# The science of preinfusion (what hydraulic conductivity and saturation does for you)



## dr.chris (Nov 13, 2014)

I started posting some stuff on TMC because someone was talking about the effects of preinfusion with their new machine, and having written it wondered if it might be of interest here aswell.

Now I'll start off by saying that I am not an expert in what happens with coffee, other than being a home user for a few years. What I am is a consulting computational fluid dynamics engineer, and what I used to be was a university researcher developing models of minerals leaching for the mining industry. This doesnt mean I am right about this stuff, but I have a good background to talk about it.

OK, so to start with its worth saying that dissolution rates are always some variation of something which is initially very fast and then tails off slowly. Depending on what is dissolving (and other factors) you can get a fairly linear response for a while after the initial peak.

Dry material is more resistant to water flow than wet, so water will go through a wet puck faster. (And its the simplest reason why if channelling starts its not going to get better). Water will move through the puck by conduction (pressure force from above) and through diffusion/capillary action (Capillary action being a pressure load based on relative saturation).

If you preinfuse a puck you are putting enough water in to diffuse through the puck to change its flow characteristics at a low enough pressure so you are not forcing things. At the same time the water itself will be in contact with the coffee particles and will be extracting the soluble compounds. This extraction will be very quick - the intial peak- as all the easiest compounds to liberate will be available, but will be limited by the volume of water and the solubility of the compounds. It could become saturated which will slow extraction.

When you then start the main extraction the first thing that happens is that the new water pushes out the preinfusion moisture, but beyond that its flowing through an evenly saturated puck (nice even flow properties) with (I am guessing) many of the main soluble compounds on the 'linear' part of the dissolution curve. I.e. its all very controllable and predictable.

If you don't preinfuse you are dependent on the behaviour of the wetting front - in other words the water may not flow through the puck evenly. If the grounds are evenly compacted and distributed and the machine is producing a nice even pressure across the top of the puck you may not notice any difference. However the puck will be resistant to flow and you are suddenly applying water and a big pressure. The water wants to find a path into the puck.

If there is any variation in resistance on the surface of the puck (uneven tamp/ ground distribution) then that is going to affect where the water goes.

As soon as the water starts to penetrate into the puck in one area it becomes easier for more water to take that route because the conductivity increases with saturation. Where the water goes next will depend on how the grounds are distributed through the puck but even if you get an even distribution of flow out of the bottom you may end up with areas at the top of the puck which stay relatively dry.

If the top of the grounds are wet then the wetting front will find it easier to penetrate the top layer of the puck and the pressure distribution (and flow) should be much more even.

Lesson done. Debate welcome, even if/especially if you can show me to be wrong


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## simonp (Nov 18, 2014)

dr.chris said:


> I started posting some stuff on TMC because someone was talking about the effects of preinfusion with their new machine


That would be me then:drink:


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## dr.chris (Nov 13, 2014)

Yes of course it would


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## Eyedee (Sep 13, 2010)

MY immediate conclusion from taxing my grey matter to grasp that is----- pre-infusion is good. (I like it if it's simple).

Ian


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## simonp (Nov 18, 2014)

Eyedee said:


> MY immediate conclusion from taxing my grey matter to grasp that is----- pre-infusion is good. (I like it if it's simple).
> 
> Ian


That was my shortened response on TMC too LOL


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## dr.chris (Nov 13, 2014)

simonp said:


> That was my shortened response on TMC too LOL


My problem is that like with anything like this there is a set of jargon which becomes natural to you if you spend too long working in the area, and stripping that out to the point where it becomes comprehensible isnt easy.

So yeah - I should of just said that preinfusion is a great idea ^^


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## simonp (Nov 18, 2014)

dr.chris said:


> My problem is that like with anything like this there is a set of jargon which becomes natural to you if you spend too long working in the area, and stripping that out to the point where it becomes comprehensible isnt easy.
> 
> So yeah - I should of just said that preinfusion is a great idea ^^


It's always good to back up an opinion with explanation and facts though, as a design engineer I learned that one fast....


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## Vieux Clou (Oct 22, 2014)

So pre-infusion on a simple machine such as my Lelit would consist of what?

- switching the pump on briefly and then off again;

- never worry, it'll do it anyway while it's building up to full pressure.

And not that it's relevant (maybe), but once the extraction is running, will the pressure gradient across an ideal puck be linear?


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## Obnic (Jan 14, 2014)

Ah ha! With that skill set we ought to bring up the subject of profiled pressure extractions using machines like Vesuvius.

Preinfusion certainly makes for more even flow through the puck but what about manipulating the rate of flow as the shot progresses?

