high gravity brewing
From The Oxford Companion to Beer
is a term that refers to the process of preparing a type of strong wort with a high original gravity, intended to produce a beer with a high alcohol content. A normal gravity wort is typically in the range of 10°–13° Plato and will result in a beer containing 4%–6% alcohol by volume (ABV).
High gravity brewing is performed by brewers for two reasons. First, it is performed to make a style of beer that has a high alcohol content, such as a German bock, Belgian tripel, or British barley wine. These types of beers are specialty styles and are generally not produced in high volumes.
The second reason a brewer might perform high gravity brewing is to meet high production demand when available brewing capacity is limited. In this regard, a brewer can make a high gravity wort that will become a high-alcohol-base beer to be used as a type of concentrate. At the final finishing steps, the brewer then adds deoxygenated brewing water to dilute the high-alcohol beer to a normal strength. Using this method, it is possible to brew a volume of beer and later increase that volume by up to 100% by dilution. Many major brewing companies and some large craft breweries use high gravity brewing to meet production demand. The technique is technically demanding, because high gravity fermentations typically produce a range of flavors and aromas that are not produced in lower gravity fermentations, and these can skew the desired flavor profile of beers produced by this method.
This definition is from The Oxford Companion to Beer, edited by Garrett Oliver. © Oxford University Press 2012.