Source : Webster's Revised Unabridged Dictionary (1913)

Sourness \Sour"ness\, n.
   The quality or state of being sour.

Source : WordNet®

sourness
     n 1: the taste experience when vinegar or lemon juice is taken
          into the mouth [syn: {sour}, {tartness}]
     2: the property of being acidic [syn: {sour}, {acidity}]
     3: a sullen moody resentful disposition [syn: {sulkiness}, {sullenness},
         {moroseness}]