There's a body of 'trial and error' experience on tapering pressure profiles. I for one try to keep a uniform rate of flow (crudely measured by eye and brew weight change) as the shot develops which means tapering with a long tail.

However, should we be able to extract more with low pressure (longer contact times slow flow) or finer grind and high pressure? Or should we be trying to tweak the profile to match relative solubility of oils at different points in the extraction?

Can we tax your expertise... please


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## dr.chris (Nov 13, 2014)

Vieux Clou said:


> So pre-infusion on a simple machine such as my Lelit would consist of what?
> 
> - switching the pump on briefly and then off again;
> 
> ...


I think the point of preinfusion is that its at a lot lower pressure than the main extraction, so it depends on how your machine builds pressure. The aim is to get the puck, or at least the top, damp, without forcing water all the way through. What that takes is a little bit of pressure and a few seconds.

When the extraction is running the pressure gradient will depend on the shape of the basket. You might be able to make an argument for fines movement making the puck more dense towards the bottom but its probably a marginal effect


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## Vieux Clou (Oct 22, 2014)

dr.chris said:


> I think the point of preinfusion is that its at a lot lower pressure than the main extraction, so it depends on how your machine builds pressure. The aim is to get the puck, or at least the top, damp, without forcing water all the way through. What that takes is a little bit of pressure and a few seconds.


It always takes two or three seconds for the pressure gauge to start moving - longer than it takes water to appear when I run it with no PF in place - so I guess it is pre-infusing, intentionally or not.



> When the extraction is running the pressure gradient will depend on the shape of the basket. You might be able to make an argument for fines movement making the puck more dense towards the bottom but its probably a marginal effect


Right enough. It'd be interesting to be a fly on the wall in the basket development dept.


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## simonp (Nov 18, 2014)

Vieux Clou said:


> It always takes two or three seconds for the pressure gauge to start moving - longer than it takes water to appear when I run it with no PF in place - so I guess it is pre-infusing, intentionally or not.


If it is an E61 (and some others) group then it will have some degree of pre-infusion built in. Vibe pumps also take a while for the pressure to build up.


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## dr.chris (Nov 13, 2014)

Obnic said:


> Ah ha! With that skill set we ought to bring up the subject of profiled pressure extractions using machines like Vesuvius.
> 
> Preinfusion certainly makes for more even flow through the puck but what about manipulating the rate of flow as the shot progresses?
> 
> ...


The big question is what exactly we are extracting and how fast it dissolves, because there is a healthy range of compounds in coffee and they are all going to come out at different rates. For instance, as far as I understand, caffine is extracted more slowly - as press pots and filter coffee has more caffine than espresso.

I have been wanting/aiming to set up a model to do just this for a long time but its always fallen into the category of stuff to do when you have time. I also dont have access to the code I wrote to do the minerals leaching, so its a question of using spreadsheet models and/or implementing code to do a similar job in the commercial software I use now.

There is probably some generic comments I can give right now, some of which may be obvious.

Smaller particles leach faster than large. That also means they will be leached out faster

Smaller grind size has lower conductivity than large, and higher packing. Water flows through the gaps









An even grind size will have higher conductivity than an uneven grind size, where small particles fill the gaps around big ones.

After a certain point the rate of extraction of each compound will fall off. To maintain the concentration of 'stuff' in the water coming out of the puck you might want to think about reducing the flow rate. A common mistake in the mineral industry is to think that because if you apply X amount of chemicals to some ore and you get Y products out, that if you apply 2X you will get 2Y. Quite often, and for reasons that aren't relevant and would take too long to explain, you will get less than Y.

The amount of chemicals dissolved in the water will effect the rate at which new chemicals dissolve. There are solubility limits and they can compete.

I'd add that right now I dont know how pressure and temperature influence extraction rates. Dissolution can be highly dependent on temperature, just because of the rate kinetics.

I can see there are reasons why conductivity (and therefore pressure needed) might change through extraction but I'll have to think about that one. It could be easy to reach the wrong conclusions.

I dont know enough about changing profiles through extraction as I have my much loved E61 HX machine but all this talk makes me think more about a lever


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## jjprestidge (Oct 11, 2012)

Regarding pressure profiling, it was the next big thing commercially for a few years with the Strada, Synesso and Slayer, but almost everyone seems to be using a similar profile: long preinfusion, ramp up to 9+ bars, constant pressure for a little while, then a declining pressure towards the end.

JP


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## Mrboots2u (May 10, 2013)

Oh well that's sorted that then , no need to experiment with pressure profiling


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## garydyke1 (Mar 9, 2011)

Longer softer pre infusion means a faster resulting flow rate, hence you go finer , hence you extract higher ..... discuss!


