Drauflassen (derauflassen) is German for “to put on top of,” and, when applied to brewing, refers to putting one batch of wort “on top of” another. The technique behind the term refers to topping up fermenting wort with new oxygenated wort while the fermenting wort is at its peak of fermentation, 24 to 72 hours after its start. The oxygen in the fresh wort will cause the yeast briefly to revert its metabolism back from the anaerobic mode to the aerobic mode, where it multiplies. Once the added oxygen is depleted, the yeast switches back to anaerobic fermentation, converting the fermentable sugars into alcohol and carbon dioxide.
The most common scenarios in breweries calling for the use of the drauflassen technique are the final stages of yeast propagation, when a rather small volume of new yeast is transferred from the laboratory or propagation plant into the main fermentation vessels. Drauflassen then ensures a quick multiplication of the quantity of yeast. The technique may also be used as an emergency procedure in case of a severe lack of sufficient quantities of vital and clean pitching yeast as it enables fermentation of large volumes of wort with small quantities of pitching yeast. The drauflassen stage can be repeated if the starting quantity of yeast is very low. Due to the metabolic stress on the yeast caused by its switching back and forth between the anaerobic and aerobic stages, drauflassen can cause taste and flavor differences to beer fermented in a normal one-stage fermentation.
For some breweries, however, drauflassen is a standard operating procedure, most often used when the size of the brewery’s fermenters greatly exceed that of its kettle. In such cases the brewery will usually fill half the fermenter on the first day, followed by the other half on the second day.