Estimation of crude protein and amino acid contents in whole, ground and defatted ground soybeans by different types of near-infrared (NIR) reflectance spectroscopy

This study was designed to examine the ability of near-infrared (NIR) spectroscopy to determine amino acid and crude protein concentrations in soybean and explore the effects of particle size, fat content and types of NIR spectroscopy instruments on the predictive ability of the NIR models. Whole, ground and defatted ground soybean were scanned by a diode array NIR analyzer and defatted ground soybean were scanned by a Fourier transformed NIR analyzer. Samples were analyzed for crude protein and amino acid contents using reference wet chemistry methods. The NIR calibration models of crude protein and most amino acids showed acceptable coefficient of determinations (R2c = 0.605–0.952), while the NIR models for Cys, Met and Trp were less accurate (R2c = 0.498–0.667 for Cys and R2c = 0.482–0.615 for Met and R2c = 0.406–0.481 for Trp). The R2c values in NIR calibration were found to relate to the correlation between amino acids and crude protein. The grinding process and lipid extraction were found to improve the NIR spectroscopy in crude protein and amino acids prediction. The Fourier transformed NIR analyzer was found to perform similar to the diode array NIR analyzer for defatted ground soybean analysis. No significant differences were found between crude protein and amino acid contents predicted by these two NIR spectroscopy instruments (P > 0.05).