Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

souring
     n : the process of becoming sour