[Lipoxygenase activities and beer ageing]

Ageing of beer involves changes in flavour impression. Chemical reactions of compounds during brewing lead to formation of an oxidized flavour. A papery pasty off-taste arises in many beers, this is also referred to as cardboard flavour, produced by trans-2-nonenal. The production of stale off-flavours caused by trans-2-nonenal during beer storage has to be related with lipid oxidation during wort production. Controlling oxidation during the wort production is considered to be important for flavour stability. Next to auto-oxidation, the enzymatic oxidation caused by malt lipoxygenase (LOX) seems to be very important. Although the two LOX isoenzymes contribute to the nonenal potential in wort, it is LOX-1 that produces mainly the 9-hydroperoxide of linoleic acid, a precursor of trans-2-nonenal. This paper deals with an improved extraction method of lipoxygenase and a separation method for LOX-1 and LOX-2. LOX-1 is thought to be the key enzyme in beer staling process. LOX-2 is only detected in germinating barley, while LOX-1 is present in the barley grain. The activity of both isoenzymes increases during germination and decreases during kilning. Only a small portion of the remaining LOX is reported to be extracted into the mash. LOX remaining into the non extracted material, can produce more hydrophylic hydroperoxides precursors that can solubilize into the wort. Several methods have been proposed to control and reduce the beer staling. Most of these methods are focused on the control of LOX at different stages of the malting and brewing process, including development of LOX during mailing, oxygen uptake during milling, oxygen levels in the mash, temperature and pH of mashing-in, extraction of lipids and LOX during mashing, LOX remaining into the non extracted material, wort separation. To inhibit the oxidation of beer, natural antioxidants of barley should be protected and the production of new antioxidants in situ could be favoured. Also the fermentation conditions and the selection of the yeast variety can have an influence on the reducing capacity of the final beer. Ageing of beer is the result of the interference of the LOX-1 content in the malt, the extractibility and stability of LOX during brewing, the LOX activity in the non extracted material, the oxygen levels during brewing and the reducing capacity of the malt.