Towards a competitive solid state fermentation: Cellulases production from coffee husk by sequential batch operation and role of microbial diversity

Abstract

The cost of cellulases is the main bottleneck for bioethanol production at commercial scale. Solid-state fermentation (SSF) is a promising technology that can potentially reduce cellulases cost by using wastes as substrates. In this work, a SSF system of 4.5L bioreactors was operated continuously by sequential batch operation using the fermented solids from one batch to inoculate the following batch. Coffee husk was used as lignocellulosic substrate. Compost was used as starter in the first batch to provide a rich microbiota. Two strategies were applied: using 10% fermented solids as inoculum in 48h batches (SB90) and using 50% solids in 24h batches (SB50). A consistent and robust production process was achieved by sequential batch operation. Similar cellulase activities around 10 Filter Paper Units per gram of dry solids were obtained through both strategies. Microbial diversity in the starting materials and in the final fermented solids was characterized by next generation sequencing. Microbial composition of both fermented solids was similar but the relative abundance of families and species was affected by the operation strategy used. Main bacteria in the final solids came from compost (families Sphingobacteriaceae, Paenibacillaceae and Xanthomonadaceae), while main fungi families came from coffee husk (families Phaffomycetaceae, Dipodascaceae and two unidentified families of the class of Tramellomycetes). There was a high presence of non-identified mycobiota in the fermented solids. Main identified species were the bacteria Pseudoxanthonomas taiwanensis (12.3% in SB50 and 6.1% in SB90) and Sphingobacterium composti (6.1% in SB50 and 2.6% in SB90) and the yeasts Cyberlindnera jardinii and Barnettozyma californica (17.8 and 4.1% respectively in SB50 and 34 and 9.1% in SB90), all four previously described as lignocellulose degraders. The development of these operational strategies and further biological characterization of the end product could eventually benefit the process economics by providing a standard and specialized inoculum for a continuous SSF for cellulases production.

Design Aspects of Solid State Fermentation as Applied to Microbial Bioprocessing

“…Spouted bed bioreactor show improved fermentation performance, including higher product titers, yields, and productivity [124]. They studied the spouted-bed bioreactor with intermittent spouting with air, which achieved high production levels both of total protein and enzymes.…”

“…The result was similar to packed-bed bioreactor, but spouted-bed had uniformity and had no solids-handling problems. However, it was found that continual spouting was found to be unfavourable to this SSF, possibly because of shear impact damage to fungal mycelium during spouting [124]. The design scheme of the spouted-bed bioreactor is illustrated in Figure 7.…”

Microbial Strategies for Cellulase and Xylanase Production through Solid-State Fermentation of Digestate from Biowaste

“…Cellulases and xylanases' production is mainly associated with microorganisms that are able to fully degrade lignocellulosic materials; it is, therefore, is of great relevance to determine the most-adequate strain or specialized inoculum to enhance cellulase production [11]. In this context, several inoculation strategies have been proposed for the production of different enzymatic compounds; they include the use of autochthonous microbiota for protease production [12], development of a specialized mixed consortium [13], and the use of specific cellulose-producing strains (mainly fungi) such as Trichoderma reseei [14][15][16][17][18]. T. reseei is a widely-referred to fungus for cellulase production using agroindustrial or agricultural wastes with a significant fibre content (cellulose > 25%) as substrates [13].…”

Valorisation of digestate from biowaste through solid-state fermentation to obtain value added bioproducts: A first approach

“…Another interesting alternative to be assessed is the development of different operational strategies such as working in a sequential batch configuration thus allowing the adaptation of the autochthonous microbiome as reported by Cerda et al (2017a). Another approach would be to work with specific strains to overproduce, by instance, proteases, as successfully reported by El-Bakry et al (2016).…”