The Oxford Companion to Beer definition of
Vorlauf is German for “recirculation.” When a mash is transferred to a lauter tun or when the mash rest has finished in an infusion mash tun, some particles of grain remain in suspension under the vessel’s false bottom. These materials come mostly from the grain’s embryo, which is rich in lipids, and its husk, which contains phenolic compounds. See lipids and polyphenols. If they were boiled in the kettle, they would contribute unpleasant flavors to the finished beer. These particles are therefore sent back into the grain bed so that they never enter the wort stream. During the vorlauf sequence, wort is drained by simple gravity from underneath the false bottom into a collection vessel called a grant, from which it is pumped over the top (or just below the surface) of the grain bed. See grant. In the process, the grain bed serves as a filter that traps all floating particulate matter. Brewers continue the vorlauf until the recirculating wort runs clear. Only then is the wort lautered, that is, drained into the kettle. Although the technique is standard in the United States and Germany—from whence Americans derived the word “vorlauf”—not all brewers feel that the wort needs to be fully clarified. Some worry that reintroduction of the fine particulate matter into the mash bed will impede the run-off, thus causing more problems than are solved.See also kettle and lautering.