Analysis of near-infrared spectra of complicated biological fluids by two-dimensional correlation spectroscopy: Protein and fat concentration-dependent spectral changes of milk

Generalized two-dimensional (2D) correlation spectroscopy has been applied to analyze near-infrared (NIR) spectra of milk with different protein and f at concentrations. The NIR spectra of milk show rather poor signal-to-noise ratios compared with those of a protein or fat solution and have changing baselines from one spectrum to another. Poor signal-to-noise ratio and vari ations in baseline are common problems for NIR spectra of real-world sample s. This study aims at expanding the utility of generalized 2D correlation s pectroscopy to complicated multicomponent biological systems. In order to o vercome the above two problems, we have employed multiplicative scatter cor rection (MSC) and smoothing as pretreatment procedures of the milk spectra selected for the calculation of 2D NIR correlation. 2D synchronous correlat ion spectra in the 2000-2400 nm region constructed from protein or fat conc entration-dependent spectral changes of milk sharply enhance bands assignab le to proteins or fats, respectively. It has been found that a power spectr um along the diagonal line in a synchronous spectrum very effectively shows the contribution of a particular component to the NIR spectra of milk. In fact, for example, the power spectrum for the fat concentration-dependent s pectral changes of milk is very close to an NIR spectrum of fat itself. Two -dimensional asynchronous correlation spectra demonstrate the existence of bands that cannot be identified even by calculation of second derivatives a nd chemometrics analysis of the spectra, The asynchronous spectra also eluc idate interaction between fats, proteins, and water.