Structural Changes of Milk Components during Acid-Induced Coagulation Kinetics as Studied by Synchronous Fluorescence and Mid-Infrared Spectroscopy

Dynamic oscillatory experiments and front-face synchronous fluorescence spectroscopy and mid-infrared (mid-IR) spectroscopy have been used to investigate structure evolution, at the macroscopic and molecular levels, during milk acidification kinetics. The studies were performed using skim milk, at two different temperatures (30°C and 40°C), to which was added glucono-δ-lactone (GDL) to generate different structural changes in casein micelles and gels. Synchronous fluorescence spectra were recorded in the 250–500 nm excitation wavelength range using an offset of 80 nm between the excitation and emission monochromators for each system during the 300 min acidification kinetics. The change in the fluorescence intensity at 281 nm reflects the pH-induced physicochemical changes of casein micelles and, in particular, structural changes in the micelles in the pH range 5.5–5.0. Regarding mid-infrared spectroscopy, the region located between 1700 and 1500 cm⁻¹, corresponding to the amide I and II bands, and the 1500–900 cm⁻¹ region, called the fingerprint region, were considered for the characterization of milk coagulation kinetics. Changes in the absorbance at 1063 cm⁻¹ as a function of pH for kinetics recorded at 30°C and 40°C reflected pH-induced phosphate dissolution in the pH range 5.5–5.0. Compared to rheometry, which reveals microstructure changes only in the gel state, spectroscopic methods make it possible to monitor molecular structure changes in micelles throughout the acidification processes.