The traditional purpose of a grant was three fold:
(a) to avoid a potential vacuum in the lauter or mash/lauter tun during wort pumping for re-circulation or filling the kettle, which could seal the mash to the false bottom, thus causing turbid wort or stuck mashes;
(b) to allow the brewer to asses wort clarity and wort flow;
(c) in larger brewery systems with multiple lauter tun outlets, to determine whether all parts of the grain bed flow sufficiently well or require raking or other measures to improve flow-through. The grant thus serves as a flow buffer. In old tower-style breweries, where brewing vessels are stacked one on top of the other and wort flow is by gravity only, there is, of course, no need for a grant as a vacuum break, but its other uses remain valid.
The simplest grant design is essentially an open-top cylindrical can with wort flowing in from the bottom and being pumped off through a side port. This minimizes wort splashing and thus oxygenpickup. In traditional breweries, however, especially in Germany and the Czech Republic, the grant is typically a long, shallow copper tub, bordered in regal brass, and fed golden streams of wort by a dozen or more swan-necked lauter run-off tubes.
It is a strikingly beautiful design, part of a copper brewhouse that is sure to quicken the heart of any romantic brewer. Unfortunately, it is also a cause for worry, because traditional grants allow plenty of contact between wort and air, and modern brewing tends to eschew hot wort aeration, which can initiate staling reactions. In the Czech Republic, wort oxidation from the run-off into traditional grants is partially responsible for the deeper gold coloring than is normally seen in German “pils” beers.
Modern brewing systems usually have no grant at all. Instead they have special valves, often controlled by sensors and complex electronics, to allow wort to flow directly from the lauter tun to the kettle without the risk of a lauter tun vacuum or oxidation. The in-line sight glass used to check for wort clarity is then sometimes called a “grant,” although that designation is undeserved by either beauty or function.
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