Physicochemical properties and the extraction process of natural melanin from Auricularia polytricha

Auricularia polytricha (A. polytricha) is a kind of edible fungus with high nutritional value, and melanin is one of its main active components. However, the research and application of melanin in A. polytricha has been neglected for a long time. The purpose of this study was to analyse the physicochemical properties of the melanin from A. polytricha and establish a cellulase-ultrasonic synergistic extraction process of A. polytricha melanin by orthogonal array and response surface design. Scanning electron microscopy (SEM), ultraviolet-visible spectroscopy (UV-Vis), infrared spectroscopy (IR), electron paramagnetic resonance spectroscopy (EPR) and elemental analysis were used to characterize the melanin. The stability and antioxidant activity of the melanin were also measured. The results showed that A. polytricha melanin is an eumelanin and displays a bulk amorphous morphology. The melanin had good stability in heat and light, and it also exhibited good antioxidant activity. The optimized extraction process of A. polytricha melanin was as follows: enzyme-tosubstrate ratio (E/S) of 1.4%, enzymolysis temperature of 35°C, enzymolysis pH of 4.0, enzymolysis time of 80 min, NaOH concentration of 0.68 mol/L, solid-to-liquid ratio of 1:40 (g/mL), ultrasound power of 350 W, ultrasonic time of 62 min, and ultrasound temperature of 70°C. The results will be helpful for the efficient production of A. polytricha melanin and its application as a food colouring as well as antioxidant agent.