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?
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. ;)