Hope you can help; I am, occasionally, creating an unwanted, bitter curry sauce. Can you tell me why, how to avoid it and, if possible, how to recover the sauce, please?
 
To create a curry sauce I often use pureed garlic, onion, ginger and a little water as a base. This is fried gently for 10-15 minutes until it starts to take on some colour, but not too much. I then add a can of tomatoes with curry powder, cumin, salt, pepper, etc. Sometimes the sauce turns out bitter/sour. Adding sugar does not help.

So far as I know, I am not burning the garlic/onion/ginger - no burnt smell or colour. All ingredients are fresh.
 
I think it is something to do with the onion/garlic/ginger mix, because this can smell odd when cooking - very soapy smell, I would say.

This sounds daft, but could it be some sort of chemical reaction between the base ingredients?