Imperial IPA's are also called "Double" IPAs. This uniquely American style takes the craving for hops and runs with it. These usually use double or even triple the typical amount of hops for a Pale Ale, but also add more malts to balance. The resulting beer has huge hoppy highs and deep malty depths with an high ABV to match.
Imperial (Double) IPAs are a stronger, very hoppy variant of IPAs that typically have alcohol content above 7.5% by volume. The style is claimed to have originated with Vinnie Cilurzo, currently the owner of Russian River Brewing Company in Santa Rosa CA., in 1994 at the now-defunct Blind Pig Brewery in Temecula, California. The style has been embraced by the craft brewers of San Diego County, California, to such an extent that some refer to double IPAs as "San Diego pale ale".
The term pale ale originally denoted an ale that had been brewed from pale malt. The pale ales of the early 18th century were lightly hopped. By the mid-18th century, pale ale was mostly manufactured with coke-fired malt, which produced less smoking and roasting of barley in the malting process, and hence produced a paler beer. One such variety of beer was October beer, a pale well-hopped brew popular among the landed classes, who brewed it domestically; once brewed it was intended to cellar two years.
Among the first brewers known to export beer to India was the Bow Brewery, on the Middlesex-Essex border. Bow Brewery beers became popular among East India Company traders in the late 18th century because of the brewery's location and Hodgson's liberal credit line of 18 months. Ships transported Hodgson's beers to India, among them his October beer, which benefited exceptionally from conditions of the voyage and was apparently highly regarded among its consumers in India. During the same period, several Burton breweries lost their European export market in Russia when the Tsar banned the trade, and were seeking a new export market for their beer.
At the behest of the East India Company, Allsopp brewery developed a strongly-hopped pale ale in the style of Hodgson's for export to India. Other Burton brewers, including Bass and Salt, were anxious to replace their lost Russian export market and quickly followed Allsopp's lead. Perhaps as a result of the advantages of Burton water in brewing, Burton India pale ale was preferred by merchants and their customers in India, but Hodgson's October beer clearly influenced the Burton brewers' India pale ales.
Early IPA, such as Burton brewers' and Hodgson's, was only slightly higher in alcohol than most beer brewed in his day and would not have been considered a strong ale; however, a greater proportion of the wort was well-fermented, leaving behind few residual sugars, and the beer was strongly hopped. The common story that early IPAs were much stronger than other beers of the time, however, is a myth.
Demand for the export style of pale ale, which had become known as India pale ale, developed in England around 1840. Some brewers dropped the term "India" in the late 19th century, but records indicated that these "pale ales" retained the features of earlier IPAs. American, Australian, and Canadian brewers manufactured beer with the label IPA before 1900, and records suggest that these beers were similar to English IPA of the era.
Imperial (Double) IPA Notes:
History: A recent American innovation reflecting the trend of American craft brewers “pushing the envelope” to satisfy the need of hop aficionados for increasingly intense products. The adjective “Imperial” is arbitrary and simply implies a stronger version of an IPA; “double,” “extra,” “extreme,” or any other variety of adjectives would be equally valid.
Aroma: A prominent to intense hop aroma that can be derived from American, English and/or noble varieties (although a citrusy hop character is almost always present). Most versions are dry hopped and can have an additional resinous or grassy aroma, although this is not absolutely required. Some clean malty sweetness may be found in the background. Fruitiness, either from esters or hops, may also be detected in some versions, although a neutral fermentation character is typical. Some alcohol can usually be noted, but it should not have a “hot” character.
Appearance: Color ranges from golden amber to medium reddish copper; some versions can have an orange-ish tint. Should be clear, although unfiltered dry-hopped versions may be a bit hazy. Good head stand with off-white color should persist.
Flavor: Hop flavor is strong and complex, and can reflect the use of American, English and/or noble hop varieties. High to absurdly high hop bitterness, although the malt backbone will generally support the strong hop character and provide the best balance. Malt flavor should be low to medium, and is generally clean and malty although some caramel or toasty flavors are acceptable at low levels. No diacetyl. Low fruitiness is acceptable but not required. A long, lingering bitterness is usually present in the aftertaste but should not be harsh. Medium-dry to dry finish. A clean, smooth alcohol flavor is usually present. Oak is inappropriate in this style. May be slightly sulfury, but most examples do not exhibit this character.
Mouthfeel: Smooth, medium-light to medium body. No harsh hop-derived astringency, although moderate to medium-high carbonation can combine to render an overall dry sensation in the presence of malt sweetness. Smooth alcohol warming.
Overall Impression: An intensely hoppy, very strong pale ale without the big maltiness and/or deeper malt flavors of an American barleywine. Strongly hopped, but clean, lacking harshness, and a tribute to historical IPAs. Drinkability is an important characteristic; this should not be a heavy, sipping beer. It should also not have much residual sweetness or a heavy character grain profile.
Comments: Bigger than either an English or American IPA in both alcohol strength and overall hop level (bittering and finish). Less malty, lower body, less rich and a greater overall hop intensity than an American Barleywine. Typically not as high in gravity/alcohol as a barleywine, since high alcohol and malt tend to limit drinkability. A showcase for hops.
Ingredients: Pale ale malt (well-modified and suitable for single-temperature infusion mashing); can use a complex variety of hops (English, American, noble). American yeast that can give a clean or slightly fruity profile. Generally all-malt, but mashed at lower temperatures for high attenuation. Water character varies from soft to moderately sulfate.
Commercial Examples: Russian River Pliny the Elder, Three Floyd’s Dreadnaught, Avery Majaraja, Bell’s Hop Slam, Stone Ruination IPA, Great Divide Hercules Double IPA, Surly Furious, Rogue I2PA, Moylan’s Hopsickle Imperial India Pale Ale, Stoudt’s Double IPA, Dogfish Head 90-minute IPA, Victory Hop Wallop
Source: 2008 BJCP Style Guidelines, http://www.bjcp.org/2008styles/style14.php, http://www.thekitchn.com/whats-the-difference-pale-ales-142695, http://en.wikipedia.org/wiki/India_pale_ale