Inside Domestic Espresso Machines

To begin with, the pump. 18 Bar, 15 Bar, 9 Bar...what's the difference? In reality, none. ALL the pumps used in domestic machines are vibration pumps, usually made by either Eaton or ULKA, and all of them are basically identical. As long as the machine has a pump, you can ignore the "Bar" ratings. The rating itself relates to maximum output under ideal conditions, but in reality most domestic machines have extraction pressures between 9 to 13 bar at the group.

Then comes the heating technology, and it's here where fundamental differences which profoundly affect espresso production occur.

"Thermoblock" technology means that the water is heated in a multi channel metal block with an exterior element. In the "flat" thermoblocks used in most machines the interior water volume is quite small, and stable water temperature is theoretically achieved by adjusting the flow rate of water through the block, in a similar fashion to "on demand" household hot water systems. Practically speaking, the limited water volume of most domestic espresso machine thermoblocks make them very temperature unstable, which in turn makes it extremely difficult to achieve a good espresso shot. An exception to this is the recently developed Sunbeam EM6900 machine, see http://www.coffeeco.com.au/articles/sunbeam.html  .

By using a large, stainless steel lined thermoblock and electronic temperature control in the EM6900 machine Sunbeam have been able able to overcome most of the limitations, leading to reasonable shot quality, but as yet there are no other mass market machines using the same technology. Of course, the Sunbeam thermoblock is itself a development of an earlier Krups thermoblock.

Thermoblock steaming is accomplished by using the pump to pulse small bursts of water into the superheated block, which is then supposed to flash into steam. Again in practice steam production tends to be feeble and "wet". Even the Sunbeam machine, which has a dedicated steam thermoblock and separate pump, has yet to be overcome this limitation.

Thermoblock machines in general are cheap, and will produce average espresso and reasonable milk drinks given good technique, but anyone who wants better than average should avoid them.  Another downside of aluminium thermoblocks is that they corrode, giving them a limited lifetime and introducing a fair bit of aluminium into your drink.

Boiler machines tend to fall into definite categories:

Chinese made specials, they can be anything from low volume stainless steel (120ml or so) to 250ml teflon lined aluminium.
Gaggia, which have a proprietary aluminium boiler with 2 external heating elements and a boiler volume of 120ml;
Stainless steel with copper element, boiler volume 180ml;
Marine brass with copper element, boiler volume 300ml.

All boiler types have sufficient water volume to keep reasonable temperature stability throughout a double shot. When heated to steam temperature, the limited volumes of the smaller boilers can mean slightly less steam time and power, but all have enough for average use. In my experience the larger volume boilers have a faster "recovery time" between shots and that bit of extra steam, so that you can produce 6 cappuccinos in 5 minutes for your dinner party.

Again in my experience the Gaggia boilers tend to corrode internally. They are an aluminium upper part attached to a brass group, and the combination of dissimilar metals and hot electrically conductive water creates a galvanic battery which simply eats the aluminium. This can also happen with aluminium thermoblocks, which is why some of the more upmarket ones are now lined with stainless steel.

Finally, the larger brass boilers tend to have higher power elements than the smaller stainless steel ones, again giving better thermal stability and faster recovery.

Lelit's 300ml brass boiler. The standard S/S boiler on the left, and the Silvia boiler on the right. Big, isn't it? Inside an 18 month old Gaggia boiler. The aluminium top has corroded, the chromed brass base is in perfect condition. This close-up shows the extent to which the metal has been eaten away.

The GROUP is the bit of the machine that the portafilter locks in to, where the water is pumped through a "showerscreen" on to the coffee. All of the thermoblock machines I've seen have aluminium groups with stainless steel shower screens. The smaller stainless steel boiler machines also have aluminium exterior groups and S/S screens. Gaggias have a brass group with a S/S screen, brass boiler machines have brass groups with S/S screens. Of all domestic machines, only the Rancilio Silvia has a commercial brass group and S/S screen setup.

Portafilters and pressurized filter baskets are another point of differentiation, with (again) thermoblock machines tending to have cheap aluminium portafilters and quality boiler machines chromed brass portafilters. All this concentration on metal types may seem a bit strange, but excellent espresso production relies on thermal stability of all the components in the process. Copper and brass are best, which is why you find them in commercial machines.

Both Gaggia and Rancilio use commercial portafilters and baskets on their domestic machines, although the recent Gaggia Cubika has an aluminium portafilter. Solis and Imat have brass portafilters but non-commercial filter baskets. Saeco uses its patented pressurized portafilter, made mostly of plastic but with some brass, as does Krups on its higher end machines. The Saeco portafilter pops up on other brands of machines such as La Pavoni and Spidem, so it's a good bet that these brands are "rebadges", Saeco inside another manufacturer's body design.

Pressurized portafilters and pressurized filter baskets come in many guises, but they basically have a single purpose: to allow the customer to get the appearance of acceptable crema from stale, poorly ground supermarket coffee. Rather than buy a machine with a pressurized portafilter or filter basket, I would ALWAYS recommend that you buy a decent burr grinder. See http://www.coffeeco.com.au/newsletter/september2004.html and http://www.coffeeco.com.au/newsletter/november2005.html .

Solenoid "3-way" valves are only available on high end machines, they relieve pressure in the portafilter immediately after brewing so you can pull consecutive shots. Machines with solenoid valves are always built to a higher standard, and the fact that the valve is there tells you that the manufacturer knows something about real espresso machine requirements.

Now we get down to the hard part, what do I recommend, and why?

The single most important thing to think about when contemplating a domestic espresso machine is "Which grinder will I buy?" because if you are serious about making espresso based drinks at home, the grinder is the most important component. There is simply no substitute for fresh, correctly ground coffee in espresso making!