Guinness FAQ and Folklore

(c) Alan Marshall                   archive name: beer-guinness.faq                       Revised, March 16, 1995

There has been much posted about Guinness to this group, and much
of it could be used to spawn a new group alt.guinness.folklore.  I
find a great deal of irony in this:  there is probably more
"folklore" about Guinness than any other beer, yet it was the
Guinness people that underwrote the famous "Book of Records" to
settle arguments often based on rumours rather than fact.  So, in
an attempt to "set the record straight" I offer: 

                     Guinness FAQ and Folklore


I.     Guinness Facts
II.    Guinness FAQs
III.   "Black and Tan" FAQs
IV.    Guinness and Health
V.     Specifications of the Various Types of Guinness
       Appendix I --  A guide for the un-initated to buying Guinness
                      in an Irish pub.
       Appendix II -- Patent Information on the Smoothifier

                         I.  Guinness Facts

~References:         MJBC -- Michael Jackson's Beer Companion (1993)
                    MJPG -- MJ's Pocket Guide to Beer (1991, 1994)
                    RADA -- Real Ale Drinker's Almanac (Protz)

"No other brewery uses as much roasted barley as Guinness, which
prepares its own."  (MJBC, p 181)

"The brewery...uses several varieties (of hops), among which
Goldings are perhaps the most influential." (MJBC, pp 180-181) 

Guinness "...uses its original yeast, though this has been selected
down from several strains to one, which was arrived at in about
1960.  It works at high temperatures (around 25'C/77'F), and it is
very dispersed, having neither risen to the top nor sunk to the
bottom, when it is removed by centrifuge."  (MJBC, pp 181)

"Dublin makes five or six principal versions of Guinness, in a
total of 19 variations, and exports around 40 percent of its
output." (MJBC, p 181)

All the grain used by Guinness is grown in Ireland.  The non-malt
grains used in the domestic product is 25% flaked barley and 10%
roasted barley. (MJBC p 181)

                         II.  Guinness FAQs

Q:     I've seen Guinness sold in Germany even when the
       Reinheitsgebot applied to imported beers.  How can that be?

A:     "In line with the German Purity Law, the barley is replaced
       with pale and roasted malt in Continental Europe." (MJBC p

Q:     What types of Guinness are there?

A:     Michael Jackson states that there are 5 or 6 main types in 19
       variations.  So far, we've been able to identify (Detailed
       specifications are listed in the last section of this FAQ):

       Draught Guinness (Ireland, NA), is smooth full-bodied and
       creamy.  Despite its body, it is a rather mild beer (OG
       1039, 3.4% abw, 4.2% abv).  This beer is pasteurized. 
       There are many local variations of this product.  The
       canned "Pub Draught Guinness" using the draught-flow (tm)
       system is a packaged version of this beer.

       Bottle Conditioned Guinness (Ireland) had similar
       specifications to the Draught Guinness, but the presence
       of the yeast provides for a "...spicier, fruitier, drier,
       more complex and lively, fresh character." (MJBC, 182)

       Bottled Guinness (Britain) has similar specifications to the
       above two beers, but it is pasteurized.

       All Malt Guinness (Continental Europe) is available both
       draught and bottled.  It is slightly stronger and also
       has a little more of the characteristic stout bite.

       Guinness Extra Stout/Guinness Original (bottled, available
       widely) is stronger with even more stout bite.  MJ
       reports this to be 4.8% abw, 6.0% abv, but one poster
       says that in advertising from Guinness Imports, the
       strength is listed as 5.6%.  This product is licensed for
       production widely around the world.

       There is a strong bottled version (Belgium and others) that is
       somewhat sweeter and stronger (7.5% abv)

       Foreign Extra Stout (primarily tropical countries) is also
       about 7.5% abv.  However, this is blend of Guinnesses,
       included one aged for three months.

       In Nigeria, which bans barley, Guinness is made from sorghum.

Q:     What is a Black and Tan?

A:     Guinness (or some other stout) mixed with another lighter
       coloured beer such as a pale ale or lager.  There is now a
       separate section of this FAQ on Black & Tans.

Q:     What is Black Velvet?

A:     Guinness and Champagne in equal proportions (MJBC p 176)

Q:     Are there other Guinness drinks?

