I cooked a base for indian sauce based on lots of onion, homemade ginger garlic paste, some veggies (I used carrot, a little napa cabbage, bell peppers) and spices. After cooking, the sauce had an unfortunate bitter aftertaste. Nothing burned, I cooked over careful heat.
First I thought it wasn't cooked enough, so I cooked some more over low heat. The bitter taste did not disappear.
After considering this some more, my suspicion turned to my homemade ginger garlic paste. It turned green after grinding the ginger and garlic, but that didn't bother me, it's a fairly common phenomenon. But I fried some paste in two batches, and then tasted the result. One was fried until it browned. The other was just fried for 2-3 minutes, and remained quite green. Here's a picture showing (clockwise from lower right) raw, lightly cooked and browned ginger garlic paste:
The browned batch was sweet, followed by the pungent ginger. The lightly fried batch was quite bitter, and also pungent.
So I think the bitterness in my sauce was caused by my ginger garlic paste. But why? What happened?