Understanding Espresso: Pressure (Episode #6)

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"Grains of Dust" by Red Licorice

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[benstrong]

Got my job as a Barista because of this guy! ☺️

[miguelantoniogonzalezceden]

I started watching this channel for a French Press Recipe. 3 SCA Certifications and a Pandemic Later, I am about to start a part-time job as barista while finishing a Master Thesis in Chemistry. Still come to this channel to listen to the British soothing voice of James and enjoy coffee stuff :)

[CoffeeReviews]

This is a really impressive breakdown. You can hear the knowledge, passion and enthusiasm for a pretty complex subject. Amazing work as always James

[williamadam]

Hi James, another explanation for pre-infusion is as follows: First of all, each puck of coffee has a voids, moisture, and solids. The moisture content is likely really low to start off so lets just consider the voids. These voids are only air-filled to start and thus are susceptible to compaction under any pressure. The 9 barr pressure will do this. However, from my knowledge, if water was filling these voids(pre-infusion), this would have a closer to equal hydrostatic pressure pushing back which would then be replaced over and over by the pressurized water passing through the puck. This is only from my limited knowledge of fluid mechanics and how sands in foundations pack together but hopefully, this helps a little bit.

[econnormist]

I know you said it was the last episode of the series, but I'd be super interested to know how the coffee itself is a variable. Particularly how the variety, process, age and roast all effect the other variables you covered. Maybe a future weird science episode?

[tallbikercat]

As an engineer, pressure is a result of flow against some sort of constriction (Cv). This Cv is usually used to determine pressure drop within a system (piping), but is also influential on the standing pressure behind the source of constriction. In the case of your espresso machine you have two constrictions...the primary constriction being the puck, and the secondary constriction being the orifice prior to the puck (for your pre-infusion). The orifice is a fixed constant as the size of the orifice never changes, scaling aside. The puck on the other hand is a highly variable constriction point and can be influence by a multitude of variables...grind size, grind uniformity, tamping force, oil content in the beans, moisture content of the beans, roast, etc. The result is that you can have a wide variance in flow rates at a given pressure depending on the variables affecting the constriction seen at the puck. If extraction is primarily a function of contact time, and to a minor degree pressure (as pressure affects solubility of various components, as does temperature) I would intuitively think that flow (your output) would be your primary control point regardless of other factors. Fine tuning the other factors (grind, tamp pressure, etc) affecting you puck will only improve you extraction from there, but get the flow proper first regardless of the pressure.

[MrHarlequin]

This time last year I thought coffee was a hot drink and certain high street chains did posh versions.

Then I saw James saying “don’t get an espresso machine unless you want a new hobby”.

It’s not a hobby, it’s an obsession and with each video I’m going deeper down the rabbit hole. However, I honestly believe that it, and in particular your videos have helped keep me (relatively) sane through this period, so thank you.

Off to pull levers and dream of an iPad version of the Decent ☺️

[person]

Now that i understand espresso, i need to learn how to make enough money to get an espresso machine

[SebastiaandeBeer]

Andy Serkis as Barista: ‘MY PRESSURES!’

[itsme]

Me on my Aeropress: ahh yes im reaching 9 bars

[KyleRowsell]

Excellent video as always James!
I flow profile my espresso daily on my Lelit Bianca. It has a needle value Flow Profiling paddle to adjust flow at the group head similar to a slayer steam. I love long pre-infusions, then ramping pressure up and finally backing down to about 6ish bars for the final 5-10 seconds to reduce over extraction. It’s been working well.

[McTAnGuS]

Anyone else have family and friends that ask for a coffee then when they oh wow that was amazing you dive into the details and they are like yeh next time I'm going to ask for tea. Thanks James ☺️

[pirminborer]

Watching this while pulling a shot on my la pavoni, while I was preinfusing and then ramping gently the pressure up, then maintaining flow rate, hoffman starts to talk about pressure profiling and adjusting for flow rate. Wondering if one day an automatic machine could do that. It's not just flow rate but you can see if you got good crema, chanelling or uneven extraction. I adapt for it. Certain shots you can maintain the pressure quite high, with evenly distributed flow with tons of crema. You feel and see when you pull that perfect shot. Other times you have to compromise, but it still can taste very good. Even if it's more work a manual lever, the feedback you get is priceless and you really learn a ton about coffee extraction intuitively.

[bspark]

Can’t believe James mentioned fines migration. This is a fairly well understood concept in Earth Science (Soils and Geology) and just reinforces the idea that Coffee, Espresso in particular, really is a science as well as an art.

[treyrees]

I would love a retro review series where you show us all the important espresso machines from throughout the decades. We'll be able to see how the machines developed over the years to achieve better espresso

[edmichaelson]

Whew!!
Here is my take:
(BTW Enjoyed the video)
There are so many variables
-Different beans
-Age of bean
-Storage of bean
-Grind size
-Grind amount
-The roast
-Weight
-Volume
-Type and shape of basket
-Type and hardness of water
-Temperature of water
-Tamping force
-Extraction time
-Extraction pressure (and pressure/flow profile over any given extraction time)
So you see where I'm going here
You're either a lumper or a splitter or somewhere in between
Practically speaking there are too many variables to address or over which you may not have control
You can get a good shot out of most beans making adjustments if necessary as the beans age
In fact you can get almost identical shots with different sets of settings
Sort of like cooking good results roasted, grilled, fries broiled, sauteed, sous vide
Not totally unlike wine e.g. vintage year, storage temperature, grape, color, nose, bottle size and with hints of black current, raspberry blah blah
Bottom line you either like it or you don't (although someone else might)
If you don't - tweak

[tlniec]

I really resonated with James' discussion on early lever machines and how they produce what I'd call serendipitous pressure profiling. It's always interesting when this phenomenon is seen in engineering - a new innovation makes an obvious improvement by design (faster brewing in this case), but also has unforeseen off-target effects that make it even better.

[BiggFanDDD]

I got a Flair (first espresso machine ever) and I've gotta say I absolutely love it! James's recommendation and review was excellent! Since I'm a total espresso noob, I don't know too much about pressures, but I have noticed that with higher pressures (around 9-10 bar) I often get quite a bitter brew, but with 6-8 bar I get the perfect cup.

[JanHaHe]

The new upload frequency is such a treat. Love your content James!

[ruisearts]

My country is in ruins, our currency is devalued in record-breaking hyperinflation, and I'm here listening to James explain pressure, learning moreover what I have studied. Like a scene in a film where it's the end of the world outside the window, and I'm inside listening to a beautiful record.