Weighing it all up Part II; Going with the flow

Bear with me on this one.

I recently visited an Olive oil factory in Italy and stumbled upon a happy coincidence. The factory owner took olives in from all the local farmers, washed them and sorted them and lumped them all into a massive mulcher which fed them into a giant press. You can probably see where I’m going with this, but try and act surprised.

So, via my translator (thank you Giovanni, kind Sir) I asked him how much he took from the olives. He replied that it depended on how many olives were in the vat. “Ok, so in terms of percentages, how much Olive Oil would you yield from a batch” I asked. (this actually happened). He paused, thought for a moment and replied. “On average, depending on the farm, olives, time of year and other variables we would usually look to yield maybe 18-20% of the overall weight.

I shit you not.

This astounding revelation led to incredulous looks from my host once I revealed why I was so excited by this common target. I felt we had somehow been united as brothers in the battle for great taste. He just thought I was a mental foreigner and carried on working away. Some battles aren’t worth fighting I suppose.

Anyway, once I had regained his attention I questioned him a bit more on this process and he outlined how the olives are actually re-pressed afterwards to make a cheaper commodity grade oil but the really astonishing stuff, the peppery apple-like green oil is found at this 18-20% mark. Anything after that just isn’t as tasty.

Now, what was really interesting is that he emphasised how important it is to ensure that all olives are pressed equally because in (say) a 100kg batch if some are pressed dry and some are left untouched it is still possible to yield 19kg of oil but that oil wont taste as good as 19kg of oil yielded from olives that were pressed equally and consistently.

So, by this stage you should realise where I am going with this simile but as always it isn’t as simple as that. Anyone who has ever played with sieves will realise that espresso does not work with a uniform grind, it either gushes or chokes depending on where your peak lies. Sometimes it does both as the initial resistance is eventually eroded.

A two peak grind (where the vast majority of the coffee grounds are two sizes, one much larger than the other) is best suited to espresso as this facilitates extraction using the design of the machines that we use (e.g. 9 bars, 58mm, 93c etc.). Whether this is by error or design is another argument. It is what it is.

I often liken this to a brick wall in rural Ireland, made mostly from certain size rocks but backed up with many other sizes with sturdy and solid results for all.

The resistance this packing creates allows sufficient resistance to slow the water flow through the puck to allow the necessary extraction. Thats all espresso is really.

Here’s where it gets complicate though. The smaller pieces will not only extract more than the big ones, but they will also force their way to the bottom of the puck and clog the flow.

Resistance builds and eventually the water finds a way out and then you get nasty, acrid, watery streams from your espresso. The danger in all this is not only that it tastes bad, but also that it adds weight to your espresso (should you be weighing it!)

Still with me on this? Ok.

So, this long meandering point is that just weighing out you 1:1.55 ratio does not guarantee you a great shot because that weight could be made up of a lot of things. The nature of espresso grinds is such that we can’t guarantee an even extraction across all particle sizes but a degree of relative consistency is definitely achievable until the single peak espresso machine/grinder combo comes along.

Imagine a scenario where our initial flow is high TDS but fine migration leads to a “clog and spit” which then adds watery streams to our cup coming via channels through the same area of grounds. In theory, and often in practice, this can yield us the “perfect” 19%/11% shot. We’ve over-squeezed some olives and left others untouched.

We recently took out the pre-infusion chamber plug in our Aurelia, thus increasing pre-infusion by 3 seconds (?) and which has visibly and tastily improved our shots. Essentially it wets the puck through before we reach 9 bars of pressure.

It seems like a massive u-turn in the context of the last post but just watching the flow and its ease of passage has really aligned the tastiness with the flow. Admittedly this has been done with a scale under the cup but my understanding of flow and pressure and its effect on extraction has really escalated in the last 2 weeks.

Facilitating this flow and guiding your water through your puck is the most crucial job of the barista and undoubtedly the real driver behind pressure profiling, levers, paddles et al.

A lot of this post is purely a brain dump for me to get my head around this idea. I often come up with ideas when I’m half way through explaining them, it just happens that way sometimes. I also wanted to dissuade people from using the 1.55 ratio as a one-stop-shop for perfect espresso.

I stand by the 1.55 but only in the correct context. I can’t speak for my esteemed colleague in the olive factory but I’m sure he would agree in his own peculiar way.

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7 thoughts on “Weighing it all up Part II; Going with the flow

  1. Colin, how do you feel about pressure-profiling (for lack of a better term)? I see a tiny bit of an analogous relationship between roasting with profiles and, to use your term, facilitating the flow and guiding the water through the puck, so I’m curious about your thoughts on the whole to-do about that (pressure-profiling). A few folks have chimed in, especially David Schomer, but I’m not convinced that he — or anyone, for that matter — has enough experience with this technology to really give it a thumbs-up or thumbs-down.

    Ironically enough, I keep seeing parallels between pour-over brews, V60s in particular, and espresso extraction. After reading this post, I continue to hold to that (even though I think many would disagree with me).

  2. Tumi Ferrer says:

    Nice post, Colin. I’m not kidding, I read the whole thing with my jaw dropped down in amazement about the olive oil factory. Kind of makes one think if we’re dealing with something universal…

    Anyway, I think essentially we should never stop tasting the outcome; extraction numbers are only to help us pointing out what’s wrong. I’ve always been a little bit worried about how technology makes one feel like the machine can do everything for you – like in movies where the bad guy is a computer genious, and therefore able of bringing down a helicopter via internet.

    But we’ve grown up since then 😉

    I agree with you on weighing the espresso; in my mind that’s really having a better understanding of that parameter.

  3. Mark Dunne says:

    Fascinating, learned loads from this post. Thanks.

  4. Chris Capell says:

    Fantastic reading again, Colin. Your understanding mirrors my own, but you’ve provided some really great new analogies and metaphors I can use when training staff.

    I’m extremely interested in hearing more about the removal of the pre-infusion plug from your Aurelia, and the effects you’ve seen since on flow. I’ve been using one at a new shop for two months now, and I’m not entirely happy with the pre-infusion and the (un)evenness of the flow during a shot. I’d love to email with you about it or talk on the phone if you have time, but I’m also sure there are other Aurelia users who would benefit greatly from you sharing your how-to and resulting impressions in a blog post — again, your time permitting!

    Thanks again!

  5. Ed Buston says:

    Great post and fascinating about the olives. I can’t seem to understand the concept that to get the best espresso we need to use an uneven grind. Surely an un even grind is going to equal an uneven extraction making the TDS reading unaccurate/not helpful.

    Everything about espresso points towards even extraction even this post!, surely this is only possible with consistently sized coffee grinds. Daily I use a versalab M3 grinder that grinds the coffee through both a conical blade set and a flat burr set giving a very precise grind I also use a Mahlkonig Tanzania that also creates a unimodal grind size, both these grinders produce tastier espresso than the more traditional Mazzer grinders I use. My limited experience of using sieved grinds has also produce superior shots.

    My advice up until now has been get a good grinder and keep your blades sharp as blunt blades lead to a greater variability of grind sizes.
    Do I have to make a U turn too?
    Thanks for getting my brain working again!

  6. Hugo says:

    Great wall… they don’t build ’em like that ‘ere.

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