Comparative study on occurrence characteristics of matrix water in static and gas double-dynamic solid-state fermentations using low-field NMR and MRI

Review of research progress on the production of cellulase from filamentous fungi

Cellulases have been widely used in many fields such as animal feed, textile, food, lignocellulose bioconversion, etc. Efficient and low-cost production of cellulases is very important for its industrial application, especially in bioconversion of lignocellulosic biomass. Filamentous fungi are currently widely used in industrial cellulase production due to their ability to secrete large amounts of active free cellulases extracellularly. This review comprehensively summarized the research progress on cellulases from filamentous fungi in recent years, including filamentous fungi used for cellulase production and its modification strategies, enzyme compositions, characterization methods and application of fungal cellulase systems, and the production of fungal cellulase includes production processes, factors affecting cellulase production such as inducers, fermentation medium, process parameters and their control strategies. Also, the future perspectives and research topics in fungal cellulase production are presented in the end of the review. The review helps to deepen the understanding of the current status of fungal cellulases, thereby promoting the production technology progress and industrial application of filamentous fungal cellulase.

Bidirectional Solid-State Fermentation of Highland Barley by Edible Fungi to Improve Its Functional Components, Antioxidant Activity and Texture Characteristics

In food industry, the characteristics of food substrate could be improved through its bidirectional solid-state fermentation (BSF) by fungi, because the functional components were produced during BSF. Six edible fungi were selected for BSF to study their effects on highland barley properties, such as functional components, antioxidant activity, and texture characteristics. After BSF, the triterpenes content in Ganoderma lucidum and Ganoderma leucocontextum samples increased by 76.57 and 205.98%, respectively, and the flavonoids content increased by 62.40% (Phellinus igniarius). Protein content in all tests increased significantly, with a maximal increase of 406.11% (P. igniarius). Proportion of indispensable amino acids increased significantly, with the maximum increase of 28.22%. Lysine content increased largest by 437.34% to 3.310 mg/g (Flammulina velutipes). For antioxidant activity, ABTS radical scavenging activity showed the maximal improvement, with an increase of 1268.95%. Low-field NMR results indicated a changed water status of highland barley after fermentation, which could result in changes in texture characteristics of highland barley. Texture analysis showed that the hardness and chewiness of the fermented product decreased markedly especially in Ganoderma lucidum sample with a decrease of 77.96% and 58.60%, respectively. The decrease indicated a significant improvement in the taste of highland barley. The results showed that BSF is an effective technology to increase the quality of highland barley and provide a new direction for the production of functional foods.

Advances and perspectives on mass transfer and enzymatic hydrolysis in the enzyme-mediated lignocellulosic biorefinery: A review

Enzymatic hydrolysis is a critical process for the cellulase-mediated lignocellulosic biorefinery to produce sugar syrups that can be converted into a whole range of biofuels and biochemicals. Such a process operating at high-solid loadings (i.e., scarcely any free water or roughly ≥ 15% solids, w/w) is considered more economically feasible, as it can generate a high sugar concentration at low operation and capital costs. However, this approach remains restricted and incurs "high-solid effects", ultimately causing the lower hydrolysis yields with increasing solid loadings. The lack of available water leads to a highly viscous system with impaired mixing that exhibits strong transfer resistance and reaction limitation imposed on enzyme action. Evidently, high-solid enzymatic hydrolysis involves multi-scale mass transfer and multi-phase enzyme reaction, and thus requires a synergistic perspective of transfer and biotransformation to assess the interactions among water, biomass components, and cellulase enzymes. Porous particle characteristics of biomass and its interface properties determine the water form and distribution state surrounding the particles, which are summarized in this review aiming to identify the water-driven multi-scale/multi-phase bioprocesses. Further aided by the cognition of rheological behavior of biomass slurry, solute transfer theories, and enzyme kinetics, the coupling effects of flow-transfer-reaction are revealed under high-solid conditions. Based on the above basic features, this review lucidly explains the causes of high-solid hydrolysis hindrances, highlights the mismatched issues between transfer and reaction, and more importantly, presents the advanced strategies for transfer and reaction enhancements from the viewpoint of process optimization, reactor design, as well as enzyme/auxiliary additive customization.

