Use of solid digestate for lignocellulolytic enzymes production through submerged fungal fermentation

Studies were performed on the use of the solid fraction of digestate (D) for the production of lignocellulolytic enzymes (endo- and exo-glucanase, xylanase, β-glucosidase and laccase) by fungi, in comparison with wheat straw (benchmark) (W). To date, this is the first report on the use of such an inexpensive substrate in a liquid environment. Submerged instead of solid state fermentation was applied to overcome pH inhibition and increase surface accessibility. A total of 21 fungal strains were tested: the most performing ones were Irpex lacteus DSM1183 for both β-glucosidase (52 IU/g with D, + 400% compared to W) and endo-glucanase (236 IU/g with D, + 470% compared to W), Schizophyllum commune CBS30132 for xylanase (715 IU/g with W, + 145% compared to D) and Pleurotus ostreatus ATCC96997 for laccase (124 IU/g with D, +230% compared to D). Cultures from S. commune and P. ostreatus were analyzed at the beginning and at the end of the growth test to determine soluble COD, total (TS) and volatile (VS) solids. COD was always lower at the end of the test suggesting a faster uptake than hydrolysis. P. ostreatus evidenced a higher VS reduction (-11% rather than -32%), suggesting a more effective growth of this strain on D. Results may open up new avenues for the utilization of solid digestate, an inexpensive agricultural by-product, for the production of value-added products as well as to increase biodegradation of lignocellulosic materials.