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]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "soured":
   acerb, acerbate, acerbic, acescent, aggravated, amplified, annoyed,
   augmented, baffled, balked, betrayed, bilious, bilked, bitter,
   blasted, blighted, blown, chapfallen, choleric, crab, crabbed,
   crestfallen, crossed, crushed, dashed, defeated,
   deliberately provoked, disappointed, dished, disillusioned,
   dissatisfied, dry, dyspeptic, embittered, enhanced, enlarged,
   exacerbated, exasperated, foiled, frowy, frustrated, gamy, green,
   heated up, heightened, high, hotted up, ill done-by, ill-served,
   increased, intensified, irritated, jaundiced, let down, magnified,
   off, out of countenance, pickled, provoked, pungent, rancid, rank,
   reechy, regretful, sec, sorely disappointed, sour, sour as vinegar,
   sour-tempered, sourish, stale, strong, tainted, tart, tartish,
   thwarted, turned, unripe, unsweet, unsweetened, vinegarish,
   vinegary, worse, worsened