Gelation and network structure of acidified milk gel investigated at different length scales with and without addition of iota-carrageenan

This work investigated the formation of acid milk gel at different length scales with and without the addition of iota-carrageenan (IC) (0.5% w/w) during incubation at 30 °C, using bulk rheology and syringe compression test at the macroscopic level, passive particle tracking (particle diameter 0.3 μm and 1.0 μm) at the microscopic level and pulsed field gradient (PFG)-NMR with poly(ethylene oxide) (PEO) at the molecular level. IC induced the early formation of the gel network in the sample as demonstrated by the progressive G′ and G″ increase (where G′ > G″) in the macroscopic measurement. When repeatedly compressed at the developing stage, the formed gel network in the IC sample reassociated into non-native colloidal structures unable to contain the serum phase, which also occurred without IC but at later incubation stage. The increased PEO diffusion at 30 min in PFG-NMR reflected the emerging casein-casein network and its extensive rearrangement occurring at the molecular level in both samples. This corresponded with the mean square displacement of the probe particles with IC at 30 min but not without IC, indicating that IC altered the length scale of the network rearrangements occurring at the molecular level to be detected at the microscopic level in the particle tracking. The size-dependent particle behaviors in the particle tracking demonstrated a high degree of IC-induced network heterogeneity that continued to develop even at late incubation stage, on contrary to the drastic compaction of the formed network observed in the sample without IC.