A:     A poor-man's Black Velvet is Guinness and cider.  There are
       several variations involving black current cordial or cassis. 
       Cordial will sweeten a Guinness, and if poured in through the
       head, leaves a purple "splotch" in the head.  A Purple Meany
       is half Guinness, half bitter (the dregs of the cask if
       possible) and cordial.  (No thanks to Niall McAuley and Duncan
       Snelling for these horrible drinks!)

Q:     I once saw shamrocks on a Guinness.  What that all about?

A:     This was started back in around 1988 or so for the benefit of
       tourists.  Susan Hoban, author of "Irish Pubs Across America" 
       -  A guide to over 300 Irish pubs in the U.S., writes:  "In
       fact, one will encounter many a shamrock atop a pint of
       Guinness in America, made by the bartender 'drawing' the
       shamrock at the end of 'drawing' the pint." 

Q:     How does the draught-flow can work?  What's that plastic thing
       in the can do?

A:     The simple, non-technical answer is that the little plastic
       bladder is filled with nitrogen, not nitrous oxide or nitrous
       dioxide as claimed by a couple of posters.  Since the can's
       contents are under pressure, some of the Guinness is forced
       into the plastic bladder through a pin-hole.  When the can is
       opened, the nitrogen and beer mixture is released creating the
       lovely foamy head.

       Nitrogen is used because it creates smaller bubbles than
       carbon dioxide, which can also leave a tinny taste.

Q:     Where can I get more detailed information on this?

A:     Here.  Thanks to Joel Plutchak, who saved this information
       that someone else (unattributed) posted.  (I would be happy to
       give credit to the original poster -- email me)

       "The problem has always been the fact that draught Guinness is
       (or should be) dispensed with a mixture of Nitrogen and CO2
       gasses rather than the conventional CO2 alone.  The nitrogen
       is used because it makes very fine bubbles while it is not
       absorbed into the brew as the CO2 is, thus it does not
       "over-carbonate" the beer.  Also a special faucet is preferred
       which, in combination with the gasses, creates that wonderful
       creamy brown head which lasts to the bottom of the glass.  The
       new can combines the original kegged stout recipe with
       technology which creates the draught effect to a tee.

       "Dr. Alan Forage, creator of the technology, was on hand to
       explain the mechanics of the new can.  This is the way the
       system works:  The 16.9 ounce can (containing 14.9 ounces of
       beer) is fitted with a small plastic device (Guinness calls it
       a "smoothifier") which sits in the bottom of the can.  This
       device has a pocket or cavity which is open to the atmosphere
       via a pin hole in its top.  The can is evacuated of oxygen and
       filled with beer.  Prior to sealing the can, a dose of liquid
       nitrogen is added to the beer.  The can is closed and as the
       liquid nitrogen warms a pressure is created.  The pressure
       forces about 1% of the beer and nitrogen into the plastic
       cavity.   When the can is opened, the pressure is released and
       the small amount of beer in the cavity is forced back through
       the pinhole quite violently.  The agitation created by this
       "geyser" mixes the nitrogen with the beer in such a way as to
       reproduce the tap handle character.  Open up the first empty
       can you have in order to see what the "smoothifier" looks

       Technical details from the patent are in Appendix II.

Q:     Why does the bottled Guinness taste so different than the
       draught or canned "draught-flow" product?

A:     (U.S./Canada answer):  These are two entirely different beers. 
       The bottled Guinness if Guinness Extra Stout, while the others
       are simply Guinness.  The Extra Stout uses more roast barley
       and has somewhat higher IBUs, resulting in a harsher, sharper
       flavour.  Which you prefer is a matter of personal taste.

       (Ireland answer):  The bottled product is bottle conditioned,
       meaning the presence of live yeast.  The yeast gives it a very
       different character.

Q:     What's the significance of the harp symbol on Guinness

A:     Arthur Guinness & Sons deliberately chose the harp symbol as
       its logo or symbol to appeal to nationalist pride in Ireland. 
       The harp is also a symbol of Ireland, which appears on the
       back of their coinage.  The Irish Government and Guinness
       versions of the symbol are identical, except for the fact that
       the Guinness Harp faces left, while the official government
       version faces right.

Q:     What products besides the various Guinnesses does Guinness

A:     Arthur Guinness & Sons also own and produce Smithwicks
       (pronounced without the "w"), also sold as Kilkenny, Harp
       Lager and Macardles ales.  In England they also market
       "Guinness Draft Bitter" which uses the same draught-flow
       system and Guinness stout.

