Equilibrium pressure in keg determines amount of foam
A question of
balance
One time too much foam, the next time too little.
Dispensing a perfect glass of lager can sometimes
drive bar owners to despair. But in more than fifty
per cent of all cases it is the fault of the owners
themselves. A question of equilibrium pressure.
The carbon dioxide gas in the
beer reacts differently to the
liquid when the temperature
changes. Hans Schutt, Heineken con
sultant Export Draught Beer Opera
tions, doesn't want to go into too much
detail about the whys and wherefores.
Equilibrium pressure is related to all
sorts of complicated physical pheno
mena which are difficult to explain.
"During the dispensing operation
carbon dioxide gas is released from
the remaining beer into the empty
space in the keg. To prevent this you
need a carbon dioxide cylinder which
fills the space in the keg with carbon
dioxide gas. If there is not enough car
bon dioxide in the empty space inside
the keg, then carbon dioxide from the
beer itself will escape into that empty
space. If there is too much carbon
dioxide, then too much gas will get
into the beer", explains Hans Schutt.
In both cases the consequence is
that the beer dispensed can hardly be
served. The foam is of poor quality;
too much carbon dioxide gives the
beer a harsh, astringent taste. If there
is too little foam, then the result is a
'flat' beer with what Mr Schutt descri
bes as "the apple juice effect". The
right equilibrium pressure is depen
dent on the temperature of the keg.
The higher that temperature, the
higher the pressure that has to be
applied.
For Heineken lagers the following
rule of thumb should be applied:
measure the temperature in degrees
Celsius in the keg storage room. Then
divide that figure by ten and you have
the ideal pressure for the carbon
dioxide cylinder. If the ambient tem
perature is 15°C, therefore, the pres
sure must be 1.5 bar, at 20°C it is 2
bar, etc.
Conditions
This general rule always applies,
provided that a number of conditions
are met. A keg which has just been
rolled off the truck and into the cellar
must never be coupled up straight
away. Not only is the beer still 'over-
lively' because of the transport, but
the keg must also get acclimatised, or
be given time to reach the ambient
temperature. "The keg must be left
standing for 24 hours before it can be
connected up. Besides, the place
where the keg is kept must have as
stable a temperature as possible.
Wide fluctuations in temperature
mean that the pressure keeps on
changing and that's not good", says
Hans Schutt.
In addition, the bar owner must
pay close attention to the temperature
of the surface on which the keg
stands. The cold from the ground may
have an influence on the temperature
inside the keg, in which case the rule
of thumb will no longer apply. The
penetration of cold from below can be
prevented by placing the keg on a
wooden pallet.
Often, the bar owner sets the pres
sure of the carbon dioxide cylinder at
too low a level because he thinks that
the beer would otherwise flow too
quickly from the dispenser tap.
Particularly the rather unexperienced
dispenser operators like the beer to
flow a little more slowly from the tap.
The result is a loss of carbon dioxide
and a flat beer.
According to Hans Schutt, there is
a simple trick that can be used to find
out whether the pressure is too low.
In the beer line no carbon dioxide
bubbles must be visible. If bubbles
can be seen there, then the pressure
of the carbon dioxide cylinder defini
tely needs to be increased.
That must be done quickly, as the
bar owner who can make one keg last
several days will notice after only one
day that too much carbon dioxide has
escaped from the beer into the empty
space. The outcome: the remaining
beer will not contain enough carbon
dioxide to ensure that a beer with a
good quality head can be dispensed.
THE WORLD OF HEINEKEN
