Thursday, August 03, 2006

Espresso Puck Physics.. What Really happens?

Before I begin, I would like to say that this is in no way to be taken as anything conclusive, or as an "article" of sorts, but rather as more of a mode of thinking in text format.

I could be right, I could be close, or I could be in the wrong ballpark entirely.

I recently read that there is somewhere in the neighborhood of 540 pounds of pressure constantly pressing down on the bed of coffee in the portafilter during the 9bar extraction. What is this concept of extraction, really? Does the water absorb more solubles under the high pressure and high heat? Does the puck really SWELL under the weight of 540 pounds of water?

Honestly, I really think not. What does this say about pre-infusion? Does pre-infusion really "set" the puck to swell so it provides a more evenly distributed bed of coffee? Or does the pre-infusion force the puck to become wet more thoroughly and evenly throughout? Afer-all, "water is lazy". Water will go where water already is.

Additionally, does proportionally more water mean stronger extraction capabilities in the same pull? Why is it that more complexity is usually achieved from a lighter dose than that of a heavier dose with MORE coffee in the basket? Doesn't more coffee mean more coffee flavor?

Here's what I think. I think pre-infusion helps, not in that it swells the puck for an even extraction, but that it allows the water to find the niches and dense spots in the puck before the pressure hits at full force.

I think that the puck does not "swell" as a result of water contact. I think that the puck swells as a recoil from the pressure be forced upon it for 25-30 seconds. Under that kind of pressure, the cellular structure of the coffee particles have to be under some sort of tension, which would be released at once upon the release of pressure (a'la 3-way valve.. non-valve systems will react differently, I think).

So, we may have now established the physical shape behavior of the puck under pressure. What of the pressurized water's extraction properties?

It is my belief that rather than absorbing solubles, the highly pressurized water actually displaces many of the aromatic and flavor components within the bean structure. The high pressure forces them out.. the water does not extract, but rather pushes these components out of the cellular structure of the grind particles. A finer grind means more surface area which means more exposed components to be displaced. At the same time, this also means more resistance to the pressure, which results in a longer contact time.. resulting in components being absorbed, as well as displaced by the water. Too much contact time will result in flavor components most of us would really rather not have in our cup.

The combination of displaced solubles and absorbed solubles creates a balanced flavor. Too much of one or the other, and you end up with either a sour or bitter pull. Thankfully, much of this shift in "extracted" properties is easily seen with the naked eye.

It's not only about even pressure on (and within) the puck. It's also about even contact time with the coffee particles and the pressurized water. Dense spots will flow much more slowly than air pockets. Which brings me to my next, and final, idea.

At what point does the water become saturated with dissolved solids or solubles? Just like osmosis, less saturated water will absorb more quickly and readily than that of saturated water.

Enter: The science of the ristretto. Ristretto shots take more time. Sometimes a LOT more time. The result is a richer, more concentrated, lower volume shot of espresso. That is, the ratio of water to coffee is considerably reduced. There is no doubt that the longer contact time is absolutely essential to achieve such a result.

Here is where it gets interesting.

Wine tasters will swirl the wine in the glass to allow air to incorporate into the liquid, and "release aromatics" and allow the flavors to expand.

Beer afficionados will let a beer warm up to near, or at room temperature, and then pour it into a tapered glass that is wider on top to allow the flavors to expand.

What is the taste capacity of a single tastebud? Is it possible to cram too many flavor components into such a concentrated form that, while all flavor components are present, only a few are actually perceived by the taster?

Are the flavor components for a ristretto actually any different than those in an "under the line" normale, or are we just unable to perceive the dense culimnation of flavors in the ristretto when compared to the normale? Does the expansion of flavor components in the normale actually cause an increase in our ability to perceive more of the flavor components that are present in both, or is the coctail entirely different?

This thought process began last night. I couldn't wait to begin writing about it.

I'm sure there are many factors that a non-chemist and non-engineer and non-physicist like myself are not even aware of, but this still causes me to realize something on a very real and tangible level.

In the science and practice of espresso, we have such a very long way to go. We are nowhere near the pinnacle. On top of that, consider the science of coffee quality, and the concept of subjective opinion being brought into question by minds such as that of Jaime Van Schyndel.

Concepts and ideas such as these make me understand more and more how little we actually know. It makes me realize that we are still (after ~50 years) in the infancy of espresso theory.

I have not even begun to touch on the topic of tamping, polishing, how hard to tamp, temperature, etc.. etc.. So many factors.. so little comprehension in a single instance of time.

And now, to remind us of why we stress about these little details more than sanity would normally allow, a picture of some latte art at work earlier this evening.


Billy Wilson said...

Hey Jason,

I enjoy your blog.

