There is something about the espresso extracted from a lever machine that is clearly distinct from what we can pull of a standard pump driven espresso machine. Lever shots tend to be much softer and smoother, often sweeter. As well, those who have pulled lever shots are keenly aware of the distinct visual appearance of the espresso as it begins to appear & flow from the portafilter during the initial infusion.
So, the obvious question is why? What is happening during this lever extraction process that is different than what happens on a pump driven machine? Recently we decided to identify the difference and answer some of these questions.
Pre-Infusion. A typical level machine pre-infuses the coffee when you pull the lever down, which raises the piston and allows the water from the boiler to enter the cylinder and flow through the screen onto the coffee. This water comes directly from the boiler at roughly 235 – 245F, depending on what your steam pressure is set to and this pre-infused water is also forced into the coffee at the pressure of the boiler, typically at 1.2 – 1.5 Bars of pressure. This is much different than what is happening on a pump driven machine. As we know, there are many ways that manufacturers are trying to pre-infuse coffee, from: Nothing (hitting the coffee at full pump pressure), LM’s Pump pre-infusion (pulse the pump for a second), E61′s spring & piston to slow and soften the pressure, Various Giggleurs to slow the flow of water pressure and Soft pre-infusion where the solenoid valve is opened (no pump) allowing static city water pressure to infuse the coffee. See the graphs below, which are only visual representations and not actual measurements. These various pre-infusion methods result in different espresso extraction results as the infusion pressure will effect the rate of water flow of water through the coffee grounds and also changes the taste of the espresso.
Temperature. Assuming we are using a PID controlled espresso machine, we are brewing somewhere between 198 – 204 F on average. As mentioned above, the water from the boiler on the lever is somewhere between 235 – 245 F. We have not modified a piston to place a thermocouple into the brew chamber to check the temp as the water hits the coffee. But, you would suspect that the water would enter the cylinder at this temperature and there would be immediate drop in temp as the cylinder walls absorb heat. So, I would suspect that the water that is initially in contact with the coffee would be less than 235F, but certainly hotter than 204F. I would also suspect that this temperature is decreasing over time as the shot is pulled vs PID @ 203F for the entire shot.
Water Distribution. On the lever, the cylinder fills and goes through a screen that has holes covering basically 100% of the area of the screen. After the cylinder is full, the boiler pressure (@ 1.5 Bars) forces the water through the coffee (pre-infusing it), the lever is released and you now have a column of water being pressed through the coffee at a decreasing pressure (see below) and a decreasing temperature. After the piston reaches its lowest point, there is a head pressure of air that eventually equalizes with atmosphere and results in a dry puck (see our earlier post Lever shot through a naked portafilter, which has a YouTube video of this process). This water column is always contacting 100% of the coffee bed 100% evenly. Contrast this with the multitude & variety of screens & spray head designs. Some screens use screws to hold them in place, blocking the flow of water, some spray heads force the water towards the outside of the coffee and when using a naked portafilter, you can see the espresso form on the outsides and move towards the center. Other spray heads have the opposite effect. Anyway you cut it, its not ideal in my books.
Brew Pressure. Typically the machine has the pump set approximately 9 Bars (this brings up a completely separate topic, the complete lack of accuracy of the supplied gauges on espresso machines which are often off by upwards of 2 Bars. These gauges are completely worthless and can not be trusted to calibrate your machine), but for the sake of argument we’ll assume that the pump has been calibrated and runs at 9 Bars for the entire shot. We also assume that there is a decrease in pressure during the lever extraction, but how do we determine this? One thought is to mount a pressure transducer onto a Scace and measure the pressure decrease during the shot. Another way is to measure the spring deflection. So, we took the springs for new Simonelli lever and a Marzocco lever (which was an older spring) and brought them to a lab in Seattle. They were able to give us results that showed the amount of spring pressure (in lbs) at various compression intervals (mm of compression). After this, we installed the pistons back into the machine and using a caliper we measured the amount of compression when the spring was at full compression (when the lever was pulled) as well as in its resting position. Example, the LM spring was compressed 26.65mm after being installed, compressed 56.22mm when the laver was pulled, giving us a total compression of 29.57mm during the lever extraction. We then determined the surface area of the lever piston, which allowed us to use a mathematic formula to calculate the beginning & ending brew pressure in Bars. See graph below – the red lines indicate the start & stop points in the brewing process.
So, to summarize, we have hotter water hitting the coffee at a lower pressure for pre-infusion. Following by a slightly higher brew pressure to start which gradually and consistently decreases, along with the brew temperature over the shot extraction all with water that is on 100% contact consistently & evenly over the shot. Easy to see why a lever shot tastes nothing like a shot from a pump machine.
What now? What are people doing with this kind of information and whats new in the frontier of espresso extraction? There are several companies who are pioneering pressure-profiling & temperature-profiling espresso machines. How they are doing this and the results will be in another post ;-) but for now, we can know that we are just entering a new-age of espresso brewing technologies that will make what we know today seem archaic.Posted under Equipment, Experiments & Tests, Lever Machines, Pressure Profiling