Two stage enzymatic pretreatment of rice straw for its valorisation using Silicase and Laccase

The present study investigated the cumulative effect of pretreatment of rice straw with two enzymes-Silicase (first report) and Laccase as initial stages. The optimal conditions for silicase pretreatment were determined to be a dosage of 100 U/10 g of rice straw, pH 7.0, temperature 40 ºC, and a treatment duration of 30 h. For laccase pretreatment, the ideal parameters were enzyme dose of 40 U/10 g of rice straw, pH 5.0, temperature 50 ºC, and a treatment time of 30 h. At optimized conditions, the silica reduction of ~ 20% was achieved by Silicase pretreatment, whereas the reduction in lignin was upgraded by ~ 29.8% after two stage pretreatment. A reduction of 28.6% in ash content of rice straw and 29.4% in silica was obtained during two stage enzymatic pretreatment. The FTIR studies of the pretreated and untreated straw also depicted the delignification, ash and silica removal of agro-waste. A peak observed at 1542 cm^-1 and 1643 cm^-1 suggests cyclic stretching in phenolic lignin, while the absorption band at 1419 cm^-1 corresponds to the bending (scissoring) of - OCH₃ in the syringyl and guaiacyl units of the phenolic composition. A notable decrease in these vibrations was observed in the silicase + laccase-treated sample, likely resulting from the removal of syringols and guaiacols during the enzymatic pretreatment. Using present outcomes, the study presented that the cumulative impact of Silicase and Laccase was proficient in preparing rice straw for industrial applications and reducing environmental barriers during its conversion.