Precipitation of calcium phosphates occurs in dairy products and depending on pH and ionic environment, several salts with different crystallinity can form. The present study aimed to investigate the effects of NaCl and citrate on the characteristics of precipitates obtained from model solutions of calcium phosphate at pH 6·70 maintained constant or left to drift. The ion speciation calculations showed that all the starting solutions were supersaturated with respect to dicalcium phosphate dihydrate (DCPD), octacalcium phosphate (OCP) and hydroxyapatite (HAP) in the order HAP>OCP>DCPD. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) analyses of the precipitates showed that DCPD was formed at drifting pH (acidic final pH) whereas poor crystallised calcium deficient apatite was mainly formed at constant pH (6·70). Laser light scattering measurements and electron microscopy observations showed that citrate had a pronounced inhibitory effect on the crystallisation of calcium phosphates both at drifting and constant pH. This resulted in the decrease of the particle sizes and the modification of the morphology and the microstructure of the precipitates. The inhibitory effect of citrate mainly acted by the adsorption of the citrate molecules onto the surfaces of newly formed nuclei of calcium phosphate, thereby changing the morphology of the growing particles. These findings are relevant for the understanding of calcium phosphate precipitation from dairy byproducts that contain large amounts of NaCl and citrate.

The structure development of a soft cheese curd model has been studied in relationship to its rheological properties and its biochemical characteristics (pH, amount and partition of minerals, casein proteolysis) at different technical steps including cutting, drawing, three turns and demoulding. Scanning electron microscopy was used to observe structural changes during the drainage of a fat-free soft cheese. The micrographs provided visual evidence of changes in the casein matrix from casein particles aggregated in clusters to uniform strands observed at the demoulding. The initial increase of loss tangent and of the exponent of the power law between G′ and G′′ and frequency (that were maximal at the second turn) was related to the solubilization of micellar calcium phosphate, while intact caseins and large casein fragments accumulated in the curd. After the second turn, the strength, Youngs' and loss moduli of the curd increased greatly. The hydrolysis of αs1-casein into αs1-I-CN f(24–199) may facilitate the rearrangement of casein particles within the curd. The pH-induced solubilization of calcium phosphate continued throughout the manufacture process but was unexpectedly incomplete at the end of the drainage. Combination of electron microscopic observations with dynamic rheological measurements and chemical and biochemical assessments provided increased knowledge about the structure of soft cheese during drainage, an important but poorly understood cheese making stage.

Proteolysis of sodium caseinate solution (24·5 g/l) induced by heat treatment at 120 °C at different pH values was studied by measuring nitrogen content and relative fluorescence intensity in the 4% trichloroacetic acid filtrates. The low molar mass peptides corresponding to the soluble nitrogen were identified using liquid chromatography/tandem mass spectrometry. Increase in proteolysis, deduced from the increase in soluble nitrogen content, was observed with heating time (10, 20 and 30 min) and pH (6·0, 7·0, 8·0 and 9·0). The fluorescence measurements showed that the release of peptides containing tryptophan was minimal at pH ∼ 7·0. In parallel, eighteen low molar mass peptides were characterized, of which four came from κ-casein, nine from β-casein and five from αs1-casein. Peptides were preferentially released under alkaline conditions.

The concentrations of cations (calcium and magnesium) and anions (inorganic phosphate, citrate and chloride) have been determined during the acidification of casein micelle suspensions in the pH range 6·7–1·5. The effects of casein concentration (27, 55, 83 and 144 g/kg) and ionic strength (0, 10 and 20 g/kg NaCl added) were investigated. Acidification resulted in solubilization of calcium, magnesium, inorganic phosphate and citrate ions. However, the solubilization curves were different and depended on the casein concentration. Increasing ionic strength by adding NaCl had no effect on acid-induced mineral solubilization. These results were compared with those obtained during milk acidification and discussed in relation to the mineral solubilization that occurs during curd acidification in cheese manufacture.

Preparation, physicochemical characterization and stability of iron-supplemented caseins were studied. When ferrous iron (final concentrations of 0·25, 0·5, 1·0 and 1·5 mM) was added to sodium caseinate (final casein concentration 25 g/l), iron binding to caseins was complete. In parallel, an increase in absorbance at 280 nm, a decrease in fluorescence intensity, modifications of reversed-phase HPLC profile, a decrease in pH and an increase in free sodium concentration were observed. Moreover, no release of iron from iron-casein complexes was found between pH 6·5 and 3·7 or after heat treatments or in the presence of inorganic phosphate. Only EDTA disodium salt and trisodium citrate had significant effects on the release of iron bound to caseins.