The true sour is a study in simplicity—of whiskey, sugar, and citrus. Lemon is most common for the latter, but juice with any kind of noticeable acidity will work well. Traditional sours nearly always called for egg white, an ingredient that added a light, frothy, textural element to the cocktail. And believe it or not, the egg white provided a much-needed boost of protein after a night out on the town, something that was very popular with the early morning drinkers in the sporting set who made the cocktail so popular in the 19th century. —Erik Lombardo
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