does this make sense?

Figure 1. Protein Analysis - Determination of the protein concentration of casein in an unknown sample after the addition of the biuret reagent from the results over test tubes 1 to 5.

What's casein?

type of potato extract

I think it sounds fine.

Yes, the statement you provided appears to make sense. It describes a protein analysis experiment involving the determination of the protein concentration of casein in an unknown sample. The experiment involves adding the biuret reagent to test tubes 1 to 5 and obtaining results from these test tubes. The purpose is to determine the protein concentration of casein in the unknown sample based on these results.