Applicability of Near-infrared Spectroscopic Method to Unfreezable Water Measurements in Egg White Lysozyme and Soluble Starch

Abstract

Near-infrared spectroscopy (NIR) scanning from 1100 to 2500 nm was applied to measure the unfreezable water bound to egg white lysozyme and soluble starch. Two characteristic absorption bands corresponding to water molecules were changed into the second derivative difference spectra. In the changed spectra, the peaks at each lower and higher wavelength might be attributed to the freezable and unfreezable water, respectively. When the correlation coefficients between absorbances at wavelengths with maxima in the peaks attributable to the unfreezable water and the results of differential scanning calorimetry used as a reference analysis method of unfreezable water were calculated, the correlation coefficients at 1908, 1952 and 1992 nm assigned to the combination of the stretching and bending vibrations of OH were above 0.91 in both lysozyme and starch. Therefore, it can be concluded that the NIR second derivative difference spectra, in the region assigned to the combination of the stretching and bending vibrations of OH in water molecules, have potential for the measurements of the unfreezable water bound to egg white lysozyme and soluble starch used as model samples of food materials.