Strange Combination of Questions: D-value

enigl at enigl at
Fri Nov 22 13:15:02 EST 1996

In article <199611202220.RAA11276 at>,
yersinia at GATE.CYBERNEX.NET (Yersinia) writes:

>2. This one is about D-values and spore strips. If you have, a spore 
>strip with, as an example, a D-value of one minute and 10^4>>

Sic: 10^5 is 100,000

> (100,000) 
>bugs, therefore killing 90,000 bugs in that one minute, the next minute 
>will only kill 90% of the remaining 90,000 - since 90% of 90,000 is less 
>than 90,000, why don't *all* the bugs die in the next minute, while one 
>minute was sufficient to kill the first 90,000?

Because the bacteria not only _grow_ logarithmically but they also _die_

Therefore, with a D-value of 1 minute:

1 log reduction: 10^5 = 1 minute
2 Log reduction: 10^4 = 2 minutes
3 log reduction: 10^3 = 3 minutes
4 log reduction: 10^2 = 4 minutes
5 log reduction: 10^1 = 5 minutes
6 log reduction: 10^0 (which will still one survivor) = 6 minutes
7 log reduction: <1 spore survivor per strip = 6 minutes 
(NOTE: If you have more than one spore strip placed in the same spot
inside your autoclave you STILL can get  survivors).
8 log reduction: <1 survivor per 10 strips  = 7 minutes
9 log reduction: <1 per 100 strips = 8 minutes
10 log reduction: <1 per 1,000 strips = 9 minutes
11 log reduction: <1 per 10,000 strips = 10 minutes

My autoclave is set up for an 11 log reduction of _B_. stearothermophilus_
spores by the overkill method of calculation.  That gives me a sterility
assurance factor that is acceptable and yet does not destroy the products
I need to sterilize.  

When I need to be more exact I use the internal temperature probe and
place it where our autoclave validation has found to be the coldest spot
and use the F_0 (F sub zero) method of kill probability at 121 C based on
the Equivalent Kill at 121 C integration during the entire cycle of
heatup, hold and cooldown.  This reduces the time of sterilization and
still maintains the correct acceptable sterility assurance (the F_0 method
of calculation.)

How can I say this nicely?. . . Your University should have an overhaul of
its microbiology courses if they didn't teach the above facts.   Basic
microbiology texts should also teach this.  These are questions
microbiologists should be taught.  Good questions.


Davin C. Enigl, MS-MEAS, President-Microbiologist
HACCP Validations-sm  Hazard Analysis and Critical Control Points for the
Food, Cosmetic, Pharmaceutical, and Nutritional Supplement Industry
Voice: (916) 989-8264
Web site:

November 22, 1996
7:33 am

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