Alain Riaublanc

Self-Assembly and Foaming Properties of Fatty Acid−Lysine Aqueous Dispersions

We report on dispersions of fatty acid-lysine salts in aqueous solutions which are further used to produce foams. The alkyl chain length is varied from dodecyl to stearic. In aqueous solutions, the lysine salt of the dodecyl chain yields an isotropic solution, probably micelles, whereas for longer alkyl chains, vesicles formed but crystallized upon resting at room temperature or when kept at 4 degrees C. Solid-state NMR showed that in vesicles fatty acids are embedded in a lamellar arrangement passing from a gel to a fluid state upon heating; the transition temperature at which it occurs was determined by DSC. Those results are confirmed by small-angle neutron scattering which also give additional information on the bilayer structure. Incredibly stable foams are obtained using the palmitic acid/Lys salt whereas for other alkyl chain length, poor or no foam is formed. We conclude that the foamability is related to the phase behavior in aqueous solution.

Interfacial properties of fractal and spherical whey protein aggregates

Fractal and spherical aggregates of whey globular proteins are formed under conditions that coupled heating and shear flow in a plate heat-exchanger at high temperature and for short holding time. Their properties upon adsorption and spreading at the air–water interface have been studied at neutral pH and two subphase conditions. The surface activity of mixtures of aggregates and residual proteins is enhanced and the adsorption behaviour depends strongly on the denatured native-like monomers and the charge screening. After long hours of adsorption, they form a weakly dissipative viscoelastic network, with strong interactions. When spread at the air–water interface, protein aggregates dissociate and form monolayers whose properties are described by scaling laws in the semi-dilute regime. The scaling exponents found in charge-screening subphase conditions correspond to values for polymer in θ-conditions, whereas the values determined in long-range repulsion subphase conditions are intermediate between an ideal (θ-conditions) chain and a chain in good solvent.

Evolution of Fat Crystal Network Microstructure Followed by NMR

Model systems composed of tristearin in solid state and tricaprin in liquid state with different solid-fat content (SFC) and storage time have been investigated by relaxation NMR and NMR diffusometry. The T(2) relaxation of the tricaprin in the melt exhibited a bimodal distribution as previously observed. The SFC had a major effect on the T(2) relaxation. This effect was explained according to the fast diffusive exchange model in porous media. According to this model the changes in T(2) relaxation as a function of the SFC and storage time were explained by the decrease of the surface-to-volume ratio of the crystal induced by Ostwald ripening. The diffusion coefficient D of the tricaprin in the melt decreased for higher SFC. Since no significant variation of D was observed for different diffusion time, D reflected the long-range connectivity and the tortuosity was calculated. During storage the diffusion coefficient remained constant.

Self-similar assemblies of globular whey proteins at the air–water interface: Effect of the structure

We investigated the structure of heat-induced assemblies of whey globular proteins using small angle neutron scattering (SANS), static and dynamic light scattering (SLS and DLS), and cryogenic transmission electron microscopy (Cryo-TEM). Whey protein molecules self-assemble in fractal aggregates with a structure density depending on the electrostatic interactions. We determined the static and dynamic properties of interfacial layer formed by the protein assemblies, upon adsorption and spreading at the air-water interface using surface film balance and interfacial dilatational rheology. Upon spreading, all whey protein systems show a power-law scaling behavior of the surface pressure versus concentration in the semi-dilute surface concentration regime, with an exponent ranging from 5.5 to 9 depending on the electrostatic interactions and the aggregation state. The dilatational modulus derived from surface pressure isotherms shows a main peak at 6-8 mN/m, generally considered to be the onset of a conformational change in the monolayer, and a second peak or a shoulder at 15 mN/m. Long-time adsorption kinetics give similar results for both the native whey proteins and the corresponding self-similar assemblies, with a systematic effect of the ionic strength.

Study of triacylglycerol polymorphs by nuclear magnetic resonance: effects of temperature and chain length on relaxation parameters.

It is very important to monitor the characteristics of triacylglycerol crystal network in fats, as these crystals have an impact on many food properties such as texture, sensory taste, and extended shelf life. Although time‐domain NMR (TD‐NMR) is now the reference technique to determine the solid fat index in food, the entire possibilities of this technique are not used. Some NMR studies have been performed to determine its power for the discrimination of polymorphism. In this study, extended investigations proved that TD‐NMR could evaluate triacylglycerol (TA) polymorphism, independently from temperature and chain length. Study of the dipolar interactions through second moment M2, which is characteristic of proton mobility in solid‐state samples, provided a new understanding of the structural organization of crystal molecules. Proton spin–lattice relaxation, which has been proved to be a true probe of polymorphism, has provided information on crystal networks. Combination of the two techniques revealed two very interesting kinds of results, i.e. the presence of a minimum spin–lattice relaxation time T1 for tristearin α, which is a characteristic of a dynamic molecular process, and differences in behavior between long and short chain lengths, both at a molecular and a crystal level. Copyright

Calibration Adjustment of the Mid-infrared Analyzer for an Accurate Determination of the Macronutrient Composition of Human Milk

