Sour \Sour\, v. i. [imp. & p. p. Soured; p. pr. & vb. n. Souring.]

To become sour; to turn from sweet to sour; as, milk soon sours in hot weather; a kind temper sometimes sours in adversity.

They keep out melancholy from the virtuous, and hinder the hatred of vice from souring into severity. --Addison.

Source: Webster's Revised Unabridged Dictionary (1913)