Q:     Is there anything I need to know when I go into a pub for a

A:     First, you don't "go into a pub for a Guinness".  A bird
       doesn't fly with one wing!  You always have two!  One of the
       classic posts to the r.f.d.b newgroup was Alan Clinton's "A
       guide for the un-initated to buying Guinness in an Irish pub." 
       It is reproduced in Appendix I.

Q:     What do the numbers on the Guinness labels mean and why isn't
       there a set of numbers on the Pub Draught (tm) cans?

A:     We are still looking into this, but we believe there are
       licence numbers for the different styles of Guinness.  (Ernest H. Joynt, III) offered the following

       Guinness Extra Stout (purchased in the US)
             L/A1    821212
       Guinness Foreign Extra Stout (made in Trinidad)
             L/AU    771712
       Guinness Special Export Stout (8.0% abv, made in Dublin)
             A/RM    571012

       The letters and numbers preceding the six-digit number are
       actually written with the first letter over the other two. 
       For example, the Guinness Extra Stout is "L over A1".

       The Pub Draught Guinness (tm) does not have this type of
       numbering on the can.  We do not know why.

Q:     Is the Guinness family still involved with the Guinness

A:     The family still has a large financial stake in Arthur
       Guinness & Son PLC, but have not been directly involved in the
       management since 1992.

Q:     Why are there so many Irish pubs springing up?

A:     Well, there are two types of people in this world:  Those who
       are Irish, and those who want to be!

       In fact, Guinness has set up a business that will totally
       outfit and "Irish Pub" for a would-be publican.  All the
       publican has to to is provide the site.  In Germany, they are
       opening at the rate of about 4 per week.

Q:     Someone told me that Guinness intentionally added sour
       Guinness to their beers.  Is that true?

A:     Yes, part of the process is to blend in some specially soured
       Guinness.  The following was extracted from the Homebrew
       Digest.  I believe the original author was Martin Lodahl, but
       I may be mistaken:

       "...they have a series of huge oaken tuns dating back to the
       days before Arthur Guinness bought the brewery, which they
       still use as fermentors for a fraction of the beer.  The tuns
       have an endemic population of Brettanomyces, lactic acid
       bacteria and Lord knows what else, and beer fermented in it
       sours emphatically.  They pasteurize this and blend small
       quantities of it with beer fermented in more modern vessels."

Q:     I've heard that Guinness contains oysters.  Is that true?

A:     No.  At one time, oyster shell may have been used as finings
       to rid the beer of unwanted solids, but this was not in this
       century, and may not have ever been done by Guinness.  There
       are oyster stouts that contain oyster, oyster extracts, etc.

Q:     Doesn't Guinness contain oatmeal?

A:     No.  It is a rather common misconception that Guinness (and
       all other stouts) contain oatmeal.  Oatmeal Stout is a
       distinctive style stout.  The revivalist of this style was
       Samuel Smith's Brewery in Yorkshire, England, at the request
       of its North American agent, Merchant du Vin of Seattle.

Q:     How does one spell Guinness?

A:     Never!  That's a waste of nectar!  Oh, Sorry.  I thought you
       said "spill".

       Guinness is spelled exactly as you see in the question -- two
       "N's", two "S's" and a "U" before the "I".  The "G" is always
       capitalized out of reverence.  Unfortunately, it is misspelled
       in the index of the 1991 Pocket Guide to Beer (only one "N").

Q:     Doesn't Guinness contain {insert name of animal} {insert your
       favourite word for urine}?

A:     No.  It is not part of the formulation, and there is virtually
       no opportunity for this to happen by chance.  We cannot be as
       unequivocal about Labudmilloorsons, however. ;-)

Q:     What's the proper thing to say when someone {buys you a
       Guinness/ gives you a 4-pack of Draught-flow cans/gives you
       your own cask}?

A:     Whale oil beef hooked!  ;->

Q:     Is there a Guinness smiley/emoticon?

A:     (:-{d) (licking the rich, creamy Guinness foam off the upper

                              III.  "Black and Tan" FAQs

Q:     What is a Black & Tan?  What is a Half-and-Half?

A:     The answer depends on who you ask.  The all-Guinness Black &
       Tan is Harp Lager and Guinness.  (Harp is brewed by Guinness
       at Dundalk).  Many consider the classic Black & Tan to be Bass
       Pale Ale and Guinness.  (To some Irish, they appreciate the
       fact that the Irish comes out on top!)  However, Bass Ale is
       not as available in Ireland as it used to be, and either Harp
       or Smithwicks is generally used.  However, there are many
       variations involving a stout or porter and another lighter
       coloured beer, either lager or ale.

