Untargeted metabolomics analysis of non-volatile metabolites and dynamic changes of antioxidant capacity in Douchi with edible mushroom by-products

•Changes in non-volatile metabolites and antioxidants were investigated in EMD.•More metabolites were found in Koji and the first 5-day post-fermented sample.•Flavonoids were the largest class of differential metabolites.•The antioxidant capacity of EMD gradually increased with the fermentation time.•The antioxidant capacity of EMD was closely related to flavonoid metabolites. This study investigated the non-volatile metabolites and antioxidant activity of Douchi, an edible mushroom by-product. A total of 695 non-volatile metabolites were detected using UPLC-MS/MS-based metabolomics analysis, and the greatest impact on metabolite composition was observed during Koji-making and the first 5 days of post-fermentation. Throughout the fermentation process, 366 differential metabolites were identified, with flavonoids being the most prominent followed by amino acids and their derivatives, which were found to be important for the quality of edible mushroom by-product Douchi (EMD). The antioxidant capacity of EMD significantly increased with the longer fermentation time, which might be associated with the conversion of isoflavone glycosides to aglycones, amino acids and their derivatives, free fatty acids, group A saponins, and phenolic acids. These findings suggested that different fermentation phases of EMD significantly affected the non-volatile metabolite profile and antioxidant capacity.