Inasmuch as most natural proteins contain several such groups, the biuret reaction is a very general test for proteins.
The absence of a biuret reaction would point in that direction and the question must await further study.
On now testing a portion of the clear filtrate for peptone by the biuret test, not a trace of a reaction could be obtained.
Amongst these is the biuret reaction: a pink colour obtained by adding a trace of copper sulphate and an excess of caustic soda.
biuret bi·u·ret (bī'yə-rět', bī'yə-rět')
A derivative of urea obtained by heating. Also called carbamylurea.