       A half-and-half is often just another name for a Black & Tan. 
       However, in many North American Irish pubs, the Bass/Guinness
       combination is called a Black & Tan, while the Harp/Guinness
       combination is called a Half-and-Half.

       To quote Brendan E. Molloy (

             According to "The Guinness Drinking Companion" by Leslie

                    Black and Tan = Guinness and Mild
                    Half and Half = Mild and Bitter

             The above book (now sadly out of print) was published by
             Guinness Publishing and if anyone knows they do!  The
             full Oxford English Dictionary also mentions these
             mixtures (look under Beer, Ale, Drink, etc.).

             With the decline of Mild in the UK these names are
             dropping out of use. In Dublin, where Mild is no longer
             available, you will get Guinness mixed with Bitter if you
             ask for a Black and Tan. Some publicans serve other
             combinations so it is best to check before you buy.

       Note:  Mild is a slightly sweetish 'Tan' coloured beer, that
       is typically light in alcohol. 

       However, Guinness promotional advertising in the U.S. refers
       to a Black & Tan as being the Bass and Guinness version.  Ken
       Papai described the ads as follows:

             Here is the Guinness Import Company (c) 1992 ad on their
             beer card/beer coaster that they distribute at pubs and
             beer shows:

             THE TRUE COLORS OF HALLOWEEN            (picture of black & tan
                                                     in a pint glass)

             Guinness                                (picture of a harp, the
                                                     Guinness trademark and

             Bass Ale                                (Bass logo)

             BLACK & TAN

       It should be noted that Guinness Imports Ltd., also imports
       Bass Ale.

       So, Black and Tan, Half and Half, B&T, Half'n'alf, it all
       depends on where you are and who you ask.  When your bartender
       is a 240 lb., 6'4" fellow named O'Malley, a Black & Tan (or a
       Half-and-Half) is whatever he says it is!

Q:     How do I pour a Black and Tan?

A:     Steve Glover, who has tended bar in Ireland, says that the
       layered Black and Tan is an American affectation that they
       were happy to do for extra money :-).  Otherwise, both beers
       (they used Smithwicks and Guinness) were simply poured in the
       same glass fully mixed.

       If you want a layered drink:  Carefully.  Practice.  With a
       special spoon.  There are many answers.

       I have had success pouring the Harp or Bass vigorously so that
       it develops a good head.  Using a Black & Tan spoon, pour the
       Guinness slowly over the back of the spoon.  It will remain
       layered for some time.  (You can make your own by bending the
       bowl of a spoon back so that when you lower it into the glass,
       the bowl is concave down.)

       The Guinness promotional beer coaster cited above offers the
       following advice:


             1.     Hold your pint glass at an angle, and fill just
                    over halfway with Bass Ale on tap.

             2.     Slow the flow control on the Draught Guinness tap
                    down to a trickle.

             3.     Fill to the top, letting Draught Guinness hit the
                    side of the glass.


                    Pour Draught Guinness over an upside-down spoon to
                    disperse the flow.

             4.     Let settle, and enjoy the True Colors of Halloween!

Q:     Why does the Guinness float on top of the other beer?

A:     "Quality always rises to the top" (Jon Binkley)

       Tom Otvos ( offers the following

       I have sometimes wondered about why the Guinness stays on top
       of a layered Black and Tan myself.  A friend of mine and I
       (both of us engineers) came to the conclusion that the SG of
       Guinness is not appreciably different from other beers, but
       that it is the higher viscosity of the liquid that prevents if
       from mixing.  We assumed that it was this viscosity that gives
       Guinness its body, and the perception that it is a "heavier

       David B. Sapsis  sends this

       I am virtually certain that although the very high protien
       content of this brew (what do you expect from a grist thats
       25% flaked!) does in fact make it more viscous, and this is
       born out in mouthfeel, the driving force behind Guinness
       floating on other beers is differences in density.  Not only
       is Guiness very well attenuated (I have degassed Pub draft and
       found its gravity to be 1009) it is also usually served quite
       a bit warmer than the "bottom half".  Thus, an ale served
       around 40F is at the peak of its density tempertaure curve
       (max rho at 4C) and is likely to have a higher finishing
       gravity, both raising the delta rho.  Now I do believe that
       the initial boundary layer formation is highly viscocity
       dependent, because ... if you overpush the stout, you can
       break the boundary and mixing results.  

