Describe the role of alpha-acetolactate decarboxylase in preventing diacetyl formation, detailing the specific enzymatic reaction it catalyzes.

Alpha-acetolactate decarboxylase (ALDC) plays a critical role in preventing diacetyl formation by converting alpha-acetolactate, a precursor to diacetyl, into acetoin. Diacetyl is an unwanted compound in many fermented beverages, imparting a butterscotch or butter-like flavor that is considered an off-flavor in most beer styles, some wines, and certain ciders. During fermentation, yeast produces alpha-acetolactate as an intermediate in the biosynthesis of valine and isoleucine, two essential amino acids. Alpha-acetolactate is initially non-volatile and does not contribute to the flavor profile. However, it can undergo a slow, non-enzymatic oxidative decarboxylation reaction (meaning it loses a carbon dioxide molecule through oxidation) outside of the yeast cell, transforming it into diacetyl. ALDC is an enzyme that provides an alternative, enzymatic pathway for the decarboxylation of alpha-acetolactate. Specifically, ALDC catalyzes the conversion of alpha-acetolactate into acetoin and carbon dioxide. Unlike the formation of diacetyl, the production of acetoin is desirable because acetoin has a much higher flavor threshold than diacetyl, meaning it takes a much larger amount of acetoin to be detected by taste. Moreover, acetoin can be further reduced by yeast to 2,3-butanediol, which is virtually flavorless. By expressing ALDC, yeast strains can quickly and efficiently convert alpha-acetolactate into acetoin, thereby minimizing the amount of alpha-acetolactate available for conversion to diacetyl. This significantly reduces the risk of diacetyl formation and the resulting off-flavor in the final product. Brewers often select or engineer yeast strains that express high levels of ALDC to ensure low diacetyl levels in their beers. The specific enzymatic reaction catalyzed by ALDC involves the removal of a carboxyl group (-COOH) from alpha-acetolactate, yielding acetoin and CO2.