Abstract
Saccharification of lignocellulosic biomass is a fundamental step in the biorefinery of second generation feedstock. The physicochemical and enzymatic processes for the depolymerization of biomass into simple sugars has been achieved through numerous studies in several disciplines. The present review discusses the development of technologies for enzymatic saccharification in industrial processes. The kinetics of cellulolytic enzymes involved in polysaccharide hydrolysis has been discussed as the starting point for the design of the most promising bioreactor configurations. The main process configurations—proposed so far—for biomass saccharification have been analyzed. Attention was paid to bioreactor configurations, operating modes and possible integrations of this operation within the biorefinery. The focus is on minimizing the effects of product inhibition on enzymes, maximizing yields and concentration of sugars in the hydrolysate, and reducing the impact of enzyme cost on the whole process. The last part of the review is focused on an emerging process based on the catalytic action of laccase applied to lignin depolymerization as an alternative to the consolidated physicochemical pretreatments. The laccases-based oxidative process has been discussed in terms of characteristics that can affect the development of a bioreactor unit where laccases or a laccase-mediator system can be used for biomass delignification.
Highlights
Published: 27 May 2021Lignocellulosic biomass is currently the main exploited feedstock in several value chains of the bio-based economy
The main open issues revealed by the current state of the art are: (i) the balance between high sugar yield and high sugar concentrations in the liquid hydrolysate; (ii) the variability of biomass composition, typically affecting second generation biorefinery, based on the valorization of lignocellulosic wastes that avoids a comprehensive assessment of heterogeneous hydrolysis kinetics; and (iii) the continuous use of the enzymes and difficult recycling hindered by the nature of the process involving the adsorption of the biocatalyst on the solid substrate
The attention goes from the microscale to the macroscale through the following sections, including literature analysis and discussion: (1) the kinetics of heterogeneous hydrolysis of lignocellulosic biomass catalysed by cellulases and modelling approaches as a basis for process and bioreactor design; (2) the principles of process design for saccharification and fermentation processes to maximize sugar and final product yields; (3) bioreactor design strategies to accomplish batch and continuous enzymatic hydrolysis of lignocellulosic biomasses; (4) the enzymatic depolymerisation of lignin catalysed by laccases as a potential process that may enable a fully enzymatic saccharification of biomasses
Summary
Lignocellulosic biomass is currently the main exploited feedstock in several value chains of the bio-based economy. The main open issues revealed by the current state of the art are: (i) the balance between high sugar yield (favored at diluted biomass working conditions—about 5%wt) and high sugar concentrations in the liquid hydrolysate (favored in bioreactors working at high solid loading >15%wt); (ii) the variability of biomass composition, typically affecting second generation biorefinery, based on the valorization of lignocellulosic wastes that avoids a comprehensive assessment of heterogeneous hydrolysis kinetics; and (iii) the continuous use of the enzymes and difficult recycling hindered by the nature of the process involving the adsorption of the biocatalyst on the solid substrate. The attention goes from the microscale (kinetics) to the macroscale (bioreactor and process integration) through the following sections, including literature analysis and discussion: (1) the kinetics of heterogeneous hydrolysis of lignocellulosic biomass catalysed by cellulases and modelling approaches as a basis for process and bioreactor design; (2) the principles of process design for saccharification and fermentation processes to maximize sugar and final product yields; (3) bioreactor design strategies to accomplish batch and continuous enzymatic hydrolysis of lignocellulosic biomasses; (4) the enzymatic depolymerisation of lignin catalysed by laccases as a potential process that may enable a fully enzymatic saccharification of biomasses
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