The Oxford Companion to Beer definition of
Wet Milling is a modern technique used to grind malt in preparation for mashing, and it is said to bring significant benefits compared to traditional dry milling. It is a practice recommended by the major German brewing plant manufacturers and used in conjunction with a lauter tun. See lauter tun. It is often called “continuous steep milling” and requires that milling take place in the time taken to mash a grist, normally about 20 minutes. Traditional dry milling takes place independently of the mashing process and usually takes longer as a result. In wet milling, malt is steeped in a continuous stream of warm water to bring the husk moisture content up to 15%, before the malt is ground on a pair of specially designed rollers. In wet milling, the grain husk remains mainly complete, whereas in a dry mill it can fragment. This fragmentation can slow run-offs and cause quality problems. Because of the higher moisture content during wet milling, there is no dust produced, so explosion risks are eliminated and dust removal equipment unnecessary. Because the husk remains complete, wet milling also allows a faster run-off time and a greater loading on a lauter tun (deeper grain depth), as well as reduced oxidation of the grain and the resulting wort. Wet milling systems are expensive and rarely seen in small breweries. See oxidation.