The Impact of Pre-Infusion on Espresso
When I think about the most delicious things I have eaten and drunk, one aspect of these experiences, entirely unrelated to flavour itself, always comes up… viscosity. I’m obsessed with mouthfeel. Whether it’s veal demiglace, chicken jus gras or extremely old fortified wines, the texture of the fluid inevitably plays a leading role in the wonderment of the tasting experience.
This morning I’ve spent a ridiculous amount of time searching for an online video of Joel, a Canadian barista who was based in Calgary, pulling shots with amazing and mesmerising viscosity. For whatever reason that video now seems to be unavailable on the WWW! Joel was using a Slayer Espresso machine to pull these shots. These machines distinguish themselves by engineering in two pathways for brew temperature water to pass through the portafilter. One pathway flows via a needle valve which is user adjustable and which Slayer recommends setting to a flow rate of 60 mL of water per minute, the other allows for full 9-bar extraction.
The so-called Slayer Shot involves a dual-stage approach to coffee extraction with a protracted initial pre-infusion at low flow/pressure using the needle valve, and then subsequent full pressure extraction. The low flow/pressure pre-infusion allows the barista to make one small but extremely significant change to the extraction process. This approach to pre-infusion allows for a much finer grind size to be used and for different flavour and texture characteristics to be obtained from the coffee.
I first started experimenting with this style of shot about a decade ago, and it’s the way I now make all of my espresso. I have a machine that allows me to run 2-bar pre-infusion (I do this for 20 seconds) and then move to full pressure extraction for another 12-13 seconds. I generally grind 22 grams of coffee into the portafilter and extract around 26 grams of espresso in the cup. As mentioned at the start of this post I really wanted to produce espresso shots with ridiculous viscosity, however, as with so many things, I double crossed myself. I prefer the flavours from a flat burr grinder as opposed to a conical burr grinder. Tragically conical burr grinders produce coffee with greater viscosity as a result of their bimodal grind size distribution and the production of small diameter coffee grind fines which increase the dissolved solids ratio! The trials of being a coffee nerd.
I am no engineer, and I am certainly no expert in fluid dynamics. If anyone on this distribution list is either of these then can you please educate me about the relationship between grind size, flow rate, and extraction rate for coffee please? In the mean time if you’re fortunate enough to own an espresso machine that allows you to run a long low pressure pre-infusion then I’d be interested to see if you get some interesting results using the Slayer Shot methodology!