Saw this article at the Fellow products website:
UNDERSTANDING DEGASSING: IS FRESH BEST?
The idea is that very freshly roasted beans - in the first 3 or so says following roasting - make for a worse cup. But if you wait too long, you wind up with a stale, bitter brew.
Right after roasting the beans are still releasing lots of carbon dioxide, and if you brew during that time, two things happen - lots and lots of CO2 is released all at once (it can really froth up as you wet the beans), and an uneven extraction occurs.
CO2 is released naturally as time passes - nearly half is released in the first 24 hours. This outgassing is why many brewers "bloom" their coffee at the beginning of the brewing process. Blooming is pretty easy; you wet the grounds with just enough water to get them fully damp, and then let them sit 30-90 seconds before proceeding with the brew. The length of time is due to personal preference and brewing method. When you do this, the beans really froth up, much in the same way bread dough does during the rise. The advantage is that releasing the CO2 during the bloom limits the amount that dissolves into your coffee. It's like taking a sip of unsweetened carbonated water - the flavor is quite bitter. It apparently also helps to get a more even extraction, though I wonder how such things are measured.
What I find odd, is that not long ago, the conventional wisdom was "the fresher the better." There was even a famously-failed kickstarter (though 3 years later, they are still posting updates) for a machine that roasted, ground, and brewed the coffee all within a few minutes.
I kind of wonder what is up with this. Did someone genuinely figure out something new, despite the fact that coffee has been popular for a thousand years? Or is it just a trend, and folks are jumping on the bandwagon? Confirmation bias?
I have my own theory; I suspect that espresso - which sets many coffee-making trends - might be partially responsible. I don't think (correct me if I'm wrong) that the espresso-brewing process allows for blooming. Could it simply be that non-bloomed brewing methods make for a worse cup when using very fresh beans, but that methods that do allow for blooming can result in an excellent cup, even if brewed and roasted on the same day?