I recently published a visual guide to beer bitterness which used IBUs to chart various beer styles. But what exactly is an IBU and what does it mean for beer?
What does IBU Stand For?
IBU stands for International Bittering (or Bitterness) Unit and it is used to gage the bitterness of beers.
In more precise terms, it describes the quantity of alpha acids from hop resins that have been isomerised by boiling wort. The unit of measure is parts per million, or milligrams per litre.
Before I explain more about this, let’s take a step back for a reminder of why bitterness in beer is an issue at all.
Hops and Beer Bitterness
In beer making, malt is steeped in hot water to produce a sugary wort that can be fermented by yeast.
Although fermentation reduces the sweetness of the beer, there are always residual, non-fermentable sugars that remain in the final flavour.
To provide balance to the beer, brewers introduce bitterness by adding hops.
Hops are cone-like flowers that contain oily resins, which themselves are made up of alpha and beta acids. When hops are boiled into the wort they isomerise and become part of the beer, contributing bitterness in the process.
The more alpha acids that are isomerised, the more bitter the beer.
You’ve probably noticed when you buy hops that a percentage figure is given on the packaging. This number describes the proportion of alpha acids in the hops.
Although I suppose it’s theoretically possible to use all of them in a brew, the reality is that only a fraction of the available acids will actually be isomerised.
The amount used in a brew is referred to as the utilisation.
Hop utilisation is a function of boil time and boil gravity. In general, hops boiled for longer will use more oils and bitterness, while higher gravity boils (i.e. those with more malt sugars) will utilise less.
Utilisation is important because it is one of the factors used in determining IBUs.
IBU calculations take into account various factors that can influence the amount of bitterness in a beer. These include beer gravity, boil time and vigour, temperature and more.
In simple terms, IBUs are calculated by dividing the product of utilisation and available alpha acids by the boil volume.
Although there are various brewing programmes that can calculate IBUs for you, if you’re interested in doing it manually a thorough guide can be found here.
Balance of Flavour : A Word of Warning About IBUs
The number of IBUs in a particular beer is not the only indicator of bitterness.
As I explained above, beers derive their character from the balance between sweet and bitter flavour elements. It follows that two beers with different gravities (levels of sugar) but with identical IBU values will taste differently.
Here is an example from the BJCP guide to target IBUs:
(This link will open the full IBU chart from which this extract was taken)
You can see in the illustration that the target IBUs for foreign extra and other strong stouts are similar to those for English IPAs.
However, stouts are much heavier beers and can more easily absorb extra bittering units. This is why strong stouts are not particularly bitter drinks, whereas IPA, which is much lighter, is a style that is especially known for its bitterness.
The number of IBUs may not tell you how bitter the beer tastes, but it is useful when planning recipes.
With a target IBU, you can use brewing software to develop mixes of hops that achieve the desired bitterness for the style you’re aiming for. Don’t forget that hops also contribute aroma and other flavours to beer, especially when added at the end, and that bitterness is just one of their characteristics.
As you experiment with different brews, get used to looking at IBU values in relation to the gravity of your beer. Notice the differences in taste and use them to your advantage, creating the balance of flavour that you want.