If a food does not turn Biuret solution violet do you know what nutrients the food contains?
Not really, but the only thing we know is that it can't contain protein.
The biuret solution can only test for proteins. If the food sample is put into a biuret solution and the solution turns purple/violet, then it is an indication that proteins are present in the food. If the solution remains blue, then there are no proteins present in the food. So, we can only say that, that food cannot contain protein, as a result from your observations.