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## jjprestidge (Oct 11, 2012)

Mrboots2u said:


> Oh well that's sorted that then , no need to experiment with pressure profiling


Nothing to stop you trying - I'm just noting that most places are running very similar profiles. Scott Rao mentions this in his most recent book as well.

JP


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## Mrboots2u (May 10, 2013)

garydyke1 said:


> Longer softer pre infusion means a faster resulting flow rate, hence you go finer , hence you extract higher ..... discuss!


With an aero press filter as well


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## majnu (Jan 31, 2014)

garydyke1 said:


> Longer softer pre infusion means a faster resulting flow rate, hence you go finer , hence you extract higher ..... discuss!


How many bars is softer preinfusion 2,3,4?

If you extract higher can you use a lower dose instead?


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## dr.chris (Nov 13, 2014)

garydyke1 said:


> Longer softer pre infusion means a faster resulting flow rate, hence you go finer , hence you extract higher ..... discuss!


Not when the puck is saturated. There is a limit on the hydraulic conductivity of a puck which is going to be a function of the particle size distribution and compaction (i.e. tamping and any effects from the initial water pressure load)

pre infusion is about controlling what happens when the pressure is turned up


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## garydyke1 (Mar 9, 2011)

dr.chris said:


> Not when the puck is saturated. There is a limit on the hydraulic conductivity of a puck which is going to be a function of the particle size distribution and compaction (i.e. tamping and any effects from the initial water pressure load)
> 
> pre infusion is about controlling what happens when the pressure is turned up


http://www.slayerespresso.com/2012/06/13/how-fine-can-you-go/


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## dr.chris (Nov 13, 2014)

garydyke1 said:


> http://www.slayerespresso.com/2012/06/13/how-fine-can-you-go/


Its an interesting video and discussion, but the point is you can set up the puck to the best it is going to be for water flow, but you can't go past that place.

Basic equations for flow through porous media.

Darcy's law. u = K dp/dx - u is the flow velocity, K is the hydraulic conductivity and dp/dx is the change in pressure over distance X.

Given that the relative pressure at the bottom of the puck where the coffee comes out is 0 (absolute 1 bar) (and ignoring the basket for now) then you can say that the flow velocity through a puck is going to be

u = K P/x , where x is the thickness of the puck and P is the pressure generated by the coffee machine,

K is a function of fluid viscosity, density, saturation and the intrinsic permeability of the medium

Intrinsic permeability is a function of particle size distribution, compaction and the properties of the medium, which is a combination of surface roughness and shape, and how hydrophobic/ hydrophillic it is

Particle size distribution is an interesting one in that the best flow rate is achieved through a uniform particle size because of how it packs down. A non uniform particle size tends to be more problematic. Flow is through pores in the medium and large particles create large pores which in turn are filled by small particles if they are present. (In the minerals industry they go to great lengths to remove fines from ore they are leaching as they can lead to poor conductivity and flooded regions, which you don't want in that scenario).

What this means in practice is that the finer you grind the more uniform you want your grind. Assuming water can penetrate the grind the finer you grind the faster the extraction.


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## DavecUK (Aug 6, 2013)

dr.chris said:


> What this means in practice is that the finer you grind the more uniform you want your grind. Assuming water can penetrate the grind the finer you grind the faster the extraction.


I was fine until I got to the final sentence?


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## dr.chris (Nov 13, 2014)

DavecUK said:


> I was fine until I got to the final sentence?


The rate at which the (to use a technical term) 'stuff' is going to dissolve into the water is linked by available surface area. Finer grind means a bigger surface area available for extraction for the same weight of grounds, which means 'stuff' will dissolve faster.


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## DavecUK (Aug 6, 2013)

dr.chris said:


> The rate at which the (to use a technical term) 'stuff' is going to dissolve into the water is linked by available surface area. Finer grind means a bigger surface area available for extraction for the same weight of grounds, which means 'stuff' will dissolve faster.





> Assuming water can penetrate the grind the finer you grind the faster the extraction


Ah...I sense your not talking about the "extraction" as in the shot time...but as in the solubility sense...OK now I understand what you are saying. Confusion over terms.

One of the important areas to consider is the compressive force on a puck which is mostly dry as the pressure is applied in espresso machines....vs the very big difference when those forces are applied to a puck which has been "wetted" by preinfusion. This affects the shots flow rate/extraction in quite a significant way thereafter.


