Pleurotus ostreatus bio-residues: a source of bioactives with antioxidant and antimicrobial activity

Mushrooms are known to be a rich source of functional and bioactive compounds. Among these, phenolic compounds and polysaccharides, mainly β-glucans, stand out due to their antioxidant and immunomodulatory effects, respectively. In the present work, the total phenolic compounds content, the content in β-glucans and bioactivity properties from the bio-residues obtained from mushroom production were evaluated in order to develop a circular bioeconomy approach. Pleurotus ostreatus, one of the most cultivated species worldwide, was studied. The extraction was performed using water as the only solvent to obtain the possible greener process and easy to scale up at the industrial level.
The aqueous extracts were obtained according to two different methods (1 and 2). In method 1, a hot extraction was performed (90 °C; 1 h; 500 rpm; extract 1A). In method 2, a room temperature extraction was carried out (extract 2A), and the resulting residue was subsequently extracted with hot water (90 °C; 1 h; 500 rpm; extract 2B). Extracts were freeze-dried, and the content of total phenolic compounds (TPC) was determined using the Folin-Ciocalteu method. β-glucans content was determined according to the "β-Glucan Assay Kit (Yeast & Mushroom)" assay. Moreover, the extracts' bioactivity was evaluated by assessing the antioxidant (ABTS, DPPH and ORAC) and antimicrobial (minimum inhibitory concentration – MIC) activities. The mutagenicity and cytotoxicity were evaluated by Ames and PrestoBlue assays, respectively. The TPC obtained was 11.94 ± 1.50, 9.95 ± 0.50 and 12.39± 0.57 mg gallic acid equivalent per g of dry extract for extracts 1A, 2A and 2B, respectively. Regarding the β-glucan content the results were: 10.67 ± 0.19 (1A), 7.62 ± 0.09 (2A), 5.96 ± 0.03 (2B) g per 100 g of dry mushroom. The extracts M2B and M1 were the ones that presented the highest results for the phenolic compounds and β-glucans, respectively. Furthermore, all extracts showed antioxidant activity (lower activity in the DPPH assay) and antimicrobial activity against the tested bacteria (Salmonella enterica, Escherichia coli, Pseudomonas aeruginosa, Pseudomonas fluorescens, Pseudomonas tolaasii, Pseudomonas agarici, Yersinia enterocolitica, Staphylococcus aureus, Bacillus cereus e Listeria monocytogenes), with the 2A extract standing out as an inhibitor of bacterial growth. The extracts did not show mutagenicity or cytotoxicity at the tested concentrations (40 mg/mL). Thus, the aqueous extracts from P. ostreatus bio-residues could be considered a promissory natural source of bioactives, namely phenolic compounds and β-glucans with relevant biological properties. Besides, the valorisation of these bio-residues presents an opportunity to promote the mushroom production chain sustainability.