Reducing the phytotoxicity of spent mushroom substrate (SMS) for sustainable growing media by superheated steam torrefaction: effects of temperature and residence time

The utilization of spent mushroom substrate (SMS) as growing media component holds immense promise. However, before it can be employed, it is imperative to reduce its inherent phytotoxicity. In this study, nine superheated steam (SHS) torrefaction treatments with different intensities were set up to investigate their effectiveness in removing the phytotoxicity of SMS, with a focus on changes in phytotoxicity-related substances. SHS torrefaction substantially diminished the presence of organic acids and amino acids, while significantly increased the germination index (GI). Overall, the removal of phytotoxicity escalated in line with increasing temperature and residence time. The optimal removal effect was observed at 250℃ for 40 min, and further elevating the temperature to 300℃ did not yield any additional reduction in phytotoxicity. It is concluded that SHS torrefaction can substantially reduce the phytotoxicity of SMS by modifying its chemical composition, particularly through the reduction of organic acids and amino acids. A torrefaction intensity of 250℃ for 40 min appears to be relatively optimal for this purpose. These findings underscore the potential of SHS torrefaction as a viable approach to address the phytotoxicity of SMS, thereby opening up new possibilities for its sustainable utilization as growing media component.