pita bread vs pitta bread

pita bread

noun
  • Pita; pocket bread. 

pitta bread

noun
  • A flat, unleavened bread made from wheat flour; the rounds are cut in half, and may be made into pockets or used with a dip such as taramosalata, houmous or tzatziki. 

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