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## dr.chris (Nov 13, 2014)

DavecUK said:


> Ah...I sense your not talking about the "extraction" as in the shot time...but as in the solubility sense...OK now I understand what you are saying. Confusion over terms.
> 
> One of the important areas to consider is the compressive force on a puck which is mostly dry as the pressure is applied in espresso machines....vs the very big difference when those forces are applied to a puck which has been "wetted" by preinfusion. This affects the shots flow rate/extraction in quite a significant way thereafter.


1st point - Yes - language. My fault.

2nd. Interesting about the compaction issue. I was thinking about that earlier. A dry puck may move when pressure is applied, as the air inside can either compress or just get out of the way. A wet puck probably wont (or certainly a lot less) because water wont compress and cant get out of the way as quickly.


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## Obnic (Jan 14, 2014)

dr.chris said:


> ... I can see there are reasons why conductivity (and therefore pressure needed) might change through extraction but I'll have to think about that one. It could be easy to reach the wrong conclusions.


That's Vesuvius-world for sure  it's all trial and error (often delicious but I can't pin a testable theory).

Preinfusion definitely improves evenness of extraction and allows finer grind for the reasons you expound but what comes next is the act of faith.

I think I'm looking for a table with soluble compounds in column A in order of when they start to leach, and then optimum extraction temperature and pressure data for each compound in the subsequent columns.

Then I think one could deliberately tune a profile to emphasise what gets extracted most and therefore how the shot should taste. (At this point I can hear DFK laughing at the depth of my lunacy!)



> ...all this talk makes me think more about a lever


Many men wiser than me have gone down this path including DFK.

Top thread by the way. Thanks for starting and supporting it.


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## dr.chris (Nov 13, 2014)

Obnic said:


> Top thread by the way. Thanks for starting and supporting it.


Thanks and it goes both ways. Good questions help to direct my thinking.

I spent about 10 years developing a very complex model of copper leaching for a big mining company, a process that they were carrying out over an area of around 10 square miles at their main mine. (That is a lot of sulphuric acid..). Its nice to use that experience for something useful


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## MWJB (Feb 28, 2012)

Obnic said:


> Preinfusion definitely improves evenness of extraction and allows finer grind for the reasons you expound but what comes next is the act of faith.
> 
> I think I'm looking for a table with soluble compounds in column A in order of when they start to leach, and then optimum extraction temperature and pressure data for each compound in the subsequent columns.
> 
> Then I think one could deliberately tune a profile to emphasise what gets extracted most and therefore how the shot should taste. (At this point I can hear DFK laughing at the depth of my lunacy!)


I think this might be getting a bit ambitious, especially without clarification on what you mean by compounds (there maybe several hundred flavour compounds, that will change from coffee to coffee). The important components in coffee are thought to extract in reasonably predictable proportions at typical temperatures, it seems to be more a case of how much is extracted (assuming typical proportions), rather than which.


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## dr.chris (Nov 13, 2014)

MWJB said:


> I think this might be getting a bit ambitious, especially without clarification on what you mean by compounds (there maybe several hundred flavour compounds, that will change from coffee to coffee). The important components in coffee are thought to extract in reasonably predictable proportions at typical temperatures, it seems to be more a case of how much is extracted (assuming typical proportions), rather than which.


This is indeed where things get complicated. From the point of view of creating a model you can either aim to break down the compounds into classes of things which behave in a similar way, or look at the behaviour of a small set of key compounds. We would ideally need a chemist to do that kind of classification.


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## Phil104 (Apr 15, 2014)

I don't know enough to contribute to this discussion or to ask sensible questions but I am learning from it. Eventually, could the various postings be turned into an article (by you dr chris?) that is clear about terminology etc. and then provides a more accessible reference?


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## dr.chris (Nov 13, 2014)

Phil104 said:


> I don't know enough to contribute to this discussion or to ask sensible questions but I am learning from it. Eventually, could the various postings be turned into an article (by you dr chris?) that is clear about terminology etc. and then provides a more accessible reference?


Yes I'd like to do that (maybe when we start to run out of steam) and no questions don't need to be sensible, only the answers









In other words its only going to make sense if I can write it at a level which is going to be understandable and useful


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## Obnic (Jan 14, 2014)

MWJB said:


> I think this might be getting a bit ambitious.


 that was a very measured response.

Reread my own post. I was idly musing on my own lack of scientific knowledge rather than specifying a method. Dr Chris' class of compound idea sounds much more practical.

One can measure success of a shot using extraction yield, TDS and classic tasting frameworks. The table would posit some causality: 'this profile worked better because it targeted this class of flavours'.

Ach! I'm fumbling around for something - emulate a lever profile is just a bit to accepting of lore for me.

Curse of an inquiring mind.


