Astaxanthin Microcapsules Prepared by Wet Milling and Spray Drying: Effect of Carrier Oil on Their Physicochemical Properties, Stability, and Bioaccessibility

In this study, oil-free (F1) and direct-contact oil-containing (F2) astaxanthin microcapsules were prepared by wet milling followed by spray drying, and indirect-contact astaxanthin microcapsules (F3) with excipient-emulsified oil were produced by high-pressure homogenization followed by spray drying. The microcapsules were characterized for their physicochemical properties, morphological characteristics, encapsulation efficiency, and apparent solubility. Their stability was evaluated under thermal and long-term storage conditions, and bioaccessibility was determined using an in vitro simulated digestion model. Results demonstrated that among the three types of microcapsules, F1 exhibited the highest astaxanthin payload (10.12%), middle moisture content (3.23%), favorable flowability, and a high encapsulation efficiency of 98.36%. Under scanning electron microscopy (SEM), F1 exhibited smooth and intact surfaces, and differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) confirmed the formation of a densely encapsulated structure. The apparent solubility of F1 was 6.41-, 2.30-, and 2.32-fold greater than those of free astaxanthin, F2, and F3, respectively. After rehydration, the particle size of F1 was the smallest (420.83 nm), and F2 was in the middle (441.28 nm), indicating that F1 had the best dispersion stability. After thermal treatment at 100 ℃, F1 and its redispersed solution showed 1.62- and 2.04-fold higher astaxanthin retention than free astaxanthin, respectively. Following 210 days of storage at 4 ℃ or ambient temperature, F1 retained more than 81% of its initial astaxanthin content, 1.04 to 1.08 times that of F2 and F3, respectively. After being stored for 70 days, the redispersed solutions of F1 and F2 showed higher (more than 85% and 80%) astaxanthin retention than that of F3. The bioaccessibility of F1 was 19.95 and 1.17 times those of free astaxanthin and F3, respectively, and the bioaccessibility of F2 was 1.30 times that of F3. In summary, the oil-free astaxanthin microcapsules prepared by wet milling and spray drying offer distinct advantages in terms of physicochemical properties, stability, and bioaccessibility, thereby being promising for applications in functional foods, pharmaceuticals, and related industries.