MadSci Network: Chemistry

Re: How do scientists measure the amount of protien in a meat?

Date: Fri Sep 29 15:00:57 2000
Posted By: Robert LaBudde, Staff, Food science, Least Cost Formulations, Ltd.
Area of science: Chemistry
ID: 969462071.Ch

Measuring protein is hard to do without a good chemistry lab.

Here are a number of methods used:

1. "Crude protein" is measured actually by finding out how much nitrogen 
is in a food. Only protein contains much nitrogen. So if you measure 
nitrogen, you can multiply by a factor (6.25 for meat) to find protein. 
This method requires high-temperature boiling in sulfuric acid, conversion 
to ammonia with sodium hydroxide, then distillation and titration. This 
method is dangerous even in a good lab, so it's not for you!

2. Digest the meat in dilute sodium hydroxide under heat, add Biuret 
reagent, and measure color change with a spectrophotometer. This requires 
equipment and supplies and has some danger from the hot lye.

3. Get the protein in solution and measure the absorption at the UV 
wavelength of 280 nm. This requires equipment and digestions again.

4. There is a simple method based on measuring moisture and then getting 
protein from models for different meats. You use of the microwave would 
complicate this.

5. Meat is composed almost entirely of moisture, fat and protein. If you 
measure the moisture and fat and subtract from 99.5% you will get a 
reasonably accurate protein for normal (not organ) meats. Unfortunately 
fat is just as hard as protein to do.

I suggest you change your experiment, in any event! The reason is simple: 
protein is not broken down under microwave heat. The same weight of 
protein will be there before and after. What will happen is that the 
weight of the meat will go down as the moisture evaporates, so the 
percentage protein will increase, but not the mass.

A more suitable experiment would be to measure the effects of "cooking" on 
protein. Although the protein is still there after cooking, it no longer 
has the same chemical properties, due to a change in conformation (shape 
of the molecule) called "denaturation". 

You can measure denaturation by a simple test: 

1. Make up a 5% salt solution (5 g salt per 1 L water).
2. Extract a measured amount of solution (say 50 mL) and meat (say 5 g) in 
a blender jar for 60 seconds at low speed.
3. Filter the extract through cheese cloth to remove large pieces and then 
through a coffee filter to get the turbid extract without pieces.
4. Dry the extract and find the weight of the solids left. (You'll have to 
weigh the glass container first, then after drying and subtract the glass 
5. Subtract out the salt weight found by drying an equivalent amount of 
liquid through the cheesecloth and paper.

What you will find is that the amount of solids in the extract goes down 
with the time the meat is microwaved, as you cook more and more of the 
protein. Eventually the extraction process will become very simple, as the 
liquid will easily pass through the filter paper. That's because there's 
no protein in the liquid anymore!

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