Comments on: Further Ruminations on Filter Grind

By: Chris

Chris — Wed, 14 Oct 2009 12:16:27 +0000

The interesting thing is what goes on inside a particle of coffee. Assuming transport is via small holes or pores that get bigger with extraction you could argue that the center of larger particles dont actually feel pressure effects. Once you get beyond a certain depth (particle size) the most likely scenario is that the only thing which effects extraction is time.

I actually look at a very similar process in mining as my main job so would love to find out more info on coffee leaching – but the general principle is that there are 2 competing issues on what leaches and how fast.

The first is chemical based- as in what it takes to liberate each soluble species in the coffee grain (which includes things like solubility and rate of dissolving, and probably pressure too – and crucially this will be species dependent- i.e different species dissolve at different speeds)

The second is diffusion based – the time taken for water to get into the particle and for the dissolved species to travel out. For small particles diffusion is virtually nothing, for bigger particles it dominates.

By: David

David — Tue, 14 Jul 2009 12:58:22 +0000

I’m afraid I don’t have even an approximation of the pressure, unfortunately no datasheet accompanied the aspirator.

I would debate that there is no difference between pushing and pulling.

In the case of a vacuum the grinds remain under atmospheric pressure.

With the aeropress they are subjected to an increased pressure – which has an impact on the rate and nature of extraction (obviously join the dots to espresso).

So I wouldn’t say there is no difference.

By: Stribb

Stribb — Tue, 14 Jul 2009 12:43:51 +0000

I don’t think this is a different technique in principle from aeropress, though it’s got to have a higher pressure differential. There’s no difference to the end product between pushing the coffee through by high pressure and pulling it through with a partial vacuum.

I was entertained to see the coffee boiling in the low vacuum. Do you know (even approximately) what pressure was in the bottom chamber?

By: Colin Harmon

Colin Harmon — Tue, 14 Jul 2009 09:16:18 +0000

First up, thats brilliant.
I recently pondered the same question because my attitude towards fines is “if you can’t beat them,join them”. I was also asked recently why coffee is brewed for 4 minutes. These two questions made me come to the conclusion that;
Coffee was originally ground coarser because of the sludgy nature of brewed coffee when it was ground finer and a 4 minute brew time was the result of this, not an intended aim.
Coffee loses more flavour to the burrs and through oxidation once ground. One must bare in mind that the vast majority of places do not grind fresh.
I think these two factors perhaps set the coarse grind standard.
Its also interesting to note that the available technology at the moment is pointing everyone away from modern updosing methods and back to a proper extraction at a lower dose. Are we merely rediscovering what others discovered before? (another example of a short brew time and fine grind would be the ibrik, the ooldest and most prevalent brewer perhaps?)
Great article, great vid, we need more information!

By: Stephen Leighton

Stephen Leighton — Tue, 14 Jul 2009 00:20:05 +0000

I so need time to get my head around this but I hate you in so many ways for making me think about stuff I thought I already knew. Its like telling someone oxygen inst what we breath , discuss.