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Nutation: how to do it right

Postby drdna on Fri Dec 04, 2009 4:02 pm

I have been perplexed for a while by the use of the term "nutating" in espresso preparation. Nutation is defined in various ways here or here or even here.

It used to apparently be a fancy way of describing the nodding of the head.

In botany, it refers to the slight bend a plant may make as it grows.

In astronomy, it is the wobble in the oscillation of the precession of the axis of a rotating body.

None of these terms fully capture the idea of the motion I think we all assume we understand in our colloquial use is such phrases as "nutating tamp," but perhaps one of the reasons we do not get consistent results is because we are all doing our "nutating" tamp a bit differently.

In truth, to really be a true nutating motion when we tamp, we would have to wobble the tamper from side to side a bit while rotating. Okay, yes, that DOES probably happen, but not by intention, I don't think. Unless, I am wrong, most people are simply spinning or rotating the tamper while applying pressure. The major variables here are the amount of pressure applied and the amount of spinning/rotating done.

My personal experience is that any form of true nutation (i.e., wobble) is undesirable are it makes the puck uneven. I have also found that excessive pressure with the relatively asymmetric and dynamically changing forces of rotation can lead to channeling. So, what I tend to do is more akin to light polishing. Essentially I use only the weight of the tamper for my final "nutating" step and I simply rotate the tamper. This reveals any asymmetry because if the puck is not level, the tamper will wobble a bit as it rotates, which I suppose is actually the nutation.

So in fact I am really doing an anti-nutating step, I guess.

Anyway, I would be very interested to hear other people's thoughts about this topic, what they do, what their experience has been, and what the thoughts are on standardizing what is meant by nutating, which may not be nutation at all.
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Postby danetrainer on Fri Dec 04, 2009 4:48 pm

drdna wrote:In astronomy, it is the wobble in the oscillation of the precession of the axis of a rotating body.


Indeed...I think this description comes the closest to describe what is happening when I apply it to my tamp.
I grind straight into the basket, weigh for the amount I want. The grounds from my Macap are light and fluffy, no clumps...I give the basket a slight shake side to side to level the grounds. Then with the tamper and lightly contacting the grounds the "nutulating" occurs (with a slight rotation of the tamper) as it begins to compress the coffee. As I approach the point (determined by feel) of compression, the tamper is completely leveled out and the final tamp applied.

Basically it is moving the grounds to points that have "pockets" as you are lightly compressing...performing the distribution without all the other fuss.
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Postby HB on Fri Dec 04, 2009 4:51 pm

For those who are scratching their heads after reading the above, here's a video:


From the Tamp and Dose Techniques Digest

PS: What's up with tapping the portafilter, Dave? :lol:
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Postby another_jim on Fri Dec 04, 2009 5:06 pm

The motion on Dan's video is rather understated. Think of flipped coin that has landed on the ground, but not yet settled, as it wobbles along its edge.

You roll the tamper, so the top of the handle describes a circular motion above the basket. The bottom edge of the tamper presses down at one spot on the edge of the puck, and that spot rotates around the entire edge as the motion proceeds.

Here is a rough illustration:

Image

When you are done, you've leveled the coffee, and sealed the edge of the puck. IMO, it's the fastest way to do this.

It is easier to to use a curved base tamper when doing this.
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Postby drdna on Fri Dec 04, 2009 6:30 pm

another_jim wrote:When you are done, you've leveled the coffee...

This is the key in my opinion, and this is why my motion is a very subtle one, as I said more akin to a simple rotation.

Clearly the motion being described, as I said, is not nutation, but in fact precession.
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Postby another_jim on Fri Dec 04, 2009 7:21 pm

Um, if you say so.

But in actual mathematics, precession is 2D physics, the simple rotation of the axes of an elliptical orbit; while nutation is 3D analytical geometry, the motion of a tilted annulus touching a plane at one point and having its axis describe a circle parallel to that plane. This motion will inscribe a circle on said plane whose radius is the cosine of the tilt angle times the radius of the annulus.

I picked the word nutation, since that is the word for the motion I was describing. I never realized that it would give people trouble. If I had, I'd have called it the "coin flip tamp."
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Postby weasel on Fri Dec 04, 2009 7:36 pm

When I 'nutate', I first tamp the coffee in a straight down fashion to establish a fairly firm bed, then I gently press along the circumference in an effort to seal the edge.

I do not use the full weight of the tamper, rather a light touch. My goal is to prevent edge channeling without altering the even distribution (in theory) of grounds. Too much pressure will shift grounds/distribution.

After the nutation, I finish with a second straight tamp (and twist).

Once I began using this method, my pours became much longer, and I then had to adjust to a coarser grind. The espressos then became much better and more consistent. I believe this is largely due to a reduction in edge channeling.
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Postby cafeIKE on Fri Dec 04, 2009 8:42 pm

  • Baskets don't have an edge as shown above
  • This whole 'seal the edge' strikes me as utter nonsense
  • It's freaking near impossible with a close fit tamper

If the tamper is canted, it is not pressing the coffee at the basket side. As the tamper goes around the basket it is moving coffee away from the basket side.

Image

A water molecule is around 0.2nm. The finest fines are, what, on the order of 1µm. If the puck were made of nothing but fines, there would still be space for 100's of water molecules side by side to slide between the basket and the coffee.

At best, nutation may work by shifting the coffee back and forth, filling some voids or 'locking' the coffee into the perforated bottom, as a line of defence against channeling.

A gentle side to side shake and light rap have equal or better efficacy.
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Postby HB on Fri Dec 04, 2009 9:36 pm

drdna wrote:So in fact I am really doing an anti-nutating step, I guess.

Mathematical explanations and diagrams aside, I never gave nutating much of a chance, until I evaluated the Dalla Corte Mini. It was proving to be fussier about dose and distribution than expected. Jim suggested nutating tamp; it definitely improved the evenness of the extraction and lengthened the time before blonding. Intuitively I would expect it to make a bad situation worse, but in this particular case, it was an unmistakable improvement.

That said, nutating is not a regular part of my barista routine, though I do enjoy saying the word. :D
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Postby SwingT on Fri Dec 04, 2009 9:54 pm

drdna wrote:Unless, I am wrong, most people are simply spinning or rotating the tamper while applying pressure.


I am doing a "wobbling" rotating tamp similar to the video, (after paying attention to this mornings shots, I edited this to show I am doing)less wobble maybe, but quite similar to the video. Just let it settle in, and check the level around the edges - making sure the tamper is level all way round compared to the edge of the portafilter, probably not even five pounds of pressure - this is on the Vivaldi.

An S1 vivaldi owner suggested that I use a nutating motion.I do have a convex base tamper for the Vivaldi. seems to give a more consistent distribution of the coffee in the PF, cuts down on spritzers, channeling.

I don't however do the nutation on the Cremina - flat based tamper, doesn't seem to need it.

ETA - I got the convex tamper because that was what Chris Coffee recommended for the Vivaldi. With the screw hanging down from the dispersion screen, I suspect that the convex base may benefit by distancing the puck from the screw. I also suspect that the "nutating" may work better with the convex base tamper.

Nutating the flat based tamper with the Cremina certainly is no improvement and may well be detrimental.
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