of bitterness in hops and, eventually, in your beer. Understanding not only how they influence your beer individually, but in conjunction with each other over time will inform your hop choices and improve your brewing practices. Essential oils provide us with the fragrances we most commonly associate with hops and their unmatched complexity is one of the reasons why hops are such intriguing additions to our beers. As the science on hops continues to evolve, so will our understanding of these beautiful flowers, but no matter if you can name all alpha and beta acids or know specific oil contents of hop variations by heart, all you have to do is plunge your nose into a handful of hops to know that they are truly special ingredients.