Production And Antioxidant Activity Of Melanin By Submerged Fermentation Of Auricularia Auricula

Auricularia auricula is a traditional culinary and medicinal fungus in China and melanin is one of the main bioactive compounds in its fruiting body. However, it is too long to produce A. auricula fruiting body and melanin extraction from the fruiting body is complex with high cost. In the present study, melanin was produced by submerged culture of A. auricula, and its physico-chemical properties and antioxidant activities were studied. Firstly, statistically designed experiments, such as Plackett–Burman design, steepest ascent path and response surface methodology, were conducted for the media optimization to enhance the production of melanin by submerged culture of A. auricula. Optimal media were obtained at the following concentrations: glucose 0.90 g/L, tyrosine 6.68 g/L, peptone 6.99 g/L, CaCO3 6.75 g/L, Soluble starch 6 g/L,(NH4)2SO4 1.5 g/L, MgSO4 0.8 g/L, K2HPO4 1 g/L, CuSO4 0.005 g/L, FeSO4 0.01 g/L. The melanin yield was enhanced from 306.52 mg/L to 1008.08 mg/L under optimum conditions.Secondly, The melanin was purified from the fermentation broth of A. auricula and its physico-chemical and structure properties were analyzed. The results showed that the melanin from the fermentation of A. auricula has the similar properties with synthetic melanin: insoluble in water, inorganic acid and most of organic solvents; soluble in alkali liquor and DMSO; fading in oxidant agent but almost not affected by reducing agent; able to chelate metal ions; stable in the glucose and sucrose solution; unstable in glutamic acid, citric acid, and tartaric acid solution; It indicated the presence of phenol structure of the melanin to produce brown precipitate with FeCl3 and turn to blue when FeSO4 / K3Fe(CN) 6 was added. The UV spectra of A. auricula melanin had wide range absorption from 200 to 800 nm, and the absorption declined with the increase of absorbance wavelength. When plotted against the wavelength, the log of the optical density of melanin solution produced a linear relationship with a negative slope of-0.0030. That was the typical spectral characteristics of melanin. Infrared spectrum indicated A. auricula melanin had-OH,-NH2, aromatic ring and other functional groups. Scanning electron microscope showed A. auricula melanin had amorphous structure.Moreover, pyrogallol self-oxidation, ABTS and DPPH free radical scavenging, salicylic acid method and FRAP method were performed to evaluate the antioxidant activities of A. auricula melanin. The results showed the scavenging activities of A. auricula melanin on superoxide anion, hydroxyl free radical and ABTS cation radical were obviously stronger than that of Vc and synthetic melanin, and its scavenging activities were concentration-dependent. However, the DPPH free radical scavenging activity and reducing power of A. auricula melanin were weaker than that of synthetic melanin and Vc. This work showed A. auricula melanin is a natural antioxidant with great potential application. The results from this work are helpful for melanin production by submerged culture of A. auricula.