frankfurter vs knackwurst

frankfurter

noun
  • A moist sausage of soft, even texture and flavor, often made from mechanically recovered meat or meat slurry. 

knackwurst

noun
  • A highly seasoned scalded sausage made from beef, pork, and fatty tissue similar to a frankfurter, but shorter and thicker. 

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