Test of proteins - Xanthoproteic test

Xanthoproteic test : A few proteins give yellow colour along with concentrated nitric acid (formation of yellow stains on fingers when working along with nitric acid in laboratory). The creation of yellow colour is due to reaction of nitric acid along with benzenoid structures. So, while a protein solution is warmed along with nitric acid a yellow colour may be developed that turns orange on addition of   solution.