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Topic: FROTH:: Whence the air? (6 msgs / 98 lines)
1) From: Art Lawson
You need to look into Venturi effect, I believe.

2) From: Michael Horowitz
Assume no magic frothing attachment.
Steam is water vapor. It changes to water when it hits the colder
milk.
Where does the air come from that causes the froth?
Logic tells me that when the nozzle is brought to just below the
surface, somehow I start to inject air. Can anyone explain the process
or point me to a reference?
Frothless in Falls Church - Mike.
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3) From: Michael Horowitz
Exactly my thinking. Now, the trick is how to position the nozzle to
achieve that.
All the instructions I"m reading say "place the nozzle just below the
surface", but I don't see how that will pull air into the milk.
What are you doing up so early? - Mike
At 01:10 AM 12/14/02 -0700, you wrote:
<Snip>
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4) From: Dan Bollinger
Air is sucked past the tip into the high velocity steam jet.  Frothing is
more effective on cold milk, so make sure you concentrate on frothing first,
heating last. Put the tip in the milk, and slowly lower the pitcher until
you hear a sucking sound or the sound of cloth tearing.  Keep it at that
level as the milk rises in the pitcher.  It should double in volume.  Dan
<Snip>
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5) From: Andrew J. Lynn
The best instructions I've read say to start with the tip just below the 
surface, then lower the pitcher until the nozzle is just above the 
surface, where you start to hear the characteristic sucking noise. 
 There, air is getting pulled into the steam jet.  After you get some 
foam going you can finesse it to get microfoam, etc.
Andy Lynn
eagerly awaiting my new check valve, cause the steamed milk just isn't 
the same if you can't make the espresso to go with it
Michael Horowitz wrote:
<Snip>
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6) From: David Lewis
At 3:13 AM -0500 12/14/02, Michael Horowitz wrote:
<Snip>
Interesting question. First, as another poster mentioned, there's the 
Venturi effect: the high-speed jet causes a lower pressure at its 
outer edge that pulls in air. The second is that the volume of steam 
is about 700 times the volume of the water it's composed of. So when 
the steam condenses in the cold milk, it's going to pull a vacuum 
from the surface. I don't know what the relative contributions are.
Best,
	David
-- 
"To announce that there must be no criticism of the President, or 
that we are to stand by the President right or wrong, is not only 
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