On Sun, 31 Mar 2002 02:48:53 GMT, bbruner at uclink4.berkeley.edu (Bob)
wrote:
>As an example of a concern... how do we know the T to which the prion
>material was actually exposed? Poor heat transfer in some "corner" of
>the apparatus, and, voila, survival. If I recall, the survival of
>infectivity at 600 deg was low, so it wouldn't take much of a cool
>spot in the apparatus.
>>>bob
That is a very good point. While their method used a 10x10x23cm
furnace surrounding the covered crucibles (which do not have a totally
flat surface with quick T-equilibrium), the crucibles size was not
stated. The one gram sample seems small, but how thick? The sample
at the furnace temp. of 600C was flaming it was so hot, . . . but
without micro- or nano-TCs or something else measuring the temp., how
do we know the temperature was evenly distributed? They *did have*
one TC in an identical crucible, but was that crucible dry and empty?
If so, that would invalidate their whole experiment, in my book.
They admit a pre-heating treatment to 100C (plus or minus 10C, and
that seems too wide to me), but do not say how long that time was, yet
a 1-2 minute heat-up was stated for the test runs to experimental
temperatures? How can the sample reach that so fast? Then, after the
hold time, they cold-shock with dry-ice for "less than 30 seconds."
That sounds like a lot of cold-shock.
Now, I would intuitively think the F sub-zero might start before 100C
and certainly at 100C that time should have been controlled and
recorded and added to the F sub-zero calculus integration. This is
not hard, in fact, my old heat processing research system (1991) did
this automatically. But, this was not taken into account by Brown et
al., as far as I know.
Carbon was previously shown to protect in a steam autoclave ( not dry
heat, but I will give them that one since I have seen dry-heat
protection before), but is this protection simply insulation from
heat? Think of the heat-penetration rate through a wall of carbon
atoms, (again, how think?).
I admit, that 15 minutes intuitively should be enough for heat
transfer, but I have been very surprised before, so I want to see the
actual measurements, not just one TC in a dry crucible. I want at
least one TC in the actual crucible with an equivalent (may be not
prion infected, or you would have to destroy the TCs) tissue sample.
Compare my article, (with King and Torok, of the USDA): "Talaromyces
trachyspermus, A Heat-Resistant Mold Isolated From Fruit Juice" J.
Food Protection 56:1039-1042 (December 1993). If anything, I over
estimated the heating, in the initial isolation step. King and Torok
used a more accurate method (King and Whitehand (1990) for the heat
resistance work after isolation. They also used 15 minutes as the
test time, my initial isolation was 30 minutes at hold temp. with a
TC in an actual juice sample.
I know I have seen that spores at 340C survive for a time -- But,
again, I object, because, the D-value is more important that just
temperature. Without the D-value and a validated heat distribution
profile say, via TCs) of the apparatus, the data is definitely
suspect.
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