koげるgeru
1.

To undergo a change in color and texture due to intense heat, resulting in a state resembling carbon or ash, often accompanied by a distinct smell and smoke, just before the material catches fire.

2.

To be heated by fire to the point where the surface becomes darkened, either black or brown, indicating damage or alteration, without completely catching fire.

3.

To experience a thermal alteration that gives rise to a burnt appearance, typically occurring with substances like food or material when exposed to flames or high heat for too long.

Example:

あーあ、トーストが真っ黒に焦げてしまっているよ。