Composted versus Raw Olive Mill Waste as Substrates for the Production of Medicinal Mushrooms: An Assessment of Selected Cultivation and Quality Parameters

Two-phase olive mill waste (TPOMW, “alperujo”) is a highly biotoxic sludge-like effluent of the olive-oil milling process with a huge seasonal production. One of the treatment approaches that has so far received little attention is the use of TPOMW as substrate for the cultivation of edible mushrooms. Fifteen fungal strains belonging to five species (Basidiomycota), that is, Agrocybe cylindracea, Pleurotus cystidiosus, P. eryngii, P. ostreatus, and P. pulmonarius, were evaluated for their efficacy to colonize media composed of TPOMW, which was used either raw or composted in mixtures with wheat straw in various ratios. Qualified strains exhibited high values of biological efficiency (e.g., 120–135% for Pleurotus spp. and 125% for A. cylindracea) and productivity in subsequent cultivation experiments on substrates supplemented with 20–40% composted TPOMW or 20% raw TPOMW. Only when supplementation exceeded 60% for raw TPOMW, a negative impact was noted on mushroom yields which could be attributed to the effluent's toxicity (otherwise alleviated in the respective composted TPOMW medium). Earliness and mushroom size as well as quality parameters such as total phenolic content and antioxidant activity did not demonstrate significant differences versus the control wheat-straw substrate. The substrates hemicellulose content was negatively correlated with mycelium growth rates and yields and positively with earliness; in addition, cellulose: lignin ratio presented a positive correlation with mycelium growth and mushroom weight for A. cylindracea and with earliness for all species examined. TPOMW-based media revealed a great potential for the substitution of traditional cultivation substrates by valorizing environmentally hazardous agricultural waste.