Pasteurization, as distinct from sterilization, requires that the temperature of the product concerned be raised to such a point and held for such length of time that the vegetative growth of infecting micro-organisms is killed. It is possible for the spores of micro-organisms to resist pasteurizing conditions and remain viable, although weakened, and under favorable conditions initiate new growth. Sterilization implies the destruction of both vegetative growth and spores but requires the employment of higher temperature and/or a longer holding time.
In order to pasteurize a particular product it is necessary to hold the product at a specific temperature for a specific period of time. This time/temperature relationship is dependent upon the nature of the product and the particular organisms concerned. In general, that time/temperature relationship employed is that required to kill the most resistant organism. Rough beer with a high yeast content requires a longer holding time than a bright filtered beer at a given temperature, since there are more cells to be rendered non-viable. A temperature of 74oC (165oF) for a period of 20 seconds is generally used, but different temperatures and holding periods are often employed in the light of experience with a given beer.
In order to keep the carbon dioxide in beer in solution, a pressure must be applied to the beer. For example, a beer containing 4.7 g/litre (2.5 vol) carbon dioxide must be held at 1.52 bar (22 psi) when its temperature is 15oC. At 0oC the equilibrium pressure is 0.48 bar and at 74oC the equilibrium pressure is 7.03 bar (102 psi).
The plate pasteurizer design ensures that the beer is subject to a pressure in excess of its equilibrium pressure throughout the temperature range. There are three very important reasons for this:
- It has been found that if carbon dioxide is allowed to come out of solution while the beer is at pasteurizing temperature, that a haze will form when the beer returns to normal temperature, even though the carbon dioxide goes back into solution.
- Free gas collecting in the holding tube shortens the holding time. Thus it is possible to process at a sufficiently high temperature but not effectively pasteurize the beer, because carbon dioxide resulting for insufficient holding pressure has reduced the volume required to give the correct holding time.
- Below equilibrium pressure a release of carbon dioxide occurs and fob is formed. Fob flows very slowly and is difficult to remove once it has formed.