So in caffe espresso you are seduced by the smell of the fresh ground coffee, a culinary nymph suggesting flavors of unparalleled beauty. But just as she is tempting you she is already morphing into the old hag with the poison apple. All this delicate fragility is really quite exciting in a culinary art. It is no wonder that each shot you make, try as you might, is a little different than the last. All you can do is go through the factors affecting the process and control them as best as you can.
Perfection is always just around the next corner.
Those that would, with elaborate chemical formulations try to pin down and define espresso and the brewing reaction in terms of Chemistry. It is ridiculous, you cannot. Try to tell a French Chef you are going to analyze his best sauce and reproduce it through chemistry-he will laugh you out of his kitchen. Espresso is a culinary art, and so it shall always be.