This recent thread
about stopping shots got me thinking. If the taste of the flow as the espresso blondes is neutral; how can stopping the shot be critical? But there is lots of anecdotal evidence that changing the brew ratio slightly can create large changes in taste. This evidence is anecdotal since it takes multiple shots to change the brew ratio; and doing enough shots to cancel out shot to shot noise is a always a major project.
The answer is to copy a custom from drinking bourbon -- adding a drop to a splash of distilled water to a bourbon to "open up" the oak flavors. You can take the same shot of espresso, sip it straight, add a drop and sip, add a drop and sip, and note the changes that occur. I happen to have a jug of Zerowater, which comes with a neat little spigot, for my bourbon, tequila, steam iron and other distilled water needs. So I'm in a good position to check if this is something worth pursuing.
I've tried this two times each on all the five roasted coffees that I have available. I started each as a 100% brew ration ristretto, with the shot run until the it fully blonded to eliminate extraction yield as an issue. This is easily done with a lever or other variable pressure machine, since the flow can be retarded; it is not easy, but possible on a constant pressure machine using a very fine grind and close to choke dose.
My very interim results suggest this may be the real deal; and I urge others to give it a try.
In short, the anecdotal evidence is true, a few drops did change the taste balance on two of the coffees so that they went from a strong but non-descript bitterness to a much more open, complex and balanced flavor. The other three changed more gradually, with a splash needed to bring the fruity flavors forward and to mellow the bitters.
My feeling is that this does not presage well for making a goal of precise brew ratios. These changes of taste take place on a very narrow ledge at best to a knife edge at worst. They are not so much a result of extraction or concentration as something textural in the particular shot. I think that the bourbon method of tasting, and adding a few drops if the taste needs opening up may be the least complicated way to get the best shot.