*3.4. Near-Infrared (NIR) and Visible Spectroscopy*

In the NIR range characteristic absorption bands corresponding to the overtones and combination vibrations of proteins, lactose, fat, and water molecules are manifested (see Table 6), but these bands are weaker than in the MIR region and harder to distinguish on the background of highly intensive light scattering on the milk particles—the fat globules and casein micelles. The NIR spectra of the bulk scattering coefficient and the scattering anisotropy factor strongly depend on the particle size distribution which can vary in each sample of milk [50]. It means that samples with equal fat and protein content, but different particle size distributions have different spectral transmittance and reflectance, which makes extracting information about sample composition from the spectral data a complicated task, especially for raw, unhomogenized milk [65].

From the other side, weaker than in MIR, water absorption, especially in the visible and the short-wavelength part of the NIR region allows using longer optical path flow-through cuvettes (up to 10 mm) suitable for in-line measurements. Another advantage of NIR techniques is the possibility to employ more affordable and robust multichannel detectors-based spectral instruments with no moving parts. Despite some lack of sensitivity and selectivity, less reliable and more specific calibration compared to MIR spectroscopy, NIR techniques are now leading in milk analysis.

Different approaches and measuring schemes (Figures 5 and 6) are used for the registration of the NIR spectra of milk, including collimated transmission (CT) [84], diffuse transmission (DT) [64], diffuse reflection (DR) [85], backscattering (BS) [86].

Table 6 summarizes the applications of NIR technologies for the quantitative and qualitative determination of the main milk constituents (crude protein, casein, fat, lactose), comatic cells count, and common adulterants reported in the selected papers.

**Figure 5.** Measuring collimated transmission (**a**) and diffuse transmission (**b**).

**Figure 6.** Measuring backscattering (**a**) and diffuse reflection (**b**).



Absolute errors of determination for the concentrations of components (in %) are presented if not specified otherwise; detection limits are in italic.

In contrast to the off-line milk analyzers based on FTIR spectrometers working in the MIR spectral region, NIR technology allows developing portable devices for fast on-farm milk quality control [95] and analytical solutions for in-line applications in milking robots [96] and automated milking systems [97]. Authors claim that such devices can determine the concentrations of three major milk constituents (fat, protein, and lactose) and somatic cells count with accuracy which is quite acceptable for practical use.
