Qualitative and Quantitative Analysis of Sorghum Grain Composition Including Protein and Tannins Using ATR-FTIR Spectroscopy

Abstract

Chemical composition and grain quality traits are important factors related to the end-use quality of sorghum. Thus, rapid and non-destructive analytical methods for screening sorghum for desirable grain quality traits benefit the industries using sorghum for food, feed, and biofuels as well as sorghum breeders and seed companies working to improve the grain quality of sorghum. In the present study, a method based on Fourier-transform infrared spectroscopy (FTIR) was developed to characterize the major chemical components in sorghum flour. Sorghum flour had three major FTIR active regions from 1800 to 800 cm−1. As expected, FTIR spectra revealed that starch was the primary compound in sorghum flour, followed by protein with only minor amounts of lipids and phenolic acids. Phenolic acids were characterized by a peak at 1709 cm−1, which varied in intensity across sorghum varieties. Tannin sorghum could be discriminated from non-tannin sorghum by a shift in O–H stretch band around 3300 cm−1 and their C=C absorption peaks at 1608 and 1522 cm−1. Flour from the outer part of the kernel (corneous endosperm) had greater protein and lipid absorbance while the inner kernel (floury endosperm) had stronger starch absorbance. Pearson’s correlation analyses between FTIR peak intensities and protein/starch content showed that starch content did not have a significant correlation with infrared peaks, whereas the amide I peak at 1652 cm−1 (C=O stretching group), highly correlated with protein content (P ≤ 0.05). A calibration curve (Y = 127.79X-14.345, R2 = 0.9454) was built using peak height at 1652 cm−1 to predict protein content and this was applied to determining protein digestibility.