Why does my apple crumble develop a bitter aftertaste? I tend to make Apple Crumble in a very basic way, using Bramley apples cooked down with plenty of sugar ( enough to leave a little tartness to balance out the sweet crumble mix ) and a bit of cinnamon. 

The outcome is delicious but if I go back for a second helping, which has been known, that seems to develop a really strong sour or bitter aftertaste in the back of my mouth. 

Now I acknowledge that modest portions are probably a virtue, but this seems to be a standard pattern over several years- that first portion is so good and the second one is so bad and I'm sure there must be something wrong with the way I'm preparing the dish. What do I need to change?

The crumble recipe I use is a typical flour/sugar/butter with a few oats and sometimes crunched up nuts too. Any variance in the crumble doesn't seem to change the aftertaste.
It may be that the specific flavour has a compounding effect, in the same way that spciy heat (capsaicin) does. The first bite is great, the second one is more sour, and by the second portion it is the only thing you can taste.

It happens because of the way the mouth's flavour sensors react to certain flavour compounds, but I don't know enough to explain how it works exactly.