Question

What impurity is removed by the addition of petroleum ether in the refluxing of it and minced fish solution, resulting in a residue of "fish protein concentrate"?

Answer 1

Most of the proteins are water soluble (hydrophilic), then they are not or very little soluble in petroleum ether. I don't know exactly the procedure that you are using to isolate the fish proteins but is very probable that the re-fluxing with petroleum ether is to extract some hydrophobic contaminant. The most common contaminant of this kind can be fatty acids or lipids