What is yeast cake re-use?
Cake re-use is the simplest of all yeast re-use methods. Basically you rack the beer off your primary (whether for bottling or for secondary fermentation) and you dump a similar or heavier/darker worth right on top of the old yeast cake. Be forewarned: the resulting fermentation can take off very fast due to the large number of yeast cells. Pro: easy, fast. No starter needed. Con: cannot pitch a lighter beer on the old cake or it will pick up flavors from the previous inhabitant. If your old batch is off or contaminated your new batch is, too. Probably overpitching, leading to cleaner, less estery flavors (esters are desirable in many styles).
