Subject: Oil spot test for fat in food.

I've learnt about the oil spot test for food. However, I've come across 2 
versions of the test. Resources on the web tell me that after rubbing the 
food onto a paper bag, the presence of a translucent spot that remains 
after drying is evidence of fat. However, my teacher said that it is 
necessary to immerse the piece of paper into an organic solvent and the 
presence of fat can only be confirmed if the translucent spot disappears 
after such treatment. Which version is correct?

Re: Oil spot test for fat in food.