Effect of Non-Peptide and Non-Protein Nitrogen Compounds for the Determination of Protein Content by near Infrared Spectroscopy

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Abstract
The significance of the absorption at 2170 nm due to peptide bonds for the determination of protein content by near infrared (NIR) spectroscopy was evaluated by comparing it with absorptions due to other nitrogens (non-peptide nitrogens in protein and non-protein nitrogens). The amide group (asparagine and glutamine), guanido group (arginine), imidazole group (histidine) and amino group (lysine) in proteins did not exhibit absorption at 2170 nm. The absorptions of nucleic acid related compounds also were not observed at 2170 nm. These results suggest that a wavelength of 2170 nm is suitable for the accurate determination of protein content in foods.