Your eggplant is the most likely suspect — they develop a bitter flavor when they are past their prime. When shopping for eggplants, be sure to choose a firm and heavy fruit with a bright, glossy color and avoid any that are spongy or off-color. The skin of the eggplant can also cause bitterness, so take care to keep it out of your baba ghanoush. You can balance out any remaining bitterness by adding salt slowly and adjusting to taste.