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Isomerisation of alpha acids in the brewhouse is naturally limited to a yield of less than 50% of the dosed alpha acids.
Iso-alpha acids are also lost downstream in cold trub, through precipitation due to the fall in pH (e.g. 5.4 in wort; 4.4 in beer), through precipitation with yeast, sediment, trub and in filtration. The yield of bitter compounds should always be measured correctly on the basis of HPLC determined iso-alpha acids in the cast wort or in the finished beer, set in relation to the dosed alpha acid quantity, and not on the basis of spectrophotometrically determined beer units.
