souring
  加酸
  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.
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  Sour·ing n. Bot. Any sour apple.
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  souring
       n : the process of becoming sour