Last night I practiced some poor science and changed several variables:

I wanted to change brew temps to see how that affected taste, but in addition changed from tap water to volvic.

Dose was 16g.

For the different temps I tried pulling a shot immediately after the boiler turned off (machine still cold), at my normal time (machine warm) and after 20s of turning the steam switch on.


The cold shot was very smooth in texture (especially the crema) but very bitter.
The normal shot was amazingly smooth and actually quite balanced.
The hot shot was less smooth and burnt/bitter.

Revisiting shot 1 I was suddenly able to discern between sour and bitter, although I don't think i am able to describe the difference.

There appear to be 2 eureka moments in this for me:
1. Tasting the difference between sour/bitter.
2. Volvic appears to have transformed everything. Even on the deliberately bad shots they were smoother in texture than all of my previous attempts. I must therefore conclude there is something in my tap water which leads to harsh espresso.