Changes in sulfur content and physicochemical properties related to storage and distribution of spent shiitake mushroom substrate by fermentation

This study evaluated the effects of fermentation using a sulfur-oxidizing bacterium on sulfur reduction and storage-related quality characteristics of spent shiitake mushroom substrate (SMS). Pretreated SMS was inoculated with Sphaerotilus microaerophilus sp. nov. (FB-5) at concentrations of 0.001%, 0.1%, and 10% and fermented at 25°C for 21 days. Before pretreatment and fermentation, the moisture content, pH, total acidity, sulfur content and nitrogen contents of SMS were 34.21%, pH 4.68, 0.64%, 3.14%, and 0.32%, respectively, and no pathogenic microorganisms were detected. After fermentation, the moisture content of the FB-5-inoculated groups increased to 36.67-38.42%, which was significantly higher than that of the control group (CG). The pH and total acidity ranged from pH 4.05 to 4.11 and 0.38 to 0.51%, respectively, with no significant differences among treatments. Sulfur content was highest in the control group, whereas all inoculated groups exhibited significantly reduced sulfur levels (2.91-2.95%), particularly at lower inoculation concentrations. The nitrogen content ranged from 0.32 to 0.33% and showed significant differences compared to the control group. The population of FB-5 with depending on inoculation concentration and gradually decreased over prolonged fermentation. These results demonstrate that FB-5 fermentation effectively reduces sulfur content in SMS and suggest its potential application as a pretreatment strategy to improve the storage and distribution suitability of spent mushroom substrates.