Better Extraction, Better Espresso
The basic shot making techniques get you decent espresso, but not necessarily the best espresso. For that, you want to find which combination of pressure, temperature, grind, and finishing color works best with the blend. Also, things do go wrong—temperatures and pressures can drift, burrs get dull, machines can build up coffee oils faster than usual and require an unscheduled cleaning. Each of these mars the taste and needs to be identified. A good barista can taste an espresso and have a fairly good idea of what adjustments are required to optimize the taste. By the second or third go round of tasting and adjusting, the blend should be well dialed in. This takes experience, but here are some tips to get you started.
The bulk of this section is dedicated to extraction options since most baristas, whether pro or amateur, have little awareness of what is available to them. In fact there isn't even an agreed upon vocabulary to describe all the variations in extraction. However, before getting into extraction options, let's cover a few tips on adjusting the pressure and temperature.
Adjusting Brew Pressure
Proper espresso extraction occurs at 8 to 10 bar of pressure. The higher pressures in this range intensify the flavors.
On low end home machines the pressure cannot be set. For the type of pump in this class of espresso machines, the brew pressure is inversely proportional to the flow rate (i.e., faster flow means lower pressure, slower flow means higher pressure). In order to stay within 8 to 10 bar, you limit the extraction to a flow rate that produces 1.5 to 2 ounces in 20 to 25 seconds. On better vibration pump machines, you can set the maximal pressure via an adjustable over pressure valve (OPV, also called an expansion valve). Use a blind filter or pressure measuring portafilter and set the pressure between 8.5 and 10.5 bar. The pressure during the extraction will be roughly half a bar less for standard or ristretto shots. Rotary pumps add a constant amount of pressure to that of the mains. They are also controlled by an OPV at the pump head, and in many cases, a constant pressure valve on the suction side. Use a blind filter or pressure measuring portafilter to set the pressure exactly at the desired shot pressure.
Adjusting Brew Temperature
Proper espresso extraction occurs at 90°C to 96°C (195°F to 205°F). Lower temperatures accentuate acidic origin flavors, while higher temperatures accentuate bitter roast flavors.
Single boiler home machines with non-adjustable mechanical thermostats have their par temperature at the point the thermostat turns off the heater. You can brew at a lower temperature by flushing a few seconds of water after the heater turns off; and at a higher temperature by forcing the heat back on by turning on the steam switch for a few seconds (turn it off prior to brewing). This is somewhat hit or miss, but can be improved by calibrating the flush and steam switch time with a thermocouple thermometer.
Heat exchanger machines are set by adjusting the pressurestat, doing a sequence of eight shots or so at your normal rhythm, and seeing where the shot temperature settles. After that, you should also adjust the flush to get an idling machine to that temperature. For one-off experiments or single-shot home use, simply adjust the flush amount (see How I Stopped Worrying and Learned to Love HXs for more details).
Machines with electronically controlled brew boilers can be set from the front panel without fuss. Vapor bulb thermostats can be adjusted inside the machine. Both types should be checked with a thermocouple thermometer to confirm the actual brew temperatures.
Manipulating the Extraction Variables
You control the extraction by setting the grinder finer or coarser and ending the shot at a certain flow color, shot volume, or elapsed time. I recommend using flow color as the way of determining the end of the shot, as discussed in the previous section. Each combination of grinder setting and ending color gives a unique combination of shot time and shot volume (when using the same basket, coffee amount, blend and machine). In fact, fixing any two of the variables (grinder setting, shot color, time, and volume) fully specifies the other two.