One step bioconversion of lignin from rice husk to biovanillin by phanerochaete chrysosporium

Abstract

Rice processing produces two major types of wastes which are rice husk and rice straw. The husk is the outer coat of paddy grain while the straw is the dry stalks of paddy. Burning in landfills, the common rice husk disposal method, is a cause of serious concern on the environment and human health. Despite the effort to fully utilize all the fractions of lignocellulosic biomass, most research only focuses on the cellulose and hemicellulose, whereas the lignin was left as worthless refuse. However, interest in lignin is growing for its abundant phenolic compounds and utilisation in aromatic chemicals such as biovanillin. The research aimed to investigate the potential production of biovanillin from rice husk lignin in submerged batch fermentation using Phanerochaete chrysosporium ATCC 24725. As a product of lignin degradation, ferulic acid is a precursor and intermediate in one-step biovanillin production in batch fermentation. Four types of pretreatments, namely physical, chemical, physicochemical, and enzymatic hydrolysis using cellulase from Trichoderma virens KR259658 were applied prior to recovering lignin. Out of twenty-one types of pretreatments, the microwave method without any chemical addition was chosen for optimization using the One Factor-at-a-Time method and Central Composite Design. Because microwave radiation can heat uniformly inside the sample, it breaks down the lignin and hemicellulose structure. Furthermore it destroyed the partial crystallised cellulose and degraded hemicellulose into reducing sugars. The microwave also saved the pretreatment time and increased the lignin accessibility to hydrolytic enzymes. Three parameters were studied namely microwave irradiation time, solid loading, and microwave power. At irradiation time of 16.57 min, solid loading of 9.66%, and microwave power of 664.23 Watt, the predicted lignin recovery was at 34.91%. The effect of different nitrogen sources in biovanillin production was investigated using the General Factorial Design. To further investigate the significant factors affecting the biovanillin production, 2-Level Factorial Design was carried out using the combination of diammonium tartrate, meat peptone, and five other factors, lignin concentration, the temperature of incubation, pH of the medium, agitation speed, and size of inoculum. Out of these seven factors, only three factors - organic source, pH of medium, and agitation speed have significant impact and were used in optimization. The maximum biovanillin production was performed with the media composition of 20.0 g/L of glucose, 1.8415 g/L of diammonium tartrate, 6.0 mM of meat peptone, 0.2 g/L KH2PO4, 0.013 g/L CaCl2.2H2O, 0.5 g/L MgSO4.7H2O, 0.3 g/L of lignin and 0.0025 g/L thiamine hydrochloride, and treatment conditions of temperature at 30.64 ºC, pH 4.0 and 199.61 rpm agitation speed. The predicted R2 0.6583 is in reasonable agreement with the adjusted R2 of 0.8016. The biovanillin produced after the optimized condition equals to 0.0122 g/L. In conclusion, this work has shown the potential utilization of lignin extracted from rice husk to produce biovanillin in submerged fermentation using Phanerochaete chrysosporium.