Also known as a cream or sweet stout this beer style can trace its origins back to the practice of blending milk and beer in the United Kingdom in the late 1800s. Modern milk stouts don't use real milk but are brewed with lactose, or milk sugar, added to the kettle or fermenter. This unfermentable sugar gives the beer some residual sweetness. In the late 1800s milk beers were served at lunchtime to laborers for added strength to get through the day. In time, brewers began experimenting by adding milk directly to the fermentation stage and began touting these “milk stouts” as restorative beverages.
Many claimed that every glass contained “the energizing carbohydrates of 10 ounces of pure dairy milk,” according to British historical records. By the turn of the 20th century, doctors even went so far to prescribe milk stouts as the cure for various ailments including to nursing mothers to increase their milk production. However, the British government banned use of the term milk stout in 1946 to stem such unproven claims and to prevent any chance of the sweet beer finding its way into children’s hands. By that time there was not actually any milk in milk stouts as brewers had discovered how to produce and use lactose, or milk sugar, in the beer.
One of the few survivors of that era is Mackeson’s XXX Stout, which has been produced since 1907. Mackeson’s XXX Stout was originally called Mackeson’s Milk Stout before regulations were enacted. A milk churn still adorns the label.
History: An English style of stout. Historically known as “Milk” or “Cream” stouts, legally this designation is no longer permitted in England (but is acceptable elsewhere). The “milk” name is derived from the use of lactose, or milk sugar, as a sweetener.
Aroma: Mild roasted grain aroma, sometimes with coffee and/or chocolate notes. An impression of cream-like sweetness often exists. Fruitiness can be low to moderately high. Diacetyl low to none. Hop aroma low to none.
Appearance: Very dark brown to black in color. Can be opaque (if not, it should be clear). Creamy tan to brown head.
Flavor: Dark roasted grains and malts dominate the flavor as in dry stout, and provide coffee and/or chocolate flavors. Hop bitterness is moderate (lower than in dry stout). Medium to high sweetness (often from the addition of lactose) provides a counterpoint to the roasted character and hop bitterness, and lasts into the finish. Low to moderate fruity esters. Diacetyl low to none. The balance between dark grains/malts and sweetness can vary, from quite sweet to moderately dry and somewhat roasty.
Mouthfeel: Medium-full to full-bodied and creamy. Low to moderate carbonation. High residual sweetness from unfermented sugars enhances the full-tasting mouthfeel.
Overall Impression: A very dark, sweet, full-bodied, slightly roasty ale. Often tastes like sweetened espresso.
Comments: Gravities are low in England, higher in exported and US products. Variations exist, with the level of residual sweetness, the intensity of the roast character, and the balance between the two being the variables most subject to interpretation.
Ingredients: The sweetness in most Sweet Stouts comes from a lower bitterness level than dry stouts and a high percentage of unfermentable dextrins. Lactose, an unfermentable sugar, is frequently added to provide additional residual sweetness. Base of pale malt, and may use roasted barley, black malt, chocolate malt, crystal malt, and adjuncts such as maize or treacle. High carbonate water is common.
Commercial Examples: Mackeson's XXX Stout, Watney's Cream Stout, Farson’s Lacto Stout, St. Peter’s Cream Stout, Marston’s Oyster Stout, Sheaf Stout, Hitachino Nest Sweet Stout (Lacto), Samuel Adams Cream Stout, Left Hand Milk Stout, Widmer Snowplow Milk Stout
Source: BCJP - http://www.bjcp.org/2008styles/style13.php, All About Beer - http://allaboutbeer.com/article/milk-stout-2/