Pouring espresso into hot water versus hot water into espresso struggling to find an answer to this (if there is one), perhaps because nature of the question doesn't work so well with search engines.

Essentially, I've noticed (and so have others I've spoken to about this), that if I make espresso in a moka and want an Americano style coffee (i.e. hot water mixed with espresso -- please don't crucify me if this isn't the correct definition of Americano) then it affects the taste significantly if I make it in these two ways:


Boil water in kettle, pour water into mug, pour moka espresso into hot water in mug -- the two mix -- then drink.


v



Boil water in kettle, pour moka espresso into mug, add hot water into espresso in mug -- the two mix -- then drink.



I've noticed the first method produces a nice, smooth Americano that's nice to drink... whereas the second method (coffee in first, add water) produces a relatively less pleasant, sometimes unpleasant, bitter (perhaps?) and very inferior drink.

How is this possible? My only guess is perhaps it's a chemistry thing to do with some kind of oil in the coffee? Or it just mixes better the other way? I have no idea but I swear this difference is real and so do some of my friends, but none of us know why.

Thanks...
I would suspect that a few centigrades make all the difference here. With coffee, too high temperatures will negatively affect your flavor. Rule of thumb: Hot is good, boiling isn’t.

If you put the water in your mug first, you’ll lose just a bit of temperature as the water warms the mug and/or stands for a bit until you add the coffee. 

If you reverse the order, you are effectively scalding (some of) the coffee with the boiling water, losing volatile aromatic compounds and degrading others.