加酸
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. Souring \Sour"ing\, n. (Bot.) Any sour apple.
souring n : the process of becoming sour