Fermentation-Induced Enrichment of Phenolics and Volatile Compounds Modulates Phytochemical Composition and Antioxidant Activities of Dorstenia convexa Root and Monodora myristica Seed

This study investigated how microbial fermentation modulates the release, biotransformation, and functional properties of phytochemicals in two underutilized spices, Dorstenia convexa root and Monodora myristica seed. Methanol extract yields of plants were increased after fermentation by Aspergillus oryzae and Lactiplantibacillus plantarum , while Saccharomyces cerevisiae caused them to decrease. Fermentation with Aspergillus oryzae, Lactiplantibacillus plantarum, and Saccharomyces cerevisiae at optimal days increased total phenolic and flavonoid contents (TPC, TFC) of unfermented Dorstenia convexa root (24.01 mg GAE/g dw, 8.38 mg Rutin/g dw) by 17.58%, 12.59%, 2.54% for TPC, 20.56%, 28.43%, 19.20% for TFC, respectively. Additionally, they raised the TPC and TFC of unfermented Monodora myristica seed (6.71 mg GAE/g dw, 4.08 mg Rutin/g dw) by 31.19%, 34.97%, 6.62%, 52.67%, 58.10%, 39.91%, respectively. LC–MS/MS analysis revealed that fermentation enriched polymethylated flavonoids and promoted polyphenol biotransformation, including the hydrolysis of flavonoid glycosides into aglycones and the interconversion of phenolic acids. GC-MS profiles demonstrated substantial changes of volatile organic compounds under the identification threshold, with fermentation mainly modulating esters, alcohols, coumarins, and terpenoids, depending on microbial strain and plant matrix. Fermented plant extracts showed significantly enhanced antioxidant capacities, but exhibited a minor increase (≤2 mm) in the inhibition zones against Escherichia coli and Bacillus cereus . Notably, Lactiplantibacillus plantarum fermentation markedly improved pancreatic lipase and angiotensin-converting enzyme inhibitory activities of plant extracts, while Saccharomyces cerevisiae made them stronger in α-amylase inhibition. Overall, these findings provide instructive insights to transform phytochemicals using different fermentation strategies for improving their nutritional value.