Abstract: In order to expand the application of curcumin in foods, a food-grade dilution-stable microemulsion with high curcumin in it was prepared with oleic acid as oil phase, polyglycerol monolaurates with different degrees of polymerization as surfactants and polyols with different carbon chain lengths as cosurfactants. A suitable surfactant and cosurfactant were selected by analyzing the pseudo ternary phase diagrams. Furthermore, the effects of the mass ratio of surfactant to cosurfactant (Km), aqueous phase pH, and NaCl concentration on the phase behavior of microemulsions were investigated. The structure of microemulsions was analyzed by the conductivity method. The particle size and morphology of microemulsion droplets were determined by laser particle size analyzer and transmission electron microscope (TEM). The results showed that the best emulsification efficiency together with the largest proportion of microemulsion area was obtained under the following conditions: a 2:1 mixture of hexaglycerol and decaglycerol monolaurates as the surfactant, ethanol the cosurfactant and a Km value of 1/2. The emulsion droplets were uniformly spherical in shape with particle size of 10–100 nm. The addition of NaCl and an aqueous phase at non-neutral pH could led to a decrease in the proportion of microemulsion area and an increase in the minimum dilute ratio (DR). An infinitely dilutable microemulsion could be formed at a mass ratio of surfactant to oil phase (SOR) of 9:1. A W/O microemulsion could be obtained with water content less than 45%. The microemulsion could be transformed into double continuous type with increasing water content up to 60% and then into O/W-type at higher water content. Microemulsification could increase the solubility of curcumin to 4.87 mg/mL without influencing the biological activity of curcumin. This indicated that the microemulsion might be an excellent carrier for curcumin that could efficiently improve its poor solubility and dilutability.

Key words: microemulsion; polyglycerol monolaurate; curcumin; pseudo ternary phase diagram