Water debit. Flow rate. Gicleur. Jet. Preinfusion. Progressive preinfusion. Pre-brew.
There's more than one way to skin a cat, as they say. Espresso is supposed to be pulled at around 9 bars to achieve its distinctive texture, but of course that is far from the whole story. How you get to 9 bars, it seems, is pretty important.
Slayer's method is unique: when the brew actuator is in the middle position (pre-brew mode), water pressurized to 9 bars flows through a needle valve that allows you to set the water debit
(flow rate) as low as you want. While most espresso machines have a water debit of 8 or more grams per second, the Slayer's water debit in pre-brew mode is generally set to 1-2 grams per second. With that setting, there may be 15 seconds or more between exposing the coffee puck to water and applying pressure to it—and the rate of the pressure ramp after that is more gradual compared to a typical espresso machine's. However, when the Slayer's brew actuator is in the left position (brew mode), the water passes instead through an 0.7 mm flow restrictor. In this mode, the water debit is a more familiar 9 grams per second.
Compare this pre-brew mode to an old La Marzocco Linea or an Elektra A3/T1. These machines have either no flow restriction or a very wide flow restrictor. They apply full pressure to the coffee puck in about 2 seconds. This tends to result in a machine that is either unforgiving, picky about dose, or both. These machines are quite capable of pulling delicious espresso, but they can be frustrating to use or limited in the style of coffee that they can excel at extracting.
That said, machines with a 'gentler' rise to full pressure hardly a new phenomenon. Home espresso enthusiasts have long lauded the famous Faema E61 brew group. It features a preinfusion chamber, a pocket of air downstream of the flow restrictor that increases the time required for the incompressible water to fill the space above the espresso puck, delaying smoothing the pressure ramp considerably and resulting in a more forgiving espresso machine. This design pattern has been used in various incarnations in many well-known espresso machines, such as the La Spaziale S1, the Nuova Simonelli Aurelia, and the Kees van der Westen Speedster. Indeed, a "sipper" lever espresso machine that draws water from the steam boiler to fill the lever piston features a similar design, and that design precedes pump espresso machines entirely.
Another common design is to use a narrow flow restrictor. The smallest in common use is the 0.6 mm flow restrictor found in modern La Marzoccos and Synessos, among others. Like a preinfusion chamber, this flow restriction delays the rise to full pressure and makes for a more forgiving espresso machine.
Lastly, there is line pressure. Many espresso machines (famously, paddle group La Marzoccos and the Synesso Cyncra) feature the ability to apply water at line pressure (generally 2-4 bar) to the coffee puck prior to engaging the pump for full pressure.
Various combinations of these designs exist and have been in wide use for many years. And it is very likely that the variations in the pressure ramp and flow rates that a machine provides accounts for a significant part of what differentiates one machine's 'personality' from that of another. In that light, "preinfusion" is a rather loaded word that requires further explanation to understand in detail. However, all of these designs have one thing in common: they come to some pressure (more than 1 bar), if not full pressure, within 6-8 seconds. To differentiate their design, Slayer calls their preinfusion phase "pre-brew." In the interest of clarity and succinctness, I will use that term when referring to "Slayer style" preinfusion in this review.
In my experience, using pre-brew goes beyond enhancing the "forgiveness factor": this kind of preinfusion requires grinding so much finer, all other things being equal, that the resulting shot is very different in character. This concept is discussed at length in a previous discussion, "Pressure profiling, flow profiling, and a new rule of thirds
To illustrate how pre-brew affects the shot flow, I have prepared a series of videos. The Slayer has a brew pressure gauge on the front panel that measures pressure downstream of any flow restriction, meaning that it reads the pressure of the water on the puck during a shot. It also has an angled "shot mirror" that allows you to see how your shot is flowing through a naked portafilter without the need to bend down and look up from below. This makes capturing pressure and shot flow at the same time very convenient.
In the videos below, each shot uses a 19 gram dose, and the shot is stopped at 30 grams, for a 63 percent brew ratio. The first demonstrates a "Slayer style" shot with a long pre-brew phase (25 seconds at 1.5 grams per second flow rate) and a typical total brew time of around a minute. It also shows re-entering pre-brew phase at the end of the extraction to restrict flow, tailing pressure and flow rate at the end of the shot.Slayer style shot—long pre-brew.
For this particular coffee, a light roast of a bright, clean washed Costa Rican, a long pre-brew phase works nicely. The sweetness is ample, balancing out the tart citrus with sweet middle tones.
The second video demonstrates using a "traditional" approach simulated by performing a short 5 second pre-brew phase, using the same grind and dose as before. The result is a choker whose total time is only a little longer than the previous shot's, but whose time at full pressure is considerably longer. This tastes as you would imagine—acrid, cooked, and over-extracted."Traditional" brew profile, same fine grind.
Note that this isn't the only way to simulate such a "pressure profile." One could also loosen the needle valve to give a larger water debit (say, 6 grams per second); I have not investigated this in any detail, however.
Finally, here is a demonstration of the same coffee and dose with a much coarser grind (4 notches on my K10 Pro Barista). "Traditional" brew profile, coarser grind.
With the coarser grind, the shot flows normally and completes in about 27 seconds. This particular coffee did not work as well in this format, tasting rather sour and under-developed. In this case, if pre-brew weren't available, I would dial this shot in by lowering the dose to allow tightening the grind, and probably also use a lower the brew ratio. Note, however, that these changes would almost certainly entail sacrificing shot body and concentration to achieve taste balance.
This serves to illustrate what the Slayer is best at: making it easy to get an well-balanced, full-bodied espresso shot from a dense, light-roasted, high-grown washed coffee.
As another example of this, last month fellow forum member Mike Ivanitsky
challenged me to try a ridiculous coffee that normally wouldn't be suitable for espresso and brew it on the Slayer. I selected George Howell's La Esmeralda Mario San Jose
, a washed gesha grown at around 1500 meters. I guessed the grind based on a visual assessment of the roast degree, selected my usual ~18g dose for my basket of choice (VST 18 gram basket), and pulled a shot with about 20 seconds in pre-brew, shooting for a brew ratio of around 60-65%. The first shot was good, but a little too tart. I tightened the grind slightly, raised the temperature setting, and repeated. The second shot was amazing. The aroma was full of fragrant orange floral tones, and the shot was candied orange with tons of caramel flavor.La Esmeralda Mario San Jose espresso—a shot of fragrant orange flower honey.
I also reviewed the Chicago Trio
that Jim Schulman graciously sent us on the Slayer. They serve as good examples of when Slayer's pre-brew feature works very well, and when it's preferable to use a coarser grind and a more abbreviated pre-brew time (i.e., a more typical pressure ramp).
- C1 was a very nutty, creamy, lightly acidic northern Italian style espresso. While I was able to use long pre-brews and produce good shots, it didn't truly shine until I cut my pre-brew short and used a faster pressure ramp and coarser grind.
- C2 was another espresso blend featuring a healthy dose of pulped natural Brazil. It practically demanded a coarse grind; all the fine-ground, extended pre-brew shots came out acrid and bitter.
- C3 was easily my favorite of the Chicago trio. An SO from El Salvador, it was clean, bright, and worked extremely well with a long pre-brew phase, which made its Sweet Tart acidity pop. It also was easy to tame the acidity and bring the caramels to the forefront by lowering the dose and temperature together.