The discussion below is restricted to what's called "semi-automatic" machines: those where you grind the coffee yourself but the machine has controls designed to regulate temperature and pressure. "Super-automatic" where you put in beans and water and they put out coffee are out of scope here.
The basic principle of espresso is simple: you grind up the coffee, pack it down and then force heated water through under pressure. The difference between swill and pure liquid perfection is in the details. Moreover, if you're going to get the details right, the first thing you need to do is get them consistent; the exact procedures and settings you need differ with each coffee and each machine, but if you can be consistent then you can dial them in over time. [Aside: when I took machining in college, the first thing the instructor told me was that machining wasn't about cutting metal, it was about measurement. If you could measure accurately, you could cut accurately.] The major variables you need to control are:
- The coffee itself.
- The grind.
- The amount of coffee.
- The dispersal into the portafilter basket and the tamp.
- Water temperature.
- Water pressure.
The coffee is something you buy, so you have some control over it but not complete control. With the right grinder, you can completely control the grind and the amount of coffee. Dispersal and tamp is a matter of personal technique and practice. With the right espresso machine, you can control water temperature quite precisely and with any pump machine, pressure control should be quite good. So, as you can tell, this is primarily a matter of getting good equipment.
The grinder thing is pretty simple: get a burr grinder with enough adjustments. Don't get a doser. Get one with a timer. A little elaboration: blade grinders (the cheap canister ones that you can buy for $20-$40) don't do a good job of getting you a consistent grind. The individual grounds aren't the same size and you can't control the overall size except by grinding longer. Don't buy one. You want a burr grinder and you want one that allows you to adjust the grind finely and over a large range. Different beans require different grinder settings, so easy adjustment matters if you change beans much.
The reason you want a timer is to let you control the amount of coffee you grind. This is a parameter people usually specify by mass, but using a scale is a pain in the ass. Grind time is a good proxy here. What I typically do is make some test shots and then set the grind time on my grinder (it has 3 presets). Then when I want to pull a shot I just put the portafilter under the grinder and hit the right preset button. None of this requires much thought once you get it wired.
There are lots of good grinders. What I have is a Baratza Vario. There are two features I like about this. First, it has easy adjustments with two slides up front, one for macro (espresso versus drip) and one for micro (grind fineness once you've selected espresso). Second, it has timer presets, which, as I said earlier, is super-convenient. There's a rest for you to put the portafilter on while you grind, but you need to hold it there or it falls off. I notice that Baratza now makes a weight-based Vario W. This seems like a good idea, but I don't know how well it will work with espresso, since you don't want to grind into a hopper but right into your portafilter, and it's not clear how the scale integrates with that. One caution I would have with the Vario is that the really gross burr adjustments are done with a hex wrench (included). They're easy but kinda scary (keep turning until the motor starts to labor), so if that freaks you out, you might consider another choice.
There are a lot of choices in what kind of espresso machine you buy, but let's get something out of the way now: espresso machines have pumps. Yes, you can buy a cheap machine that works off steam pressure, but that's not what you want.
The central problem that dictates the design of an espresso machine is this: The water you use to make espresso needs to be at one temperature (~200 F). The water you use to steam your milk needs to be at steam temperatures (~250 F). If you're going to make milk drinks (I don't, but Mrs. G. does) then you need to somehow address this. There are four basic approaches that I've seen:
- Have a single boiler and a switch that selects which temperature to maintain at (a single boiler machine).
- Have two boilers, one at each temperature (a double boiler machine).
- Have a boiler set to steam temperature and use a heat exchanger to heat your water to espresso temperature.
- Have a boiler set to water temperature and an electric thermal block heating system to make steam.
Single boiler machines are basically a terrible solution for more than about one or two people if you want to make any kind of steamed milk drink. Here's what the procedure looks like if you want to make a latte: set the thermostat switch to "water"; pull a shot; set the thermostat switch to steam; wait for it to heat up; steam your milk. This is all reasonably fast because the boiler heats up fast. However, say you want to make another latte. Now you have to set the thermostat back to water and wait for it to cool down, which can take minutes. You can accelerate this some by just running water through the group head which pulls cool water out of the reservoir into the system, but basically it's a pain. I've used this kind of machine in an office setting and it sucks.
The obvious (and best) solution to this problem is to have two totally separate boilers, with one set to water and one set to steam. This is of course more expensive, especially since manufacturers seem to have decided to engage in a little market segmentation. To give you an example, Chris Coffee's cheapest double boiler is the Mini Vivaldi II at $1995. They'll sell you a Rancilio Silvia (a very nice single boiler) for $699. This isn't an uncommon pattern: many double boiler machines sell for more than twice what a good single boiler would cost. I don't know anyone who has bought two singles instead, but it's sure occurred to me.
The other two solutions are compromises. In a heat exchanger machine, the boiler is set to steam temperature and then the water for the espresso runs through a tube set inside the boiler, thus heating up on the way (good description here. The idea is that as the water is being pulled out of the reservoir and onto the coffee it heats up. The obvious problem, however, is that when you're not pulling espresso, the water in the heat exchanger tube is heating up eventually to the temperature of the steam, at which point you're back where you started, as is the heavy metal group head which provides a lot of thermal intertia. Standard procedure here is a cooling flush which means that you run some water through the (empty) portafilter/brew group to get it down below the right temperature. Then you quickly pack the portafilter and pull your shot. This all requires some coordination.
About a year ago, QuickMill came out with a new machine (the Silvano), which has a single boiler for the water and a thermoblock for the steam. This has the advantage that you can tightly temperature control the water and the group head and still get decent steam fast. The steam isn't as good as it would be if you had an actual boiler, but it's pretty good, so it's a reasonable compromise. And since the water side is temperature controlled, you get to pull a predictable shot without much messing around, which is what I, at least, am after. It shouldn't be surprising at this point that I have a Silvano, which I'm pretty happy with. Here's what it looks like pulling a shot of Four Barrel Ethiopia Welena Suke Quto (and no, those two little spurts onto the backsplash are not intended. That's evidence of tamping error.)
Oh, one more thing: the water supply for espresso machines can either be plumbed (there is a water tube coming from your pipes) or unplumbed (there is a water reservoir you have to refill). Plumbed typically only comes on higher end machines. I don't know if it's worth stepping up to one of those machines to get plumbed, but I do know that my Silvano is unplumbed and I wish it were plumbed. It's pretty annoying to have the shot already to go and realize you're out of water. Doubly annoying if it's your last shot worth of coffee.