9
PU CALCULATION SETUP
This section covers the selection of PU calculation constants and of the PU Cut-off
temperature.
Pasteurisation units
Experiments on mixed populations of the common brewery contaminants have
shown that at temperatures over about 50ºC there is a ten-fold increase in kill rate for
every 7ºC increase in temperature. For example, if the time required to kill a
population of contaminant cells at 60ºC is found to be 5 minutes then if the
temperature were to be increased to 67ºC the time required would be only 0.5
minutes. The Z Value is defined as the increase in temperature required to produce
a ten-fold increase in kill rate. The Z Value in this case is therefore 7ºC.
For beer the Pasteurisation Unit (PU) has been defined (arbitrarily) as the biological
destruction observed when beer is held for one minute at a temperature of 60ºC.
This temperature, where 1 PU per minute is achieved, is known as the Base Value:
60ºC in this case. The same definition of the PU is often used for soft drinks and
other beverages but, where there is a possibility of spore-forming bacterial
contamination being present, very much higher temperatures are required for
pasteurisation and so other definitions are in use outside the brewing industry.
Experiments have shown that Pasteurisation Units are additive in their effect in
situations where the temperature is changing such as in the tunnel pasteurisation
process.
If you select S
TANDARD
PU C
ALCULATION
then the Base Value will be set to 60ºC and
the Z Value to 7ºC. (If the temperature units have been set to Fahrenheit then the
Base value will be set to 140 ºF and the Z Value to 12.5 ºF.) These are the normal
settings for beer production.
If you select C
USTOM
PU C
ALCULATION
then you will be able to set up any Base Value
and Z Value that you require. Settings will depend on conventional practice in
various industries.
The PU calculation that you choose will apply to all PU monitors that use this RPC-
80 playback unit.
PU cut-off temperature
At temperatures below about 50ºC it is doubtful that any significant pasteurising
effect is achieved even though from a strictly mathematical point of view the correct
PU total could still be obtained, given enough time in the pasteuriser. For this reason
is it desirable to set a temperature level below which no PUs are added to the total.
This is known as the PU Cut-off temperature.
The PU Cut-off temperature is initially set to 50ºC (122 ºF) but you can set any
temperature that you require. Higher settings will give a more conservative picture of
total PU achieved.
The Cut-off temperature that you choose will apply to all PU monitors that use this
RPC-80.