■ RESULT AND DISCUSSION
Interpretation of Spectral Analyses. The FT-NIR
original spectra of the pure and adulterated onion powder
and the SNV-preprocessed spectra (Figure 1) reveal differences
in the absorption intensities between 1920 and 1980 nm
(marked) and can easily be identified. The absorption bands in
this region correspond to the O−H stretch and O−H band
combination and the H−O−H deformation combination,
which represent the starch content.21 Starch is a semicrystalline
polymer composed of two polysaccharides, amylose and
amylopectin. Amylose is a mostly linear chain consisting of
up to 3000 glucose molecules interconnected primarily by α-
1,4-glycoside linkages.13 The spectral fluctuations from 1400 to
1600 nm correspond to the first overtone of the hydroxyl
group. The precise position of these bands is very sensitive to
hydrogen bonding in the starch molecule.22