Asymmetric flow field flow fractionation for the investigation of caseins cross-linked by microbial transglutaminase

Association of Enzymatically and Nonenzymatically Functionalized Caseins Analyzed by Size-Exclusion Chromatography and Light-Scattering Techniques

“…Since supramolecular protein assemblies, namely, casein particles, represent the original building blocks of casein gel networks, the influence of casein modification on their physical and morphological properties might play an important role in the microstructure and texture of dairy products and food model systems. Abbate et al recently showed that the molecular weight of nonmicellar casein associates increased upon cross-linking with mTG. Their shapes shifted from being rather loose and elongated toward a more compact and spherical conformation.…”

“…The intrinsic properties of the individual casein types, α S1 -, α S2 -, β-, and κ-casein, like their rheomorphic conformation, the different distributions of hydrophobic and hydrophilic structure segments, as well as the varying degrees of phosphorylation, determine their special physical performance, which is dictated just as strongly by the environmental factors of the food matrix (e.g., temperature, pH, ionic strength, type of ions) . In bovine milk, the caseins naturally occur as spherical colloid particles (casein micelles) with a hydrodynamic radius ( R h ) of ∼100 nm, constructed of nanoclusters formed together with calcium phosphate as an essential compound. , In the absence of calcium, casein molecules associate mainly via hydrophobic interactions forming particles significantly smaller in size ( R h ∼ 10 nm). − These nonmicellar casein associates, which exhibit rather elongated, anisotropic conformations, , are to some extent comparable to the substructure units of the micelles . During the production of dairy foods, like yogurt, a slow acidification of milk to the isoelectric point of casein (pH 4.6) enables the proteins to form a stable, fractal gel network, where casein micelles further aggregate in clusters, chains, and strands. , Similar microstructural properties are observable for acid-induced gels of nonmicellar casein, which are often utilized as food model systems. , …”

Size Modulation of Enzymatically Cross-Linked Sodium Caseinate Nanoparticles via Ionic Strength Variation Affects the Properties of Acid-Induced Gels

“…In case of excessively cross-linked NaCn (≥24 h), rearrangements are impaired on the molecular level due to the fixation of casein molecules. In combination with the increased internal density and higher sphericity of the casein particles [24][25][26], this might increase their mobility in the gel network, as was also indicated by the higher tan δ (see Figure 3). Thus, rearrangements of casein clusters on the microstructure level might be facilitated, leading to higher forced syneresis.…”

“…It is, however, not clear, whether this fraction actually co-exists with casein nanoparticles in solution, or if its appearance in the chromatograms is rather due to measurement conditions causing some dissociation (i.e., dilution effects, shear forces during separation). No additional peak for casein monomers and/or small aggregates was observed in asymmetric flow field fractionation measurements of Cn-PB [24], or in SEC of NaCn in milk serum using a Sephacryl S-500 column [45]. With regard to the first peak in the chromatograms, Hannß et al [25] reported that it contained only low amounts of protein, but also triglycerides and phospholipids, suggesting a co-elution of large casein particles and fat droplets.…”

“…It was reported that the cross-linking of casein-bound glutamine and lysine residues by mTGase [23] occurs predominantly between casein molecules within the same NaCn nanoparticle, resulting in a change from elliptical to spherical shape [24,25] as well as in a transition from a soft particle character towards a hard sphere-like behaviour [26]. The size and aggregation number of native NaCn nanoparticles varied with casein concentration, temperature, pH and ionic strength due to modulation of hydrophobic and electrostatic interactions [27,28].…”