Dairy ingredients-based emulsions and beta-carotene delivery

Abstract

Instability and water-insolubility of many bioactive nutrients greatly limit their oral bioavailability and thus their health benefits. Therefore, the delivery of these compounds requires protective mechanisms. Emulsion-based delivery systems are becoming some of the most ideal microencapsulation carriers for these lipophilic bioactive components, and tailored structures of emulsions potentially contribute to a better control of the stability and bioavailability of instable and poorly water-soluble bioactive components. The current study mainly investigated four model O/W emulsions with different initial droplet size, oil phase compositions, emulsifiers, and water phase compositions. A representative lipophilic bioactive nutrient, β-carotene, was encapsulated into these model emulsions. Emulsion properties, and the in vitro digestion, release, bioaccessibility and absorption by enterocytes of encapsulated βcarotene were investigated. Re-dispersible dry forms of these model emulsions containing β-carotene were also prepared, and their microstructures, re-dispersibility, and the properties of their reconstitutions were characterized. A whey protein isolate (WPI) stabilized emulsion with small initial droplet size showed better creaming and pH stability and higher cellular uptake of β-carotene than that with large initial droplet size. After passing through the simulated gastrointestinal tract (GIT) digestion, initial droplet size significantly influenced the emulsion properties (e.g., droplet size and distribution and surface charge), but did not significantly affect the bioaccessibility and cellular uptake of β-carotene. Monoglycerides (MG) in the oil phase showed competitive absorption on the droplets surface with WPI, leading to reduced droplet surface charge. MG significantly increased the viscosity and creaming stability of WPI-stabilized emulsions. MG also significantly promoted the bioaccessibility and cellular uptake of β-carotene by Caco-2 cells (p<0.05). Emulsions stabilized with different emulsifiers of WPI, sodium caseinate, or tween 80, showed different droplet sizes, surface charges, creaming and pH stability, and cellular uptake of β-carotene without passing through the GIT. Selection of emulsifiers also significantly modified the emulsion properties when exposure to the GIT digestion, and the bioaccessibility and cellular uptake of β-carotene after the GIT digestion (p<0.05). Incorporation of KGM into the water phase of emulsions greatly improved the creaming and pH stability of WPI-stabilized emulsions, and significantly decreased the oiling-off of the emulsions during a freeze-thaw test. Emulsions containing KGM in the water phase showed a lower final release rate of encapsulated β-carotene then the emulsion without KGM (p<0.05), and the release rate decreased with increasing KGM content. Dried emulsions showed different morphologies and microstructures, depending on the drying method (spray-drying or freeze-drying), and the compositions of emulsions before drying. Dry emulsions showed fast re-hydration and good re-dispersibility in water. Compared with emulsions before drying, re-constituted spray-dried and freeze-dried emulsions showed shifted droplet size distribution to large and small size, respectively. Re-constituted emulsions containing KGM showed significantly decreased viscosity but increased creaming stability compared to emulsions before drying (p<0.05). Overall, the present study provided useful information about different model O/W emulsions as delivery carriers for lipophilic components, and on how emulsion structures can be designed to modify the release of health-beneficial lipophilic components and improve their oral bioavailability, which could be important in developing functional foods with sustained release, or improved oral stability and bioavailability of functional ingredients entrapped in food matrixes.