Where does the bitterness of beer come from?

The bitterness is found in the hop flower and is expressed in % α (alpha acids) as well as the essential oils that define the beer’s flavour. Some hop varieties have less essential oils and more alpha acids and vice versa.

The alpha acid content is between 3 and 17% α and depends on the hop variety. The content is indicated by the grower for each harvest. There are thus two types of hops, the bitter hops and the aromatic hops.

The hop flowers contain lupulin, a kind of yellow powder consisting of soft resin. This resin contains the alpha acids, which are insoluble. During the boiling of the wort, the alpha acids undergo isomerisation (the molecular structure of the alpha acids is changed) and become soluble to create a bitter product, the isoalpha acid. The combination of this bitterness and the malt sugars produce a beer’s complex taste.

The EBU is applied when the alpha acids are transformed into isoalpha acids, which determine the bitterness of a beer after they have been dissolved during the boiling of the wort.

The properties of hops mean that they can be used at two different times of the brewing process, of a Pale Ale for example. The hops, from which the brewer will want to extract the isoalpha acids to determine bitterness, will be added at the beginning of the hopping process. The hops from which you want to extract the essential oils for their aroma will be added no later than ten minutes before the end of the dry hopping process as they are volatile. In the case of Gueuze beers, which are neither bitter nor aromatic, old aged hops, which have lost their bitterness and their aroma will be added, to preserve the beer.

The hopping to define the beer’s bitterness always involves heat and can be enhanced by a dry hopping for aromatic purposes. In that case a bag of hop flowers is placed in the tank. The essential oils in the hop flowers will be released, adding aromatic qualities to the beer.

Some key figures :

  • 1 EBU = 1 mg of isoalpha acid/litre
  • Man can discern the bitterness after approximately 12 EBU.
  • A pilsner generally has a bitterness of 18 to 24 EBU.
  • A Pale Ale between 32 and 38 EBU.
  • A Gueuze between 8 and 12 EBU.