How about that pre-infusion huh? For about twenty years I have had an ever increasing interest in espresso and one subject that I have not gained much traction with is pre-infusion. Does it do anything? Does it yield espresso extractions that are better, or just different?

I finally decided the way to find out was to implement a controllable pre-infusion mode in my home espresso machine. The middle position of the brew lever of the E61 group on my machine opens the brew path without running the brew pump. If the machine is supplied with mains water pressure, this allows pre-infusion of the coffee until the lever is moved up to the brew position, when the pump starts. When I mentioned I intended to modify my machine to incorporate manually controlled, line pressure, pre-infusion, Richard Muhl, Technical Services Manager at Five Senses, suggested I use a timer to delay the pump activation instead. This would give much better reproduceability of the pre-infusion time. Well that turned out to be just the start of the modding exercise.

The machine started out as the first version of the Expobar Barista Minore (aka Brewtus in the USA) which included a simple digital temperature controller with a 1 degree C dead band, a vibratory pump, a pressure gauge on the steam boiler only and a ‘hot’ touch steam wand. Within a year or so I had added a cool touch steam wand, plumbed in pressure regulated mains water and a drain and also added a shot camera so I could monitor naked extractions.

A few months ago I started the major upgrades and this required a bit of a ‘tear down’ because I had to make way for a few relays, a 12 volt DC power supply, the pre-infusion timer, a brew path suitable for mains pressure and rotary pump and a lot of extra wiring. I also wanted to be able to indicate every event and process occurring within the machine using LED and neon lighting.

Following another bit of Rich’s good advice I decided to use a solenoid valve to isolate the mains water pressure from the brew boiler. This valve would only open when the brew process was initiated, rather like the steam boiler auto fill solenoid valve, which only opens when the controller circuit detects a low level in the steam boiler via the level probe. I also needed to install a thermal expansion valve to bleed off excess pressure due to the thermal expansion of cool water entering the heated brew boiler. The brew valve is only designed to handle pressure from one side (the inlet side) so a one-way or check valve was fitted to prevent brew boiler pressure bypassing the brew valve. Finally, the brew path needed a connection for the new brew gauge. After a bit of juggling the new brew path was squeezed into the bottom of the machine.

I was pretty keen to reduce the pump noise of the original machine so I chose a rotary pump and mounted it inside the espresso cabinet. The rotary pump allows me to adjust the pump pressure right at the pump head. The pump is configured for dual water outlets. The idea here is that when the steam boiler fills, there should be a negligible effect on the pump output pressure, maintaining constant brew pressure during extraction.

A brew gauge and PID temperature controller was installed to bring the machine up to the spec of later versions of the Barista Minore. I have never been a fan of the power switches on home espresso machines so I opted for a soft touch switch with a blue LED. This particular switch cannot handle the current drawn by the machine so I used a beefy relay rated for light contactor duty to switch the mains current. The red neon adjacent to the switch indicates that the contactor is working. The pre-infusion timer simply delays pump activation when the brew process is started. The E61 lever is raised and this closes the brew switch which opens the brew solenoid valve via a relay. The timer is started and when it finishes, the pump is activated via another relay.

The switching of current to the brew boiler element is isolated from the PID controller with a solid state relay as it is in later versions of the machine. Unchanged is Expobar’s use of the pressure-stat contacts to prevent brew boiler heating while the steam boiler is heating. This allows the machine to draw less than 10 Amp and means it can run from a standard (Australian) household power outlet. The original controller unit is used to handle steam boiler filling sensed by the level probe and it has the additional benefit of cutting power to the steam boiler while the water level is low.

The 12 Volt DC power supply allowed me to run a low voltage control circuit from the brew switch to control the pump and brew valve relays. Additionally, the power supply was convenient to run the LED lighting. Unfortunately the inside of a coffee machine is a bit hot for power supplies and SSRs so a cooling fan was installed to bring the internal temperature down from around 65 deg C to around 35 deg C.The first fan I tried cost about $18 and was way too noisy so I found an ultra quiet PC fan for $3 more and it produces a barely noticeable hum.

Red neons indicate brew and steam boiler heating.

Green neons indicate the state of the brew valve, pre-infusion timer, pump and steam boiler auto filling valve.

In standby mode we have a subterranean aura of blue LED lighting which changes to green during the pre-infusion phase and red during brewing.

Some small blue LEDs concealed behind the E61 group shine continuously to enhance the bling factor! And yes I think I am getting better espresso shots. I always use a naked portafilter and monitor every shot with the shot camera. I have settled on 5 seconds pre-infusion at 4 bar and then brewing at 9 bar (as indicated at the brew gauge) however, your mileage may vary, as they say. Happy extractions!

By our tech Graeme Burton

Be the first to know


Simply fill out your details to get the latest coffee news direct from us.