Seems zingibain is a specific protease, meaning it only cleaves at certain residues, in this case proline. (Link
) This is contrasted with a protease like trypsin that cleaves indiscriminantly.
With that in mind, I agree with Babcock Hall. Protease activity here is just going to give you smaller peptides, so the biuret test by itself will probably not be effective.
If you can get your hands on some filters with defined pore sizes, you might be able to take advantage of the fact that the parent protein is larger than the cleavage products. I.e., find a filter where the pore size is smaller than parent proteins but larger than the products. If you force your mixture through the filter, any parent protein will not go through.
EDIT: You may also want to consider whether this protease is effective with egg albumin. First requirement is that albumin contains proline (it does, which you can see by checking the protein databases, e.g. here
). But whether the proline is accessible given the protein's native 3D structure is also a consideration. Maybe is, but just something to think about.