What is Guinness?
Guinness is the quintessential Irish stout and is made from: water, barley malt, hops, and brewers yeast. A proportion of the barley is flaked and roasted to give Guinness its dark color and characteristic taste. It is pasteurized and filtered. Despite its reputation as a "meal in a glass" or "liquid bread", Guinness only contains 198 calories per imperial pint, less than an equal-sized serving of skimmed milk or orange juice.
The water used comes from the Lady's Well in the Wicklow Mountains in Ireland.
Draught Guinness and its canned namesake contain nitrogen (N2) as well as carbon dioxide (CO2). Unlike carbon dioxide, nitrogen does not dissolve in water, which allows the beer to be put under high pressure without making it fizzy. The high pressure is required to force the draught beer through fine holes in a plate in the tap, which causes the characteristic "surge" (the widget in cans and bottles achieves the same effect). The perceived smoothness of draught Guinness is due to the low acidity and the creaminess of the head caused by the nitrogen. "Original Extra Stout" tastes quite different; it contains only CO2, making a more acidic taste.
Contemporary Guinness Draught and Extra Stout are weaker than they were in the 19th century, when they had an original gravity of over 1070. Foreign Extra Stout and Special Export Stout, with ABV over 7%, are perhaps closest to the original in character.