Question

Which chemical is countable for bitter taste of Chillies​

Answer 1

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If that is the case here it is. Although different chili peppers do have their own flavors (some are sweet, some are lemony, some are bitter, etc.) by far any individual flavor is overpowered by a chili's “pungency” (spicy heat) caused by the capsaicin. This is what most people consider the “taste” of chili peppers.