Which ingredient in dark chocolate contributes to its bitterness?

The ingredient in dark chocolate that contributes to its bitterness is nonfat chocolate solids. These solids are derived from cocoa beans, and they contain a significant amount of cocoa mass, which includes both cocoa solids and cocoa butter. The cocoa solids contain flavonoids and other compounds that impart a strong, bitter flavor profile, characteristic of dark chocolate.

In contrast, milk solids typically found in milk chocolate do not contribute to bitterness; instead, they add creaminess and a sweet flavor. Cocoa butter, while an essential fat in chocolate, is responsible for providing a smooth mouthfeel and does not contribute to bitterness. Sugar, although it can balance out bitterness, directly counteracts it by adding sweetness rather than enhancing it. Thus, nonfat chocolate solids are the primary component in dark chocolate that conveys that distinctive bitter taste.