What makes a good espresso machine? I always suggest newbie's and expert alike start off by reading
Dr. Ernesto Illy's article on what it takes to make quality espresso. In it he specifies two variables that must be controlled in an espresso machine; brew temperature and brew pressure. Based on Illy's statements, I like to point out that any espresso machine which allows you to tune the temperature and pressure variables, is a machine capable of making fine espresso. Once those two variable have been reduced to constants,
it's then a simple matter to master the techniques involved in the production of espresso it's then up to the individual to master the techniques involved in making espresso to the best of their abilities and as far as their interest takes them.What are the differences between the types of espresso machines? First, I'll eliminate some coffee makers that are commonly referred to as 'espresso makers' but in actuality aren't capable of producing espresso. In this category I include all moka/steam pots, the Aerobi Aeropress, and any other machine that can't produce and maintain a brewing pressure of at least 9 bar.
That leaves a lot of consumer, prosumer, and commercial espresso machines to discuss. Consumer machines are fairly inexpensive, with small boilers usually made of lightweight material, and use parts that wouldn't stand up to the day-in/day-out constant beating a commercial machine must be made to withstand. A prosumer machine is more robustly built, with larger boilers and other parts, but still not as solidly built as commercial machines. Commercial machines are built to take a beating, with heavy duty frames, large boilers; overall they're built like tanks and last for decades if taken care of.
Within the three general categories of espresso machines (consumer, prosumer, commercial) are four types of machine control: manual, semi-auto, automatic, and super-auto. Manual machines are lever operated, and there are two subsets of lever machines; all-manual and spring-activated. Semi-auto machines have on/off switches to control the flow of water. Automatic machines use a flow control valve and electronics to control how much water each shot is given. The super-auto category is made up of machines with built in grinders and with electronic control of every aspect of the espresso making process.
Then there's the matter of boiler configuration: Single boiler/dual use (SBDU), heat exchanger (HX), and double boiler (DB). The SBDU is most often used in low to mid-range consumer machines. The SBDU uses the same boiler to brew the shot and heat the water to make steam for micro-frothing. To switch from brewing to steaming a switch is flipped that increases the boiler temperature from ~200°F to ~300°F. Because of the difference in temperature for the two functions, it's not possible to brew and steam at the same time.
The HX machine uses a heat exchanger built into the boiler to provide brew water at the proper temperatures. The boiler temperature is maintained at ~256°F, and fresh water pumped through the HX is flash heated to the correct brewing temperature. Because of this configuration it's possible to pull a shot and steam milk at the same time. This design is used in some prosumer machines and in the majority of commercial machines.The DB machines have two boilers; one for brewing and another for steaming. Each boiler is maintained at the ideal temperature for each function, resulting in a machine that's both simple to use and is capable of superb temperature stability. This design is used in high-end prosumer and commercial machines.
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