NOTE: In the 2015 BJCP Style Guidelines, Brown Porter has been removed. Category 13 Brown British Beer describes the modern versions of 13A Dark Mild,13B British Brown Ale and 13C English Porter. They are grouped together for judging purposes only since they often have similar flavors and balance, not because of any implied common ancestry. The similar characteristics are low to moderate strength, dark color, generally malty balance, and British ancestry. These styles have no historic relationship to each other.
Brown Porter falls into the category of "confusing beer styles". It is somewhere between an English brown ale, a mild ale, and a robust porter. Trying to pinpoint its character is easier if you imagine that these three beers form a triangle of sorts with brown porter in the center. The body of a brown porter is bigger than a Southern English brown and an English Mild ale, but less than a robust porter. It is roastier than either the mild or English brown ale but again, not as roasty as a robust porter. The malt character is closer to the mild and English brown. England has some of the finest pale malts available. These malts are full-bodied and chewy with a broad toasty malt backbone. The use of similar specialty malts creates the same chocolate notes as a robust porter but without the burnt and acrid notes.
As you can see, a brewer must walk a fine line when designing this recipe. When comparing this beer to a robust porter, it is actually brown whereas the robust porter is usually a deep red/black. The brown porter will be slightly lower in alcohol as well. Chocolate notes are its hallmark as well as the use of English hops and yeast. The lines get blurred sometimes and commercial examples are often ambiguously labeled, which makes matters worse. Keeping the roasty notes and IBU's lower and the toasty malt and chocolaty character higher may take some experimentation. The use of black malt, such as black patent, in general defines a porter, but a brown porter has much less in the grist than a robust porter. Used in low proportions, it will add color to beer without much flavor impact. When flavor contribution is desired, it will give beers a dark roasted character which some describe as being slightly burnt and smokey. It has a different character than that of roasted barley which is usually only used in a robust porter recipe. Crystal or caramel malts are used for body and mouthfeel and perhaps a bit of sweetness. Chocolate malt is almost always added for the chocolate notes typical of the style.
American versions normally use 2-row malt instead of English pale ale malt but this is OK. It allows the specialty malts to shine through. You may notice Americanized versions are a little bigger in alcohol, body and bitterness. The use of American hops for bittering will leave the tell-tale resinous, citrusy, and piney character in the background. Americans like to put their own stamp on this beer. Some brewers are adding smoked malt to the beer with great results. Others are adding oatmeal to the grist for that silky quality we love in bigger dark beers.
You may have read Brewing Classic Styles by Jamil Zainasheff and John J. Palmer. In the section on Brown Porters, p. 154, Jamil expounds on the use of classic brown malt as being an essential component of the grist. He says that it provides the "nutty, slightly roasty, gentle chocolate background note that is apparent in some commercial examples." And without brown malt, brewers would rely too heavily on the darker roasted malts, pushing the beer too far toward the robust porter end of the style. As noted, it is a difficult recipe to get just right and I believe Jamil may have found something here.
Important factors to remember when brewing this beer are the use of a good English base malt, such as Maris Otter, simple hop schedule of English variety hops, keep the roasted notes to a minimum, keeping the residual sweetness to a minimum by using an attenuative strain of English yeast, keeping the fermentation cool to reduce the amount of fruity esters, and the use of brown malt to keep the right character intact without having to resort to too many dark roasted malts which can push the beer into the robust porter style.
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