Lagering was developed in Germany for bottom-fermented lagers, and it involves a long, cold storage at low temperatures for several weeks, if not months. Although lagering refers to bottom-fermented beers, some top-fermented beers such as Kölsch and Altbiers also require periods of lagering.
For classical lagering, the temperature is held at 4 to 5 degrees C (39–41°F) until the VDK level is below 0.15 mg/L (or ppm) and then slowly cooled (Briggs et al., 2004). It is during these warmer temperatures that much of the secondary fermentation takes place as the fermentable extract is slowly depleted. The temperature is then reduced one degree per day until the beer reaches freezing, and is held at freezing for the length of the maturation period. It is important that the temperature is brought down gradually (1‒3°C/day) until the lagering temperature has been reached.
Selection of yeast with the proper flocculation characteristics is obviously important for a long aging process. With a powdery yeast, the transfer to storage and secondary fermentation would carryover too much yeast; the secondary fermentation would occur too quickly, and the yeast would not settle sufficiently at the completion of fermentation.
Some of the short-comings in using this method is the sludge at the bottom of the tank can warm up, allowing yeast autolysis and producing off-flavors in the beer.
Click on the following topics for more information on secondary fermentation.