Honeywell Minitrend QX, User Manual

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43-TV-25-30 Iss.6 GLO Aug 07 UK 297
Appendix G: F sub zero Sterilisation
The significance of F
0
The F
0
value is used in the pharmaceutical and related industries in the sterilisation of items.
A brief summary is included here to give the essence of the meaning of the terms used.
When items are subjected to sterilisation by heating, the rate at which micro-organisms are
killed is dependent on the temperature.
Traditionally items were sterilised by holding them at 250 ºF (= 121.11 ºC), and the F
0
value
for a sterilisation is simply the equivalent time at this temperature that would produce the
same effect. For many "average" micro-organisms each minute at 121.11 ºC reduces the
number present by a factor of 10, so a 15-minute hold at this temperature would reduce the
number by a factor of 10
15
.
If it were possible to heat a sample from ambient to 121.11 ºC in 1 second, hold it there for
15 minutes, and then cool it back to ambient in 1 second, the temperature / time profile
would look something like the trace in
Figure 1 on page 297 and the F
0
value would be
15, representing 15 minutes at 121.11 ºC.
In practice, of course, to reach this temperature the object would have to be subject to an
initial heating period up to that temperature, and later a cooling period back down again to
ambient temperature, more like the profile shown in
Figure 2 on page 297 . During all
this heating up, holding at temperature, and subsequent cooling, micro-organisms are being
killed at different rates, and the F
0
value is calculated by summing the effect at each tem-
perature (provided it is over some defined starting temperature - see below).
This calculation is performed by using the expression, where T is the temperature in ºC and
Δ
t is the time spent at that temperature. The z factor that appears in this equation is the tem-
perature coefficient for the destruction of micro-organisms, and is the increase in tempera-
ture, in ºC, that produces a 10-fold increase in sterilisation rate.
This summation is only performed once the sample temperature has reached some speci-
fied starting temperature. For example in
Figure 2 on page 297 if this starting tempera-
ture has been set to 100 ºC (a common value) then the F
0
summation would begin at t = 8
minutes when the temperature first reaches this value.
Figure 1
Figure 2