The other adult beverage.....

On Jan 10, 3:39*pm, Steve wrote:
On Thu, 10 Jan 2008 12:35:30 -0800 (PST), wrote:
Even a stove-top "machine" (Bialetti Moka Express style) like I use?

"Moka" pots operate under pressure, so the boiling temp is reduced.

So this phrase is incorrect?

http://en.wikipedia.org/wiki/Moka_Ex...s._drip_coffee

Or does the steam pressure that is created simply push the not-
quite-100C water up through the grounds? (as described on
coffeegeek.com)

Man, you go on more about coffee than Myron does about whisky! :-)

B

On Jan 10, 1:58 pm, Steve wrote:
So this phrase is incorrect?

http://en.wikipedia.org/wiki/Moka_Ex...s._drip_coffee

If you mean Wikipedia's claim that "Due to the higher pressures
involved, the mixture of water and steam reaches temperatures well
above 100 °C" then yes, that claim is incorrect.

I know nothing about espresso and other high-falutin' coffee machines,
but I do remember something from chemistry class -- if they really
operate at higher than atmospheric pressure, then the claim is
absolutely correct. Higher pressure corresponds to higher boiling
point. If the machine brings the water to boil under higher pressure,
then higher temps are being used.

Jon.
Doing my part to drag civilization down :-)

Surfing a few espresso and "moka" explanations, it does sound like
espresso machines operate at below-boiling-point temperatures --
around 90C is claimed to be optimal, _well_ below boiling at the very
high pressures involved. Most web sites get it right, that _water_ is
being forced through the grounds, but some mistakenly claim it is
steam.

Plenty of sites describe the "moka" contraptions as operating at
higher temps, and given that the water is reaching its boiling point
at slightly higher than atmospheric pressure, they must be. So the
wikipedia explanation for moka machines must be correct.

The claims of espresso sites that temps much higher than 90C ruin the
brewing process seem to contradict the moka brewing process -- the
only explanation I see is that the chemistry changes quite a bit at
the espresso pressures. Moka pressures are only slighty above
atmospheric.

Jon.
Not replacing my Mr. Coffee anytime soon...

"Steve" wrote in message
ews.com...
On Fri, 11 Jan 2008 08:04:41 -0800 (PST), wrote:

Moka pressures are only slighty above
atmospheric.

It's not steam pressure that forces the water through the grounds but
air pressure. Air will expand at temps far lower than 212 degrees.

Air will expand as soon as the temperature begins to increase. Assuming the
vessel is tightly sealed (as it must be for expanding air to force the water
to move) how do you keep all the water from being forced out before it gets
hot?

Wolfgang

On Jan 11, 10:10 am, Steve wrote:

It's not steam pressure that forces the water through the grounds but
air pressure. Air will expand at temps far lower than 212 degrees.

Now it's getting fun ;-) Let's say initial air temp is 20C, heated to
90C. That's 70C delta. Expansion should be proportional to temp,
relative to abs. zero. So each added degree would add 1/(273+20)
fraction of volume. My calculator says 70/293 is about .24, or 1/4. So
it would take four cups of air to push one cup water out by the time
90C is reached. Do those contraptions have that much air space? As
Wolfgang noted, this expansion doesn't wait until the water is 90C, it
happens all the way up the temp scale, so I'd think it'd be "better"
to wait and let the steam pressure push the water.

Jon.

wrote in message
...
On Jan 11, 10:10 am, Steve wrote:

It's not steam pressure that forces the water through the grounds but
air pressure. Air will expand at temps far lower than 212 degrees.

Now it's getting fun ;-) Let's say initial air temp is 20C, heated to
90C. That's 70C delta. Expansion should be proportional to temp,
relative to abs. zero. So each added degree would add 1/(273+20)
fraction of volume. My calculator says 70/293 is about .24, or 1/4. So
it would take four cups of air to push one cup water out by the time
90C is reached. Do those contraptions have that much air space?

Yeah, that sounded a little fishy to me, too.

As
Wolfgang noted, this expansion doesn't wait until the water is 90C, it
happens all the way up the temp scale, so I'd think it'd be "better"
to wait and let the steam pressure push the water.

Agreed, bearing in mind that Wolfgang also noted that one doesn't need to
bring the full volume (or anything near that much) to boiling temp, and that
pressure is relieved by the water moving through and out of the system,
thereby mooting any supposed notable increase in the boiling temperature.

Wolfgang

"Steve" wrote in message
ews.com...
On Fri, 11 Jan 2008 07:35:10 -0800 (PST), wrote:

On Jan 10, 1:58 pm, Steve wrote:
So this phrase is incorrect?

http://en.wikipedia.org/wiki/Moka_Ex...s._drip_coffee

If you mean Wikipedia's claim that "Due to the higher pressures
involved, the mixture of water and steam reaches temperatures well
above 100 °C" then yes, that claim is incorrect.

I know nothing about espresso and other high-falutin' coffee machines,
but I do remember something from chemistry class -- if they really
operate at higher than atmospheric pressure, then the claim is
absolutely correct. Higher pressure corresponds to higher boiling
point. If the machine brings the water to boil under higher pressure,
then higher temps are being used.

Jon.
Doing my part to drag civilization down :-)

You're thinking pressure cooker.
Think percolator and you will be closer. The pressure is not that
high, certainly nowhere near the 9-10 bars of pressure for espresso.
Calling moka pots "stove top espresso makers" is a misnomer. They
can't make espresso. It's not a percolator, either.

Right. The important thing to keep in mind is that it is not necessary to
bring the entire contents of the vessel to boiling temperature. A
percolator works on the principle of boiling a small quantity of water and
using the pressure generated by creating steam in a small sub-compartment
within the vessel to push up a column of water which then spills over into
the basket holding the coffee. The water in that column, early in the
process, is far below boiling temperature. Throughout the process, the bulk
of the water is steadily rising in temperature but comes nowhere near the
boiling point. Moreover, as the container is not tightly sealed, there is
no build up of pressure except, briefly and intermittently, under the curved
plate resting on the bottom; consequently, the boiling temperature of the
water remains essentially at the usual for atmospheric pressure.

I'm not sure exactly how stove top espresso makers are configured, but it's
safe to assume that something like the same principal is at
work.....pressure from the steam generated by boiling a small portion of the
water forces the rest of the somewhat cooler water through the mechanism.

Wolfgang

On Jan 11, 6:12 pm, "Wolfgang" wrote:

I had a simple steam pressure machine. Worked well and I got good
coffee.

May be of interest;

http://www.cooklikeaprofessional.com/espresso.html

http://www.sallys-place.com/beverage...so_machine.htm

MC

"Steve" wrote in message
ews.com...
On Fri, 11 Jan 2008 09:34:48 -0800 (PST), Mike
wrote:

May be of interest;

http://www.cooklikeaprofessional.com/espresso.html

From their page:
"A pump espresso machine, on the other hand, has a boiler that heats
the water to 85-90 degrees - the ideal temperature for espresso"
Um, they did a booboo.

Hm.....

Celsius?

Wolfgang

"Steve" wrote in message
ews.com...
On Fri, 11 Jan 2008 12:03:22 -0600, "Wolfgang"
wrote:

Celsius?

Has to be, but the correct range is 88 - 96.

85-90......88-96

I may or may not ever have made espresso myself (there are entire days in my
life that I no longer remember with absolute clarity) but the difference
here appears to my admittedly inexperienced eye to be well within a
reasonable range for normal disagreement. Or is there some critical factor
I'm not aware of?

Wolfgang