Yvon Le Graët

pH-induced solubilization of minerals from casein micelles: influence of casein concentration and ionic strength

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.

Iron and citrate interactions with hen egg white lysozyme

The binding of ions to proteins is of great interest in biological science (catalytic function, structural stability) and a good understanding of this relationship is needed for the control of the structure and functionality of proteins. The interaction of hen egg white lysozyme with calcium, magnesium, iron, citrate, phosphate and sulphate are studied by using the Scatchard representation or ESI-MS for iron-binding studies. This is the first time that anions have been considered and the results indicate that citrate interacts with lysozyme through electrostatic bonds. A pH of 6 favours citrate interaction to lysozyme, whereas no interaction appears at pH 9. ESI-MS studies also highlight that iron co-ordinates lysozyme, inducing the release of three protons, including one arising from an amine lateral chain.

Iron-supplemented caseins: preparation, physicochemical characterization and stability

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.

Physicochemical characterization of iron-supplemented skim milk

Abstract The physicochemical characterization and study of technological properties of skim milk samples supplemented with FeCl 2 or FeCl 3 were carried out. The iron concentrations ranged from 0 to 1.5 mM. For both salts used, iron was mainly bound to the micellar phase with a probable change of casein structure. In parallel, the mineral distribution and the casein micelle hydration changed differently as a function of the initial oxidation state of iron (Fe 2+ or Fe 3+ ). After acidification (between 6.7 and 4.0) or after heat treatments (95 °C—15 and 30 min), no release of iron was found. Rennet coagulation parameters (clotting time, aggregation time and curd firmness) of the different iron-supplemented skim milk samples were more modified when fortifications were performed with FeCl 2 than with FeCl 3 . The results were discussed in relation with proteins and minerals modifications.

Theoretical model for calculating ionic equilibria in milk as a function of pH: comparison to experiment.

Acidification is fundamental for the processing of milk into cheeses, caseins, and fermented dairy products. It is established that a pH decrease changes the ionic equilibria of milk, inducing the solubilization of micellar calcium phosphate. This study aimed to present a theoretical model calculating ionic equilibria in milk as a function of pH. From the pH and total concentrations of minerals and caseins, the model calculated the concentrations of all ionic species and their partition between micellar and aqueous phases of milk. As the pH decreased, the minerals present in the micellar phase were gradually displaced into the aqueous phase. The calculated concentrations of minerals were in a good agreement with the experimental ones determined from acidified milk. A very satisfactory accuracy of the calculations, estimated by a root-mean-square error (RMSE) value of 5% for Ca and P(i) and a slope of the plot close to a unit, was obtained. The model is proposed for the simulation of ionic equilibria and the partition of salts between the aqueous and micellar phases of milk and dairy formulations during acidification.

Determination of ammonium in milk and dairy products by ion chromatography

To control the quality and the biochemical evolution of milk and dairy products during their technological transformations, it is interesting to determine their ammonium concentrations. A chromatographic method for the determination of this compound is proposed. The method is based on the separation of ammonium by cation-exchange chromatography and its detection by suppressed conductivity. With an appropriate sample preparation, this method enabled identification and quantification of ammonium with good repeatability (relative standard deviation of about 5%). Moreover, good sensitivity (less than 0.5 mg/kg) and no interference between ammonium and other matrix components were determined. It was also shown that this method offers a very promising alternative for studying changes in ammonium concentration of milk or caseinate after their heat treatments and in different dairy products such as yoghurt and cheeses (hard cooked and mould ripened cheeses).

Resistance of β-lactoglobulin-bound lactose to the hydrolysis by β-galactosidase

An in vitro study was conducted to investigate the sensitivity of lactose-β-lactoglobulin conjugates to β-galactosidase from Kluyveromyces lactis. The hydrolysis was monitored by ion exchange chromatography. Compared to free lactose or lactulose, which were rapidly hydrolysed, protein-bound lactose was not hydrolysed by β-galactosidase even after an extended hydrolysis time. Tryptic digestion of the conjugates before addition of β-galactosidase improved the substrate accessibility and led to the release of about 50% of the linked lactose. The results strongly suggest that the resistance of protein bound lactose is linked to the globular and compact structure of lactose-β-lactoglobulin conjugates.