Resveratrol-loaded oil-in-water emulsion-based delivery systems: Impact of droplet size, pH, temperature, and ultraviolet light on stability

<title>Abstract</title> <p> Resveratrol is a bioactive polyphenol with antioxidant and health-promoting activities, but its poor water solubility and chemical stability limit its use as a functional ingredient. This study explored the impact of droplet size, pH, storage temperature, and UV light exposure on the partitioning and stability of resveratrol loaded in medium-chain triglyceride (MCT) oil-in-water emulsions. Resveratrol was loaded into the oil droplets using a pH-driven method based on the substantial reduction in the water-solubility of resveratrol when the pH is adjusted from alkaline to neutral. Initially, oil droplets with different mean droplet diameters (D ₃₂ ) were prepared by varying the homogenization method and operating conditions: small (0.62 µm), medium (1.29 µm), and large (6.79 µm). There was a slight increase in droplet size for the emulsions containing small droplets after incorporation of resveratrol. Moreover, the surface charge of small and medium droplets became more negative after resveratrol loading, suggesting some of it was located at the oil-water interface. The oil solubility, encapsulation efficiency, and oil/water partitioning of resveratrol were measured. The maximum solubility of resveratrol in MCT oil was 2.03 mg/mL at 37°C. The partitioning of resveratrol into the oil phase was highest (&gt; 74%) for the emulsions containing the smallest droplets. In general, the encapsulation efficiency of the resveratrol in the emulsions was relatively high (&gt; 93%), suggesting there was little loss of resveratrol during the loading process. All emulsions remained stable throughout 21 days storage at 4°C, with little resveratrol loss and little change in droplet characteristics. However, storage at 37°C caused some resveratrol degradation, especially in the emulsions containing the largest droplets. The resveratrol remained relatively stable when stored at pH values from 3 to 7 but exhibited appreciable degradation when stored at pH 8 and 9, highlighting the poor stability of this polyphenol under alkaline conditions. When exposed to UV light, resveratrol degradation followed second-order kinetics. The photostability of resveratrol was better when encapsulated in small oil droplets and stored under mildly acidic to neutral pH conditions (longer half-life) than when encapsulated in larger oil droplets and stored under alkaline pH values (shorter half-life). These findings show that resveratrol stability is enhanced by reducing oil droplet size and storing the emulsions under acidic or neutral pH conditions at refrigerated temperatures, offering practical guidance for the formulation of resveratrol-based functional foods and cosmeceuticals. </p>