Single Baskets & “Pressure Per Pull of the Lever”

[r-gordon-7]

While my Gaggia Factory was heating up this morning, I happened to think that I've never tried using it with its single basket... not even once. I recall reading even before my first machine arrived that single baskets are "pretty well worthless" - and accepted that as, "the way it is". So, I never even gave using the single basket a second thought. Until this morning... when I figured the time had come... So, with low expectations, I ground, dosed, tamped, pulled - and was astounded at the result... closer to "the perfect cup" than any other to date - and, most significantly, with a far more crema than any cup I've pulled before.

Suddenly, the reason (or at least what I think must be the reason) became clear - and it has more to do with the single basket's physical configuration than with volume, dosing or tamping... The taper of the single basket makes the area of the single basket's screened bottom significantly smaller than the area of the more straight-walled double basket's screened bottom. At the risk of overstating the obvious (and at the further risk not only of displaying the paucity of my "artistic skills" - but, even worse, of typing graphics that might not display properly), the difference:

|___|

vs

\__/

Assuming my measurements (and math) are close to correct, the area of the screened portion of the respective Gaggia basket bottoms are ~2,000 mm^2 on the double vs. only ~700 mm^2 on the single. Given the volume of water in the group head should be essentially the same per pull regardless of which basket is used, a given lever pull of equal speed should generate significantly more pressure using the single basket than the double simply due to the smaller area of the screened bottom...

Anyone else find single baskets to be beneficial for this reason?

r-gordon-7

[RapidCoffee]

Suddenly, the reason (or at least what I think must be the reason) became clear - and it has more to do with the single basket's physical configuration than with volume, dosing or tamping... The taper of the single basket makes the area of the single basket's screened bottom significantly smaller than the area of the more straight-walled double basket's screened bottom.
...
Given the volume of water in the group head should be essentially the same per pull regardless of which basket is used, a given lever pull of equal speed should generate significantly more pressure using the single basket than the double simply due to the smaller area of the screened bottom.

In his excellent Professional Barista's Handbook, Scott Rao discusses basket shape and extraction characteristics. He notes that a straight-walled cylinder (the shape of a typical double basket) will overextract the upper layers of grinds and underextract the lower layers. The truncated cone shape of a typical single basket forces more liquid flow per unit area through the lower layers, providing (in theory) a more uniform extraction of grinds.

Without further details, and based on just one pour, the most likely explanation is that the espresso gods smiled upon you and your single basket this morning. I hope they continue to do so.

[mayhew]

Have you tried making your grind a bit finer for the double basket? I wonder if you're not just slightly too coarse for the double basket and the single took up the slack on that.

[peacecup]

The Factory should produce more that enough pressure even with the double basket. Although some believe that espresso needs to be brewed under great pressure that's a bit of a myth. Remember the first espresso machines brewed at just steam pressure (1-2 bar). Home spring levers brew at 6-7 bar, and the espresso is exquisite.

There are a lot of factors that result in taste in the cup. I think when you hit a solo right you get clarity that is unsurpassed - maybe this is because, as John pointed out, a larger, straight-walled basket will tend to be less-evenly extracted, so some of the bitter flavors will arrive.

The simplicity of a lever machine makes it possible to reproduce your results - simply follow the same procedure (grind, dose, tamp, beans, warm-up time) and your should be able to arrive at a very narrow range of taste variation.

The single basket can be humbling because variation in any of the above factors will result in magnified effects in the cup compared to when one uses the double basket.

PC

[matthyx]

If only I could find a Pavoni triple basket with cone-shaped bottom

[r-gordon-7]

If only I could find a Pavoni triple basket with cone-shaped bottom

I was just thinking exactly the same thing! (...provided we're talking Millennium or later Pavoni, so it'll fit the Factory as well...)

r-gordon-7

[AndyS]

Given the volume of water in the group head should be essentially the same per pull regardless of which basket is used, a given lever pull of equal speed should generate significantly more pressure using the single basket than the double simply due to the smaller area of the screened bottom...

Anyone else find single baskets to be beneficial for this reason?

Are you sure you pulled the lever at the same speed for the single as you normally do for a double? Same lever speed means same flow rate out of the basket. And that would mean that, after the preinfusion period, your single ran for about half the number of seconds as your doubles.

And THAT would imply either that you really like underextracted shots (like the single), or your doubles are usually very overextracted.

BTW, lever SPEED has no direct correlation to extraction pressure; it is force on the lever that is directly correlated to extraction pressure.

[peacecup]

With manual levers it is almost impossible to exactly replicate shots because of the tendency of the human arm to compensate for differences in resistance that result from differences in grind, dose, tamp, and beans. If, as Andy says, you can train yourself to pull for exactly 20 sec. (assuming a 10 sec pre-infusion) you will always have a 30 sec shot, and brew pressure will be altered only by the puck (I guess.. or is it even more complicated?). In any event, my method is to prepare the puck so that I'm always applying as much pressure as the machine will stand, and this provides at least some level of consistency. If the shot runs too slow I back off just a little. If the shot runs too fast it is quickly obvious that the resistance is too low. The later shots always seem to taste worse to me.

BTW, I'm not sure I buy the argument that straight-walled baskets tend to over extract the upper layers. Doesn't make sense to me.

PC

[IMAWriter]

[Sniped]
The single basket can be humbling because variation in any of the above factors will result in magnified effects in the cup compared to when one uses the double basket.

