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How do those N2 widgets work in cans of beers like guineas, tetleys and Old Speckled Hen? Is the N2 in the widget and released when the can opens, or is the N2 in the beer and the widget helps mix and froth the whole thing?

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http://en.wikipedia.org/wiki/Widget_(beer)#Method

When the can is opened, the pressure in the can quickly drops, causing the pressurised gas and beer inside the widget to jet out from the hole. This agitation on the surrounding beer causes a chain reaction of bubble formation throughout the beer. The result, when the can is then poured out, is a surging mixture in the glass of very small gas bubbles and liquid.

...so, if I understand correctly, the widget contains liquid nitrogen when bottled. After bottling the widget raises in temperature and pressurizes to match the pressure of the can. When the can is opened the can drops in pressure and the nitrogen spews out of the widget, stirring the beer into a nice creamy head.

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    I am trying to wrap my head around the use of the liquid nitrogen and how the gas stays in the ball during filling and post filling. Pressure alone (at least at the can level) isn't enough to keep the nitrogen in a liquid state. So it must evaporate to a gas post fill, staying in the ball is still the mystery. But a great answer indeed. – brewchez Dec 2 '10 at 12:52

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