Biorefining spent substrates of shiitake (Lentinula edodes) and oyster mushroom (Pleurotus ostreatus): enzymatic saccharification of cellulose and xylan, with lignin recovery from residues

Spent mushroom substrate (SMS), the main by-product of mushroom cultivation, is a source of sugars that can be released by saccharification. This work aimed at investigating the enzymatic saccharification of the polysaccharides of the SMS of shiitake (Lentinula edodes) and oyster mushroom (Pleurotus ostreatus) and exploring the lignin extraction from the saccharification residues. First, analytical enzymatic saccharification (AES) with a cellulase cocktail and an experimental hemicellulase-rich preparation was applied. AES revealed higher digestibility of both polysaccharides for shiitake SMS than for oyster mushroom SMS. Using the cellulase cocktail, shiitake SMS resulted in a digestibility above 80% and 70% (w/w) for cellulose and xylan, respectively, while the maximum values for oyster mushroom SMS were 52% and 32% (w/w). The experimental enzyme preparation resulted in lower cellulose digestibility and higher xylan digestibility. Still, the saccharification trend between the two SMS types remained unchanged. To enhance the enzymatic saccharification of oyster mushroom SMS, hydrothermal treatment was applied. The treatment improved the enzymatic digestibility of cellulose by up to 84%. A validation experiment at larger scale showed that hydrothermally treated oyster mushroom SMS had a comparable overall conversion with non-treated shiitake SMS. Following a biorefinery strategy, lignin was extracted from the residues of the preparative enzymatic saccharification using the green solvent γ-valerolactone under different temperatures and holding times. The extracted product contained 98.8% lignin and did not contain cellulose or xylan. The results of this study provide the grounds for biorefinery processes enabling recovery of bioactive compounds, fermentable sugars, and high-quality lignin from SMS.