A new schema for the conversion of acetolactate in young beer.

Kamiya, T., Kajino, K., Hiratsu, H., Mawatari, M. and Inoue, T.

Abstract
The results of an investigation into the products formed from acetolactate in green beer are described and their implications regarding beer maturation and flavour development discussed. Acetolactate normally undergoes a nonenzymic oxidative conversion into diacetyl, the concentration of which in the green beer must be reduced below taste threshold level by converting it into acetoin and/or butanediol in order to achieve an acceptable flavour. It has generally been accepted that this conversion must be carried out using reactions catalysed by yeast enzymes during a secondary fermentation or maturation process. However, it has now been found that the oxidative conversion of acetolactate to diacetyl requires the presence of dissolved oxygen in the green beer, and that any acetolactate remaining after all the oxygen is used up is converted directly to acetoin without any participation by the yeast. This was confirmed by adding acetolactate to degassed, pasteurized bright beer (dissolved oxygen content under 0.1 mg/litre) under an inert gas atmosphere. After storage under strictly anaerobic conditions in the absence of yeast, a certain proportion (varying according to temperature and time from 10% after 50 hours at 2 degrees C to 80% after 10 hours at 70 degrees C) of the acetolactate was converted to acetoin. The reaction rate constant for this process was calculated and found to be consistent with the known facts about the oxidative conversion of acetolactate to diacetyl. The authors therefore propose a new model for the conversion of acetolactate, postulating that acetolactate is decarboxylated to form an unknown intermediate product, which if oxygen is available is oxidatively converted to diacetyl, but under anaerobic conditions is converted directly to acetoin. It is considered probable that at the dissolved oxygen levels usual in green beer, only a small proportion of the total acetolactate is in fact converted into diacetyl, and that some accelerated maturation techniques actually work on the decarboxylation stage of the process.
Keywords : acetoin anaerobic beer diacetyl flavour green beer maturation oxidation precursor reaction