On this website I found this claim:
Great baristas in fact will only grind beans for an espresso right before using them. 30 seconds after they have been ground, the beans are already too stale for a good espresso.
I am deeply skeptical. Is there any way to verify, quantify, or otherwise substantiate that claim, or is it superstition?
I can say, by experience, that it's not a superstition. Maybe not seconds, but for sure it's just a few minutes.
Maybe someone could find a reference to an experiment or I may encounter and add it later. But, you can easily observe it by your nose.
The aromatic components are the source of the flavor in coffee beans. When you ground them, you crack them fine. The components get free and start to evaporate. That's why you can smell them. However, we want to taste them so badly. So, we need to trap then into the water as quick as possible, before most of them evaporate.
Below is a nice chart that shows how quickly coffee goes stale when it's stored as whole beans. Ground coffee, I assume, would be similar, but so much quicker as the surface area is greater for evaporation this time.
The only way to verify that claim is by your own senses: flavour and taste in the cup. these are both very subjective, I might be a superficial espresso drinker but for me some good fresh ground beans last at least 3 days if kept sealed and in the fridge.
Ground Coffee has the tendency to absorb every oder odor in the fridge or wherever is kept (in fact one of last uses of stale or unused coffee is has a natural odor eater), so the way it is sealed is very important. Majority of ground coffee seller (Lavazza, Segafreddo, etc.) they vacuum the pack removing almost all the air (check the pack should be hard as a rock before you buy it otherwise the sealing is gone), they claim coffee will stay good even for years - again, it all comes down to what you do with it: if it is moka brew I have had it all my life and most of the times is acceptable, for espresso it is not even ground properly so I never tried.
The only exception to this is Illi sealing, which is packed in nitrogen modified atmosphere I find this ground coffee to be one of the best so far, when you open the pack your do feel the same aroma of fresh ground coffee, I wish they would sell it in an espresso grinding setting (I know it is difficult) to see how it would come out. I regularly buy the decaffeinated version which is ground finer and which I use to make the once-in-a-while-decaf with my oscar.
UPDATE
Last month I started freezing (deep freezing, at least -20) ground coffee inside sealed jars, the result is amazing:
There is no "stale" odor when you open the jar after a week or so: actually no flavor at all, I was expecting it to aggregate into bigger chunks or to develop some kind compactness but no, it is thin as it used to be;
The magic happens when you put it inside a warmed up portafilter, just before pulling the shot: you can feel everything is coming back to life and the espresso is just as it used to be: visually the crema is the same, taste-wise it seems there is more acidity, but I could be wrong and it could be happening only with this variety of beans (100% arabica, almost medium roast)
