Flavor evolution of top fermented beers

The distinctive flavours of many traditional top fermented beers include a significant estery component. It has been observed that when such beers are bottled and subsequently stored for a long period, they often suffer a significant loss of flavour active esters, especially isoamyl acetate. Two mechanisms of ester breakdown have been identified. One is chemical and can affect any beer, but the other involves the hydrolysis of the esters by yeast enzymes and only occurs in bottle conditioned beers and some unpasteurized brewery conditioned beers. The frequency and severity of this enzymic ester hydrolysis depends on the yeast strain used as well as on conditions during fermentation, maturation, bottling and storage. It appears that the esterases responsible are found only in the interior of the yeast cells so long as they remain viable, but are released by cell lysis. As a certain level of cell lysis is inevitable during fermentation and maturation (whether the latter takes place in the brewery or in the bottle), the only effective method of controlling this problem is likely to be the selection (or production by biotechnological methods) of yeast strains with the lowest possible esterase activity levelsstatus: publishe