Question: I submitted a sample of finished feed product for crude protein analysis, but I don’t understand how this value was determined. Can you help me understand what my result means?
Thank you for your question. Protein testing is one of the most common analyses performed at our Nutrition Analysis Center in Des Moines, Iowa. In order to answer this question, we first look at what proteins are, then examine how crude protein testing works in the lab.
All proteins are made up of individual interconnected amino acids, and each amino acid unit contains at least one nitrogen atom. However, differences in the side chain configuration of each amino acid lead to some amino acids containing up to three additional nitrogen atoms per unit.
At this point, you may be asking yourself why we are talking about nitrogen when you thought you were reading an article about protein. The reason is: the most common and well-recognized method for testing protein content is by measuring the total nitrogen in a sample.
The analysis looks like this – the sample being tested is combusted in a closed system and the released combustion gas is measured by a thermal conductivity detector that is configured to quantify nitrogen. Total protein is calculated by multiplying the percent nitrogen by a protein conversion factor, which is 6.25 for most foods and feeds. Other factors can be utilized in cases where the amino acid ratio in the product has an abnormally higher or lower amount of total nitrogen, and our team can report results using other factors upon request.
Although crude protein tests provide a decent idea of the amount of protein in a feed or food, a more robust measurement of “true protein” would be a full amino acid profile. Quantifying total amino acids in a product is the most reliable and accurate way of determining protein levels in a nutritional product, as it does not include adulterants such as melamine, urea, nitrates, or other non-protein nitrogenous compounds.