High-gravity beer
High-gravity beer is any of various specialty beers, either craft beers or malt liquors, brewed with a high original gravity. A beer with an original gravity of 1.070 or greater before fermentation (as wort) is generally considered to be high.
High-gravity beers are a traditional expression of beer making in Europe, and many microbreweries in the United States also produce them. High gravity beers may be intended to be more flavorful, meant to be sipped and savored, and are often paired carefully with foods. Generally, high gravity beers are more expensive to produce than mass-produced beer due to larger amounts and quality of grains and malt used.
On the other hand, high gravity malt liquor is often intended primarily for its high alcohol content. Often lower quality grains may be used, resulting in beer that is equal in production costs to other mass-produced beers, while full of more filler grains or adjuncts. This allows malt liquors to achieve a boost in alcohol content without a significant increase in production cost.
Alcohol content
High-gravity beer tends to have higher alcohol content compared to other beers due to the concentration of sugar and fermentable ingredients at the beginning of the brewing process.
Alcohol (as well as CO2) is produced when yeast cells consume sugars and other carbohydrates in the beer. To determine the alcohol by volume (ABV) for a completed batch of beer, an initial specific gravity reading is taken while the batch is in the wort stage--before the yeast has been pitched. This initial reading is referred to as the "original gravity", or OG. After the yeast is pitched, it consumes the sugars and produces alcohol and CO2 as byproducts. Different strains of yeast have different alcohol tolerances and will typically die from alcohol toxicity well before consuming all of the sugar, whereas others may continue to produce alcohol to much higher levels. When the yeast has completed its work the specific gravity is measured, again, and recorded as "final gravity" (FG).[1]
The difference between OG and FG can help determine the amount of sugars that have been metabolysed into yeasts, and therefore the final ABV level of the beer. The calculation of ABV in practice is done by feeding the OG and FG readings into one of several empirically-derived formulas.[2] For rough estimations, brewers may even use as simple a method of determining ABV as dividing OG by 0.75, assuming the majority of fermentible sugars have been metabolized.[3]
The alcohol content of beer and wine is limited by how much the yeast is able to produce and what levels it is able to tolerate before dying. In contrast, distilled spirits take the process one step further by extracting alcohol out of the mixture by heating, cooling it into nearly pure alcohol. In this way, liquors can have final ABV levels well beyond those yeast strains can survive.