Facile Alternative Sustainable Process for the Selective Extraction of Microbial Melanin

Natural melanin biopolymers have multifarious activities, but their commercial exploitation is obstructed due to their high price, unsustainable source, use of harmful solvents, and low extraction yield. Conventional techniques for the extraction of the melanin biopolymer encounter several disadvantages such as poor extraction efficiency, excessive use of acid and alkali, and lengthy time duration. Due to the heterogeneous and complex structure of melanin, it is insoluble in most of the organic solvents as well as water and is less soluble in DMSO and alkaline pH. Herein, the suitability of an aqueous solution of quaternary ammonium hydroxide known as tetrabutylammonium hydroxide (40% w/w TBAOH in water) to extract the melanin biopolymer from the endophyte Streptomyces hyderabadensis 7VPT5-5R was investigated. The aqueous TBAOH extraction method led to a 66% increase in the yield of melanin in comparison to the conventional method of extraction. The used solvent was further recycled for five consecutive cycles with melanin yields ranging from 5.54 ± 0.03 to 5.47 ± 0.02 g/L. Considering the efficiency of the aqueous TBAOH for effective extraction of melanin and its recyclability potential, it can be considered as a sustainable solvent for the large-scale extraction of pigments from bacterial culture.