grain out
From The Oxford Companion to Beer
is the physical process of removing spent grain from the mash tun or lauter tun. This is accomplished either by mechanical means (i.e., a stirring mechanism, or mash mixer, in the mash or lauter vessel) or by manual means (i.e., an assistant brewer removing the spent grain by hand).
Larger brewhouses are almost always equipped with a variable-speed, reversible mash mixer in the mash tun or lauter tun that serves the dual purpose of mixing the mash to ensure even consistency and heat distribution and, when run in reverse, pushing the spent grain out of the vessel. Other designs employ a plowing device attached to the lauter knife assembly. The spent grain can then be moved via closed pipes and pumps or via transfer bins to a holding vessel or to trucks or railcars for transport.
In smaller brewhouses, generally under 20 hl (17 US bbl), mash vessels are often not equipped with a mash mixer. In such situations, a brewer will open a manway door and physically move the spent grain into bins using a plastic shovel or similar tool.
This definition is from The Oxford Companion to Beer, edited by Garrett Oliver. © Oxford University Press 2012.