Influence of rheological properties and microstructure of microemulsions on the kinetic release of bioactive compounds

Microemulsions (ME) are one of the most effective ways to incorporate lipophilic active compounds into water-based food matrices. Great stability, colloidal domain droplets, and optical isotropy are some of the ME advantages. The present work analyzes the influence of rheological behavior and microstructural characteristics of food microemulsions on the release rate of lipophilic vitamins. A fluid ME was compared to a ME with higher dispersed phase content and a bicontinuous structure (gel-ME). Additionally, carboxymethyl cellulose (CMC) was added to fluid ME (3.5 and 10 g/100g), obtaining systems with different viscosity and microstructure. All ME were characterized using FTIR, rheological analysis, and TEM microscopy. Those systems with similar zero-shear viscosity showed significantly different viscoelastic behavior; gel-ME behaved like a concentrated suspension of macromolecules but with the largest plateau modulus. Microemulsions with CMC exhibited a viscoelastic solid type with G’ > G’’. Mechanical spectra were satisfactorily fitted with Generalized Maxwell model and the relaxation time spectra were determined. Kinetic release of vitamins E and D was studied and modeled at 37°C. Fluid and gel-ME reached higher percentages of release in a very fast manner, while CMC systems showed a matrix-driven nature of the release, in agreement with the relaxation time behavior determined from rheological experiments.