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## jjprestidge (Oct 11, 2012)

Obnic said:


> that was a very measured response.
> 
> Reread my own post. I was idly musing on my own lack of scientific knowledge rather than specifying a method. Dr Chris' class of compound idea sounds much more practical.
> 
> ...


Lots of people played with profiles when the Strada came out. I don't believe anyone was trying to emulate a lever profile from the off - they were trying everything, and ultimately a long preinfusion, gradual ramp, constant pressure then tail off was found to work best by most, with some subtle variations.

I don't know of any commercial operators who run different profiles for different coffees, although there may be some. Even with the EP it's not practical in a busy shop.

The other danger, as James Hoffmann noted a while back, is that you end up introducing yet another variable and constantly chasing your tail.

Don't let this put you off, but also don't expect to suddenly find a magic bullet.

JP


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## michaelg (Jul 25, 2013)

dr.chris said:


> This is indeed where things get complicated. From the point of view of creating a model you can either aim to break down the compounds into classes of things which behave in a similar way, or look at the behaviour of a small set of key compounds. We would ideally need a chemist to do that kind of classification.


As a former medicinal chemist in drug discovery I would say whilst a nice idea in theory it probably doesn't hold up in practice. Sometimes relatively small changes in a basic skeleton of a molecule would make large differences in how to extract (in this case partition between an aqueous solution and an imiscible organic solvent like ethyl acetate or dichloromethane) and purify (mostly load onto a packed column of fine silica and wash through into small fractions with litres of solvent (sometimes in a gradients mix of polar and no polar solvents) to separate from left over reagents and biproducts. Some might behave similarly but not guaranteed. I'd say pH of water is quite critical depending on whether the compounds you want to extract are acidic or basic.


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## Xpenno (Nov 12, 2012)

Superb post, great read and plenty to think about.

One thing I thought about with pre-infusion is that as the water passes through the puck at a slower rate that the initial compounds dissolve into the water changing it's pH (and composition in general) and making it more able to dissolve other compounds as a result. This takes out more of the oils and compounds in the early stages of the extraction and then this somehow results in more room for the water to pass through, hence quick end to the shot.


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## michaelg (Jul 25, 2013)

Xpenno said:


> Superb post, great read and plenty to think about.
> 
> One thing I thought about with pre-infusion is that as the water passes through the puck at a slower rate that the initial compounds dissolve into the water changing it's pH (and composition in general) and making it more able to dissolve other compounds as a result. This takes out more of the oils and compounds in the early stages of the extraction and then this somehow results in more room for the water to pass through, hence quick end to the shot.


I think the problem with this theory is that the potentially pH modified water would be pushed out by the influx of higher pressure water that comes post pre-infusion. I think the amount of space that dissolution of compounds would create would be minimal as I would expect the amount would be in the order of milligrams and the pre-saturation and more efficient and even compression of a saturated puck would be the cause of faster extraction.


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## Xpenno (Nov 12, 2012)

michaelg said:


> I think the problem with this theory is that the potentially pH modified water would be pushed out by the influx of higher pressure water that comes post pre-infusion. I think the amount of space that dissolution of compounds would create would be minimal as I would expect the amount would be in the order of milligrams and the pre-saturation and more efficient and even compression of a saturated puck would be the cause of faster extraction.


Yeah it was just a thought really, I have no idea what goes on in there







I like the saturated puck model described in the first post. I guess we are basically saying that the pre-infusion penetrates the puck evenly under low pressure, this then allows more water to flow through the puck once the pressure ramps. If you hit it with high pressure from the off you get more resistance at the surface of the puck (in fact the puck will be expanding back towards the water source which could be even worse) and you effectively get micro-channels and although the extraction might look normal it's actually not as complete as with the pre-infusion model.

I know a few, including myself have noticed that a light tamp can result in a much more even more comprehensive extraction, this would also work on the basis that the water can enter the puck more easily as the particles are not packed as closely together, also the puck would expand less upon contact with the hot water.


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## michaelg (Jul 25, 2013)

Xpenno said:


> Yeah it was just a thought really, I have no idea what goes on in there
> 
> 
> 
> ...


Yup, I think that video with the glass portafilter (from the Londinium forum I think) shows just how turbulent that first application of pressure can be in disturbing the surface. The initial prewetting can be seen as perhaps settling the puck before the 'onslaught' that follows.

I think whatever goes on in there, I think there is enough evidence to show that pre-infusion can only be a good thing in terms of giving better extractions overall.


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## dr.chris (Nov 13, 2014)

Good to get input from a chemist. I think there are things we might be able to look at - just to understand some of the mechanics

I had a thought about pH - and that is connected to carbon dioxide. Now when the water under pressure hits the grounds CO2 is released. Under pressure the CO2 is likely to stay in solution until it hits the bottom of the basket where it foams up.

