To subdue excess pitta, foods that cooling, calming, and detoxifying are needed.  The bitter taste comes from alkaloids that neutralize excess acidity.  Many herbs contain countless different alkaloids.  Turmeric is only slightly bitter; guduchi is seriously cleansing; neem is even more bitter, and bhumi amalaki which we list as Stone Breaker.  Most foods contain at least some sweetness.  This is, of course, anabolic so it calms the excess catabolic nature of pitta.  Astringent foods and herbs are used mainly to reduce the tendency of excess pitta to to cause bleeding.