Source : Webster's Revised Unabridged Dictionary (1913)

Tartness \Tart"ness\, n.
   The quality or state of being tart.

   Syn: Acrimony; sourness; keenness; poignancy; severity;
        asperity; acerbity; harshness. See {Acrimony}.

Source : WordNet®

tartness
     n 1: the taste experience when vinegar or lemon juice is taken
          into the mouth [syn: {sour}, {sourness}]
     2: a sharp sour taste [syn: {acerbity}]