Q:     Is there any meaning to the Black & Tan name other than its
       obvious reference to the colours of the beers?

A:     Yes.  The first known reference to the expression Black & Tan
       was in reference to a breed of beagles used as hunting dogs in
       Ireland.  The term was also used to refer to a a regiment of
       British soldiers recruited to serve in Ireland after the First
       World War.  They had a reputation for being quite brutal and
       have been accused of many attrocities against the Irish in the
       years 1919-21.

Q:     Should I order a Black & Tan in Dublin?

A:     Some have posted that this is not advisable, as you might stir
       up tempers.  Steve Glover and others post that ordering a
       Black and Tan is no problem.

Q:     Is there a Black & Tan Song?

A:     Thanks to J. N. Walsh, we can offer you:

       Come Out ye Black & Tans!

       I was born on a Dublin street
       Where the loyal drums did beat
       And those bloody English feet
       They walked all over us!
       But every single night
       When me Da would come home tight,
       He'd invite the neighbours out
       With this chorus:

       Come out ye Black & Tans!
       Come out and fight me like a man.
       Show your wife how you won medals
       Down in Flanders.
       Tell her how the IRA
       Made you run like hell away
       From the green and lovely lanes
       Of Killeshandra!

Q:     In another newsgroup, someone asked if Black & Tans were
       invented in the U.S.  Is this true?

A:     Wasn't everything worthy and of good repute, including
       justice, honour (sorry, honor) and civilization invented in
       the U.S. of A.?  ;-)

                         IV.  Guinness and Health

Q:     Is Guinness really "good for you"?

A:     Medically, possibly.  Its alcohol is low enough to have a
       relaxing effect.  The bottle-conditioned product may produce
       a mild laxative effect due to the live yeast.  Too many
       bottles of the bottle-conditioned product can have an extreme
       laxative and diuretic effect, which is not good for you.

       There is medical folklore about Guinness and stout, in
       general. "In some countries, stout is seen as an aphrodisiac,
       or as a beneficial bath for newborn babies." (MJBC p 177) 
       Since most research indicates the aphrodisiac effects are
       primarily psychological, there may be truth to the former. 
       Too many may have an adverse affect on performance, however.

       Of course, if you feel as good as I do after enjoying and
       Guinness, then there is no question that it is good for you!

Q:     But isn't Guinness high in calories?

A:     Not especially.  "Guinness is not notably high in calories..."
       (MJBC, p 177)  This refers to the draught, bottle-conditioned
       and draught flow (tm) product, which are fairly low in
       alcohol.  Calories in beers come from the alcohol and the
       residual sugars.  The higher the alcohol, the higher the
       caloric content.

Q:     Where did the "Guinness is Good for You" come from?

A:     Guinness's advertising agency (S. H. Benson) did some market
       research during the 1920's to find our what people liked about
       Guinness.  People responded that they felt good when they had
       their pint and the slogan was born.  The slogan is still used
       in some countries (notably in Africa) that do not regulate
       advertising claims as zealously as the U.K. and North America. 
       Some have even posted that the advertising features athletes
       and imply that there athleticism can be attributed to

Q:     Guinness has been dispensed in hospitals, correct?

A:     Yes.  In England, post-operative patients used to be given
       Guinness, as were blood donors.  Sadly, this is no longer the
       case in England.  In Ireland, Guinness is still made available
       to blood donors and stomach and intestinal post-operative

Q:     What about the old wives' tale about nursing mothers drinking

A:     Current medical research suggests that pregnant women and
       nursing mothers should totally abstain from any form of
       alcoholic beverage.  In pregnancy, it can lead to Fetal
       Alcohol Syndrome, characterized by slow development.  Alcohol
       does pass in the mother's milk, so nursing mother's should
       avoid Guinness and any other form of alcohol.

Q:     Guinness is high in vitamin G, isn't it?

A:     There is no vitamin G!  However, the folklore surrounding
       Guinness has often lead to it being called vitamin G. 
       Recommended Daily Allowance:  3 pints a day.

