Because each of us responds to bitterness a little differently, one might think that tasting a bitter food or beverage is a primarily subjective experience, more opinion than fact, more a feeling than a component able to be counted and benchmarked.
Nevertheless, bitterness is a fully quantifiable flavor component. In the same way that one can quantify the heat of a pepper by measuring its capsaicin levels and converting the number to either Scoville or American Spice Trade Pungency units, one can objectively rank relative bitterness by measuring the level of alpha acid units in a food or beverage and converting the number to IBU, or International Bitterness Units.
Although brewers first used hops as a stabilizer, rather than flavoring agent, over time, and with improved processing technology and refrigeration, hops have become far more important as a flavor than a preservative.
Beer gets its bitterness primarily from hops. Hops are flowers shaped like pinecones; the base of their petals contains lupulin glands that excrete an oil high in the alpha acids we recognize as bitter.
Rate the Hops: IBU Indicates How Bitter a Beer will Taste
Hops are rated and ranked by their levels of alpha acid. A beer with a high alpha acid rating will tastes bitter and receives a high IBU rating. A Weizenbier, lightly hopped, is one of the least bitter beers; an American IPA or Barley Wine is the polar opposite, highly hopped and, for some, unbearably bitter. A typical Weizenbier exhibits roughly 10 IBU, while a formidable American IPA will be rated at about 50 to 70 IBU’s.
Alpha acids, when heated, isomerize to form iso-alpha acids. When we taste bitter beer, we mostly taste iso-alpha acids. A brewer creates iso-alpha acids when he adds hops to the boiling wort, the malt sugar liquid he later ferments.
If the specific gravity of the wort is very high, its sugar concentration permits less alpha acid to dissolve into the wort. However, in general, the longer a brewer boils the wort-containing hops, the more alpha acids isomerize and, in general, the more bitter the result.
The earlier hops hit the boil, the more they will contribute to the eventual IBU score. If a brewer knows the IBU rating for a batch of hops, he can better control the bitterness they contribute by adjusting boiling times or using multiple varieties of hops with differing IBU scores.
Brewers occasionally introduce hops much later in the brewing process to add less bitter flavor components, too, but if you consider yourself a hophead, you definitely dig bitter.