Tartness, n.
The quality or state of being tart.
Syn. -- Acrimony; sourness; keenness; poignancy; severity; asperity; acerbity; harshness. See Acrimony.
Shades of meaning
Fernald has no entry of its own for tartness; he lists it under acrimony.
He lists it without comment; the entries above explain its neighbors.