Frédéric Gaucheron

Combined effects of temperature and high-pressure treatments on physicochemical characteristics of skim milk

Abstract The combined effects of temperature (4, 20 and 40 °C) and high-pressure treatments (250, 450 and 600 MPa for 30 min) on the physicochemical characteristics of skim milk were studied. At 4, 20 and 40 °C, the effects of high-pressure treatments were similar (except for the treatment at 250 MPa), leading to an increase in protein hydrophobicity, a decrease in lightness, a decrease in average diameter of particles and slight solubilization of calcium and phosphorus from the colloidal to the aqueous phase of the milk. At the same time, denaturation of β-lactoglobulin probably occurred. At 40 °C, and especially at 250 MPa, the effects were very different because protein hydrophobicity remained unchanged and the average diameter of particles increased with the presence of two distinct populations of casein micelles. These results are discussed in relation to the effect of high-pressure treatments on protein structure.

Iron fortification in dairy industry

Iron deficiency is usually the result of insufficient dietary intake of iron, poor utilization of iron from ingested food, or a combination of the two. Direct addition of iron to milk or dairy products might be an effective mean of increasing the dietary intake of iron to the general population. However, iron fortification of milk or dairy products induces several biophysicochemical modifications with important consequences. This report reviews general informations on the iron fortification of milk and dairy products.

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-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.

Acidification of pressure-treated milk

Abstract The behaviour of reconstituted pressure-treated milk towards acidification with glucono-delta-laclone (GDL) was studied. After high-pressure treatment at 250, 450 or 600 MPa for 30 min, GDL was added for 20 h at 4 °C and soluble minerals, 1H NMR relaxation rate and solvation changes with pH were studied, as well as the buffering properties of milk. Solubilization of calcium and phosphorus with pH, changes in a 1H NMR relaxation rate and solvation with pH were enhanced by the pressure treatment between pH 6.8 and 5.2. Between pH 6.8 and 6.1, the changes appeared to be independent of the pressure level, while between pH 6.1 and 5.2, they became dependent on the pressure level. The results are discussed in terms of pH and buffering changes due to the compression-decompression cycle.

Identification of low molar mass peptides released during sterilization of milk

Abstract Heat-induced proteolysis of casein micelle suspensions in salt solution, in milk ultrafiltrate and in milk during sterilization (120°C; 10–20 and 30xa0min) was studied. The increase in the non-protein nitrogen content and the amount of the released peptides soluble in 12% trichloroacetic acid with heating times indicated proteolysis. The low molar mass peptides were identified from HPLC/MS/MS analyses. Ten peptides were characterized, of which three came from κ -casein, two from β -casein and five from α s1 -casein. No major influence of the dispersing phase on the heat-induced proteolysis was detected.

Quantitative and qualitative variability of the caseinolytic potential of different strains of Pseudomonas fluorescens: Implications for the stability of casein micelles of UHT milks during their storage

Pseudomonas fluorescens grows at low temperature and produces thermo-resistant protease(s) that can destabilize UHT (Ultra High Temperature) milk during its storage. The consequences of contamination of microfiltered milk with 9 strains of P. fluorescens on the stability of the corresponding UHT milk during storage had been investigated in this study. The strains were classified in two groups according to their ability to destabilize UHT milk. For the group of highly destabilizing strains, sedimentations of UHT milks, low values to phosphate test and the presence of aggregates were observed. Zeta potential and hydration of casein micelles decreased, whereas non casein nitrogen (NCN) and non protein nitrogen (NPN) contents increased. The analyses of NCN fraction by liquid chromatography coupled to mass spectrometry indicated that the different casein molecules were hydrolyzed in a similar way for the destabilizing strains suggesting that the same enzyme was implicated. For the group of slightly or not destabilizing strains no visual and biochemical alteration were found. This study showed that destabilization of UHT milk by P. fluorescens was highly variable and strain-dependent.

Selective determination of phosphopeptide β-CN(1–25) in a β-casein digest by adding iron: characterization by liquid chromatography with on-line electrospray-ionization mass spectrometric detection

A beta-casein tryptic digest has been analysed by reversed-phase high-performance liquid chromatography (RP-HPLC) with on-line electrospray-ionization mass spectrometry (ESI-MS). Analyses of peptides were carried out before and after addition of iron(II) to the peptides in solution. In both cases, the majority of peptides were identified by the determination of molecular masses by ESI-MS and by prior knowledge of the amino acid sequence of beta-casein, and thus of its corresponding tryptic peptides. In the presence of iron(II), only phosphopeptide beta-CN(1-25) was able to bind iron to form different complexes that have increased retention times on the RP-HPLC column and that also absorbed at 280 nm. The method presented here appears to be selective for peptides containing phosphoseryl cluster(s).

Influence of pH on the heat-induced proteolysis of casein molecules.

Proteolysis of sodium caseinate solution (24.5 g/l) induced by heat treatment at 120 degrees 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 pepticles 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 approximately 7.0. In parallel, eighteen low molar mass peptides were characterized, of which four came from kappa-casein, nine from beta-casein and five from alphas1-casein. Peptides were preferentially released under alkaline conditions.