The Collaborative International Dictionary of English v.0.48:

Sourness \Sour"ness\, n.
   The quality or state of being sour.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "sourness":
   Faustianism, acerbity, acidity, acidulousness, acridity, acridness,
   acrimony, anger, animosity, animus, asperity, astringence,
   astringency, atonalism, atonality, bad blood, bad humor,
   bad temper, bile, biliousness, bitingness, bitter feeling,
   bitter pill, bitterness, cacophony, causticity, choler, clinker,
   cold comfort, corrosiveness, cuttingness, disappointment,
   discontent, discontentedness, discontentment, discord, discordance,
   discordancy, disgruntlement, disharmony, dissatisfaction,
   dissatisfiedness, dissonance, dissonancy, dissonant chord,
   divine discontent, edge, envy, feud, flatness, gall,
   gall and wormwood, hard feelings, harshness, ill blood,
   ill feeling, ill humor, ill nature, ill temper, ill will,
   inharmoniousness, inharmony, keenness, mordancy, off note,
   peevishness, petulance, piquancy, poignancy, pungency,
   querulousness, rancor, rebelliousness, resentment, restiveness,
   restlessness, roughness, severity, sharpness, soreness, sour note,
   spleen, stridor, sulkiness, tartness, trenchancy, tunelessness,
   uneasiness, unfulfillment, unhappiness, unharmoniousness,
   unmusicality, unpleasure, unsatisfaction, untunefulness, vendetta,
   venom, vexation of spirit, virulence, vitriol, wolf