Well, just back from a side trip without access to cyber space, so a little late jumping in. However, staling, aging, maturing, etc. is an interesting subject from the point of view of the chemist. There are many foods such as cheeses and wines that benefit enormously from a careful aging. My thought is that most of these processes are the result of slow oxidation and/or hydrolysis. Benign chemical changes that impact differently with time on the palate.
When it comes to taste, I don't think that anyone would argue that it is subjective, and often associated with some custom or tradition. For example, the 'ideal coffee' varies wildly from country to country. My conclusion is that a lot of it is simply what you are accustomed too.
I am convinced that the C02 content in the coffee is the key that makes espresso brewed coffee unique to any other brewing process. The reason is that brewing under temperature and pressure allows the C02 to dissolve in the brewing water. As Dan noted in the first note, the dissolved carbon dioxide forms carbonic acids which in turn lowers the pH of the brewing water.
I have often read that coffee is made up of more than 1000 compounds that can be extracted from the roasted beans. I imagine that carefully prepared 'cowboy coffee' would get most of these compounds out for a cup of full bodied coffee that might lead to the Brokeback Syndrome, but I digress. On the other extreme, I have often read that in espresso coffee, we usually will only extract about half of the extractibles from the beans. Which of these compounds and the relative amounts is carefully controlled by us with brew temperature, volume, time, etc.
Back to the lab. Most chemical analysis, and even some commercial purification of compounds, is done by chromatography, which usually involves a stationary phase (the coffee grounds) and a mobile phase (brewing water). As the mobile phase passes over the stationary phase, compounds will dissolve in the mobile phase and be carried off with the mobile phase. As the mobile phase picks up compounds, its ability to further dissolve other compounds is mitigated until some kind of equilibrium is reached.
Too much information? Perhaps, but once you can 'visualize' this process you can begin to see how things can change with the amount of C02 present. For example, all solubilities are pH dependent. Therefore, as you modify the temperature and/or pH of the brewing water, you will modify which of the available compounds will preferentially extract. Depending on which flavors you prefer, you will find a 'peak performance' for you based on days from roasting.
Since our discussion with Dan last Friday, I have thought of one other probable factor associated with the C02 content. You can imagine that the chromatography process that I mentioned is occurring at the interface between solid and liquid. Therefore, the polarity of the mobile phase will greatly impact the extraction profile. Difficult to do in text format, but if you consider that water, H20 actually looks somewhat like this: H-O-H. Then consider CO2 which would look more like: O-C-O, which in turn forms carbonic acid, etc. Therefore, beyond the effect of pH, the amount of CO2 dissolved will further impact the extraction profile.
I'm afraid there is still more..... Since the coffee grounds are porous, the mobile phase must 'wet' or penetrate into the bean matrix to complete the extraction. The fresher the coffee, the higher the content of CO2 which is already trying to boil off a room temperature. Well, hit it with water at around 190 deg. F and you can imagine how violent the outgassing is. This is bound to be a one way process which delays the wetting phase. Since we are only extracting for about 25 seconds, this is significant enough to also change the extraction profile.
Conclusion: if everyone is paying a premium for aged cheese and wine, why are we paying a premium for fresh coffee?