Why is liquor a little bitter?

Fusel (oil) (= fuseoil)

Fusel is an important part of liquor and one of the main sources of bitterness of liquor (regardless of acid). The total amount of fusel in liquor is large and there are many varieties.

aldehyde

Furfural is considered to have the bitter taste of coke, while acrolein and crotonaldehyde are extremely bitter.

Polypeptide: If there are peptides in liquor, it will be bitter, because most peptides are bitter peptides.