Composition
Guinness stout is made from water, barley, roast malt extract, hops, and brewer's yeast. A portion of the barley is roasted to give Guinness its dark colour and characteristic taste. It is pasteurised and filtered. Making the product requires knowledge in the sciences of microbiology, mycology, bacteriology, and thermodynamics. Despite its reputation as a "meal in a glass", Guinness only contains 198 kcal (838 kilojoules) per imperial pint (1460 kJ/l), fewer than skimmed milk or orange juice and most other non-light beers.
Until the late 1950s Guinness was still racked into wooden casks. In the late 1950s and early 1960s aluminium kegs began replacing the wooden casks; these were nicknamed "iron lungs".
Draught Guinness and its canned counterpart contain nitrogen (N2) as well as carbon dioxide. Nitrogen is less soluble than carbon dioxide, which allows the beer to be put under high pressure without making it fizzy. The high pressure of dissolved gas is required to enable very small bubbles to be formed by forcing 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 its low level of carbon dioxide and the creaminess of the head caused by the very fine bubbles that arise from the use of nitrogen and the dispensing method described above. "Original Extra Stout" contains only carbon dioxide, causing 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 1.070. Foreign Extra Stout and Special Export Stout, with abv of 7.5% and 9% respectively, are perhaps closest to the original in character.
Although Guinness may appear to be black, it is officially a very dark shade of ruby.
Bottle conditioned Guinness Extra Stout was available in the UK until 1994, and in Ireland until early 2000.
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