What can cause the biuret test to produce an orange colored solution?

What can cause the biuret test to produce an orange colored solution?


I did the biuret test to a few number of foods in a school lab assignment. The test with tofu turned orange. I wonder why, since the biuret test is supposed to give violet as positive. I tried looking it up on google with no success.


An explanation I came up with is this: during my search, I found out the composition of the biuret and Fehling reagent is quite similar ([Fehling reagent](https://en.wikipedia.org/wiki/Fehling%27s_solution) and [biuret test](https://en.wikipedia.org/wiki/Biuret_test)). Maybe the orange result has something to do with this similarity, as the Fehling test gives orange as the positive result. But the thing is, the Fehling test I did on the tofu showed a negative result. At first, I suspect I took the test tube with Fehling for the biuret. But this is not the case since my classmates got the same orange result with the biuret.


