Estimating the amino acid composition in milled rice by near-infrared reflectance spectroscopy

This study was conducted to develop near-infrared reflectance spectroscopy (NIRS) equations to predict the amino acid and nitrogen content of milled rice powder. The samples were scanned by NIRS and analyzed for amino acid composition and total nitrogen by HCl hydrolysis–HPLC methodology and Kjeldahl method, respectively. The NIRS equations of 15 different amino acids, except for cystine, methionine and histidine, showed high coefficients of determination (RSQ=84.8–97.5%) and low standard errors in calibration (SEC) with 3 g samples for NIRS scanning, while the calibration models of cystine and histidine could explain less variation (RSQ with 77.7 and 65.0%). Calibration for methionine was not suitable to estimate methionine because of its very low RSQ (10.2%). The equations for total amino acids and nitrogen also showed high RSQ and lower SEC, respectively. Furthermore, calibration equations developed with only about 500 mg samples showed similar accuracy and reliability to those with the full cup by using the same calibration set. The equations developed for relative contents of total amino acids did not show good, effective calibration and cross-validation. Only eight different amino acids can be predicted using the equations because their RSQs of calibration were higher than 50.6% (50.6–73.9%). The others cannot be estimated with confidence by their relative contents due to lower RSQ in calibration. Moreover, their relative contents can be calculated from their absolute contents estimated by NIRS calibration.