Roles of water in improved production of mycelial biomass and lignocellulose-degrading enzymes by water-supply solid-state fermentation of Ganoderma lucidum

The liquid phase was proved to be a significant influencing factor among the three phases in solid-state fermentation (SSF), which determined water control was crucial. However, obvious water loss was caused by microbial utilization and moisture evaporation. Super absorbent polymer (SAP) was utilized to supply water in SSF owing to its high water-holding capacity. Adding 0.15% SAP could significantly increase the biomass of Ganoderma lucidum by 33.59% and promote filter paper activity (FPA), endocellulase activity and laccase activity by 27.11%, 29.14% and 47.39%, respectively. Water states of fermentation substrates were detected by the low-field nuclear magnetic resonance (LF-NMR). Results revealed that water present and lost was dominated by the capillary water. At the end of fermentation, the capillary water content (C_cw) in water-supply SSF was 20.48% and 17.20% higher than that in static SSF and cold-model SSF. The relaxation time of the capillary water was reduced by 56.53% in water-supply SSF and by 53.40% in static SSF, but it just reduced by 6.82% in cold-model SSF. In addition, the C_cw in SSF had a high correlation with the biomass and lignocellulose-degrading enzyme activities of G. lucidum. These results clearly demonstrated that capillary water played a very important role in improved production of G. lucidum.

High Productivity Ethanol from Solid-State Fermentation of Steam-Exploded Corn Stover Using Zymomonas mobilis by N2 Periodic Pulsation Process Intensification

Solid-state fermentation, featured by water-saving, eco-friendly and high concentration product, is a promising technology in lignocellulosic ethanol industry. However, in solid-state fermentation system, large gas content inside the substrate directly leads to high oxygen partial pressure and inhibits ethanol fermentation. Z. mobilis can produce ethanol from glucose near the theoretical maximum value, but this ethanol yield would be greatly decreased by high oxygen partial pressure during solid-state fermentation. In this study, we applied N₂ periodic pulsation process intensification (NPPPI) to ethanol solid-state fermentation, which displaced air with N₂ and provided a proper anaerobic environment for Z. mobilis. Based on the water state distribution, the promotion effects of NPPPI on low solid loading and solid-state fermentation were analyzed to confirm the different degrees of oxygen inhibition in ethanol solid-state fermentation. During the simultaneous saccharification solid-state fermentation, the NPPPI group achieved 45.29% ethanol yield improvement and 30.38% concentration improvement compared with the control group. NPPPI also effectively decreased 58.47% of glycerol and 84.24% of acetic acid production and increased the biomass of Z. mobilis. By coupling the peristaltic enzymatic hydrolysis and fed-batch culture, NPPPI made the ethanol yield and concentration reach 80.11% and 55.06 g/L, respectively, in solid-state fermentation.

Humidity Control Strategies for Solid-State Fermentation: Capillary Water Supply by Water-Retention Materials and Negative-Pressure Auto-controlled Irrigation

Solid-state fermentation (SSF) has regained interest owing to its advantages in solid waste treatment and fermentation industries. However, heterogeneous heat and mass transfer are often caused by the absence of free water and noticeable water loss from microbial utilization and moisture evaporation in SSF. It is necessary to explore more effective ways to solve issues of water loss and water supplement in SSF based on online capillary water monitoring, because capillary water is the dominant form of water that is present and lost in substrate. Two novel capillary-water supply strategies were proposed, established and evaluated using three selected reference strains, including water-retention materials and negative-pressure auto-controlled irrigation (NPACI). This study employed superabsorbent polymer, a kind of water-retention material to enhance enzyme productivity with the most significant increase of 2.47 times. Moreover, the combination of NPACI and 0.1% superabsorbent polymers increased productivity by 2.80-fold, together with lowered gradients of temperature, moisture and products. Furthermore, a modified liquid-supply SSF was constructed through successful capillary water control by proposed humidity control strategies. This modified SSF system could address the shortcomings of inhomogeneous culture of traditional SSF.

Water dynamics during solid-state fermentation by Aspergillus oryzae YH6

Water is crucial for microbial growth, heat transfer and substrate hydrolysis, and dynamically changes with time in solid-state fermentation. However, water dynamics in the solid substrate is difficult to define and measure. Here, nuclear magnetic resonance was used to monitor water dynamics during the pure culture of Aspergillus oryzae YH6 on wheat in a model system to mimic solid starter (Qu or Koji) preparation. During fermentation, overall water content gradually decreased from 0.84 to 0.36 g/g, and water activity decreased from 0.99 to 0.93. Water content in different state (bound, immobilized and free) changed differently and all moved to more "bound" direction. The internal water distribution over the substrate matrix also showed a faster reduction inward both in the radical and axial direction. Our findings provide the prerequisites for optimal processes where water dynamics in solid-state fermentation can be monitored and controlled.