          V.  Specifications of the Various Types of Guinness

Draught Guinness (Ireland & North America):

OG:          1039 (9.75 Plato)
Alcohol:     3.4% abw, 4.2% abv
BU:          45
Colour:      EBC - 130

"Draught-flow" Canned "Pub Draught Guinness"

Similar specifications as the Draught Guinness, above.

Alcohol:     3.3% abw, 4.1% abv

Bottled Guinness (U.S.) -- This is an "Extra Stout"

Alcohol:     4.8% abw, 6% abv (Although one poster noted that they had
             seen 5.6% in their promotional materials. Michael Jackson
             reports these figures.)
BU:          50+

Continental Guinness (Draught and bottled)

Alcohol:     "a little more than 4 percent by weight, 5 by volume"
             (MJBC p 181)

"strong" Bottled Guinness

OG:          1073 (18 Plato)
Alcohol:     6% abw, 7.5% abv
BU:          50+

Guinness Extra Stout (England, according to Eckhardt)

OG:          1052 (13.2 Plato)
Alcohol:     4.4% abw, 5.5% abv
BU:          50
Colour:      9.5 (Eckhardt's Scale)  This would translate into
             something around or over 50 SRM and about 130 EBC

Guinness Extra Stout (Ireland, according to Jackson)

(bottle conditioned)

OG:          1039 (9.75 Plato)
Alcohol:     3.4% abw, 4.2% abv

Bottled Guinness (Ireland)

"similar" specifications as Draught Guinness

OG:          1040
Alcohol:     3.5% abw, 4.3% abv
Contents:    Pale malt, flaked barley, roasted barley, English and
             American whole hops (RADA)
Note:        This may be the same beer as the previous listed

Foreign Extra Stout (this is a blend of Guinnesses):

OG:          1073 (MJPG91)
BU:          mid-60s

Guinness Extra Stout (Ireland in 1901, according to Eckhardt)

OG:          1075 (18.2 Plato)
Alcohol:     6.3% abw, 7.9% abv
BU:          90 (!)

                              Appendix I


1.     Choose your pub carefully. A pint of Guinness does not
       appreciate loud music, loud people or bright flashing lights.

2.     Ask politely for a pint of Guinness. Depending on the pub, it
       is possible to catch the barmans eye and mouth the word
       "pint", he will translate this accurately.

3.     The barman will fill the glass between 70% and 80% capacity.
       It will then be put to the side for a few moments to allow it
       "to settle". Once the brownish liquid has almost turned to a
       solid black the barman will then fill the rest of the glass.
       NB: do not under any circumstances take the glass before it is
       filled. Some virgins seem to think that the settling stage is
       the final stage and walk away with an unfinished pint. At this
       point we Irish DO understand the predicament, but I assure you
       it causes endless mirth as well.

4.     Once you have received your pint, find a comfortable stool or
       seat, gaze with awe into the deep blackness, raise the pint to
       your mouth and take a large mouthful. Be firm. 

5.     A good pint can distinguished by a number of methods. A
       smooth, slightly off- white head is one, another is the
       residue left on the inside of the glass. These, surpise
       surprise, are known as rings. As long as they are there you
       know your're okay. A science of rings is developing - the
       instance that comes to mind is determining a persons
       nationality by the number of rings (a ring is dependent on a
       swig of Guinness each swig leaving it's own ring). An Irishman
       will have in the region of 5-6 rings (we pace ourselves), an
       Englishman will have 8-10 rings, an American will have 17-20
       (they sip) and an Australian won't have any at all as they
       tend to knock it back in one go!

6.     As you near the end of your pint, it is the custom to order
       another one. It is a well known fact that a bird does not fly
       on one wing.

                    - Alan Clinton (

                              Appendix II

                 Patent Information on the Smoothifier

Robert James  posted:

I was doing some patent search work at the Washington library in
Chicago the other day and I ran across no. 4,832,968, the U.S.
patent for the Guinness in a can stout.  The basics:
Inventors:          Alan J. Forage & William J. Byrne
Assignee:           Arthur Guinness Son & Co., Ltd.
  -    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
  -    A charge of liquid N is added to the stout
  -    Can is sealed
  -    As liquid N boils off in can during pasteurisation (60 oC 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
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 gases 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.

-- Alan Marshall            "It's a lot of work to get up in front of
   AK200032@SOL.YORKU.CA    a class and make it look like you know
   York University          everything there is to know about something
   Toronto, Canada          you know nothing about."  Prof. Anonymous