Comparative Analysis of Milk Souring by Reflective Infrared Spectroscopy

Introduction: To improve the efficiency and competitiveness of the dairy industry, it is important to control the quality of dairy products. Infrared spectroscopy demonstrates significant potential for its application in composition measurement, detection of falsifications, and control in technological processes. At the same time, there is no information on the use of IR methods for the analysis of milk souring during storage and processing.

Purpose: The study of optical properties in the infrared spectrum of milk during souring to justify the choice of the spectral range and the most informative control parameters during storage and processing.

Materials and Methods: For measurements, drinking pasteurized milk with a mass fraction of 4.01% fat, 3.37% protein, and 4.94% lactose was taken. The spectral characteristics of reflection and absorption α(λ) were measured in the ranges of 400-2500 nm and 2.5-18.0 microns according to a generally accepted method.

Results: In the near-infrared region, there are absorption maxima at wavelengths of approximately 980 nm, 1200 nm, 1455 nm, and 1930 nm. The integral absorption coefficients calculated both in the entire spectrum and in the maximum regions vary slightly (less than 10%) and non-systematically with an increase in acidity by 6 times during the souring process. Statistical parameters also change slightly and non-systematically. All spectral curves have pronounced left-sided asymmetry and moderate flatness. In the medium-wave absorption spectrum, there is a single maximum at about 9400 nm in the 8000-12500 nm region. The dependence of the absorption coefficient on acidity in the range of 8000-12500 nm is increasing and can be statistically reliably approximated. Statistical parameters do not change systematically with changes in acidity in the mid-IR range. All spectra have right-sided asymmetry and pronounced flatness.

Conclusion: It is most advisable to control the acidity of milk during storage and processing during souring by absorption in the middle IR range, and the most informative spectral region is 8-12.5 microns. The statistical parameters of the absorption spectra of milk in both the near and middle infrared ranges vary non-systematically and, often, insignificantly. All spectra are flat–topped, in the near-IR range they have left-sided, and in the middle IR region they have right-sided asymmetry.

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