HVG Germany: Hopfen

Wort boiling

Hop bitter substances dissolve poorly in a watery solution, as they are mainly non-polar. The solvency of the alpha-acids with a pH of 5.0 is around 40 mg/l. The solvency of the beta-acids is even less with wort pH. It is around 1.5 mg/l. Only traces of beta-acids are found in the beer but their oxidation products such as the hulupones are stable and soluble and remain in the solution up until the end product. 

The formation of the smallest drops of resin with a large surface is prerequisite for the resins dissolving well in the wort.

Due to thermal reactions the humulones isomerise into isohumulones. The isohumulones are then present in the solution as cis- and trans-stereo-isomers.


In the wort the ratio between cis- and trans- isomers is about 7:3. A small part of the isohumulones can be found in the wort as allo-isohumulones, abeo-isohumulones and spiro-isohumulones. The spatial structure of the molecule influences its behaviour in wort and in the beer.

There are solubility, isomerisation and aging differences between the various iso-humulone fractions and their cis- and trans-stereo-isomers. Only the dissolved iso-alpha-acid imparts the bitterness typical for beer.

Cohumulone can enter the solution more easily due to its slightly higher polar character as opposed to the n- and adhumulone. A faster isomerisation could not be determined. Iso n- and iso-adhumulones are less polar, are enriched in the froth and are probably responsible for a creamy foam.

The iso-alpha-acids are sensitive to oxygen and form a unspecific amount of oxidation products while the wort is boiling. These may also be relevant for the aroma.