I thought of something today as I was having my morning espresso. I’m used to a small shift in flavor from the more acidic end when I first start sipping it to a richer, more chocolate taste at the end.
Could this be due to the layer of crema on top? My understanding is that at least part of the crema is due to CO2 being released during the extraction, and CO2 certainly will have an acidic taste to it. So it makes sense that the first sips will have more crema because it’s up top, and there will be less as I drink the rest of the espresso.
I also realize that part of the taste shift I am experiencing would be due to the drop in temperature as the espresso cools a bit.