|MadSci Network: Chemistry|
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 (http://www.nal.usda.gov/fnic/foodcomp/). 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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