The role of nitrogen sources and caffeine for growth of Pleurotus ostreatus (oyster mushroom)

The use of spent coffee grounds (SCG) for cultivation of Pleurotus ostreatus has become a popular way to reuse this type of waste. However, it is unclear if high mushroom yields can be achieved or if the caffeine content of residues can be substantially decreased for safe disposal of SCG. To address this lack of knowledge, this study provides key information related to the fate of caffeine during cultivation of P. ostreatus on SCG. Using agar and in liquid culture, a wide range of nitrogen sources (including caffeine) and extracts from fresh and spent coffee grounds were evaluated for their ability to support vegetative (mycelial) growth. It was found that inorganic N was the best source for mycelial growth and that caffeine, while toxic at high concentrations, also promoted growth at low concentrations. Pleurotus ostreatus was also grown on SCG-amended substrates to evaluate the effect of caffeine during both vegetative and reproductive phases. In two trials, P. ostreatus was grown in treatments ranging from pure SCG (SCG100) through to pure sawdust (sawdust100) and with intermediary mixtures of these substrates. In a laboratory-scale study, three of the four treatments became fully colonized (SCG100, SCG25+sawdust75 and SCG50+sawdust50) but only SCG100 and SCG25+sawdust75 developed mushrooms. Caffeine degradation by P. ostreatus occurred when grown on SCG (with and without sawdust) with caffeine and its degradation products detected in both the substrate and fruiting bodies. In a commercial-scale study, full colonization was observed for SCG25+sawdust75 and sawdust100 and mushrooms developed on both. Again, caffeine degradation was detected and there was a decrease in caffeine content of the SCG. All of the compounds that have been previously described for fungal degradation of caffeine were detected, identified and a likely degradation pathway was suggested