Technical note: Fourier transform infrared spectral analysis in tandem with P nuclear magnetic resonance spectroscopy elaborates detailed insights into phosphate partitioning during skimmed milk microfiltration and diafiltration.

Our previous study identified peaks in the P nuclear magnetic resonance (P NMR) spectra of skim milk, denoting the interaction of different phosphate species such as inorganic and casein-associated phosphate during the separation of colloidal and serum phases of skim milk by microfiltration (MF) and diafiltration (DF). In the current study, we investigated the same samples generated by the aforementioned separation using attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy analysis. The results confirmed that the technique was not only capable of differentiating between the mineral equilibrium of the casein phosphate nanocluster (CPN) and milk serum, but also complemented the application of P NMR. An ATR-FTIR broad band in the region of 1,055 to 1,036 cm and a specific band at 1,076 cm were identified as sensitive to the repartitioning of different phosphate species in milk in accordance with the P NMR signals representing casein-associated phosphate and inorganic phosphate in the serum. A third ATR-FTIR signal at 1,034 cm in milk, representing precipitated inorganic calcium phosphate, had not previously been detected by P NMR. Thus, the results indicate that a combination of ATR-FTIR and P NMR spectroscopies may be optimally used to follow mineral and protein phase changes in milk during membrane processing.