The Collaborative International Dictionary of English v.0.48:

Acidulate \A*cid"u*late\, v. t. [imp. & p. p. Acidulated; p.
   pr. & vb. n. Acidulating.] [Cf. F. aciduler. See
   Acidulous.]
   To make sour or acid in a moderate degree; to sour somewhat.
   --Arbuthnot.
   [1913 Webster]

WordNet (r) 3.0 (2006):

acidulate
    v 1: make sour or more sour [syn: sour, acidify,
         acidulate, acetify] [ant: dulcify, dulcorate,
         edulcorate, sweeten]