pressure is what makes espresso espresso it allows us to brew coffee in seconds
rather than minutes and produces the concentrated liquid gold shots that has come to define coffee
around the world so isn't it a little strange that we just tend to set and
forget the pressure on our espresso machines what if there is the perfect
pressure that we've been missing this whole time today we're gonna see what we can find out
this is part three of our series answering the most common questions on espresso extraction
this time we're hoping to answer the question what is the ideal brew pressure for espresso?
if you're anything like me when i started working as a little barista i was always taught that the
machine should be set at the sacred nine bars of pressure this is how it's always been done okay
so this is normally the point where i say it's time to shatter the mystic traditions with some
cold hard facts but this time our tests do suggest that the old nine bar tradition still holds up so
if you're in a hurry there's your answer based on our testing we recommend a brew pressure of 9 bars
of course there's a whole lot more to it than that but you're probably not interested in that so...
before we dive into the results let me set some things up with a quick note on pressure
in most modern espresso machines an electric pump forces water throughout the ground coffee
at a set pressure on older lever machines this pressure is created by a lever and the massive
biceps of the barista a lever machine is a very simple example of what baristas sometimes call
pressure profiling pressure profiling refers to the process of manipulating the pressure of the
water throughout the shot unlike an electric pump which maintains a constant pressure with
a lever machine the pressure starts high and then gradually drops throughout the shot as the spring
returns back to its original position another example of this is what we call pre-infusion this
is where the line pressure of water runs for the very first few seconds to allow the ground coffee
to get wet and saturated before getting smashed by the full pressurized force of the water and the
goal here is to prevent channels from occurring in the bed of coffee during extraction now for
these tests we didn't focus on pre-infusion lever machines massive biceps or any other ways in which
pressure can be varied throughout the shot no we tried to keep things focused on our mission
of finding out is there an ideal pressure to set your espresso machine to and with that out of the
way here's the tests that our engineers cooked up we measured espresso shots brewed at 6 bars
9 bars and 12 bars of pressure using 14 grams 17 grams and 21 grams of ground coffee we then
measured the extraction time that is how long the espresso took to brew and we measured the TDS or
the total dissolved solids using a refractometer this tells us the overall strength of the espresso
we also monitored the flow rate using our shiny new flow telemetry system oh yes and we tasted
the coffee too you know old school methods now it is worth noting that the pressure was measured
using the dial on the front of the machine which in this case was the La Marzocco Linea PB and KB90
now pressure measured at the dial will always be higher than the pressure at the group head the
pressure at the group head will also vary depending on the flow rate of the coffee
so for us we decided that the most practical way to measure pressure for our purposes is using the
dial and this is what most users will see from day to day finally we use the same coffee for
all of these tests it's a light medium roast made up of natural processed coffees which we rested
for a week after roasting well except for one of the tests but i'll get to that in a moment
the first thing that we found was that as pressure increased the extraction times also increased in
other words the more pressure we used the more slowly the shots poured now logically you would
expect the opposite to happen so what is the deal here our guess was that the carbon dioxide
gas produced by the fresh coffee was producing more and more resistance to the water as the
pressure increased so to test this theory we tried the same experiment with the same blend of coffee
except this time it was stale yum the reason we chose stale coffee was that as coffee gets
older the trapped gases slowly escape the roasted coffee beans and that's why style coffee produces
less crema sure enough with stale coffee we found that the extractions behave the way that you would
expect so it does seem likely that the gas theory is the reason that the high pressures result
in slower pouring shots this also seems to be supported by looking at the rate
of water flowing through the bed of ground coffee using the flow telemetry device the
flow device graphs the pulses from the machine's flow meter which gives a visual guide to the flow
rate of each espresso that we made the shots brewed at 12 bars quickly saturate the bed of
ground coffee but then immediately drop down to a slower flow rate than the lower pressure shots
this indicates that this resistance is occurring really early on in the process
the next thing that we found was that the TDS that is the strength of the espresso had more
to do with the extraction time than the pressure when extracting times were low so was the TDS
when extraction times were higher so was the TDS regardless of the pump pressure now that's
not to say that the pump pressure doesn't have an impact as we noted earlier when using fresh coffee
increasing the pressure help to increase the extraction time which might be a good
thing if you're using a lighter roasted coffee that's harder to extract from of course grinding
finer will also cause the extraction times to increase but the grind size will change
not only the strength but also the overall balance of the flavour extracted from the coffee
speaking of flavor let's get to the point which one tastes the best we started with our standard
brew recipe for this coffee the pump set to 9 bars of pressure and use that as our benchmark
we then kept everything else the same and reduced the pressure of the machine to six bars
the results weren't great our tasters describe these shots as watery and thinner in texture
compared to the control of course as we already discovered lowering the pressure also made the
shots run quicker so we did try a few adjustments to compensate for this we tried grinding finer
and then we tried increasing the dose and then we tried a combination of both the end result
was that they all came up short in flavor and texture compared to the shots brewed at 9 bars
then we tried the same process again at 12 bars of pressure this time our results were mixed
some tasters preferred the thicker texture and bigger bolder flavors of these shots
some felt that they were starting to taste over extracted and preferred the balance at 9 bars
none of us had regularly made espresso at higher pressures in a cafe setting so we thought we would
push this little experiment just a touch further the head barista of our espresso bar will agreed
to run his machine at 12 bars for a day just to see what the impact had on a variety of different
coffees and get some feedback from a broader mix of people and the feedback from both baristas and
customers was less than enthusiastic baristas told us that they struggled to get the coffee's tasting
right and customers told us that the coffee tasted different in milk and the end results of all of
this is that we're gonna stick with nine bars now for the home or pro barista our suggestion is this
if your roaster recommends a specific brewing pressure for their coffee then go for it otherwise
your best bet is probably still gonna be nine bars now there's definitely more to be investigated on
using lower pressure for darker roasts and using higher pressures for lighter roasts
perhaps there's some wins in there but that's gonna have to wait till next time okay so how
do you adjust the pressure on your machine well it does depend on the type of machine that you have
commercial machines and some high-end home machines have a rotary pump rotary pumps
are larger and quieter than the vibrating pumps found on many smaller home machines
if your machine has a rotary pump then the process is fairly simple locate the pump and
find the screw that controls the pressure load up a fresh shot of espresso hit go and adjust the
screw until the dial reaches the pressure that you desire for machines with a vibrating pump which
is usually found in more affordable brands then the pressure is generally not adjustable without
getting into some diy hacks if you missed part one or two of this series then click right here
and watch those next you'll definitely enjoy them otherwise i will catch you guys next time