This is not a conversation about preinfusion. But only because preinfusion is a misnomer. This topic isn't remotely new. Anyone who has followed the LMLM threads knows that different sized gicleurs provide different water debit flow rates and that folks who prefer classic comfort blends likely love the shots produced by the higher water debit of the stock gicleur, while those who prefer lighter roasts will prefer the characteristics of the smaller 0.6mm gicleur.
Water debit is a common measurement to describe the flow from the shower screen of an espresso machine when there is no back pressure on it. In general, rotary pumps have higher water debit measurements than vibe pumps. Equipment manufacturers generally equip machines with restrictors often known as gicleurs to adjust the water debit down from what it would be without one. Smaller gicleurs result in lower water debit measurements. The large majority of Home Baristas accept their machines for what they are and live within the confines of the manufacturer's gicleur selection. Small changes in water debit are inconsequential to the results of the espresso produced from a given machine. However, there are now more and folks using machines that can alter their water debit in the fly.So what. Who cares?
By now, most of us should be familiar with the charts below from Espresso 101: How to Adjust Dose and Grind Setting by Taste
These are both wonderful, but they are 2-dimensional.
Dimension 1: Dose.
Dimension 2: Grind
Enter dimension 3: Water Debit.
3 dimensional charts are tricky. The charts above are a "slice" of the 3d chart with water debit held constant. Since most machines feature a fixed water debit, there has been no driving force to explore what happens when you change it. Certainly little mention of how to use it
to control the flavor of your shot. So options for a quick and easy tool are to hold grind constant and put together a "Dose vs Water Debit" chart, hold dose constant and put together a "Grind vs Water Debit" chart or to build a 3d "Rubik's Cube Chart" showing all 3. I like Rubik's Cubes, but I think they make most folks' brains hurt. These charts would be no different, but the beauty of a 3d chart is that it puts it all together in such a way that you can experiment with dose, grind and water debit in a fashion that yields predictable results with minimal waste.
I dont have the charts ready yet but I can begin the discussion with results that have been observed from a handful of folks with machines capable of tweaking water debit at will.Neutral Water Debit
This is somewhere between 4 and 8 ml/s. With a "normal" dose that yields roughly 2mm of headspace between the tamped puck and shower screen, and a "normal" grind this will act like a standard E61 with respect to pressure ramp at the puck, dwell time to first drips, etc.Decrease Water Debit
As you decrease the water debit below 4 ml/s, you start developing puck saturation before pressure builds and the flow rate of the shot increases
. Decreasing water debit takes longer to get first drips, but it does the same thing to the overall timing of the shot as lowering the dose or using a coarser grind. Moreover, it allows one to use a finer grind with a given dose, or use a higher dose with a given grind than a higher water debit would allow. It's important to note that with the current rising tide of flow profiling machines, water debit can be adjusted along a continuum, just like grind and dose. Increase Water Debit
As you increase water debit above 8ml/s, you begin to compact the puck faster than water permeates through the puck. Pressure builds at the puck very quickly and the shot time will [b]increase[b]. Increasing water debit can increase or decrease time to first drips, depending on the grind and dose, but the thought is that more fines migrate to the bottom of the puck and the resulting pour will generally be slower, much like increasing the dose or tightening the grind would. Likewise, increasing the water debit allows one to use a coarser grind with a given dose or a lower dose with a given grind than a lower water debit would allow.
In and of itself, I'm not sure that water debit does much to impact the flavor of the coffee. What it does is allow you to access combinations of grind and dose that would otherwise be outside of your normal brewing parameters. It also gives you an added level of control in case your dose is a bit high or low or if you accidentally forgot to adjust the grind tighter after your last gusher, you can just bump your water debit up or down accordingly and enjoy a great shot.
This is already too long for most reasonable folks to read.