Diacetyl is a key cause of off-flavor in beer. Diacetyl reduction by yeast happens during storage. That process needs longer maturation times. Acetolactate decarboxylase prevents the formation of diacetyl. That allows brewers to shorten maturation times by up to 14 days for reduced cellar costs.
Diacetyl and 2,3-pentanedione, also known as vicinal diketones (VDKs), are flavor components. They're generated at the beginning of fermentation. If they exceed a specific flavor threshold, VDKs will give beer an undesired off-flavor. Diacetyl is the main contributor to these off-flavors. It has a butterscotch or buttery flavor.
During storage, yeast reduces VDKs. This reduction is one of the parameters used to evaluate a beer's degree of maturation. Brewers use acetolactate decarboxylase to prevent the formation of diacetyl.
Acetolactate decarboxylase reduces vicinal diketone formation. It converts its precursors into acetoin and 3-hydroxy-2-pentanone. These are flavor free.
Acetolactate decarboxylase catalyzes the following reaction: (2s)-2-hydroxy-2-methyl-3-oxobutanoate <=> (3r)-3-hydroxybutan-2-one + CO2.
By reducing diacetyl levels, acetolactate decarboxylase can cut maturation times by 2 to 14 days. That means lower cellar costs. It also means brewers don't need to reprocess so many batches because of high diacetyl levels.
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