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Why does milk stout contain lactose?

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Six small glasses of craft beer in varying amber and golden hues are arranged on a metal tray atop a wooden bar counter in Brussels, Belgium. — Beer Glasses of beer in a bar in Brussels.

Milk stout contains lactose because it is used as a nonfermentable sugar to provide sweetness to the beer.1

During the brewing process, the ale yeast used to ferment the beer is unable to digest lactose. Consequently, the sugar remains in the final product, which contributes to the characteristic sweetness and creamy mouthfeel associated with this style of stout.21