Structure-Function Relationship in Citrus-Fiber-Based Emulgels for Controlled Curcumin Delivery

Biphasic systems able to effectively release bioactive molecules along the gastrointestinal tract (GIT) are receiving growing interest. In this work, emulgels structured with citrus fiber, a digestion-resistant structuring agent, were produced using two types of edible oils (Miglyol^® 812 N and rice oil). Samples with 3% w/w of fiber were loaded with curcumin. The rheology of emulgels, reference hydrogels, and oil phases was studied. Complex modulus (G*) and viscosity (η) increased with increasing fiber fraction, whereas the phase angle (δ) was fiber fraction-independent (p < 0.05). Dynamic and flow behaviors were modeled using weak gel model and modified Cross model, respectively. Samples with rice oil were more consistent and viscous than samples with Miglyol^® 812 N because of the higher G* and η of rice oil. Curcumin does not affect the rheology of oils, whereas it modifies the emulgel behavior. In emulgels, curcumin does not change (p < 0.005) both weak gel parameters. Gel strength (A) was 750 ± 40 Pa s^z again 760 ± 40 Pa s^z and 597 ± 2 Pa s^z again 604 ± 4 Pa s^z for the system with rice oil and Miglyol^® 812 N, respectively, and network extension (n) resulted to be 14.13 ± 0.03 for all samples. Curcumin slightly increases the phase angle δ, 5.83 ± 0.09° again 7.0 ± 0.2° and 5.5 ± 0.1° again 7.10 ± 0.08° for the system with rice oil and Miglyol^® 812 N, respectively. This suggests a reduction in the structure of the fiber network. Curcumin has an oil-dependent influence on the zero-shear-rate viscosity (µ0) and on the time constant (m), while it does not affect the shear-thinning index (n), which resulted to be statistically independent of all systems (p < 0.05) yielding an average value of 1.616 ± 0.007. According to in vitro release studies, the percentage of cumulative released curcumin at 24 h was 15 ± 1% for emulgel with Miglyol^® 812 N, whereas for the sample with rice oil, it was 18 ± 1%. Overall, results suggest the attractiveness of these systems for potential applications in the sustained oral release of curcumin.