Free and conjugated phenolic compounds and their antioxidant activities in regular and non-darkening cranberry bean (Phaseolus vulgaris L.) seed coats

Postharvest darkening in dry bean (Phaseolus vulgaris L.) negatively impacts consumer preference. Regular-darkening (RD) and non-darkening (ND) cranberry beans were profiled for seed coat phenolics using HPLC–DAD/LC–ESI–MSn and assessed for antioxidant activity using oxygen radical absorbing capacity (ORAC) and ferric-reducing antioxidant power (FRAP) assays. Flavonoids were exclusive to free phenolic extracts of RD beans. Total phenolic content (TPC) was significantly higher (p <0.05) in RD seed coats (52.41–56.43 mg GAE/g) compared to ND seed coats (2.00–2.06 mg GAE/g). Alkaline hydrolysis revealed the presence of phenolic acids and flavonoid conjugates in RD beans. Only p-coumaric and ferulic acids were found in ND beans. Overall antioxidant activity was stronger in RD seed coats (125.28–287.87 µmole TE/g and 20.71–54.01 µmole AAE/g) compared to ND seed coats (2.11–10.09 µmole TE/g and 0.73–1.88 µmole AAE/g). This is the first report of specific flavonoids in cranberry beans.