sweet potato vs white sweet potato

sweet potato

noun
  • A tropical perennial American vine, Ipomoea batatas, having a fleshy tuber. 

  • The tuber of this plant cooked as a vegetable. 

  • An ocarina. 

white sweet potato

noun
  • A variety of sweet potato (Ipomoea batatas) that has a relatively white skin and flesh 

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