FTIR analysis of physiochemical changes in raw skim milk upon concentration

Increasing milk solids concentration by evaporation induces several important changes in the system, which affect stability during further processing. This study aimed to fingerprint such structural changes during concentration of raw milk from 9 to 17 or 25% total solids, using FTIR spectroscopy, and relate them to physicochemical changes of the system. An increase in β-sheets (1632 cm⁻¹) and unordered structures (1645 cm⁻¹) at the expense of intermolecular β-sheets (1618 cm⁻¹) was observed in concentrated samples. Changes in conformational stretching of proteins with concentration also affected the intensities of spectral regions corresponding to lactose and minerals. Increased formation of unordered/random structures was also supported by SDS-PAGE analysis of concentrated skim milk and its supernatant, which demonstrated dissociation of the caseins into the serum and increased association of β-lactoglobulin with the micelle. The results were further validated with the principal component analysis (PCA) and concentration-normalized spectra. Proteins in unconcentrated milk systems undergo subtle but important structural changes during concentration, likely making them more prone to destabilisation during downstream processing. The results from this study suggest that FTIR analysis could be used for prediction of the stability of the concentrated milk systems during downstream processing.