I buy into the ideas that you have about pre-infusion, and in fact the puck does not swell during extraction, but only after the pressure is released.

about your thoughts on ristretto. what you could do is pull a normal shot, and compare it to a ristretto. put both shots in water so that each drink equals 3 oz. (you should be putting a little more in the ristretto drink). cup them out. if you find that they are the same in flavor (i don't believe you will) then you are right in thinking that the flavors are too concentrated for us to taste, and a ristretto is exactly the same as a normal shot. I think that other things are going on though. I think more insoluble solids are getting into the brew with a faster shot.

for sure though... stretching our your shots with a little water is a fantastic way of judging your espresso.... long live the italiano!!!

Jason Haeger said...

Thanks, Billy. Good ideas.

I'm fairly certain you're right about the water dillution and flavor profiles.

I also think you're right about more insoluble solids.. which would explain the flavor profiles of a (too) quick pull.

Maybe this leads to a better understanding of the fine balance of timing, water volume, and amount of coffee. Unfortunately, the entire field of temerature exploration has been completely ignored.. and that can only magnify this can of worms.

Phil said...

to throw another variable in there, with regards to testing your theories:

I pull nothing but ristrettos all day long every day, that's how I make all my drinks. I can taste a ton more stuff in a ristretto than a normal shot, i believe, because it's what I'm used to. Having been pulling quite a few normal style shots after hour recently (for one of many experiments), i've noticed I'm now tasting more and more complexity in normal style shots.
There's a huge degree of personal tasting ability that comes into all of it.

It's similar in wine tasting, experiments in wine tasting having proven that people that drink relatively cheap wine all the time, simply can't taste the complexities in more expensive wine and therefore don't see what all the fuss is about. Vice versa expensive wine drinkers can't enjoy cheap wines. But, given several months of drinking just the opposite type of wine, both these groups found that their taste buds (or brain probably) adapted and they could enjoy it.

While this perhaps isn't directly comparable to the normal espresso vs ristretto discussion, it's worth taking into account.

Jason Haeger said...

Phil, Perhaps it's not merely our perceptions, but the "field" in which our perceptions lie.

I've been pulling more ristrettos myself, lately, and I find that I don't usually prefer it, but for the specific blend at our shop, the flavor is much improved. I have known this beforehand, but had never really done a taste comparison.

I think the blend has MUCH to do with how the dose impacts the flavor. For instance, there is MUCH more to be found in Rocket's Classic Espresso blend with a lighter dose than with a ristretto, however, the flavor is ruined with AAH! Coffee's Super Tuscan when pulled from a lighter dose than from a heavier one.

A variable not even mentioned yet:

The specific coffee being used in the process.

I do believe that there are many blends and many single origins that benefit greatly from a heavier dose, or ristretto. I also believe that I get more out of blends designed for a lighter dose than I do out of blends designed for a heavier dose.

Then again, that could just be a sign of my lack of experience.

Jaime van Schyndel said...

You could have broken this down into a few smaller posts ;-)

Send me an email when you get a chance @ barismo account. I have some cupping notes to share and some commentary about competitions I want to leverage on you.

I like the train of thought. Challenge everything.

Owain said...

Hi guys,

It's an interesting discussion. But some fundamental principles should be observed. If you have a container under a pressure of 9 bars - every part of the container is under 900,000 Newtons per square meter - in every direction. 9 bars is equivalent to 130 pounds per square inch. A standard filter basket is about 2.3 inches in diameter. Pi x radius squared gives an area of 4.15 inches squared. 130 x 4.15 ~ 540 pounds of pressure covering the 'top of the puck' - minus the minimal pressure relief due to the release of the extracted coffee from the system.

This would be true if the top of the coffee puck were solid, and offering equal and opposite force - but it is not. The reality of pressure means that every particle of coffee is under equal pressure IN EVERY DIRECTION - except the bottom of the filter where the pressure is released. This is why coffee is found up in the group and stringent back flushing is required to clean out the excess from solenoid valves. If "540 pounds of pressure were constantly pressing down", back-flushing would certainly not be required. The reality is that it is also pressing up... and left... and right!

If the coffee has not been over-tamped/over-ground - the grounds are practically suspended while the pump is operational - hence the expansion.

Pre-infusion is for two large reasons. One is to 'soften up' the grounds which have been poorly ground, over ground and tamped too heavily. The other is to raise the temperature of the coffee slowly. Immediate, constant, high temperature/pressure water being applied to the grounds scolds and burns the grounds. Much in the same way as cooking... type "raise temperature slowly" (with the quotes) in to google for a raft of chemical reactions which benefit from the same treatment.

I hope this helps to develop the debate further... anything for a better cup of coffee!