Imperial Stout style beers have a black colour an ABV between 5% and 7.5% and a high bitterness
Imperial Stout characteristics
Imperial Stout is a dark beer, ranging from dark brown to pitch black, and a well-formed head. Aroma and flavour tend to be rich and complex, with presence of roasty malts or dried fruits and touches of coffee, caramel or chocolate. Hop flavour can be from medium to high. Imperial stout is a good choice for barrel-aging, making the aromas and flavours more balanced and smooth.
Imperial Stout—also called Imperial Russian Stout—traces its origins to early 1700s in the UK. One of the most recurrent questions when trying to define this style is about difference between a Porter and a Stout. Stout was a term used in the UK in order to name strong porters, and the legend is that most of it was brewed in order to be exported to the Russian court. This trade was finished due to Napoleonic wars, and then the style became popular among the UK locals. The high ABV is said to be a way of avoid the freezing of beer under low temperatures.
The historical facts is that since 1822, strong porters that were exported to the baltics had a excise tax applied that made them commercially uncompetitive, so English brewers started focusing in brewing and exporting India Pale Ales to the British Indian colonies. While the Imperial Stout did became popular during the 19th century in the UK, it was already popular long before the tariffs were imposed, and as early as in 1785, stout was already being sold in the UK and even advertised in the newspapers.
In the 1800s the use of the terms porter and stout had a geographical bias, being porter mostly used in the Manchester region and stout mostly in the London region, but the term stout is also found in Manchester newspapers, always referring to a strong kind of porter.
About the quantity of alcohol, it is a myth that the ABV was high in order to survive the freezing, as the transport to the baltic ports was always made by ship, and sea water would be frozen at higher temperatures than beer. On the other hand, it is indeed true that Imperial Stout kind of beer was heavily exported to Russia—and other Baltic countries as well—until taxes became too high. But the term Imperial Stout didn't appear until as late as 1821, but the popularization of the terminology in the UK didn't happen until 1830, when the Beerhouse Act lowered the taxes for beer producers, allowing to small brewers to start producing commercially, and also making big brewers to be interested in new styles in order not to lose their share of the market. Still, the term Russian Stout didn't appear until 1889, being applied to Stouts exported to Russia, and being a different brew than the Imperial Stout
All the exports to Russia fell then into the hands of A. Le Coq, who had virtually a monopoly until 1912 when they were planning to open a brewery in Estonia, but their plans were crashed by the WWI.
After the war, the name Russian Stout was used by Barclay's brewery, only changing the name to Imperial Russian Stout in the 1970's. Nowadays, Imperial Stout are regarded as slighty stronger than Porters, but the ABV seems not to be the main difference, but a more sweet-chocolate character in the porters vs a more bitter-fruity touch when it comes to Imperial Stouts.
Back And Forth interview
Bodegraven (Netherlands), 28/02/2017
John Brus (De Molen), “We are all about making beer, not about naked ladies or dragons or whatever on the label. This is beer!”