Paradigms of Human Memory - S2-E21
Trivia: In a very subtle nod to co-star Donald Glover's rap act "Childish Gambino", the song that Duncan is playing on the piano during the "Glee" parody is actually the same tune as Glover/Gambino's song "Not Going Back."
Answer: It was part of the plot that when the boiled yam was dropped it looked like it was stepped on (which it never was). So there was no way to tell the difference between the flesh being soft because it was boiled or because it was stepped on (in the show that is, I don't know if one could tell the difference in real life). Plus, the rest of the yam wasn't boiled, so there was no evidence the yam was sitting in boiling water.
Bishop73