Appendix I Background Information - Pentair HAFFMANS RPU-352 Instruction Manual

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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.
RPU 352, RPU 353, RPC 80, RPC 50
HAFFMANS RPU 352, RPU 353, RPC 80 RPC 50
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