NOTE: In the 2015 BJCP Style Guidelines, Oatmeal Stout is now Style 16B in Category 16 Dark British Beer which contains average to strong, bitter to sweet, modern British and Irish stouts that originated in England even if some are now more widely associated with Ireland. In this case, “British” means the broader British Isles not Great Britain.
Oatmeal stouts are very popular with homebrewers, and for good reason. If you love stouts, and who doesn't, or are new to the style, an oatmeal stout is a good introduction.
The flavors are of dark roasted-grains with a bitter chocolate and coffee and cream notes. It's like the English version of a dark milkshake.
It was a very popular beer in the 1800's when it was thought to be a nutritious supplement to people's diet. In actuality, most oatmeal stouts only contain about 5% oatmeal in the entire recipe due to problems with astringency when more is added.
Even so, the marketers jumped at the chance to claim their beers were healthy, and the beers were even prescribed by physicians for lactating mothers and sick children.
Oatmeal stouts was popular in the late 1800's but declined soon after as the last one was brewed before World War I.
Samuel Smith Old Brewery revived the style in 1980, just in time for the craft beer movement in the U.S. Homebrewers adopted the style along with all beers English and it has remained popular ever since.
Oats give the stout a rich creamy (most often described as "silky") mouthfeel and a huge long-lasting head.
Oats can add a nutty, grainy, or earthy flavor, but too much can make the beer seem astringent (although the other big flavors in a stout can sometimes hide this character).
One other problem with using a little too much oats in the grist is that the mouthfeel can go from "silky" to "oily". It seems that around 10% of the grist is the threshold for this oiliness and an intense flavor.
Some things to consider when brewing this beer are that when most people, judges included, think of an oatmeal stout, they imagine a beer that is very rich in body and mouthfeel with a big nutty or biscuity flavor from the oats.
To get this impression in the beer, you can roast your flaked oats in the oven at around 300 degrees F (149 degrees C) until they begin to turn brown and you can smell the oatmeal cookie notes.
To make sure you get enough of the nutty-biscuity character, a small amount of Victory malt helps as well.
To get the correct amount of mouthfeel and body, use flaked oats and mash at around 154 degrees F (68 degrees C).
Use a yeast that does not fully attenuate the beer, such as the popular Wyeast 1968 London ESB or White Labs WLP002 English Ale to allow some residual impression of sweetness.
Keeping the hops bittering in check will also help with the impression of sweetness in this beer.
References: Information for this article was adapted from Brewing Classic Styles by Jamil Zainasheff and John J. Palmer and the 2008 and 2015 BJCP Style Guidelines.
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