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]

The Collaborative International Dictionary of English v.0.48:

Souring \Sour"ing\, n. (Bot.)
   Any sour apple.
   [1913 Webster]

WordNet (r) 3.0 (2006):

souring
    n 1: the process of becoming sour

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "souring":
   acescence, acetification, acidification, acidulation, aggravation,
   amplification, annoyance, augmentation, contentiousness, deepening,
   deliberate aggravation, deterioration, embittering, embitterment,
   enhancement, enlargement, exacerbation, exasperation, fermentation,
   heightening, increase, intensification, irritation, magnification,
   provocation, sharpening, worsening