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High-gravity beer is any of various specialty beers, either craft beers or malt liquors with an increased specific gravity. A beer with an original gravity 1.070 is generally considered to be high. High-gravity beers are a traditional expression of beer making in Europe, but only a small percentage of microbreweries in the United States produce them. They may be more expensive than mass-produced beer due to larger amounts of high quality grains or malt used, or in the case of malt liquor lower in cost or equal to other mass-produced beers, and full of more filler grains or adjuncts. They may be more flavorful, intended to be sipped and savored, and are often paired with carefully prepared foods, or in the case of high-gravity malt liquor, they may sacrifice some flavor for an inexpensive boost of alcohol level.
High-gravity beer tends to have higher alcohol content compared to other beers due to the concentration of sugar and flavor-enhancing ingredients at the beginning of the brewing process. Steel Reserve is one example of a high-gravity lager beer containing 8.1% alcohol.
Alcohol is produced by yeast consuming sugars and other carbohydrates in the boiled and cooled mixture (called "wort"). To determine the alcohol by volume (ABV) for a batch of beer the "original gravity" (OG) is measured after the wort has been cooled and before the yeast has been "pitched". After pitching the yeast, it consumes the sugars and produces alcohol and CO2 as byproducts. Different strains of yeast have different alcohol tolerances and will die before consuming all of the sugar, where others may continue to produce alcohol to much higher levels. When the yeast has completed its work the specific gravity is measured, again. This number becomes the "final gravity" (FG). The FG will be lower than the OG, and the greater the difference the more alcohol has been produced. A typical FG number is around 1.012. A calculation is performed comparing the OG to the FG (many online calculators are available for free) to determine the actual ABV that was produced.
Note that the alcohol content of beer and wine is what the yeast is able to produce and survive, where distilled spirits take the process one step further and boil the alcohol out of the mixture and cool it into nearly pure alcohol. This is why liquors can have ABV levels well beyond yeast's ability to survive.