Laccase Mediated Lignin Removal From Lignocellulosic Biomass for Bioethanol Production.

Bioenergy, in the form of bioethanol, is a viable potential source but several limitations need to be overcome. Bioethanol can be prepared from sugars and starch, but this would not be practical from socio-economic perspective because it competes for food sources. Lignocellulose, being one of the most abundant organic sources, has been considered as a potential raw material for bioenergy production. Lignocellulosic materials contain both cellulose and hemicellulose, present in a matrix of lignin. Pretreatment is a necessary step to remove lignin and several pretreatments such as wet oxidation and steam explosion are used to open up the lignocellulosic material for the subsequent hydrolysis to take place. These pretreatment methods are limited by loss of cellulose and hemicellulosic sugars, as well as by production of microbial inhibitors and environmentally undesirable chemicals. In this study, we aimed at the development of a novel approach to use lignolytic enzymes produced by white-rot fungi to pre-treat lignocellulosic material. Such an approach is inspired by the fact that natural degradation of lignin is carried out in the environment by certain white-rot fungi, which solubilize and mineralize lignin with the help of lignolytic enzymes. Two lignocellulosic materials, switchgrass and sweet sorghum (300 mg), were subjected to enzymatic pretreatment consisting of laccase enzyme (activity 20 units mL^-1) along with one of the three mediators; HBT (1-hydroxybenzotriazole), VLA (violuric acid), and ABTS (2,2’ –Azinobis-(3-ethyl-benzthiazoline-6-sulphonate)) for 24 h at 25 ⁰C.  The mediators were applied in a wide range from 0.25 to 1.5 mM to optimize the mediator concentration for maximum lignin reduction. Laccase mediator system was effective in significant reduction of lignin. Different mediators had different optimum concentration and different effectiveness to reduce lignin content.

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