As best I can tell, Milk Stout began as something like the Brewer's version of fortified water. In an attempt to make their stouts appear healthful, brewer's advertised the nutritious qualities of milk, and touted the health benefits of stouts including sugars derived from it. Surely a marketing gimmick, but also a delicious beer.
Left Hand Brewing Co. Milk Stout skips the nutrition angle, opting to hook drinkers with the delicious angle. They also play up the (relative) oddity of the style, claiming that their Milk Stout challenges conventional opinion of stout in general.
Milk Stouts are created by adding lactose, or milk sugar, to the beer during the brewing process. As the other sugars are converted into alcohol by yeast, lactose remains untouched, since the fermenting little buggers can't digest it. Remaining in the beer throughout the brewing process, the unfermented lactose adds body and sweetness, balancing out the bitter and roasted qualities typical of most stouts.
When poured, Left Hand's version is deep brown, with a vaguely transparent character that glows a dusky amber when held in front of a light. The beer shows little carbonation, and develops only a thin, creamy, tan head, which dissipates rapidly.
As expected, the beer exhibits deeply roasted aromas, conjuring up the scent of spent espresso pucks (ask a barista; extracted coffee smells markedly different than fresh espresso). There are also hints of chocolate with butter and vanilla overtones, as well as a subtle nuttiness at play, like the milk solids that toast when you make browned butter.
The flavor profile repeats the aromas, though everything is smoothed out and more subtle. All of the other flavors are framed in a well-rounded roasted flavor befitting a dark stout. The butter returns as toffee, and the coffee notes are a bit cleaner, reminiscent of chocolate-covered espresso beans. Lingering in the background is the unmistakable, nutty and subtly sweet taste of evaporated milk.
This makes sense when you think about process. Evaporated milk is made by heating milk to very high temperatures, which also has the effect of caramelizing some of the milk's sugar. That sugar is the same lactose found in Milk Stout, and is subjected to similarly high temperatures during the brewing process. I also detected an interesting tang, and I can't help but wonder if this is attributable to the lactose as well, as I know that lactose will ferment into lactic acid in the right conditions.
A slight bitterness tugs at the edges of all of these flavors, acting as a counterweight for all of the sweet-seeming notes. None of the flavors really takes a dominant role, aside from the roasted quality that underpins and creates structure for everything else. The result is a beer that combines depth of flavor with an undeniable smoothness. The traditional flavors of stout are there, but they're tempered and evened out by the slight sweetness. The relatively mild bearing also allows more subtle notes to shine through, making for a beer with plenty going on at the surface, and even more going on underneath.