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