Background: Human milk composition analysis seems essential to adapt human milk fortification for preterm neonates. The Miris human milk analyzer (HMA), based on mid-infrared methodology, is convenient for a unique determination of macronutrients. However, HMA measurements are not totally comparable with reference methods (RMs). Objective: The primary aim of this study was to compare HMA results with results from biochemical RMs for a large range of protein, fat, and carbohydrate contents and to establish a calibration adjustment. Methods: Human milk was fractionated in protein, fat, and skim milk by covering large ranges of protein (0-3 g/100 mL), fat (0-8 g/100 mL), and carbohydrate (5-8 g/100 mL). For each macronutrient, a calibration curve was plotted by linear regression using measurements obtained using HMA and RMs. Results: For fat, 53 measurements were performed, and the linear regression equation was HMA = 0.79RM + 0.28 (R2 = 0.92). For true protein (29 measurements), the linear regression equation was HMA = 0.9RM + 0.23 (R2 = 0.98). For carbohydrate (15 measurements), the linear regression equation was HMA = 0.59RM + 1.86 (R2 = 0.95). A homogenization step with a disruptor coupled to a sonication step was necessary to obtain better accuracy of the measurements. Good repeatability (coefficient of variation < 7%) and reproducibility (coefficient of variation < 17%) were obtained after calibration adjustment. Conclusion: New calibration curves were developed for the Miris HMA, allowing accurate measurements in large ranges of macronutrient content. This is necessary for reliable use of this device in individualizing nutrition for preterm newborns.

Functional Properties of Protein Fractions Obtained from Pumpkin (Cucurbita Maxima) Seed

Protein fractions were extracted from pumpkin (Cucurbita maxima) seeds with deionized water, salt solution, and alkali solution, and characterized under alkaline (pH 8) and ionic strength conditions for protein solubility, interfacial pressure, and functional properties. The supernatants obtained from the protein fractions after hydration and centrifugation were designed deionized water supernatant, salt solution supernatant, and alkali solution supernatant. All protein fractions showed poor foamability and very low stability. The designed deionized water supernatant presented the highest emulsifying properties. In terms of stability, designed deionized water supernatant emulsions flocculate during storage but with low rates of coalescence compared to the two other fractions.

Transition from fractal to spherical aggregates of globular proteins: Brownian-like activation and/or hydrodynamic stress?

We report on the structure of whey protein aggregates formed by a short heating coupled to shear at high temperatures (80-120) and neutral pH in scale-up processing conditions, using gel filtration chromatography, light scattering, small angle neutron scattering, and cryogenic transmission electron microscopy. The results are interpreted in terms of coexistence of residual non-aggregated proteins and aggregates. The characteristics of aggregates such as the size, the aggregation number and the shape evidence two different morphologies. Whereas aggregates formed at 80 °C show a selfsimilar structure down to a length scale of the monomer with a fractal dimension typical for reaction limited cluster aggregation (D~2.2), aggregates formed at higher temperature show a spherical morphology, with the structure from small angle neutron scattering data best modelled with the form factor of a polydisperse sphere. We compare the structure of these aggregates to that of aggregates formed in quiescent conditions at lab scale. The structure transition is interpreted in terms of a non-trivial interplay between three perturbation factors: interparticle interaction, temperature and shear.

The phase and charge of milk polar lipid membrane bilayers govern their selective interactions with proteins as demonstrated with casein micelles

The biological membrane surrounding fat globules in milk (milk fat globule membrane; MFGM) is an interface involved in many biological functions and interactions with the surrounding proteins or lipolytic enzymes in the gastro-intestinal tract during digestion. The MFGM exhibits lateral heterogeneities resulting from the different phase states and/or head-group charge of the polar lipids, which were both hypothesized to drive interaction with the casein micelles that is the major milk protein assembly. Atomic force microscopy (AFM) imaging was used to track the interactions of casein micelles with hydrated supported lipid bilayers of different composition, phase state and charge. Zeta-potential and Langmuir isotherms of the different polar lipids offered additional information necessary to interpret AFM observations. We showed that the negatively-charged casein micelles did not interact with milk sphingomyelin in the gel or liquid-ordered phases but did interact with polar lipids in the liquid-disordered phase (unsaturated polar lipids and milk sphingomyelin above its melting point). A wide intermolecular distance between polar lipids allowed protein adsorption on the membranes. However, the presence of the anionic polar lipids phosphatidylserine and phosphatidylinositol prevented any interaction with the casein micelles, probably due to electrostatic repulsion. These results open perspectives for the preparation of tailored emulsions covered by polar lipids able to modulate the interfacial interactions with proteins.

The molecular organisation of dairy matrices influences partitioning and release of aroma compounds

Abstract Partition coefficients of 12 aroma compounds from a strawberry aroma blend were investigated in dairy matrices containing skim-milk powder and 1.5 or 5% (w/w) of low melting point fraction or high melting point fraction anhydrous milk fat (AMF) acidified to pH≈4.6 with glucono-δ-lactone. The release of aroma compounds was markedly reduced as the fat level increased, notably for the more hydrophobic compounds. The partition coefficients were significantly higher over matrices with high melting point than with low melting point fraction milk fat, probably due, in part, to the solid fat content of the lipid fraction. They were also, to a lesser extend, higher in emulsions than in stirred gels. These results demonstrate the role of structure and molecular organisation of the matrix constituents on aroma partitioning.