On the 30/6/2003, Podargus wrote:
On the Science Show
on Saturday, and of course as I write, high pressure for killing micro
organisms in food was/is a major topic.
It became the
topic at our face to face Science Discussion Group today, but although
this group has a few members who could be expected to have some idea of
why the high pressure would kill the micro organisms. Only a few
guesses were offered.
But of course
someone here will know;-)
David
Maddern replied:
It
was interesting to hear what he said about oysters. The toxins
produced by microalgae would not be destroyed, nor viruses.
Science
Show.. ABC Saturday 28 June 12.10pm
Having
said that I would be confident that the oysters he was talking about
(South Australian, from OYSA) would have been good, as no record exists
of people getting sick from commercially grown oysters in SA (as
opposed to NSW) and there is an aggressive water quality monitoring
program for toxic Microalgae and pristine lovely coasts.
Podargus
responded:
Google
did the job. It appears from the URL below that it is the rise in
temperature that does the job. Flash heating and cooling?
That is assuming I have read it correctly.
http://www.siu.edu/~foodsafe/PDEHP.html
David Maddern replied:
I
cant see how temperature would be the only thing.
That
Google reference is a research proposal
if
it were temperature then oysters would have to be cooked to kill spores
After
6 minutes in the chamber they aren't, the proteins aren't coagulated
So
there must be another mechanism
They
said on that Science Show interview that they slowly rose the pressure
and then quite quickly release it. I know from other sources that
they go up to 40,000 lbs./sq. inch (hydraulics go up to about 10,000
lbs./sq. inch)
They
also said that vodka perfuses an orange
One
possible mechanism is that gasses dissolve in the tissues to an
increasing extent during the ramp up, and then bubble out to a non
bubbly state back at 1 atmosphere
That
would rupture bacterial cells
How
does that sound?
David Allen commented:
It sounds good if you
can explain why it doesn't rupture all the oysters cells as well.
Podargus
replied:
This
URL gives some idea, and more or less discounts the (flash)
heat/cooling from pressure.
http://www.cfsan.fda.gov/%7Edms/a2z-uvw.html
How
It Works: Using specially-designed equipment, packaged food is exposed
to 50,000 to 100,000 psi (pounds per square inch) of pressure for a
short time. The ultra high pressure interferes with the metabolism and
structure of bacteria and destroys these living cells, but does not
crush the food.
As
long as the food is mostly air-free and contains water, hydrostatic
pressure doesn't crush food because the water in the food protects it
from physical damage. However, living bacteria are destroyed by the
effects of high pressure on their cellular functions.
Donald Lang
added:
Rude intrusion:
I like the discussion but it sounds suspicously like the theory that
grass is green because the atoms of grass are green. Bacteria are
killed by pressure because their internal workings don't function under
pressure.
I might look
forward to the next round in which it is explained that the internal
workings of bacteria are incompatible with high pressure because
bacteria never learnt to live under high pressure.
OK, a sentence
or two more and then you can shoot me... Not too long ago a few people
outside the correct set of disciplines were suggesting that life
evolved way down deep, because bacteria had been found with the ability
to live under very high temperatures and what might be called very
higher pressures.
So I will tune
in next week...
And I may keep
my head down...