I seem to remember that CO2 in solution forms a weak acid. This will keep, or at least shift, the pH in the puck down until the CO2 is flushed out.


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## Big Tony (Dec 18, 2012)

Ok, so I worked out that pre infusion is beneficial and kind of lost interest in the rest (bit over my head tbh).

I'm expecting an Expobar Brewtus next week and I know it has pre infusion built in. All I want to know (in laymans terms) is whether you can suggest anything that would improve the pre infusion process that isn't already covered in the manufacturers handbook?

thanks in advance


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## Xpenno (Nov 12, 2012)

Big Tony said:


> Ok, so I worked out that pre infusion is beneficial and kind of lost interest in the rest (bit over my head tbh).
> 
> I'm expecting an Expobar Brewtus next week and I know it has pre infusion built in. All I want to know (in laymans terms) is whether you can suggest anything that would improve the pre infusion process that isn't already covered in the manufacturers handbook?
> 
> thanks in advance


If you are plumbing in then you can lift the lever until just before the pump activates and pre-infusion will take place at your local water pressure. You can add a pressure regulator in line before the machine which will allow you to set this pressure to 2bar/3bar etc...

If you are not plumbed then the machine doesn't technically pre-infuse. A small amount of water will be forced out of the brew boiler by the pressure but it's not enough to saturate the puck and there is no (well very little anyway) pressure.


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## michaelg (Jul 25, 2013)

dr.chris said:


> Good to get input from a chemist. I think there are things we might be able to look at - just to understand some of the mechanics
> 
> I had a thought about pH - and that is connected to carbon dioxide. Now when the water under pressure hits the grounds CO2 is released. Under pressure the CO2 is likely to stay in solution until it hits the bottom of the basket where it foams up.
> 
> I seem to remember that CO2 in solution forms a weak acid. This will keep, or at least shift, the pH in the puck down until the CO2 is flushed out.


Makes me think of buffering although to be honest it's something I did at school rather than uni or work so not so fresh in my mind but links in with a lot of the research on the effect of different waters.

I think a lot of the compounds extracted from coffee as basic so perhaps they offset the acidity of the dissolved CO2.

I remember in final year browsing one of those Wiley books all about Coffee (the chemistry thereof) - wish I had liberated it now!


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## michaelg (Jul 25, 2013)

Couldn't find it on Amazon but maybe this would be interesting;

Coffee: Volume 1: Chemistry https://www.amazon.co.uk/dp/9401086931/ref=cm_sw_r_awd_0tIIub1R5VQQX


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## dsc (Jun 7, 2013)

I always wondered what happens to fines in a coffee puck, we often hear the term 'fines migration' which I'm guessing refers to fine particles being pushed downwards into the puck and when saturated swelling, binding with other larger particles and slowing the brew. With preinfusion at low pressure these might simply stay in place, swell up and clog the puck less? just a thought really.

I'm also curious how much preinfusion is bad? I'm guessing at some point the top of the puck which is saturated first is overextracting the coffee, this is then pushed downwards not extracting anything more as the solution probably cannot take anymore and being flushed on the bottom. Or is that all wrong?

As for pressure profiling it brings too much flexibility in my opinion. You can do so much with pressure in those 25-30sec and I bet people experimenting found that some overcomplicated profiles give a similar output to a generic preinfuse-brew-lower-pressure.

T.


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## dr.chris (Nov 13, 2014)

dsc said:


> I always wondered what happens to fines in a coffee puck, we often hear the term 'fines migration' which I'm guessing refers to fine particles being pushed downwards into the puck and when saturated swelling, binding with other larger particles and slowing the brew. With preinfusion at low pressure these might simply stay in place, swell up and clog the puck less? just a thought really.
> 
> I'm also curious how much preinfusion is bad? I'm guessing at some point the top of the puck which is saturated first is overextracting the coffee, this is then pushed downwards not extracting anything more as the solution probably cannot take anymore and being flushed on the bottom. Or is that all wrong?
> 
> ...


I don't think coffee swells that much, but I could be wrong. The coffee chemistry book above does look interesting but not 60 pounds interesting I do have the Illy book somewhere which does some basic info.

Fines. In general I dont know. Fines is a big issue in minerals recovery but the mechanics are different. The thing is they will start evenly distributed through the puck.

If they do move then they will move with the wetting front and will build up in concentration, but when they hit the bottom of the basket they are going to be washed out.

Less likely but If they only start moving when the flow is full on then again they are going to be washed out the bottom

I think you may be right about 'over' pre infusion, but suspect its only a very small amount of water compared to what ends up extracted and will therefore not be noticable


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## AndyS (May 12, 2012)

Hi dr chris, interesting stuff.

