Advances and Challenges in Biocatalysts Application for High Solid-Loading of Biomass for 2nd Generation Bio-Ethanol Production

Abstract

Growth in population and thereby increased industrialization to meet its requirement, has elevated significantly the demand for energy resources. Depletion of fossil fuel and environmental sustainability issues encouraged the exploration of alternative renewable eco-friendly fuel resources. Among major alternative fuels, bio-ethanol produced from lignocellulosic biomass is the most popular one. Lignocellulosic biomass is the most abundant renewable resource which is ubiquitous on our planet. All the plant biomass is lignocellulosic which is composed of cellulose, hemicellulose and lignin, intricately linked to each other. Filamentous fungi are known to secrete a plethora of biomass hydrolyzing enzymes. Mostly these enzymes are inducible, hence the fungi secrete them economically which causes challenges in their hyperproduction. Biomass’s complicated structure also throws challenges for which pre-treatments of biomass are necessary to make the biomass amorphous to be accessible for the enzymes to act on it. The enzymatic hydrolysis of biomass is the most sustainable way for fermentable sugar generation to convert into ethanol. To have sufficient ethanol concentration in the broth for efficient distillation, high solid loading ~<20% of biomass is desirable and is the crux of the whole technology. High solid loading offers several benefits including a high concentration of sugars in broth, low equipment sizing, saving cost on infrastructure, etc. Along with the benefits, several challenges also emerged simultaneously, like issues of mass transfer, low reaction rate due to water constrains in, high inhibitor concentration, non-productive binding of enzyme lignin, etc. This article will give an insight into the challenges for cellulase action on cellulosic biomass at a high solid loading of biomass and its probable solutions.

A novel sucrose-inducible expression system and its application for production of biomass-degrading enzymes in Aspergillus niger

“…The candidate transformants were first screened on esculin plates to test the BGL production. The size of the halo around the colony was positively correlated with the BGL activity [ 20 , 21 ]. Among the transformants, FBL9 and FBL20 showed much larger black halos around the colonies than the parental strain ATCC 20611 (Fig.…”

“…BGL plays a vital role in lignocellulose degradation by undertaking the rate-limiting final step of hydrolyzing cellobiose, which is an intermediate product and also an inhibitor of cellulase activities, into glucose [ 21 , 22 ]. As such, supplementing the T. reesei cellulase preparation with exogenous BGLs is of crucial importance for efficient biomass degradation [ 20 , 23 , 24 ]. The addition of purified BGL to the T. reesei Rut-C30 cellulase preparation has been proven to bring about an 80% increase in glucose yield during the saccharification of pretreated cornstalk [ 23 ].…”

“…Nevertheless, enzymatic degradation of biomass into monomers that can be subsequently utilized by the fuel or chemical industries still presents significant barriers, such as low efficiency and high costs of biomassdegrading enzymes [18,19]. Thus far, the commercial cellulases for cellulose degradation are predominantly from filamentous fungi, of which, Trichoderma reesei is extensively used as a source of cellulases but has a less abundance of β-glucosidase (BGL) [18,20]. BGL plays a vital role in lignocellulose degradation by undertaking the rate-limiting final step of hydrolyzing cellobiose, which is an intermediate product and also an inhibitor of cellulase activities, into glucose [21,22].…”

Subcritical Water Pretreatment for the Efficient Valorization of Sorghum Distillery Residue for the Biorefinery Platform

“…Inedible plant biomass or lignocellulosic biomass predominantly involves grasses, agricultural residues, and woody materials. The estimated annual production of lignocellulosic biomass is around 170 billion metric tons [11]. It is not easy to use and valorize LCB for biofuels and other value-added products due to the rigid, recalcitrant cell walls of plants that protect them from fungi and other pathogens [3,12,13].…”