Acid-Induced Gelation of Enzymatically and Nonenzymatically Cross-Linked Caseins—Texture Properties, and Microstructural Insights

Abstract

Casein gels consist of a fractal organized network of aggregated casein particles. The gel texture thereby depends on the structure, the spatial distribution, and the interaction forces of the network’s elementary building blocks. The aim of this study was to explore the technofunctional consequences of a possible specificity of Maillard reaction-induced cross-linking reactions on casein with respect to texture and microstructure of acid gels. Therefore, sodium caseinate glycated with lactose in the dry state (60 °C, a w 0.5) was compared with casein samples cross-linked with methylglyoxal, with glutaraldehyde, or via microbial transglutaminase, respectively, at similar levels of protein cross-linking as confirmed by size-exclusion chromatography under denaturing conditions. Casein gels prepared by acidification with glucono-δ-lactone were characterized concerning pH kinetics during gelation, mechanical texture properties under large deformation, and water-holding capacity, while viscometric properties of casein suspensions were obtained prior to gelation. The gel microstructure was captured by confocal laser scanning microscopy and evaluated by means of image texture analysis. All protein cross-linking reactions studied led to an enhanced gel strength which was accompanied by an increased interconnectivity of the gel network and a decrease in apparent pore sizes. Gels with more densely packed strands, as was the case for enzymatically modified casein, exhibited pronounced mechanical stability. The spontaneous destabilization of the gel network upon prolonged glycation reactions, which was not obviously displayed by microstructural features but connected to an increased viscosity and pronounced pseudoplastic flow of the unacidified suspension, suggests a limitation of particle rearrangements and the weakening of interparticle protein–protein interactions by additional structure attributes formed during the early Maillard reaction (glycoconjugation).

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

“…Susceptibility of acid-induced gels from NaCn to syneresis is shown in Figure 6 as a function of incubation time with mTGase and for the different ionic conditions. Consistent with previous research [35,53], moderate cross-linking (1 and 3 h incubation) decreased the forced syneresis of the gels significantly, whereas excessive cross-linking (≥24 h) increased syneresis again. The addition of salts to cross-linked NaCn had no significant effect on susceptibility to syneresis, except for the 1 h sample where it was slightly reduced by the addition of NaCl.…”

“…Therefore, denser microstructures were previously interpreted as a prevention of these rearrangements by NaCl [55] as well as by covalent cross-linking of caseins by mTGase [56]. More recently, Hannß et al [53] reported a comparable microstructure for acid-induced gels from uncross-linked Cn-PB and Cn-PB cross-linked by mTGase to a PD of~60%, while forced syneresis was significantly decreased. This suggests that rearrangements of casein clusters might still be possible when casein is only partially cross-linked, while covalent isopeptide bonds contribute to the water holding capacity of the gel network.…”

What are melanoidins, polymers in food?

“…Although melanoidins are involatile, they make an effect on the flavor of foods through the adsorption of flavor compounds [19,20]. Protein glycation with polysaccharides through the Maillard reaction forms the structure like melanoidins, boosts emulsifying features, and improves solubilities of proteins [21,22]. Melanoidins also affect the formation and stabilization of foam [23][24][25].…”