I believe there are significant differences between your mineral models and an espresso extraction:

1. coffee particles swell significantly as they hydrate

2. fresh coffee liberates a great deal of CO2 which makes the hydrated puck more compressible than it would be otherwise

3. coffee particles appear to partially clog the perforations in the filter, producing more resistance

4. increased pressure increases flow rate only up to a point, after that the flow decreases

5. preinfusion at the beginning leads to a faster flow rate later on.

Since you have the Illy book, you probably would be interested to read section 7.3 where he talks about fines migration and the puck's "time-dependent geometry." It certainly appears that in espresso preinfusion there is a lot more going on than simple hydration of the material.

As others have pointed out, a gentle preinfusion ends up increasing the flow rate once full pressure is applied. There is widespread use of this technique nowadays to allow finer grinding. This means that lighter, harder to extract roasts can be successfully prepared as espresso. Previously the light roasts were considered unusable.


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## jjprestidge (Oct 11, 2012)

michaelg said:


> Makes me think of buffering although to be honest it's something I did at school rather than uni or work so not so fresh in my mind but links in with a lot of the research on the effect of different waters.
> 
> I think a lot of the compounds extracted from coffee as basic so perhaps they offset the acidity of the dissolved CO2.
> 
> I remember in final year browsing one of those Wiley books all about Coffee (the chemistry thereof) - wish I had liberated it now!


You might find this interesting:

http://pubs.acs.org/doi/abs/10.1021/jf501687c

JP


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## michaelg (Jul 25, 2013)

jjprestidge said:


> You might find this interesting:
> 
> http://pubs.acs.org/doi/abs/10.1021/jf501687c
> 
> JP


Thanks! Just need to find a friend or colleague with access to the whole article now! Nice to see my cups in the pic with the article!


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## Hoffmonkey (Apr 28, 2014)

Its linked from the top of the summary: http://pubs.acs.org/doi/full/10.1021/jf501687c


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## Xpenno (Nov 12, 2012)

michaelg said:


> Thanks! Just need to find a friend or colleague with access to the whole article now! Nice to see my cups in the pic with the article!


Yeah, just click the PDF (or whatever you want) button. The paper mostly discusses the dissolution of flavour compounds in the puck basked on the solvents (in this case Ca+/Mg+ ions) found in the water. Unfortunately it doesn't cover much outside of that at this stage. Still would be useful for someone to read who can remember anything from their Chemistry degree i.e. not me!


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## michaelg (Jul 25, 2013)

Xpenno said:


> Yeah, just click the PDF (or whatever you want) button. The paper mostly discusses the dissolution of flavour compounds in the puck basked on the solvents (in this case Ca+/Mg+ ions) found in the water. Unfortunately it doesn't cover much outside of that at this stage. Still would be useful for someone to read who can remember anything from their Chemistry degree i.e. not me!


Cheers, will take a look at a proper computer once I am done running about signing things prior to a house move next Tuesday


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## dr.chris (Nov 13, 2014)

AndyS said:


> Hi dr chris, interesting stuff.
> 
> I believe there are significant differences between your mineral models and an espresso extraction:
> 
> ...


Thanks for posting that there is some reading for me to do. I did like the Illy book but from faint memory there are some contradictory numbers which reduced my faith in all it had to say.

You are right about differences between minerals and coffee but much of the underlying science is the same. Its more a question of trying to understand the different mechanics and how they feed in to the basic rules.

The general aim of what we are trying to do here so far and what I was doing before are obviously different. The minerals leaching model I developed was looking in detail at local processes but was primarily concerned with variables we could change, which meant skating over aspects that we could not adequately or reliably model. What we were looking for was a way of determining if approach A was better than approach B. Here, so far, I have been just describing what I think is going on based on the knowledge I built up in the past.

As for point 2 above I would suspect the analogy is much closer to what happens when you open a can of fizzy drink. I dont know how much gas is going to be present in the puck itself but suspect that due to the pressure it wont appear until it hits the bottom of the basket.

Point 1 and point 4 could well be related. Compaction too. I do some work on soild models (but not enough on the theory - yet) so may be able to get some insight there, some time







. Hydraulic conductivity is also the limit to the amount of flow that can pass through a porous region. If you apply a pressure greater than that needed to match the conductivity all that work is going to go directly into pushing on the puck.


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## AndyS (May 12, 2012)

dr.chris:

I think you are wise to be skeptical about the content in the Illy book. A lot of the content there does not pass close scrutiny, however many interesting concepts are proposed.

