twice-baked bread vs zwieback

twice-baked bread

noun
  • A type of sweet raised bread baked for a second time in order to render it brown, hard, and crisp. 

zwieback

noun
  • A usually sweetened bread enriched with eggs that is baked and then sliced and toasted until dry and crisp; considered easy to digest and therefore given to the ill and used as a teething food for toddlers. 

How often have the words twice-baked bread and zwieback occurred in a corpus of books? (source: Google Ngram Viewer )