Whey protein concentrate/pullulan gel as a novel microencapsulated wall material for astaxanthin with improving stability and bioaccessibility

Astaxanthin (AST) has various biological activities such as antioxidant, antitumor and anti-inflammatory, but poor water solubility, instability and low bioaccessibility greatly limit the development of its industrial application. In this study, whey protein concentrate (WPC) and pullulan (PUL) were used to prepare WPC/PUL gels under different conditions, the microstructure, drying activation energy and powder hygroscopicity were investigated. The WPC/PUL gel formed by heating 10% WPC and 8.0% PUL (pH 7.0) at 80 °C was selected as wall material to encapsulate AST by freeze-drying. The highest encapsulation efficiency achieved 93.14% when the volume ratio of AST solution to WPC/PUL gel was 1:2. Compared with free AST, the stability of AST encapsulated in WPC/PUL gel was significantly enhanced during simulated gastrointestinal digestion and the bioaccessibility was increased up to 5.1 times. AST retention was significantly improved after 26 weeks of storage at 4 °C and 25 °C, with half-lives of 822.38 d and 516.17 d, respectively. The results of X-ray diffraction and differential scanning calorimetry showed that AST was encapsulated in WPC/PUL gel in an amorphous state. The incorporation of AST caused a decrease in the surface hydrophobicity of WPC/PUL gel. It was confirmed by UV, Fourier transform infrared and fluorescence spectra that AST changed the protein conformation of WPC/PUL gel, and the formation of the WPC/PUL/AST complex was mainly due to hydrophobic interaction and hydrogen bond. This study provided a microcapsule based on WPC/PUL gel as a wall material to encapsulate AST, which has a promising future in food processing.