PC

Boy, don't I know that!
I had a wonderful single last week (Toscano), which usually I do 15grams in the Elektra basket.
Next single....ugh!
I did stuff about 10 grams in that single...only an 8th inch, maybe 2-3 mm from the lip. Lighter tamp.YYMV

[IMAWriter]

Follow up.
I just 3 straight singles on my my my MYYYY Cremina.
The difference is that I used a SO, as opposed to a blend. Someone more scientific than me will prolly be able to explain my results, compared to the relative lack of success using a very fine blend.
The result:
Other than a bit of temperature rise (should have cool-toweled my group after the 2nd shot, but by then I was fried on caffeine) my singles were out of this world. A more "open" taste experience. I relate this to music...less cluttered. Oddly, imagine a really wonderful cup of coffee, distilled and concentrated into 1oz.
OK, I cheated. A Little.The coffee was a home roasted (to city+) Ethiopian Sidamo Koratie, a berry, flowery, and slightly tea-like masterpiece. The bean, not my roasting expertise!
I can only imagine how that famous Biloya would taste.
Maybe the consistent bean density of a SO somehow allows a more even extraction?
I'm starting to agree with those here who have extolled the virtues of the single basket.
One last thing. Both my tampers are slightly larger than 49mm. One is 49.2, the other 49.4
Both are wee bit too tight for the single Cremina basket.
I'm going to guess Richard Penney may have an answer as to whether the Elektra single basket might afford me a bit more room. I don't want to be forced to break out the Dremel tool!

[RapidCoffee]

BTW, I'm not sure I buy the argument that straight-walled baskets tend to over extract the upper layers. Doesn't make sense to me.

In Ch.3 of The Professional Barista's Handbook, Scott Rao argues convincingly that "the upper layers of the coffee bed contribute a much greater percentage of solids to the cup than do the lower layers" (in a cylindrical basket). There are several reasons for this. Extraction begins at the top of the coffee bed, and extraction is faster at the top because the incoming water is hottest and contains the fewest dissolved solids. In addition, particle movement is impeded in the lower bed by fines migration. I'd suggest you get a copy of the book if you're interested in a more lucid explanation.

[peacecup]

I have not read the book, nor do I know anything of Scott's technical background. A hydrodynamic model in capable hands ought to be able to make reasonable predictions of these things. It seems to me the the top layers would quickly heat, and once extracted, do little saturate the remaining water as it passes through, allowing it to extract the lower layers nearly as much as those on top were. Fines definitely do migrate to the bottom - this can be seen when the puck is removed and any larger particles are resting on top. On the other hand, fines are more readily extracted, so they're being at the bottom ought not prevent them from extracting.

In theory, I guess, a single layer of grounds across a very wide puck, would extract most evenly. But then there would be no pressure....

Things are very much not as simple as they seem....

But my espressos have been VERY good of late,

PC

[RapidCoffee]

I have not read the book, nor do I know anything of Scott's technical background. A hydrodynamic model in capable hands ought to be able to make reasonable predictions of these things.

Scott references a paper entitled "Lattice Boltzmann model for coffee percolation". Is that technical enough?

[AndyS]

It seems to me the the top layers would quickly heat, and once extracted, do little saturate the remaining water as it passes through, allowing it to extract the lower layers nearly as much as those on top were.

Well, sure, but how much is "nearly as much?" 90%? 75%? 60%????

You should read that section of the book. Scott didn't offer any experimental evidence, just an intriguing theory suggesting that a tapered basket might extract more evenly. And with the constant mania of the last two years focusing on even extraction in the basket, his idea can't be so easily dismissed....

[Nickk1066]

Scott references a paper entitled "Lattice Boltzmann model for coffee percolation". Is that technical enough?

That's actually quite a simple model what it doesn't do is:
* vary the lattice node sizes over time (the puck expands, altering the shape of the lattice)
* vary the temperature through the lattice (ie link the heating effect of the fluid flow throughout the puck)
* vary the rate of extracted compounds based on temperature and pressure at the particular node over time.
* vary the rate of flow depending on the viscosity of component extracted (ie oils)

[peacecup]

I agree that the model misses those factors (though to me its not simple!).

One example is, if fines are extracted more quickly (which they most certainly are), then wouldn't we want a gradient of particle sizes in the basket, ranging from coarse on top to fine on the bottom?

Another question is the SHAPE and SLOPE of the relationship between extraction vs. temperature. Is it a straight, steep line (meaning a little temp. diff. could be a lot of extraction diff.) or a flat line (meaning a +/- 5 degree diff. means little diff. in extraction). Does anyone know? Does it vary by bean type? Which part of the extraction tastes best?

Its good that people are thinking about these things, and developing and testing models. If the results are like those in most scientific discipline, however, each test raises more questions than it answers.

PC

[RapidCoffee]

That's actually quite a simple model what it doesn't do is:
* vary the lattice node sizes over time (the puck expands, altering the shape of the lattice)
* vary the temperature through the lattice (ie link the heating effect of the fluid flow throughout the puck)
* vary the rate of extracted compounds based on temperature and pressure at the particular node over time.
* vary the rate of flow depending on the viscosity of component extracted (ie oils)

Even worse IMHO is the lack of experimental verification of the model. The only measurements are flow rate (volume vs. time plots in Fig.3-4; Fig.5 is just a simulation). Nonetheless an impressive beginning.