The birth of Porter

Porter was a hot new beer style in the early 18th century. It was a fairly simple beer, brewed from 100% brown malt and a development of the Brown Beer which had been brewed in London for a century or two. The big innovation of Porter wasn’t how it was brewed but how it was conditioned.

Until the Porter revolution London brewers had shipped out their beer at the end of primary fermentation. Any ageing took place in a haphazard manner, either by publicans or third parties. With Porter, brewers took the ageing process in house.

Initially they aged all their Porter for a medium length of time – no more than six months. As they became more accomplished at ageing beer, brewers realised that Porter matured more evenly and more reliably in larger vessels. Soon they were building vats with a capacity of thousands - or even tens of thousands – barrels.

The aged flavour which brewers sought – and which drinkers loved – derived from a long, slow secondary fermentation with Brettanomyces. Brewers at the time had no idea how this process worked, not realising that another type of yeast, other than the one that they pitched, was inoculating their worts.

Obadiah poundage’s letter of 1760
Obadiah Poundage was the author of a famous letter which explained the early history of Porter. His real identity is unknown, but it’s assumed that he had worked as an outdoor cooper for one of the large Porter brewers. At outdoor cooper being a brewery employee who went around pubs checking up their cellars.

Here's how I interpret his letter:

• many drinkers liked the taste of aged beer, to get this they drank either all old beer or old beer mixed with young
• before Porter, breweries sent out all their beer young and to get old beer publicans either had to age beer themselves ("start butts") or buy it from middlemen
• brewers had the idea of a partially-aged beer themselves to:

• generate greater profit

• make life easier for landlords

• the first Porter was Brown Beer aged for 4 or 5 months
  by 1760 customers and publicans expected Porter to be clear

Richardson's hydrometer experiments in the 1770s give us the first hard data on the strength of Porter:

Richardson's hydrometer trials early 1770s
Beer	OG	FG	ABV	App. Atten-uation
Strong ale	1110	1052	7.67	52.73%
Common ale	1075	1025	6.61	66.67%
Porter	1071	1018	7.01	74.65%
Table beer	1040	1004	4.76	90.00%
Source:
“A History of Beer and Brewing” Ian S. Hornsey, 2003 p.436.
Note:
ABV my calculation from the gravity drop.