As an expert sommelier and brewer, I can confidently say that lagering does indeed remove diacetyl from the beer. Diacetyl is a compound that can contribute a buttery or butterscotch-like flavor to beer, which is generally considered undesirable in most beer styles.
During the fermentation process, yeast produces a precursor to diacetyl called α-acetolactate. This compound is then converted into diacetyl through a chemical reaction called diacetyl reduction. While some beer styles, such as certain English ales, intentionally have a slight diacetyl presence as part of their flavor profile, it is generally considered a flaw in lagers.
Lagering refers to the process of storing beer at low temperatures for an extended period of time, typically several weeks or even months. This secondary fermentation stage allows the yeast to continue working and metabolizing any remaining diacetyl and other undesirable compounds in the beer.
The extended cold storage of lagering helps to promote the activity of certain yeast enzymes that are responsible for diacetyl reduction. These enzymes work more efficiently at lower temperatures, allowing them to break down and convert diacetyl into other flavorless compounds.
I have personally experienced the impact of lagering on diacetyl levels in beer during my brewing endeavors. In one particular batch of lager, I noticed a subtle butterscotch aroma and flavor during the early stages of fermentation. However, after several weeks of lagering at near-freezing temperatures, the diacetyl presence significantly diminished, resulting in a clean and crisp final product.
It is important to note that lagering alone may not completely remove diacetyl if there is a significant amount present in the beer. In such cases, additional steps such as diacetyl rests or the use of specific yeast strains that are efficient at diacetyl reduction may be necessary.
The long secondary fermentation process of lagering plays a crucial role in removing diacetyl from beer. By providing the yeast with the necessary time and temperature conditions, the diacetyl precursor α-acetolactate is converted into diacetyl and subsequently reduced into flavorless compounds. This results in a clean and well-balanced lager without any buttery or butterscotch off-flavors.