Comparative performance of commercial and laboratory enzymatic complexes from submerged or solid-state fermentation in lignocellulosic biomass hydrolysis

Abstract

The aim of this study was to compare the hydrolysis performances of four lignocellulolytic complexes from commercial or laboratory origin and produced either by solid-state fermentation or by submerged fermentation. To evaluate their potential, saccharification tests were performed on cellulose, as model substrate, and wheat bran, as lignocellulosic substrate, using either the same filter paper unit or the same amount of protein to introduce these enzymatic complexes. A great difference was observed for the laboratory enzymatic complex produced by solid-state fermentation, which has shown a greater efficiency of cellobiohydrolase on cellulose and better conversion capacity on wheat bran, probably due to the presence of side activities. This comparison has proved that solid-state fermentation could be a promising technology to overcome the biomass recalcitrance and lower the cost of conversion step.

Introduction

Over the last three decades, lignocellulosic biomass conversion has received a great interest, mainly due to the potential use of carbohydrates and lignin as sustainable sources of bioethanol or bioproducts precursors in a biorefinery concept (Kamm and Kamm, 2004). Exploitation of the lignocellulose polysaccharides requires breaking them down into fermentable sugars. However, the major components of lignocellulosic biomass (cellulose, hemicelluloses and lignin) may have variations in their proportions and structural arrangements, depending on the type and age of the plant, resulting in a particularly recalcitrant structure to enzymatic hydrolysis (Himmel et al., 2007).

Among the different feedstocks, wheat represents one of the most important crops produced worldwide with 653.6 million tons produced in 2010 and the most widespread on earth with 217.2 million hectares harvested in 2010 (FAOSTAT, 2012). Wheat milling industry generates a large amount of bran, a co-product corresponding to the outer part of the grain, which generally accounts for 14–19% of the total mass. The main components of this fibrous material include (glucurono)arabinoxylans, mixed linkage β-glucans, cellulose, proteins, lignin and variable proportions of starch dependent on the milling conditions (Maes and Delcour, 2001). This agro-industrial residue remains an important source of lignocellulose with low value-added.

Currently, technical feasibility of the biochemical conversion processes from lignocellulosic biomass has been proved, but economical feasibility has not been yet demonstrated due to the low yield and high cost of enzymatic hydrolysis step (Talebnia et al., 2010). The wide diversity of plant-cell-wall-degrading enzymes necessary for an efficient saccharification, especially cellulases, hinders the competitiveness of the processes. Existing research with the GMO approach to reduce the conversion cost represent a considerable and very complicated work on many enzymes, which shows limited results for the moment. Development of new strategies is therefore requisite to overcome the biomass recalcitrance, to improve the cellulose accessibility and to make the conversion step cost-effective, but also the processes cost-competitive.

Among the different cellulolytic strains, Trichoderma reesei species are recognized as the best adapted for the production of enzymatic complexes, and more particularly the QM 9414 and Rut-C30 public access mutants (Cen and Xia, 1999). Two technologies are currently utilized to achieve the production of enzymatic complexes: traditional submerged fermentation (SmF), which occurs in the presence of excess water submerging nutrients and microorganisms, and solid-state fermentation (SSF), which happens on insoluble solids in the absence or near absence of free water. Most enzymes companies apply classical SmF technology to manufacture their biocatalysts because this process is regarded to be more easily controlled. However, SSF technology could be an interesting alternative pathway, which is nowadays underexploited, to generate enzymatic complexes. One of the main SSF advantages is the direct use of raw materials (i.e., lignocellulose), which are employed to induce the production of a broad range of enzymes. This technology has also been recognized to have lower consumption of water and energy, high side activities, and it has been generally claimed that product yields are higher in SSF than in SmF (Singhania et al., 2010, Pandey, 2003).

Many studies have been realized in order to compare SSF and SmF in terms of product yields and characteristics (Hölker et al., 2004). Unfortunately, there has been few studies comparing these technologies on their performance in application. Consequently, the aim of this study was to fill this lack, and thus saccharification tests have been initiated on cellulose and wheat bran.