Effect of 2,2-azobis (2-amidinopropane) dihydrochloride oxidized casein on the microstructure and microrheology properties of emulsions

ArticleinFood science and biotechnology 25(5):1283-1290 · October 2016with 9 Reads
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Abstract
The impacts of protein oxidation on the droplet size and microrheology properties of casein emulsions with 20% oil content were investigated. The degree of protein oxidation was indicated by carbonyl concentration. The droplets in the emulsions of different-oxidation-degree casein had bimodal distribution, but their size altered due to oxidation. The effects of protein oxidation on the morphology, motion type, viscoelasticity, and stability of droplets were also investigated by microrheology analysis. The droplet motion was blocked by protein oxidation due to mean square displacement slope results. Solid–liquid balance values provided the liquid behavior dominating these emulsions. Oxidation of carbonyl concentration 16.72 raised the primary droplets, increased the elasticity, decreased the viscosity, and promoted the droplet motion rate, resulting in better stability of emulsions. Further oxidation promoted the aggregation of droplets and resulted in poor stability.

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  • Article
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    The exceptional surface-active and stabilizing properties of casein have been recognized from the earliest days of colloid science, and casein has long been employed as a functional ingredient in food processing. With its unique physico-chemical properties, this natural polymeric surfactant has potential for even greater use in formulation science and nanoscale processing technology. Gels may be produced from solutions of sodium caseinate, from micellar casein dispersions, and from casein-stabilized emulsions by a variety of treatments, including heating, acidification, and high-pressure processing. The temperature dependence of the interactions and the state of aggregation can be controlled by systematically varying the ionic composition of the system. By incorporating air bubbles into gradually flocculating caseinate-stabilized emulsions via simultaneous whipping and slow acidification, aerated emulsion gels of good foam stability can be formulated.
  • Article
    The sol-gel transition of a model dairy system (sodium caseinate solution) which undergoes gelation by acidification has been studied by conventional bulk rheology and particle tracking microrheology, via confocal microscopy. The Brownian diffusion of fluorescent microspheres (0.21, 0.32, 0.5, and 0.89μm in diameter) with different surface coatings (polyethylene glycol, carboxylate groups and polystyrene) was used to probe spatial mechanical properties of the gels at the scale of microns. The microrheological results are compared with the macroscopic viscoelastic properties (storage and loss shear modulus) measured in a concentric cylinder rheometer (double gap, at shear strain of 0.005 and frequency of 1Hz). At pH values close to pI of the caseins, where formation of a protein network, i.e., gelation, became obvious from the confocal microscopy and bulk rheological measurements, all the particles had a tendency to adhere to the network. In spite of this, the microrheological values of the moduli were only slightly lower than the macroscopically determined values and the gel points calculated via both techniques tended to be in good agreement. However, the particle tracking method has higher sensitivity and can detect changes in the structuring of the system before these are registered by the bulk rheological measurement.
  • Article
    Sedimentation coefficients were determined for association products in systems containing SH-kappa-casein, SH-kappa-casein-alpha(S1) or beta-caseins (1:1, by weight) and SH-kappa-casein-alpha(S1)-casein-beta-casein (2:1:1, by weight) under a variety of environmental conditions. As the concentration of NaCl is increased or, as the concentration of CaCl2 is increased at 0.05 M NaCl, the system containing SH-K-casein alone acts quite differently from those which also contain the Ca2+ -sensitive caseins. It is concluded that electrostatic as well as hydrophobic attractions are important between SH-kappa-casein monomers. Viscosity and sedimentation measurements combined indicate that nearly spherical polymers containing 25-30 monomers each and with radii of approx. 10 nm are formed in all systems. It is considered that these polymers are micelle subunits in which the non-polar portion of each monomer is oriented radially inward while the charged acidic peptides of the Ca2+ -sensitive caseins and the hydrophilic, carbohydrate-containing portion of kappa-casein are near the surface. Asymmetric distribution of kappa-casein in a micelle subunit results in hydrophilic and hydrophobic areas on the subunit surface. In this situation, aggregation through hydrophobic interactions forms a porous micelle. Micelle growth is limited by the eventual concentration, at the micelle surface, of subunits rich in kappa-casein. This model successfully accounts for the properties of equilibrium and nonequilibrium micelle systems, for the destabilization of the micelle by dephosphorylation of Ca2+ -sensitive caseins or by high concentrations of NaCl or CaCl2 and for the events associated with clot by rennin action.
  • Article
    Organisms are constantly exposed to various forms of reactive oxygen species (ROS) that lead to oxidation of proteins, nucleic acids, and lipids. Protein oxidation can involve cleavage of the polypeptide chain, modification of amino acid side chains, and conversion of the protein to derivatives that are highly sensitive to proteolytic degradation. Unlike other types of modification (except cysteine oxidation), oxidation of methionine residues to methionine sulfoxide is reversible; thus, cyclic oxidation and reduction of methionine residues leads to consumption of ROS and thereby increases the resistance of proteins to oxidation. The importance of protein oxidation in aging is supported by the observation that levels of oxidized proteins increase with animal age. The age-related accumulation of oxidized proteins may reflect age-related increases in rates of ROS generation, decreases in antioxidant activities, or losses in the capacity to degrade oxidized proteins.
  • Article
    The interactions between the classical cationic surfactant dodecyltrimethylammonium bromide (DTAB) and 2.0 mg/mL casein were investigated using isothermal titration calorimetry (ITC), turbidity, dynamic light scattering (DLS), and fluorescence spectra measurements. The results suggest that the cationic headgroup of the surfactant individually binds to the negatively charged amino acid sites on the casein chains because of the electrostatic attraction upon the addition of DTAB. When the surfactant concentration reaches a critical value c1, DTAB forms micelle-like aggregates on the casein chain, resulting in the formation of insoluble casein/DTAB complexes. Further addition of DTAB leads to the redissolution of casein/DTAB complexes because of the net positive charge on casein/DTAB complexes and the formation of DTAB free micelles. The addition of salt screens the repulsion between the surfactant headgroups and the attraction between casein and surfactant molecules, which weakens the binding of surfactant onto the casein chain, favoring the formation of free surfactant micelles.