When we serve beer either in cans or at festivals, the most common question we get asked is what IBU means. It is an acronym for International Bitterness Units; it is a measurement of a specific component of hops that directly correlates with a perceived bitterness in beer. The more hops in the beer the higher the IBU, the more bitter the beer will be. So far, so simple right?
Except that doesn’t fully encapsulate bitterness in beer; it is rarely straightforward.
Hops have three main functions in beer: bitterness, aroma/flavour (i.e. taste), and preservation. When, during the brewing process, hops are added will affect the two formers. The latter is a natural result of the hop plant increasing acidity, which helps extend shelf life; in the age before refrigeration, hops were essential in all beer recipes for this reason. Many beer consumers may not like the taste of “hoppy” beer, but hops are a fundamental part of (the vast majority) of all beer recipes.
The biggest factor in determining IBUs is when hops are added during the recipe. The first way is during the boiling process, where hops are broken down by the excessive heat and their chemical structure changes into a form that’s directly correlating with known bittering compounds. This is where the bulk of the contribution to IBU comes from.
In the simplest terms, IBUs are the measurement of alpha acids – examples include humulone – that become isomerized. Isomerization, in this scenario, is when the original biochemical structure is broken down during excessive heat and reforms with the same number of atoms, but in an altered physical structure. These isomerized alpha acids are what is being calculated.
The formula states:
IBU (alpha acid in mg/L) = [Alpha Acid % of Hop * Weight of hops added in mg * Utilization Rate (0.01-0.35 depending on when the hops were added in the boil)] / The Volume of the Kettle in L
If you add hops later in the process, usually near the end of the boil, it imparts more flavour and aroma, but less to IBU. The final main method of hop dosing is known as ‘dry-hopping’; contributing hops after fermentation is complete. Dry-hopping will impart no IBUs but far more flavour, as there is no heat involved. What flavour is imparted will depend on the type of hop, ranging from citrus and floral, to piney and resinous.
However, hops can give other chemical compounds associated with bitterness. This is where beer’s bitterness gets complicated. Tannins and polyphenols are well known bittering agents that can’t be directly measured like alpha acids. They can be increased with hops if a dry hop session lasts several days, or simply by exposing beer to the hop oils that don’t break under excessive heat. Furthermore, some malts, like black or roast malt, will impart an acidic bite to a recipe due to higher levels of these same compounds. Bitterness comes from a lot of different ways.
When we add hops to our recipes, it’s important to try to match the amount of IBUs to the style. Once that’s complete, we brewers have to decide what additional hop additions are needed to match a desired taste/aroma. There are further complications come the brewday: were the chosen hops available? Were they fresh enough? Will they mix properly with the recipe as designed? Most importantly, how will consumers perceive bitterness when there may be multiple factors that influence it?
A big question among brewers is what the point of IBUs, if the scale is not the authority on bitterness? Yet many, including us, insist on labelling our products with this number. If you’re curious about the arguments of this debate, tune in next time where we’ll look into how taste/bitterness is perceived and how the industry responds.
Until then, cheers!