Re: Darcy's Law: it eventually comes up in most discussions about espresso percolation, but it is utterly useless in this application. IMHO, of course.


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## dr.chris (Nov 13, 2014)

AndyS said:


> dr.chris:
> 
> I think you are wise to be skeptical about the content in the Illy book. A lot of the content there does not pass close scrutiny, however many interesting concepts are proposed.
> 
> Re: Darcy's Law: it eventually comes up in most discussions about espresso percolation, but it is utterly useless in this application. IMHO, of course.


Its not useless, its just how its used. Things that people think should be constant (i.e. the hydraulic conductivity) are not. There is a lot more maths that can be applied but it just makes explaining what is happening more scary than it needs to be. Darcy's law on its own is nice and simple.

From the science point of view dealing with saturated flow is fine, unsaturated flow is 'interesting'. Dealing with the change from unsaturated to saturated is 'hard'.


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## AndyS (May 12, 2012)

dr.chris said:


> Its not useless, its just how its used. Things that people think should be constant (i.e. the hydraulic conductivity) are not. There is a lot more maths that can be applied but it just makes explaining what is happening more scary than it needs to be. Darcy's law on its own is nice and simple


Please don't get me wrong, I have nothing against Mr. Darcy. In fact he was my favorite character in the book and the whole time I was hoping he would get together with Lizzy. The whole time.

But as far as his LAW is concerned, trying to apply it to our espresso application is a waste of time. To wit: preinfusion dramatically influences the hydraulic conductivity, the hydraulic conductivity varies over time, the flow is not proportional to pressure, the coffee grains interact with the perforations at the bottom...the list goes on and on.


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## dr.chris (Nov 13, 2014)

AndyS said:


> Please don't get me wrong, I have nothing against Mr. Darcy. In fact he was my favorite character in the book and the whole time I was hoping he would get together with Lizzy. The whole time.
> 
> But as far as his LAW is concerned, trying to apply it to our espresso application is a waste of time. To wit: preinfusion dramatically influences the hydraulic conductivity, the hydraulic conductivity varies over time, the flow is not proportional to pressure, the coffee grains interact with the perforations at the bottom...the list goes on and on.


What the 'law' states is that there is a direct relationship between pressure applied to a porous medium and the rate of flow through it, which is the conductivity. That is fact. Because the conductivity varies (and we know some of the reasons why) the flow rate varies during the course of extraction, regardless of pressure. But at any single point in the puck at any point in time the local flow is a direct product of the local pressure and the local conductivity.

Darcy's law is a useful tool to get you to think about the nature of that relationship, and its the basis of more complicated mathematics that you *can* use to build a model which may be useful in getting insight. To account for all the physical phenomena isnt easy but it is possible. Just asking the questions you need to build up a model can be more informative than you'd think, because it forces you to quantify the behaviour that you observe. That kind of thing has been my day job for most of my working life.

We can say there is a relationship between grind, pressure and time on one side and flow rate on the other. We know that from observation. There are limits on these relationships which we can define. The grey area is the bit in the middle and understanding that bit takes time and sometimes getting things wrong.

Its not a waste of time, but its almost certainly not a simple hand calculation either.


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## AndyS (May 12, 2012)

dr.chris said:


> What the 'law' states is that there is a direct relationship between pressure applied to a porous medium and the rate of flow through it, which is the conductivity. That is fact. Because the conductivity varies (and we know some of the reasons why) the flow rate varies during the course of extraction, regardless of pressure. But at any single point in the puck at any point in time the local flow is a direct product of the local pressure and the local conductivity.
> 
> Darcy's law is a useful tool to get you to think about the nature of that relationship, and its the basis of more complicated mathematics that you *can* use to build a model which may be useful in getting insight. To account for all the physical phenomena isnt easy but it is possible. Just asking the questions you need to build up a model can be more informative than you'd think, because it forces you to quantify the behaviour that you observe. That kind of thing has been my day job for most of my working life.
> 
> ...


When someone comes on the forum with a great deal of experience in a specialized, related field, it is a good thing. I really do appreciate what you've said.

The reason for my satirical attitude about Mr Darcy is that almost all the time, when people talk about espresso percolation and bring up Darcy's Law, they have almost no understanding of the subject; they're simply trying to convince themselves that they know something by sounding "sciency."

I don't feel that's the case with you at all, you obviously know a lot more about flow through a porous medium than I do, and I respect your experience.

But...the flow of water through an espresso puck has many unexpected little nuances. My still-skeptical version of Darcy's Law vis a vis espresso percolation would be this: "if you increase the water pressure on an espresso puck, the flowrate will increase, unless it decreases."


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