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ABV is Alcohol By Volume, the most common way of describing how strong a non-distilled alcoholic beverage is. (The strength of distilled beverages is typically given as proof; to determine ABV, divide proof by 2; the other measure for non-distilled beverages ABW, Alcohol By Weight, was used in the US, but most brewers now use the ABV standard.)
A beer's alcohol content is determined by two things:
- The amount of grain in the mash, which provides the sugar that the yeast have to consume to turn into alcohol.
- The strain of yeast used. Since the yeast, in producing alcohol, are essentially releasing a toxic substance into their environment, they can eventually poison themselves, but different varieties of yeast have different levels of alcohol tolerance. Most beer yeast top out in the 5-7% ABV range, although some strains will go as high as 10-12%. Champagne yeasts reach 15-16%, and strains have been bred that will actually ferment up to 40%!
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If the yeast hits its tolerance, and there's still sugar left unconsumed, that sugar will also contribute to residual sweetness. Using a low-tolerance yeast could give you a weak, but sweet, beer.
IBUs are International Bittering Units, the standard for describing a beer's hop bitterness. The formula for this is rather arcane, but it boils down (pun intended) to three factors:
- The variety of hops used. There are dozens of different varieties of hops, and each is rated for its alpha acid content, which is the amount of lupulin resin it contains as a percentage of the weight of the hop flower. When hops are boiled, the lupulin is isomerized, that is, converted into the acid that makes beer bitter. (There's a second acid, called beta acid, that contributes a negligible amount of bitterness.) Most hop varieties fall in the 3-6% alpha acid range, but growers continue to develop new breeds, some pushing 20%.
- How much hops are used. Pretty self-explanatory.
- How long the hops are boiled. The length of time that the hops are boiled determines how much of their alpha acid is isomerized. It takes about 75 minutes to completely isomerize all of the lupulin; and, as they're boiled, other flavor and aroma characteristics that the hops possess are boiled away. So, typically, brewers will add hops at different points in the boil - some in the very beginning that they'll boil for the whole time, just to bitter the beer; some in the middle of the boil, to add flavor characteristics and a little more bittering; and some near the end, to provide more volatile aromatics, but almost no bittering. Brewers can also add unboiled hops to the fermented beer, called dry hopping, for an extra dose of aromatics. In recent years, craft brewers have started producing wet-hopped beers. Hops are typically dried before they're added to beer; wet-hopping adds fresh, undried hops to the finished beer, for an even more intense hop aroma. Some really crazy hop-heads like to run their beer through fresh hops right on their way from the keg to the glass, with gadgets like Dogfish Head's Randall the Enamel Animal.
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