Date: Wed, 17 May 2006 09:30:02 -0700 (PDT)
Reply-To: WDAList@yahoogroups.com
To: wdalist@yahoogroups.com
Subject: [beer] Guiness widget thingy.

Drinking a Guiness with a friend awhile ago, we got to
talking about the little widget thing that they put
into their cans and bottles.  So, if you've ever
wondered how exactly it works or what it's supposed to
do, here it is.  
-coot

Inventors: Alan J. Forage & William J. Byrne 
Assignee: Arthur Guinness Son & Co., Ltd. 
Process: 
The gas pod in the can is blow molded with nitrogen
(N) 
A laser zaps a hole in the pod. (they experimented
with holes between 0.2mm and 2.5mm finding that 0.61
mm as ideal) 
Pod is inserted in the bottom of can 
Can is filled with CO2/N supersaturated stout. N is
present at 1.5% v/v min up to 3.5% v/v. (FYI, vol/vol
is the number of volumes of gas which are dissolved in
a unit volume of beverage at 760mm of Hg & 15.6 oC)
CO2 is present at between 0.8 and 1.5% v/v. 
During filling, foam rises to top of can. This clears
the air. 
A charge of liquid N is added to the stout 
Can is sealed 
As liquid N boils off in can during pasteurisation
(60'C for 15-20 min), top of can pressurizes and
forces the stout into the pod, thus compressing the
ambient pressure N in the pod. 
Equilibrium is reached at about 25 psi 
As I interpret the patent, this is what happens when
the can is opened: 

The can quickly depressurizes to ambient pressure. The
pod thus expels the stout contained in it (about 10-15
ml) at high velocity through the orifice. This causes
high local strain of the stout at the plane of the
orifice. This strain exceeds the cohesive forces
holding the gas in solution. As a result, the N/CO2 is
liberated from the stout at the plane of the orifice.
The millions of tiny N/CO2 bubbles then become the
foam head. So contrary to my initial belief, while
some of the N gas in the pod escapes directly into the
stout, it is actually the "ripping apart" of the stout
as it exits the pod which produces the bubbles, hence
the creamy head.