"Food of love" is a line from William Shakespeare's "Twelfth night". Austen uses this reference because like Elizabeth, the Duke in "Twelfth Night" is too only concerned with looks and first impression. Throughout the novel Austen uses references to illustrate the importance of the theme of first impression/prejudice.
Twelfth Night: Entire Play." The Complete Works of William Shakespeare. Web. 26 Mar. 2010.