Lethal Rate: can be calculated by
L(Tref,Z)=10
(T-TRef)/Z
Where
T= Temperature of item
being heated
T ref =
Reference Temperature
Z = Z value of
Challenge organism (10oC if unknown)
Lethal rate is an
exponential function; hence a small change in temperature difference can have a
significant effect on delivered lethality.
For a BI system witha
z value of 10oC and T ref -121 oC, decrease in
lethality by decrease in 1 oC temperature can be calculated as:
L=10 (120-121)/10= 10-0.1 =
0.79
It means for a BI with
a Z value 10 oC one minute at 120 oC is equivalent to
0.79 minute at 121 oC in terms of lethality.
Microbial destruction at any temperature (120 oC in example) can be related to equivalent
minutes at a reference temperature (T ref ; in example
121 oC).
Fphysical /F
value:
means the equivalent amount of time, in minutes at T ref, which has
been delivered to a product by the sterilization process".
F
value is a measurement of lethality of a process.
F value is the calculated equivalent of time in
terms of lethality at a reference temperature T ref and a temp coefficient
Z that is delivered to item being sterilized.
The F value is the
integration of the lethal rates throughout the process.
Fref =
d(∑L)
Where:
d= the time increment
between each temperature reading
L= Lethal rate
calculated for each temperature reading.
F0 Value: is the number of
equivalent minutes of steam sterilization at a temperature of 121 oC
delivered to a container or unit of product using Z value of 10 oC.
F0 of 8
minutes means sterilization effectiveness of that cycle is equivalent to 8.0
minute at 121 oC.
F Bio: The term F bio represents the delivered
lethality measured by the actual kill of microorganism or in a BI System
F bio = DT×LR
DT - D
value of the BI at reference temperature
LR- Log reduction of
BI population achieved during the cycle
______________________________________________
Article Reference: PDA Technical Report No.1 “Validation of moist heat sterilization process. Cycle design, development, qualification and ongoing control”
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