Characterization of Bioactive Compounds in Spent Mushroom Substrate: A Metabolomic Perspective on Its Untapped Potential

Ergothioneine is an emerging natural product with anti-aging activity and highly sought after. In this study, we examined spent pink and pearl oyster mushrooms substrate for ergothioneine levels. However, we found that the ergothioneine contents were quite low, ranging from 1.66 to 9.14 μg·g^-1 and 2.00 to 10.38 μg·g^-1, respectively, with no significant increase compared to the unused substrate. Untargeted metabolomic analysis revealed that many low-molecular-weight compounds, including polyphenols, were reduced in spent substrates after mushroom cultivation, likely due to absorption, degradation, or utilization by the fungal mycelium. Interestingly, the spent substrates from both mushroom species were found to be enriched in certain compounds, particularly cerebroside B and ganoderenic acid D. These compounds are potentially valuable products that may be extracted from SMS. However, further targeted analytical validation is required to confirm the identity of and quantify these compounds.