Slayer – Leveraging Brew Pressure

support

Mar 03, 2009

Does anything provoke stronger feelings and louder opinions right now than the mere mention of pressure profiling?  In specialty coffee, I don’t think so.   For this reason alone the topic merits some discussion here.  It also merits discussion as Slayer is on the verge of introducing a machine that takes full advantage of brew pressure deltas, by which I mean the ability to brew coffee with more than a flat pressure profile, to enhance coffee quality.

Since this is a blog about the broader aspects of our industry, and not simply an advertisement for Slayer, I want to reflect on my own relationship with brew pressure profiling first, then relate it to Slayer.  So here goes. . .

How do we know fixed pump pressure is actually stable?

The concept of pressure profiling is at least as old as the development of Treuh (now Synesso).  Back then we talked about this idea mainly from two perspectives.

First, it was exciting to think we could shatter the orthodoxy of nine bars of fixed-pressure that seemed to exist mainly to accommodate the technical specifications of existing equipment, including our own.  We were already breaking the hold of “engineered mediocrity”  with brew temperature stability, so why not take on another challenge?

And second it was bracing to imagine that we could take the industry over a thresh-hold that had never been crossed, potentially offering something new and important.  Imagine a little upstart like Treuh introducing such disruptive technology into a market of well-established brands?  If you are associated with an independent, Third Wave cafe you know exactly what I’m talking about.  It’s great to take boutique coffee to the next level, while the powers-that-be seem flat-footed or indifferent.

Oddly, the reason the subject of brew pressure came up at all, was that we found ourselves in a workshop where the house water pressure was so high, we didn’t even need a pump to pull shots on the espresso machines we were building and testing.  This being the case we wondered what deflections in house water pressure might be doing to the quality and consistency of our shots?

We wondered if anyone even thought to measure the precision of their rotary pumps? Was everyone just assuming that their pumps were supplying pressure at nine bars to an unlimited degree of precision?  That is 9.0000000 etc, and not 9.0 +/- 1 bar, for example.  Were we relying too heavily on a $3 gauge to tell us everything we needed to know about pressure in the machine?

There is where it stopped, however.  Events unfolded.  Sales were made.  The industry embraced the machine just the way it was.  So life went on. . . nine bars, nine bars, nine bars. . . of temperature-stable brewing at fixed pressure.

Pressure Profiling. . . the new “brew temperature stability”

Now lots of people have taken pressure profiling beyond the “what if” stage and given it a try.  The technical sorts that are into these things know their stuff.  They know what it takes to control pump speed and thus regulate pressure.

However, it seems some experimenters have also hit a road block in trying to figure out how a single pump can provide variable pressure on a multi-tank brew system with all groups activated at the same time.  Most approaches to the problem so far rely on a bank of pumps and motors.  I think this is a lot of stuff, not so elegant. 

At the same time, not everyone recognizes that without a good control mechanism, pressure profiling could introduce a whack-a-mole component into brewing, which seriously undermines quality instead of of enhancing it, because it allows immediate, unlimited crazy, on-the-fly, variability.  Some camps yearn for this rheostat functionality.  Others worry about what this might mean in real cafe operations.  Still, the purists see any sort of automation, computerization etc, that interferes with the Art of the espresso, as a huge step toward the automation, that ultimately leads to a big old Thermoplan sitting on the counter, and the diminishment of the most powerful force in specialty coffee, the skill, passion, and potential of the human being. 

For sake of analogy consider this.  The iPod is a really great device.  But it’s just a hard drive with ten thousand music files without the easy interface and smart software that defines how it is used.  Memory isn’t the breakthrough.  Control is.  

Enter Slayer. . .

This is where the story of Slayer enters the picture.   As noted in earlier blogs, we started playing with single origin coffee to see if we could make these on an espresso machine instead of brewing them French press style.   Our curiosity took us down a rabbit hole of coffee discovery and adventure.

When we finally emerged on the other side, we realized that we could brew these coffees, despite early experiences strongly to the contrary.  And, we could brew seriously amazing purpose-blended espresso, by allowing ourselves the degree of pressure control we used for pure varietals.

We wanted to share this with the world at once, but the machine which we dubbed the Frankenbot, was not the right platform for sharing.  Frankenbot was a wild conglomeration of stuff that Dan Urwiler wired, plumbed, welded, soldered, propped, tipped and balanced to create a technical masterwork that though revolutionary in our eyes, was more Road Warrior or installation art than espresso machine, at first glance.

We created proto-Slayer to incorporate these ideas. In other words, we reversed engineered the outrageous flavor and body we were getting in the cup, back to a machine.

How Slayer leverages brew pressure

Slayer enables the user to set ramp-up pressure independently for each group.  We intentionally differentiate “ramp-up” pressure from “pre-infusion”, since our most successful brews rely on a much lower pressure than most pre-infusion systems are set to.   I have typically experienced excellent results at 2.5 bars.  However, Slayer can be set as low as 0.5 bars to achieve specific results.  Full adjustability is a hallmark of Slayer.

We find that this first stage is an integral part of creating the idealized espresso we keep harping about.  Slayer’s ramp up  stage is not a three second wetting of the grounds.  It is part of the brew band, a integral part of the shot extraction.  We employed a shot reflector on Slayer to allow us to watch the shot develop to the optimal point under the basket before higher pressure brewing is applied, if desired.

When you move Slayer’s brew actuator to the second position it achieves “full brew pressure”.  We have set this to 9 bars in most cases.  Originally we thought that mimicking the higher pressure of lever machines might add something. But we found this to be the case for certain coffees only.

Since Slayer is based on a mechanical valve, when brew pressure is returned to the ramp pressure position, back pressure results in a soft fall to the lower pressure.  This ramp down can be seen on the pressure gauge.  We installed high-value Omega gauges at each group for just this purpose.

Returning the actuator to the off position results in an immediate pressure drop to zero.

If this seems absurdly simple, that’s good. The machine is meant to be extremely intuitive and provide an upper and lower pressure limit to play within.  We have found that there are several brew pressure geometries that give us optimal results–for me, this number is five.  I have posted a sketch I made of Slayer brewing here (Figure 1).

Figure 1.  Slayer Brew Geometry

As noted earlier, Slayer allows the user to engage all groups simultaneously without affecting the performance of shot development on the other groups.  Slayer uses a pressure reserve approach, so the pump is fully activated whenever the machine is brewing.

It should perhaps be noted that Slayer has no timers.   Not surprising for a semi-automatic, I know, but I have been seeing plenty of these popping up on machines recently. My feeling is that once you have profiled your roaster’s coffee, you can probably get away without a shot timer.  Use your eyes to visually match the taste profile as you brew. But that’s just a note to my barista friends, and just my dumb opinion.  

Finally, Slayer has a modular group design.  As noted on Coffeed, updates to Slayer’s groups will retrofit to existing models, as part of the open source philosophy behind the machine.

Eric Perkunder

in Seattle