One of the holy grails in espresso has to do with how the hot water initially touches the carefully ground and tamped coffee in the basket. Before the pump starts (or when it runs slowly at first), the puck may be pre-wetted, or preinfused. After this, the real infusion starts: the hot water pushes through the coffee. This results in a more even extraction.
(The temperature of the water, and how stable it remains throughout one or more extractions, is an other holy grail, covered in many other articles. There’s also a lot of articles on preinfusion. Search and read.)
It’s possible to approach a humble type of preinfusion with the world’s most popular home espresso machine: the Rancilio Silvia. At least, with my version, from 2001. This is the MK1. There’s no modification needed. (Other Silviists have written about installing a switch at the pump, to pause it at the beginning of extraction for preinfusion purposes.)
It’s not as refined as the pressure profiles facilitated by manufacturers like La Marzocco, Kees van der Westen or Synesso, but these machines are in different league as the Silvia (and come with quite a different price tag).
In the boiler of the Silvia, Rancilio made a kind of throughput between the group and the steam pipe. When the three-way solenoid valve remains closed, and the pump starts, and the only open output is the steam valve, then only water will run through the steam pipe. But when the solenoid valve opens (when you hit the brew button), and the steam valve is open, then water emerges from both the group head and the steam pipe. This made me think about preinfusion.
I discovered it some time ago, when I accidentally left the steam valve open, while flushing the group. Water emerged from the steam pipe, as well as from the group. It took a while, before I connected this phenomenon to preinfusion possibilities.
A tightly packed pouch of fragrant, freshly ground beans offers quite some resistance to water pressure. Would you open the steam valve with a filled portafilter inserted in the group, and flick the brew button, you’d see only water escaping violently from the steam pipe. (You can stop the brew button instead of closing the steam valve. Inspect the slightly wetted puck: this really works.)
Now, when you close the steam valve slowly while the pump is running, this is what happens inside: the puck is slightly wetted (with open steam valve) and the water pressure on the grounds builds up gradually.
Try experimenting with this variation: with or without the steam nozzle (the piece with the small hole in it that screws in the end of the steam wand). This can make a slight difference in pre-wetting power.
Step by step:
- Prepare your extraction as usual
- Flush, preheat cup, etc.
- Insert prepared portafilter in group
- Open steam valve (half a turn will do)
- Flick the brew (coffee) switch
- Slowly close the steam valve, and voilà: there’s your own preinfused extraction
- Stop brewing when ready
Enjoy.
I’d love to receive your comments and suggestions: by mention on Twitter, or you can send me an e-mail.
My other articles on espresso (in Dutch):
Tips for better espresso: studiobronts.com/2012/02/26/tips-voor-betere-espresso/
VST baskets: studiobronts.com/2012/03/29/vst-filtermandjes-een-fluwelen-revolutie-in-espressoland/
New espresso space ship for yellowBird: studiobronts.com/2012/06/26/achter-de-schermen-nieuwe-espresso-voor-yellowbird/
–
UPDATE: On the Home Barista forum, VanBoom more or less describes a similar technique. Although we thought it up independently, he did it a few years earlier and deserves the credit: here’s the thread of VanBoom at the Home Barista Forum describing his VPT.