Why do fresh mandarin oranges get very bitter when baked? Why do fresh mandarin oranges get very bitter when baked?

What compound in mandarin oranges turns bitter when heated?
The white, pithy part is a source of the bitterness. Either peel more thoroughly, or trim off with a knife.

The pulp of some oranges (of the navel variety), can turn bitter when exposed to air.
http://www.preparedpantry.com/CitrusQuickGuide.htm