The Senseo Crema Mystery
Posted by deCadmus on 24 Mar 2005 at 11:00 am | Category: Coffee
The Mystery: What’s up with the Senseo, Anyway?
The machine produced by Philips and Douwe Egberts has been rather aggressively marketed as the “coffee machine with the delicious crema layer”. 
I have been asked more times than I can count [and I count fairly well… rarely even have to take off my socks] with questions like,
a) is this espresso? b) is it really crema? c) if it’s not crema, what is it? and, d) how does the Senseo make that stuff?
The answers:
Is it espresso? Don’t be silly. ;)
Is it really crema? No. Crema is… well, let’s defer to Dr. Illy:
“Crema, the dense, reddish-brown foam that tops an espresso, is composed mainly of tiny carbon dioxide and water vapor bubbles surrounded by surfactant films. The crema also includes emulsified oils containing key aromatic compounds and dark fragments of the coffee bean cell structure.”
The foam produced by a Senseo is *not* an emulsion; the coffee in the Senseo pod [or pad] is not ground fine enough, nor is the pressure in its brewing great enough to release the non-water soluble oils and lipids to create such an emulsion… and those few oils that *might* be released would be trapped in the filter material of the coffee pod itself. [This is confirmed in left-handed fashion by Philips/Douwe’s FAQ: “The SENSEO coffee brewing process is very efficient leaving hardly any oil in the brew.”]
Further, it’s unlikely that the coffee found in a Senseo pod is fresh enough, or been packaged well enough that the delicate aromatic compounds, or even carbon dioxide — both such an important part an espresso’s crema — remain.
So what is this stuff? It’s *foam*. Bubbles. Mostly air bubbles, and water vapor, and probably some CO2, encapsulated by the brewed coffee solution. Again, it’s not emulsified oils.
There are a number of compounds in coffee that make lovely bubbles… long, complex protein chains that have some remarkable [even improbable] properties, surface tension being only one of them. [The physics of coffee rings is a story for another day.]
So how’s the Senseo make that foam? Well, this is where the Senseo’s designers got pretty clever!
At the bottom of the pod carrier [a little tray that holds either one or two pods… think of it as a device-specific coffee basket and portafilter if you like] is a barrel-shaped nozzle. Embedded in that nozzle is a small metal disk. This disk has a very small orifice or aperture at its center… 1mm, maybe 1.5mm in size.
While brewing, the machine’s pump pushes water through this assembly under pressure… we’re not talking espresso-like pressure here, just something on the order of 1.5 to 2 times atmosphere, or 1.5 to 2 bars [by way of reference, espresso is brewed at 9 bars].
Here’s where some junior-varsity physics comes in…
One of the interesting properties of fluids is that, when under pressure and presented with a wee, little aperture as a way to escape, the fluid will first form a little vortex or funnel above the orifice itself, trapping anything that’s *not* a fluid [air, water vapor, CO2, etc.] in its center. For an example of this, look no further than your bathtub… pull the plug on a tub full of water and watch the vortices spin. And listen to the sucking sound as air is trapped in the vortex.
When the pressurized fluid [and its trapped gasses] emerges on the *other* side of the wee, little aperture and is suddenly no longer under pressure, the gasses are *encapsulated* by the fluid in a series of amazingly uniform bubbles. The size of these bubbles can be regulated by varying the amount of pressure, or the size of the orifice, or the surface tension of the fluid solution itself. So, if you want your bubbles to be *extremely* tiny [as you would for, say, an ink-jet printer - which uses this very same principle of fluids] then the aperture would be tiny, indeed.
In the case of the Senseo brewer, then, the designers tuned the size of the orifice to the typical surface tension of brewed coffee and to the amount of pressure delivered by the pump and lo… bubbles. Lots and lots of bubbles. And lots and lots of bubbles is foam… it’s still not crema.
So there you are.
As an interesting aside, I think the fluid dynamics at play here have some interesting implications for why espresso brewed with a bottomless portafilter seems to have a more textural quality to it… but that, too, is a story for another day.
