HAFFMANS RPU 352, RPU 353, RPC 80 RPC 50
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RPU 352, RPU 353, RPC 80, RPC 50 Page 97 of 112
APPENDIX I BACKGROUND INFORMATION
PU calculation
The Pasteurisation Unit (PU) is defined as relating to the sterilising effect
observed when the product is held for one minute at a temperature termed the
Base value. At this temperature therefore, 1 PU per minute is achieved.
Experiments on various mixtures of the common brewery biological contaminants
showed that at temperatures over about 50°C there i s an approximately ten-fold
increase in sterilising effect for every 7°C increa se in temperature. For example,
if the time required to kill a population of micro-organisms at 60°C is found to be 5
minutes then if the temperature were to be increased to 67°C the time required
would be only 0.5 minutes. The increase in temperature required to produce a
ten-fold increase in kill rate is termed the Z value. The Z value in this case is
therefore 7°C.
For beer the PU is normally defined by a Base value of 60°C and a Z value of
7°C. The same definition is often used for soft dr inks and other beverages.
Where there is any possibility of spore-forming bacterial contamination being
present (for example in tomato juice) very much higher temperatures are required
for pasteurisation. In these situations the definition of Base value 80°C and Z
value 10°C is often used. Other definitions are a lso in use outside the brewing
industry.
At product temperatures below about 50°C it is doub tful that any significant
sterilising 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 PU’s are added to the total calculated by a PU monitor. This is termed the PU
cut-off temperature.
Set the PU cut-off temperature at least 5°C lower t han the planned product
temperature in the holding zone of the pasteuriser. Higher values will give a very
conservative figure for PU achieved and will reduce the repeatability of the result
because the product temperature will be changing only slowly as it crosses the
cut-off temperature threshold. For beer the cut-off is not normally set below
50°C.
The next three screens allow you to adjust the PU calculation which will apply to
both temperature channels.
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