Band Assignment of Near-Infrared Spectra of Milk by Use of Partial Least-Squares Regression

Abstract

Band assignment of near-infrared (NIR) spectra of milk has been investigated by the partial least-squares (PLS) regression method in the spectral regions of 1150–1850 and 2030–2380 nm. The shorter wavelength region (1150–1850 nm) was divided into three parts (1150–1350, 1350–1650, and 1650–1850 nm), and each part was analyzed separately. The band assignment was made through the analysis of the shapes of loadings and spectral and concentration variances explained by them. Bands at 1722 and 1754 nm and that at 1208 nm are assigned to first and second overtones of CH₂ stretching vibrations of milk fats, respectively. It is revealed from the PLS regression of milk samples with constant protein content and varying fat content that two kinds of water bands exist in the 1350–1650 nm region; the first group of water bands at 1400, 1440, and 1520 nm is dependent upon the fat content, while the second group of bands at 1406 and 1486 nm seems to depend on the protein content. For all three parts in the 1150–1850 nm region it is found that other milk components such as proteins and lactose do not interfere significantly with the fat bands. For the longer wavelength region (2030–2380 nm), bands at 2302 and 2340 nm are assigned to a combination of CH₂ symmetric stretching and symmetric bending vibrations of fats. No interference from other bands with the fat bands appears, although this region is rather rich in protein bands. For the proteins, only weak bands are identified at 2056, 2160, 2316, 2340, and 2368 nm. The band at 2056 nm is assigned to a combination of amide A and amide I modes, while the band at 2160 nm is due to that of amide B and amide II modes. The bands at 2316, 2340, and 2368 nm arise from combinations of CH₂ stretching and bending modes of protein side chain groups. Other protein bands are strongly masked by baseline slope.