harden vs torrefy

harden

verb
  • To make something hard or harder (tough, resistant to pressure). 

  • To become or make (a person or thing) resistant or less sensitive. 

  • To become or make (a consonant) more fortis; to (cause to) undergo fortition. 

  • To strengthen. 

  • To modify (a website or other system) to make it resistant to malicious attacks. 

  • To unpalatalize or velarize. 

  • To become hard (tough, resistant to pressure). 

torrefy

verb
  • To subject to intense heat; to parch, to roast. 

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