Chemistry- Testing Amount of Protein in a food Sample

Question

Hi. want to find out grams of protein sample. It is supposedly Whey Protein Isolate, but i have my doubts. I know how to test for the presence with the agent. However, what I really want to do is to find out the weight of the protein in a sample.So I end up with so and so grams of protein in a so and so gram serving.

Answer 1

Why do you doubt that it's really whey protein isolate?  Knowing more about the problem might let us give better answers. :)

Also, if the question is whether it's whey protein vs some other protein (soy protein?), simply knowing the protein content may not tell you anything useful to make that conclusion.  If it might be soy protein, try analyzing for isoflavones instead.

That said, here's the answer to the specific question...

I'm assuming that you have access to some kind of UV/vis spectrophotometer.  If you don't have one of those, this is really going to be difficult; the only other reliable way I know requires special glassware.  (It's a Kjeldahl titration, which are a pain in the wherever to get working.)

If you do have a UV/vis, then I recommend the Bradford assay:  Mix a small sample of the whey powder in a known volume of water.  (Like, 0.5 g in 250 mL.)  Mix the sample solution with the assay solution (see the link at the end), and measure the absorbance at ~600 nm.

You'll also need an independent source of protein, whose concentration you DO know, to use as a standard.  A common standard in labs is BSA (bovine serum albumin).  Prepare a series of solutions with different concentrations of BSA (or other standard protein), mix with the same amount of assay dye, and measure the same way.  The absorbance at 600 nm should have a linear dependence on the protein concentration.  From that, you can work out the concentration of protein in your whey sample solution, and then back to the amount of protein per serving of whey powder.

If you DON'T have access to the UV/vis, I really recommend finding someone around there who can assay it for you. (They may well charge money for it.)  Again, you'll get better results if you tell them the whole problem, including what else you think the powder might be.  Sometimes the answer we think we want isn't really the information we need...

And thanks for asking this, this'll make a great question to present to my instrumental analysis class :D