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.  
I've noticed this with stewed apples as well. The texture changes as it cools but also an aftertaste develops. I assumed it was something to do with pectin. It's usually better when reheating in the oven though?  I changed how I do crumbles in the last few years. I now make a compote with apples and say blackberries and initially cook he crumble separately. I then combine the two when the compote is cool and then bake for a very short time. This keeps the fruit tasting fresh because its not baked to death. 