The Collaborative International Dictionary of English v.0.48:

Acidulous \A*cid"u*lous\, a. [L. acidulus, dim. of acidus. See
   Acid.]
   Slightly sour; sub-acid; sourish; as, an acidulous tincture.
   --E. Burke.
   [1913 Webster]

   Acidulous mineral waters, such as contain carbonic
      anhydride.
      [1913 Webster]

WordNet (r) 3.0 (2006):

acidulous
    adj 1: being sour to the taste [syn: acidic, acid,
           acidulent, acidulous]