The structural characterization and color stabilization of the pigment protein-phycoerythrin glycosylated with oligochitosan

Phycoerythrin (PE) is a kind of natural and nontoxic pigment protein with a distinctive phycobilin, and this pigment is easy to be destroyed in the process of food application, which makes its stability weak and application limited. In this work, PE was extracted by freeze-thaw method from Porphyra haitanensis, and the trans-glutaminase (10 U g-1 protein) was used to catalyze the glycosylation reaction between PE and oligochitosan with molar ratios of 1:1 to 4:1 (oligochitosan/PE) to obtain the oligochitosan-modified phycoerythrin complexes (OMPC). The glycosylation of oligochitosan to PE was proven by the polyacrylamide gel electrophoresis and was evidenced by the glucosamine and free amino analysis. When the molar ratio of oligochitosan to PE was 3:1, the glucosamine content of OMPC was the highest, which was 31.403 +/- 0.004 mg g-1 protein. The OMPC samples were studied by infrared spectrum, fluorescence spectrum, and scanning electron microscopy. The results showed that the secondary structure and tertiary structure of glycosylated products changed, and a compact polymer appeared. In addition, compared with untreated PE, the surface hydrophobicity and emulsifying activity of OMPC decreased, while the solubility, emulsifying stability, thermal stability, photostability, and pH stability were significantly improved. This study reveals an innovative method to stabilize PE by a glycosylation approach, which will advance PE usage as a natural colorant in food applications.