MadSci Network: Chemistry

Re: How can I test how much protein is in the meat of chicken?

Date: Tue Nov 21 16:31:29 2000
Posted By: Phyllis Stumbo, Staff, Nutrition, University of Iowa
Area of science: Chemistry
ID: 973548487.Ch

Testing food for nutrients usually requires a relatively complex chemical 
assay.  Protein is usually determined in a food by measuring the nitrogen 
content of the food and applying a factor to the amount of nitrogen to 
estimate the protein content.  Nitrogen is measured using a procedure known 
as the Kjeldahl procedure.  Kjeldahl is the name of the Danish scientist 
who developed the assay.

Nitrogen is a unique chemical constituent of protein (it is missing in 
carbohydrate and almost absent in fat.)  Therefore if you know the amount 
of nitrogen in a food you can very accurately determine the amount of 
protein in that food.  The general factor of 6.25 is applied to the 
nitrogen content of food, a food with 1 gram of nitrogen contains 6.25 
grams of protein.

The only constituents in foods that are present in large amounts are 
carbohydrate, protein, fat, alcohol, and water.  These are called "macro" 
nutrients because there are large amounts of them in food, as opposed to 
vitamins and mineral which are present in micro (small) amounts.   In 
chicken meat the largest constituents are first water, second protein, and 
third fat.  There is very little or no carbohydrate or alcohol in meat.  In 
chicken breast there is almost no fat.  In practice most people would weigh 
the food and look it up in a food table to determine the protein content.  
This is a very accurate procedure as we have very good food tables to use 
as a reference.  The USDA has a Web Page where you can look up the protein 
content of over 6000 foods (  
Choose "Search" on this page and you will be directed to their food 
composition search engine.

If you look up chicken breast, raw, meat only, on the USDA web site, you 
will find that 100 grams of this meat contains 75 grams water, 23 grams of 
protein, and 1 gram fat and 1 gram "ash" which is the mineral content.  If 
you look up this same food after it is roasted it will contain less water 
and more protein.  This is a good example of how the water content of food 
varies, thus affecting the content of other nutrients.

Phyllis Stumbo
University of Iowa

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