Solid-state bioconversion of fava bean by Rhizopus oligosporus for enrichment of phenolic antioxidants and l-DOPA

Abstract

Fava beans are a rich source of levo-dihydroxy phenylalanine ( l-DOPA) the precursor of dopamine, and are being investigated for use in the management of Parkinson's disease. The present research was conducted to enrich the fava bean substrate with phenolic antioxidants and l-DOPA via solid-state bioconversion system using the food grade fungus Rhizopus oligosporus. The l-DOPA content in the fungal-grown fava bean increased significantly to approximately twice that of control, accompanied by moderate phenolic-linked antioxidant activity and higher fungal superoxide dismutase activity during early stages of growth. This indicated that l-DOPA can be mobilized and formed from fava bean substrates by fungal bioconversion and is contributed to the antioxidant functionality of such extracts. A high superoxide dismutase (SOD) activity during early and late growth stages indicates the likely oxidation stress of initial fungal colonization and later due to nutrient depletion. High levels of soluble phenolics were observed during late growth stages. During the course of solid-state growth there was an increase in β-glucosidase activity, which correlated to an increase in total phenolic content during the late stages. This suggests that the enzyme may play an important role in the release of phenolic aglycones from fava bean substrate and thereby increases the phenolic content and accompanying antioxidant activity. The implication from this study is that solid-state bioconversion of fava bean by R. oligosporus can significantly improve the phenolic antioxidant activity and Parkinson's relevant l-DOPA content.