In the world of beer, some things are very clear. Lagers. Everyone can identify one and probably agree, more or less, what composes one. The same with a hefeweizen. The same with a Belgian wheat. The same with a lambic.
But when it comes to porters and stouts, there's no unanimous agreement. Oftentimes, these two styles are confused with another because, oftentimes, they are exactly the same.
This was not always the case. Porters came first. Their earliest mention is sometime in the early eighteenth century. As a style, porters revolutionized the brewing industry, likely because they were developed right around the time the Industrial Revolution brought about advances in the production and distribution of various goods. Porters were the first style of beer to be aged properly right at the brewery. So the story goes, the beers got their name because they were popular with London's street and river porters.
Originally, porters were brewed with inexpensive brown malts, an offshoot of English brown ales. They were aged up to a year-and-a-half in large vats until it was discovered that just a third of the porter need be matured and could be mixed with freshly made porter to yield the aged taste. This cut down on brewing cycle times. Since porter took longer to spoil and was cheaper to manufacture than other beers it became the first beer style to be brewed on a massive scale.
Just as today, the trend for stronger alcoholic content was evident. Recipes were tweaked. Porters were marketed under names like Extra Porter, Double Porter, Single Stout Porter, Double Stout Porter, and so on. There was no distinct Stout style yet. Stout merely meant what it currently means when used as an adjective. A stout porter was a heavier, more alcoholic porter. As the nineteenth century dawned, a stout was the strongest alcoholic beverage in a porter range, a fuller and creamier porter in body and strength.
Brown malts were, at that time, key to any porter recipe. The brown malts gave the porter its characteristic color and flavor. While the brown malts were cheap, they did not contain a high percentage of fermentable sugars, so a lot of brown malt was required to brew a porter in the acceptable 6%+ ABV range. Pale malts contained a third more fermentable sugars and would have been preferable in order to keep overall costs down, but pale malts in a porter could not produce the desired colors or flavors. Not until 1817, when Daniel Wheeler developed a process of roasting malted barley at over 200 degrees Celsius to create a patented black malt that would work just as well in porters for the proper color/flavor profile. In 1760, a porter would have been made with 100% brown malt. By 1800, this had dropped to 40% and, just twenty years later, to 20%.
With different malt bills now able to go into a porter, the style split into further subcategories: Brown, Robust, Baltic. Sometime in the mid-nineteenth century, the stout porter subcategory dropped the porter suffix and became referred to as stouts. Stouts subdivided as well. There were dry stouts, milk stouts, oatmeal stouts, sweet stouts, foreign extra stouts, and Russian Imperial stouts.
Although stouts were, technically, a type of porter, a general distinction arose between them based on the way most of them were brewed. Porters were made with malted barley. Stouts, which had more of a coffee- or chocolate-like flavor, incorporated unmalted roasted barley. Beer Judge Certification Program (BJCP) guidelines to this day use this as some general rule to differentiate the two.
From the late nineteenth century onwards, porters continued to decline in popularity as drinkers become less fond of its aged taste. Grain shortages in Britain during the First World War curtailed the strength of both British porters and stouts. A British stout circa 1945 was about the same strength as a porter in 1914. As porters became more watery, there was even less of a reason to drink them.
Guinness' immense popularity with stout helped seal porter's fate on a global scale. For most of the last two hundred years, Guinness also brewed porters. Though it renamed its Extra Superior Porter to Extra Stout in 1840, it continued to brew an Irish porter recipe up until 1974 when the plug was finally pulled. Porters were seen as an ancient beverage of a bygone generation.
Like many beer styles that bordered on extinction (Berliner weisses, lambics, hard ciders in the USA), porter recipes were dusted off and re-tweaked once craft breweries sought to distinguish themselves from their competitors. Paste Magazine published a list of thirty-five of the best American porters. These new offerings can differ markedly from the porters of yore. One contains maple syrup (Funky Buddha Maple Bacon Coffee Porter), another vanilla beans and smoky malts (Stone Smoked Porter with Vanilla Bean). Great Lakes Brewing Company's Edmund Fitzgerald includes unmalted roasted barley, making it more like a stout than a traditional porter.
In fact, with all the experimentation going on in the craft brewing world, there doesn't seem to be a huge difference between porter and stout anymore. The BJCP continues to distinguish porters from stouts by an old measure called Standard Reference Method (SRM), which gauges how much light is lost as a beam of light passes through a centimeter of beer. Porters traditionally have lower SRM values. But SRM values don't tell a complete story. Founders Porter from Founders Brewing Company has a fair amount of chocolate malt, Munich malt, and crystal malt. It's as dark as any stout. Other delineation markers go out the window, too. Porters were once considered sweeter and lighter than stouts. Yet a robust porter is heavier than an American stout and dry stout. And a Baltic porter can be more bitter than a sweet stout
Great Lakes Brewing once brewed a winter stout. After fermentation, one of the brewers suggested the beer would sound better if it were called a winter porter instead, and that porter magically became a stout. Says Carl Singmaster of popular Portland bottle shop Belmont Station, "The only difference between a porter and a stout is what the brewer decides to call it."