My thanks to Don Holly and Lindsey Bolger who let me tinker with a Senseo in the coffee lab, and to Don especially for sharing my enthusiasm for discovering how things work by destroying them. ;)
Thanks for explaining it! It is pretty nice though–the “crema.”
Hmmm, still a little confused. Reason being-I bought the “My Pod” to make my own Senseo coffee.
I’ve tried it several times with all different grinds of coffee and voila—NO “crema”. Why might this be?
While I’m not familiar with the “My Pod” device, a quick view of the photo of one on Amazon suggests that it replaces the entire pod carrier assembly. Without the senseo pod carrier in place there’s no longer that teeny little aperture for your coffee to squeeze through, and thus, no foam.
How come some brands of pod come with ‘crema’ writ large on the packet? Our current theory is that its some chemical they add to the ‘100% type X coffee’ in the pod. I haven’t seen more or less ‘crema’ from one brand or another, but it seems strange that the pod itself would claim to be ‘crema’ or not. Also… What is that extra large brown pod holder (seems to have a different mechanism) that comes with the senseo? Perhaps that one makes less ‘crema’!
So, if I don’t want the foam I can take out that little metal disc somehow? I hate the foam and always scrape it off with a spoon.
Would love to know your answer… please post it and send it by email.
Thanks for your help with this mystery!
Belinda
That’s correct, Belinda. If you don’t want a foamy head on your cup simply punch out the metal disks at the bottom of the pod carrier device.
DISCLAIMER: Have a care… flying metal pieces may be hazardous. You could put yer eye out! ;)
So, when you say “punch out the metal disk” you probably mean to use something like a screwdriver? I wonder if I’m strong enough to punch through that little wire mesh thingy. Well, if I’m serious about not having foam I’m going to have to try it… unless you warn me otherwise.
Thanks!
Recommended method:
1 hammer
1 hole punch or sturdy screwdriver
1 solid surface on which to do the punchin’.
Optional (but recommended)
1 pair pliers.
A third hand.
Gripping the pod carrier in pliers, hold securely against a solid surface… say a kitchen counter which you’ve covered with a cutting board or a piece of plywood — something that can take a good whack.
Hold a screwdriver or hole punch against the metal disk and give a solid tap with the hammer. Your objective is *really* to make the hole in the disk larger so as to disrupt its foamy proclivities; what’ll probably happen is that the metal disk(s) will just pop right out.
Really, you are TOO funny! And I wish you could come be my “third hand” when I do this. There is a plastic funnel piece at the bottom of the pod carrier which I’m afraid will break off when I give it a solid whack… but, we’ll see. I don’t have a hole punch but I did find a big ol’ nail (not rusty) that should do the trick. Or, I might just check out all the gadgets on my Swiss Army knife or multi-tool thingies.
Will keep you posted!
Belinda
Well, I’m sure the world has been waiting to see what has come of my/our experiment to un-bubble-ize a senseo machine.
I decided to try a less forceful way of piercing the pod holder mesh screen thingy… I found a small screw and screwed it in carefully/strategically in a few spots at the bottom through the mesh. Then I made coffee.
Well, I think I need to screw a few more holes in it but it was definitely less foamy and more bubbly. It was actually drinkable without me having to scoop off the foamy 1 inch layer at the top of my coffee cup. So, that’s an improvement!
If you want to hear more… just let me know!
Keep us posted! (Well I was waiting anyway!)
Dan.
I think most anything you do to stretch the very small holes will disrupt the bubblage to some extent. And you never know, you may stumble upon a finely tuned point somewhere in between that makes the coffee pour like honey and taste like cherry wine.
Or not. ;)
TA-DA!!!!!!!!
I am now the proud imbiber of bubble/foam-free senseo processed coffee!
I kept at it with yet another screw that widened the pod holder mesh thingy even more and …. voila! No more scooping off the foam or bubbles at all. There are just a few bubbles around the edges when the coffee gets pressed through and I’m a happy woman!
I performed this experiment on my double pad pod holder thingy and so my single pod holder still does the foam, for my guests who can’t live without senseo foam.
It worked!
Belinda
Congrats, Belinda. And no third hand needed, huh? ;)