Style Question: How does a stout become a porter?

Given that historically there has been very little distinction in terms of style, "Stout" usually just referring to a stronger beer, there's no real all-encompassing distinction between the two. So the canonical answer is: "There is technically No Difference".

Even if you want to argue that somehow Stouts were somehow bigger or stronger, one then needs to cherry-pick the results anyway. This is because generally English beers fell in strength due to tax increases from both WW1 & WW2. When your average brown beer pre-war is commonly about 7+% AbV, how can a 6% brown beer be "stout" post-war?

Lately, most professional brewers I've listened to, and articles I've read, suggest that it's the inclusion of significant amounts of roasted barley{1} that, in modern times, has become the distinguishing feature of a stout.

Where this leaves Robust Porter, I couldn't say.

{1} Roasted Barley - not barley malt, just roasted un-malted grain.

The lines between the two blur a bit due to historical evolution porter was a dark raosty and smokey brew. When brewers made them stronger they were referred to as Stout porters to indicate strength. In time the two have seperated slightly enough to warrant different catagories for each.

Today Porters tend to be drier with a somewhat more acrid or ashy roasted malt character. Stouts will be roasty but in comparison you find the roast quality smoother and often accompanied with a higher finishing gravity as well as some chocolate malt character. Gravities and ibus tend to overlap quite a bit depending on the sub styles of each when in head to head comparison.

Lastly, and this is just my opinion, homebrewers have largely lost touch with straight up porter because we tend to add things like bourbon vanilla cherries spices etc etc. more frequently than we do in stouts. So the lines between them get difficult to compare when one becomes so abused with extra flavor additions.

Going from Stout to a Porter or a Porter to a Stout happens in the recipe and the ingredients being used. Less crystal malt and higher attenuation are good practices for Porter making. Going for some residual sweetness and smoother roast character is where a Stout would fall. But in the brewing process of one or the other changing the gravity with dilution doesn't necessarily get you from one to the other. A grain bill for a robust Porter is going to really cut it as a Stout. Likewise an oatmeal stout will never make it as a Porter.

While there are some similarities in flavor and appearance the brewing of modern day versions of either of these are fairly different.

In a nutshell, it's to do with strength. Just to contra the downvote, Brad Smith, author of beersmith has blogged about porter, and links to several recipes. I've looked over the recipes, and the darker ones could easily be taken as stout recipes, and the lighter ones, brown ale recipes.

I remember the guide telling at the Dublin Guinness brewery that they used to brew Porter. That was their main seller for a while, but in time started brewing "Stout Porter" - a stronger porter to remain competitive.

Wikipedia confirms this:

The name "stout" for a dark beer is believed to have come about because a strong porter may be called "Extra Porter" or "Double Porter" or "Stout Porter". The term "Stout Porter" would later be shortened to just "Stout". For example, Guinness Extra Stout was originally called "Extra Superior Porter" and was only given the name Extra Stout in 1840.

So, a stout and a porter historically are the same style, but with the stout moniker implying that it's stronger. The same wikipedia page gives these names and gravities as a guide:

• Porter - 1.055
• Single Stout Porter - 1.066
• Double Stout Porter (Guinness) - 1.072
• Triple Stout Porter - 1.078
• Imperial Stout Porter - 1.095

So your 1.086 Imperial Stout has become a Single Stout (Porter).

Stout doesn't have to be jet black - a commercial example is Murphys', which is noticably lighter than Guinness, so don't worry over the lighter SRM.

When the brew is done, evaluate the hop balance, since that's quite a reduction in gravity - more boil utilization and less residual sweetness to balance. I'm sure the beer will be drinkable, especially given some time to mellow. I recently made a Stout with an unwanted biting hop bitterness, possibly from miscalculation/misweighing, but this has mellowed to a very drinkable beer after 3 months.

EDIT: although the same styles historically, there are are some recipe tweaks that are more typical of a porter: use of flavor and aroma hops, since stouts have typically bittering hops only, and the use of black (patent) malt for bitterness, where stouts tend to use roasted barley. But as with any rule, there are plenty of exceptions.

Just one more thing on the subject, i don't think i have seen Black roasted Barley on any Porter recipe and most Stout recipes do.

There's no difference between a stout and a porter really - Martyn Cornell waxes lyrical about his here: http://zythophile.wordpress.com/2009/03/19/so-what-is-the-difference-between-porter-and-stout/

Basically it has come to pass that a stout is typically higher gravity than a porter. Both are black roasty beers though.