Underletting is a technique used by brewers during run-off or lautering as a method of freeing up a mash bed that has collapsed, “set,” or “stuck.” Recovery of wort from a mash involves straining the wort through a complex filter bed created by the grain itself. Collecting the wort too quickly can result in the grain bed compressing, preventing liquid from flowing through it. In this case it may be possible to free the blockage by pumping hot brewing water through the mash from below. This can free up the mash bed, but it also has the disadvantage of replacing the wort underneath the vessel false bottom with water rather than wort and possibly diluting the wort before it is collected. Underletting will cause the wort to become turbid, and the brewer usually must then recirculate the wort until it runs clear. See vorlauf. In general this may be a tactic used by brewers as a “last resort,” when wort has stopped flowing through the mash bed altogether. Often brewers will simply wait for slow- flowing wort to collect without risking diluting it with the underletting technique. In rare cases, underletting may be part of the actual intended brewing technique, especially with very heavy or thick mashes.

See also lautering and stuck mash.

Steve Parkes