Closing the loop of cereal waste and residues with sustainable technologies: An overview of enzyme production via fungal solid-state fermentation

The circular economy is one of the leading sustainable development strategies that can help utilize waste to produce new products. Cereal food waste and residues represent a significant resource of million tonnes globally and a feasible feedstock for enzyme production via environmentally-friendly solid-state fermentation (SSF). This study presents a targeted review of 487 examples of quantified enzyme production from 209 studies reported on Science Direct. The collection of data enabled the identification of the key substrates and enzymes for potential production pathways. The main substrates were wheat (53 experiments), rice (22 experiments), and corn (22 experiments), while the most common enzymes reported were xylanase, CMCase, FPase with 92, 83, and 61 examples, respectively. Their activities showed a wide range with xylanase activities 7-10 000 U/gds, CMCase activities 1.4-950 U/gds, and FPase activities 1.7-438 U/gds. Other enzymes reported were β-glucosidase, Laccase, β-xylosidase. Both single substrates and mixed substrates were included, and several mixtures showed a synergistic effect for higher enzyme activities. Xylanase from wheat bran, rice straw, and their combination showed great potential for reaching high activity and promising revenues. Scale-up experiments were reported and demonstrated increased production of enzymes as a crucial step towards industrial application. Identified improvements in transparency and knowledge sharing for cross-disciplinary research are: i) harmonized enzyme names ii) expression of enzyme activity per mass unit specifying moisture content as found in 52% of studies. Only the results using the same unit (or enabling unit conversion) can be compared and used to identify the most promising production.