The secrets of the moka pot - How to video

Coffee preparation techniques besides espresso like pourover.
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naked-portafilter
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Postby naked-portafilter » May 16, 2014, 2:42 pm

Hi There,

I'd like to invite you to discuss the moka pot. I'm experimenting with it some months ago.

This is our new youtube video about this brewing method and about my measurements.



Would be happy to read your thoughts, comments, critics, questions. Of course it would be great to read them below the youtube video but here as well. Don't hold anything back!

Thank you!

Gábor

jpender

Postby jpender » May 16, 2014, 3:16 pm

Very well done! It's great to see a technical review of the lowly moka pot.

A question: Do you think the relative internal dimensions of the Brikka are comparable to those of standard moka pots like the Moka Express?

I have also probed my own moka pot (an old stainless 3-4 cup model). Here's one example (using cold water) where it is very apparent when the water first hits the temperature probe that's in the center of the grounds:

Image


Here is another one from a paper by Navarini et al (2008), showing pressure, flow and bottom tank water temperature. They used a 3 cup Bialetti Moka Express:

Image

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naked-portafilter
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Postby naked-portafilter » May 16, 2014, 3:39 pm

Hi John,

Great measurements! I thought I was the first one who got a sensor in the basket :-). O.k. At least the first video about this this really interesting topic.

I think the moka express has the same dimensions as the brikka. There is a 2 cups and a 4 cups version. The 2 cups version is the ideal size I think for this brewing method.

"lowly moka pot"

I think it's a simple but great brewing method with predictible temperatures.

Kind regards

Gábor

samuellaw178
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Postby samuellaw178 » May 16, 2014, 8:35 pm

Nice experiment! For the longest time I have wondered about the temperature in Moka pot and how to control it delibrately.

I supposed the temp sensor are correctly calibrated right? The maximum temperature you get in the coffee cake seems to be at 124 Celsius for Brikka and 118 Celsius for Moka(preheated). Water doesn't exceed 100C unless the pressure is higher than 1atm**. Which means, indirectly, your experiment indicates that the pressure in Brikka goes up to 2.3 bar and 1.9 bar for the Moka... very enlightening...

**http://www.engineeringtoolbox.com/saturated-steam-properties-d_457.html

And it's a surprise that the coffee doesn't taste burnt when brewed at that high temperature (even if the brew period is short).. :shock:

jpender

Postby jpender » May 16, 2014, 9:39 pm

samuellaw178 wrote:The maximum temperature you get in the coffee cake seems to be at 124 Celsius for Brikka and 118 Celsius for Moka(preheated). Water doesn't exceed 100C unless the pressure is higher than 1atm**. Which means, indirectly, your experiment indicates that the pressure in Brikka goes up to 2.3 bar and 1.9 bar for the Moka... very enlightening...

**http://www.engineeringtoolbox.com/saturated-steam-properties-d_457.html

It doesn't work quite that way because there is also air inside the moka pot. The pressure is the sum of the water vapor and air partial pressures. In the espresso world this goes by the name "false pressure". During the brew cycle the water vapor pressure keeps rising and the air pressure drops, but never to zero. This is the reason H. Barista said in his video that initial water volume has an impact on water temperature.

The other thing is that the steam tables assume liquid-vapor equilibrium. In the moka pot the temperature in the head space is significantly lower than in the water. I measured differences of 10-15°C between water and vapor; Navarini shows similar values. It turns out that the vapor temperature is a better indicator of the pressure when using the steam tables. This by itself would lower the pressure estimate by at least 0.6 bar. But taking into account the air pushes it back up part of the way. Navarini measured about 1.5 bar gauge pressure in his experiments.

It's a very interesting system from a thermodynamic point of view.

EDIT:
When the pot sputters (Navarini called this the strombolian phase) the water, vapor and grounds temperatures converge. So at the end the water temperature actually is a good indicator of vapor pressure. Here is Navarini's plot of measured pressure with calculated air and vapor pressures superimposed. The green line is liquid flowed.

Image

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naked-portafilter
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Postby naked-portafilter » May 17, 2014, 1:35 am

Thanks John! Great explanation. It's a good idea to measure the vapor. Later. I need some peace now :-).

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Bob_McBob

Postby Bob_McBob » May 20, 2014, 1:18 pm

James Hoffmann posted your video on his blog :o

http://www.jimseven.com/2014/05/20/video-back-moka-pot/
Chris

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canuckcoffeeguy

Postby canuckcoffeeguy » May 20, 2014, 2:29 pm

Great video, and love the nod to BTTF!

As a fellow Brikka(2 cup) owner, I appreciate your analysis of its inner workings and effort to demystify its brewing. It's no small feat getting a shout-out from James Hoffmann. And while I'll never give up drinking espresso -- my magic elixir of choice -- time and time again I pull out my trusty Brikka for a magical brew. It makes a great cup, and has a loyal following, as evidenced by this long and storied CG thread:
http://coffeegeek.com/forums/coffee/machines/220503

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naked-portafilter
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Postby naked-portafilter » May 20, 2014, 3:30 pm

Hi Ryan,

Thanks for your feed back and for the info to open James Hoffmann's blog :-).

Kind Regards!

Gábor

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naked-portafilter
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Postby naked-portafilter » May 20, 2014, 3:47 pm

Sorry Chris! As I saw the last post from Ryan I opened instantly the blog. Thank you for the hint. Kind words from James.

Gábor