HomeRoast Digest


Topic: remaining volume in evacuated containers. (3 msgs / 109 lines)
1) From: Kevin DuPre
Actually, it's not the O2 molecules left behind - if
you're pulling a vacuum there is nothing left but
negative pressure with respect to the gas pressure
within the beans. While it is true that some O2 will
be left behind, after pulling a vacuum it is miniscule
compared to the volume of CO2 that will result from
outgassing. It's the volume of that negative pressure
that determines how LONG the pressure differential
exists between the beans and the evacuated void in the
container.
In a perfectly sealed RIGID container with a vacuum
drawn on it and an outgassing material inside of it,
and a gauge reading the vacuum, you would observe over
time that the vacuum reading in a few hours or days
would not be the same as when you left it.  
This is because the gas (in the beans' case, CO2) is
leaving the interior of the beans in an attempt to
equalize the pressure between them and the voided
container. Smaller volume containers will reach this
equalization sooner than larger ones thereby reducing
the rate of flow of the outgassing of the beans.
It's an exponential relationship - as the beans outgas
into the vacuumed space, the pressure differential
decreases, decreasing the rate of degassing or
outgassing material. It stands then that the smaller
evacuated volume will continue degassing at a slower
rate than the larger one.
You don't want to degas completely because as already
discussed, once the protective envelope of CO2 is gone
the game is over. 
You'd be better off as someone suggested to fill the
remaining volume with something inert if you weren't
going to fill it with beans.  Glass is one of the
lowest outgassing materials to fill such volume - I 
haven't seen the effects of glass outgassing until you
start pulling down to 0.1 micron (which is what we
called perfect vacuum, from a scientific standpoint).
You won't even come close with a foodsaver.
My suggestion is to fill the bottom of the canister
with marbles, put a metal divider that comes very
close to the sidewalls of the canister, and then pour
your beans on top of that.  The easier route is to use
a smaller canister if possible.
<Snip>
inevitably leave 
many
<Snip>
canisters, the 
one with
<Snip>
number of O2 
molecules.
<Snip>
=====
--
Kevin DuPre
obxwindsurfhttp://www.virtualfusion.com"The real voyage of discovery consists not in seeking new landscapes but in having new eyes -- Marcel Proust"
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2) From: EskWIRED
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<Snip>
The easiest route is to use the bags, which are the smallest container
possible. They tightly conform to the beans and pack them together as tight
as a brick.
<Snip>
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3) From: Rick Farris
EskWired wrote:
<Snip>
When sealing roasted coffee, they don't stay tight very long.  I roast 75g
batches (1/6 lb) and I seal them in an 8" wide by 6" long section of bag.
That is big enough that I can smooth out the beans to be about one layer
deep.  It's also big enough that even after a week, or so, the outgassing
doesn't create a positive pressure in the bag.
It *does* blow the bags up, but there is still some room to expand, so the
beans are essentially at room pressure.
And personally, I think that a bag full of CO2 is the perfect way to store
beans.  Maybe not quite as good as nitrogen, but a lot easier to come by.
If you were to make the smallest possible package and then seal it, then I
think you would quickly fill the bag and create a positive pressure.  At
that point you would be inhibiting the outgassing, and I don't think that is
a good thing.
-- Rick
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