acerbate vs embitter

acerbate

verb
  • To make bitter or sour. 

  • To exasperate; to irritate. 

adj
  • Embittered; having a sour disposition or nature. 

embitter

verb
  • To cause to be bitter. 

How often have the words acerbate and embitter occurred in a corpus of books? (source: Google Ngram Viewer )