The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         They keep out melancholy from the virtuous, and hinder
         the hatred of vice from souring into severity.
                                                  --Addison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

soured
    adj 1: having turned bad [ant: unsoured]