On 31 Mar 2002 11:40:54 -0800, rgregoryclark at yahoo.com (Robert Clark)
wrote:
> This spore retains 10% viability even after heating for 2.5 hours at
>150 C.
So, that would be a 150 minute D-value at 150C. It would be even more
interesting to know the z-value because then you could predict the
survival time at 600C and 1000C, there *will* be some sort of time
value, you might need 10^9 to measure it however.
>Then much higher survival would be expected at only 5 to 15
>minutes.
Well, sure witha D-value of 150, this is a merely a first order
reaction. Of course there is an initial non-linear bump, explained by
A. Douglas King's work at the USDA.
>Given the much higher survivability of this spore than other
>bacteria it would be expected some fraction could survive exposure 370
>C for several minutes.
Sure, that merely depends on initial counts. That is my initial
point, the D-value is more important than the temperature.
> In the refs. I cited in the post there was still an unresolved
>question of how the spores were able to survive these high
>temperatures.
I guess they have not heard of D-value -- it is only about 120 years
old (Hunt Brothers meat canning validation used to convince the US
government safe commercial canning could take place. I was a senior
microbiologist at Hunt's).
>One possibility could be the unusual honeycombed coat
>possessed by the spore.
The standard explanation is just that we have a thermodynamic
first-order reaction. The interesting thing is the growth curve:
1) Lag,
2) log-growth,
3) stationary, then
4) log-death phase.
Bacteria live and die logarithmically. In fact, the log death happens
with thing other than heat. Cosmetic (chemical) preservatives also
kill with a characteristically linear logarithmic decline. There are
some rather interesting causes for exceptions to this, however.
>If the interior of the spore is encased in
>this coat shaped somewhat like a buckyball then heat might be
>communicated to the interior only through the thin sides of the coat,
Interesting, but there is no experimental data supporting this as far
as I know. When we did spore clumping experiments and looked at SEMs
of the spore coat destruction, there was no significant effect on the
D-values from this factor. The IR radiation seemed to penetrate
straight through even the biggest clumps we tested. The inner cells
did not have a lower probability to die over the ones on the outside
of the clump, as far as we could tell. This may not have been good
enough method to tell, however.
>thus limiting the amount of heat communicated to the interior.
> I'm thinking here of the case for example of space shuttle tiles that
>can be held at the edges while white hot at the interior.
Ok, that is dry. We tested most things wet, that is a big difference
-- deep sea vents are presumably wet in this ng thread. Dry heat
always takes longer. And, we had this discussion a few months ago on
heating anthrax spores with microwaves -- heat-loss calculations are
needed. It is amazing how much energy is needed to heat something up
when it is "dry" and not touching anything else.
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