Pump pressure

Pressure profiling used to be quite the thing as only a few machines were advanced enough for this feature. Nowadays, the technique is more widely accessible, but people seem to be in two minds about it. I’d like to go into more detail about how pressure profiling can be used, and how changing a stable brew pressure may affect your espresso brew.

Pre infusion

Pre infusion is probably the most widely available pressure profiling paramter nowadays. Some machines have the option of hard pre infusion, where the group head releases water at the main infusion pressure, pauses for a certain amount of time, and then continues to brew. Soft pre infusion, on the other hand, means a gradual increase in brew pressure. Hard pre infusion is something I tend not to recommend because it covers part of the acidity of a coffee, which should be something unique and pleasant if the coffee is well roasted. However, some experiments with extreme hard pre infusions have produced very funky and delicious brews. I will elaborate more on this in a different journal entry.

Soft pre infusion is a tricky thing. A gradual increase in pressure means less channeling from the start of the shot when using low doses. It also allows the fines in your puck to get more settled, resulting in an increased flow rate afterwards which is desirable. This is one of the things that makes a Slayer espresso machine capable of very high extractions.

I personally think that soft pre infusion can definitely help with some coffees that are a bit too harsh or difficult to extract, but it should not be necessary. It gives you more freedom in the coffees you choose, more freedom in brewing recipes and more leniency in techniques. So in a way, this innovative technique that only recently became almost standard in high-end espresso machines is mostly suitable for users that use mediocre coffee or have unskilled baristas working.

Main infusion

Since pressure influences the speed of your extraction, it is only logical that a higher brew pressure should lead to quicker extraction. However, extraction speed is not the only thing we go for when brewing espressos. One of the things about espresso is that a regular brew should not take more than roughly 30 seconds, since after that time the core of the coffee particles will start giving off flavours, disturbing your total extraction. We therefore generally try to extract as much as possible within 30 seconds without inducing off flavours.

For regular espresso brews, I recommend using a brew pressure that gives the highest flow rate. This allows you to grind finer to reach the 28 – 30 second mark and therefore increases your extraction potential and evenness. You will notice that as the pressure increases, your flow rate increases, but only up to a certain point. Generally, from 9 bars and up, the initial water flow puts so much pressure on the coffee puck, that the resistance is actually increased. If you measure shot times with varying brew pressures, you may find results similar to the following table:

Pump pressure in bars Average shot (30 sec) mass in grams
7,5 30
8 33
8,5 35
9 31

From this experiment, you can see that brewing at 8.5 bars gave a higher flow rate than brewing at 9 bars. You can take this into as much detail as possible to reach an optimal brew pressure.

Of course, working on lower brew pressure is also an option. For some coffees, this may result in completely new flavour profiles or a smoother body. However, a brew should generally only become better at these pressures if there’s something wrong with the coffee or the roast. For instance, a flicked roast may have some harsh, bitter flavours that can be reduced down by working at a lower pump pressure. A well-roasted, high-quality coffee, however, should usually be extracted as highly as possible, which requires a higher pump pressure.

Post infusion

Lowering the pump pressure at the end of your shot may create a bit more evenness and less perceived bitters in the cup. The last bit of water tends to extract mostly bitter flavours, and extracting these less efficiently (with a lower pressure) will cause fewer bitters to be extracted. Also, at the end of your shot, the coffee puck tends to have withered so much that to a certain extent, channeling is already happening. This leads to increased and uneven flow rates. A lower pump pressure may reduce the impact this has on the brew quality.

As a generic guideline, one can assume that a stable flow rate throughout the entire espresso shot is best. Tests with the Decent Espresso Machine and the Sanremo Opera have produced stellar quality brews with this hypothesis. Moreover, it creates more uniformity between shots, making it easier to dial in a coffee.