around 1800, all London Porter was aged in large (really gigantic) vats for between 6 to 18 months before being siphoned to smaller casks for delivery to the pubs.
The demand soon began to outstrip the supply. Some enterprising publicans found that they didn't have to use all "aged" beer in their pours. They could mix the highly matured beer with two parts fresh or "mild" porter to produce a beer of very similar flavor to that of the "aged" beer.
As time wore on in the 19th century, tastes again changed and the style was more often sold "mild". Many discontinued making it all together as lagers and pale ales took their place in the brewery.
Porters were made entirely out of brown malt prior to the advent of hydrometers and thermometers.
Once the hydrometer began being used by the brewers, they quickly noticed that brown malt would yield only about 2/3 as much fermentable sugar as pale malt. And with higher taxation to pay for the Napoleonic War, brewers had an incentive to use less malt.
London brewers soon began mixing pale and brown malts to increase yield and profits. But in 1816 England passed a purity law which allowed only malt and hops in the beers sold there.
It just happens that in 1817 Daniel Wheeler of Charles Street, Drury Lane designed and began using a roaster, similar to a coffee roaster, which would roast malt without burning it. He named the new malt "patent malt" or black patent.
Brewers could now use up to 95% pale malt and 5% patent malt to create a palatable product much cheaper than before.
Because of the huge popularity of the porter beer style, brewers made them in a variety of strengths. The beers with higher gravities were called "Stout Porters".
Of the stout porters being produced, the lightest, at around 1.066 OG were simply called Single Stout Porter, often designated by a single "X" on the label.
Double Stout Porter (of which Guinness is an example) came in at 1.072 OG and had two "XX" on the label.
The Triple Stout Porter came in at 1.078 OG and Imperial Stout Porter began at 1.095 OG and went up from there.
As time passed, the suffix "Porter" was dropped and these beers were called Stouts. British brewers continued using the term Porter to designate both Porters and Stouts.
During the first World War, grain shortages in England led to restrictions on the roasting of dark malts and on the production of strong beer.
With less restrictions, Irish brewers like Guinness took advantage of the lack of competition and began to dominate the bottled Stout market even though English brewers continued to brew draught stouts through the second World War.
Guinness continued brewing their Porters up until 1974 when the style was discontinued. Stouts grew into their own style but there was still division and debate on whether Stouts should be a separate style. Usually the only deciding factor is strength.
After the invention of black patent malt, Irish brewers discontinued the use of brown malt altogether, using only pale malt and black patent for color and flavor.
English brewers, however, continued using brown malt in their grain bills for a while, differentiating the Irish Stouts and Porters from the English versions.
With the rise of pale ales and clear lagers, the beer's hold on the brewing industry declined sharply. Porters, as a style, became pretty much nonexistant.
During the craft beer movement in the US, homebrewers, microbreweries and brewpubs brought back the style. Modern Porters are usually brewed with pale malt, black malt, crystal, and chocolate malts for character and color.
There are three sub-categories of Porter listed by the 2008 BJCP Style Guidelines. These are:
References: Information for this page was adapted from the 2008 BJCP Style Guidelines, The Wikipedia, The Free Encyclopedia article entitled Porter (beer), the article What the Hell is a Porter? written by Alstrom Brothers and published on Beeradvocate.com, and the article Black Patent Malt and the Evolution of Porter written by Kihm Winship and published in Zymurgy magazine in the Summer 1987 issue.
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