Adjective : his acidulous criticism of the book.    From Dictionary.com. 

A mild acidulent, usually vinegar, may be added in order to lower the pH and create a selective environment that favors the growth of the tempeh mold over competitors.    From Wordnik.com.  [Fried Tempe] Reference

Abbe Moudon starts forward; with anxious acidulent face, twitches him by the sleeve; whispers in his ear.    From Wordnik.com.  [The French Revolution] Reference