According to some literature (e.g. Bender 2005) , ‘Crude protein’ is defined as N × 6.25 (Kjeldahl determination). Others mentioned like for wheat and most cereals it is 5.8; rice, 5.95; soya, 5.7; most legumes and nuts, 5.3. I analyzed several samples of six-row food barley grains for N content using ECS 4010 CHNSO Analyzer. I wonder if anyone gives me additional information which factor is appropriate to convert grain N content into protein.
On the basis of early determinations, the average nitrogen (N) content of proteins was found to be about 16 percent, which led to use of the calculation N x 6.25 (100/16 = 6.25) to convert nitrogen content into protein content. The use of a single factor, 6.25 is confounded by two considerations; First, not all nitrogen in foods is found in proteins, nitrogen is also contained in other compounds, such as amides, free amino acids, peptides, nucleic acids, nitrogenous lipids, ammonium salts, nucleotides, nitrates, creatine, choline and secondary compounds, where it is referred to as non-protein nitrogen (NPN). The conversion from Total Kjeldahl Nitrogen to protein depends on the type of protein present in the sample and what fraction of the protein is composed of nitrogenous amino acids, like arginine and histidine. Different proteins need different correction factors because they have different amino acid sequences. The correction factor for barley is 5.83.
Yes i too agree with Sangeeta reply. Better you use 5.8 as a conversion factor for converting into barley protein. 6.25 is the average conversion factor and it should not be used for barley. You can use the average conversion factor of cereals for converting barley N into barely protein
It depends entirely on plant species, the method of analysis is the same. I personally use the method of Lowery for the determination of soluble protein.
I agree with Drs Khetarpal, Kalaivanan and Abbas, its variations among the crop species and analytical methods
http://www.fao.org/docrep/006/y5022e/y5022e03.htm
you might check the first paragraphs of this page for further explaination and quotations.
The conversion factor considers the average content of Nitrogen in foods according to the abundance of every aminoacid. As you are asking about grains, there are differences in the conversion factors according to this (the values are accepted so you can use the same published in literature). For example, as you can see from FAO table listed before by Julian, if you are analysing whole wheat the best conversion factor would be 5.83, while 5.7 is applied for flour (not whole one), that consists of the endosperm only.
About the method, you are applying a combustion method, that is not different in principle from Kjeldahl or Dumas: they all "burn" everything and then quantify the amount of Nitrogen, therefore I believe that the same conversion factor should be used indipendently from the method applied.
Protein content vary from species to species and also depends on methods. Everybody should mention the method which was used to determine protein content of a sample.
The information earlier provided by Drs Rahman, Ali, Khetarpal, Abbas are correct. The conversion constant 5.83 is more appropriate for barley when Kjeldahl approach is used.
Hi, I just wanted to ask if anyone here has more information on nitrogen conversion factor for specific amino acids, for example for arginine.
Is this conversion factor completely not applicable to a purified protein? the assumption that the nitrogen content in a protein is 16% is based on what? For albumin, the nitrogen content is 8.5%.
Sangeeta Khetarpal Could you provide reference for the conversion factor of 5.83 for barley? Thanks.
I think this is useful: https://www.tandfonline.com/doi/abs/10.1080/10408390701279749?journalCode=bfsn20
Article Leaf protein contents and nitrogen-to-protein conversion fac...
This might helps
Kjeldhal total nitrogen determination measures "total" nitrogen in a given sample. Nitrogen is one element not minding how the nitrogen is found in a chemical compound. I am of the opinion that a single factor could be used.
Researchers may try this document for conversion of nitrogen to grain at 10.1080/10408390701279749
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