The bitterness comes from the charred skin of the eggplant, so you’ll want to make sure all the peel doesn’t end up in the mashed eggplant. Also, look for whole eggplants without brown streaks. These indicate an over-mature eggplant and that tends to be more bitter. If your mixture still tastes bitter once it is prepared, you can add a half teaspoon of baking soda and taste. If it is still bitter, you can add more baking soda. This will neutralize the bitterness.