Classification of the Inside of a Grapefruit
According to Wikipedia:  


  In chemistry, a heterogeneous mixture consists of either or both of a)
  multiple states of matter or b) hydrophilic and hydrophobic substances
  in one mixture  


The inside of a grapefruit is clearly made up of multiple states of matter (solid and liquid), and is thus heterogeneous. Text books sometimes contain errors, and that appears to be the case here.