Impact of physicochemical changes in milk ultrafiltration permeate concentrated by reverse osmosis on calcium phosphate precipitation

This study aimed to characterize and compare the mechanisms of calcium phosphate precipitation in skimmed milk ultrafiltration permeate (MP) and MP preconcentrated by reverse osmosis (ROMP). The effects of different physicochemical parameters such as the pH (8.0), the heating time (60 or 120 min at 60 °C) and the seeding of samples with dicalcium phosphate (DCP) were tested. The concentration of salts (K, Ca, Na, Mg, and P) in the freeze-dried precipitates was measured using inductively coupled plasma (ICP). The amount of remaining ionic calcium was also monitored. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analysis were used to characterize the type of calcium phosphate precipitates that formed. The morphological structure of particles was determined using scanning electron microscopy (SEM). The chemical analyses showed that RO increased the rate of precipitated ions, especially Ca and P in MP, while alkalinization to pH 8.0 and heating at 60 °C significantly increased the precipitation of salts, with the calcium phosphate structure changing into complex forms such as hydroxyapatite (HAP) and whitlockite. MP preconcentration by RO paves the way for improving the precipitation yield of milk salts in the form of HAP for Ca fortification in various foods. It offers an original way to valorize the milk salts contained in the high volumes of MP generated by the cheese industry.