Enhancement of phenolic profile and antioxidant activity of black glutinous rice (Oryza sativa var. glutinosa) due to tape fermentation

Black glutinous rice (BGR) naturally contains phenolic compounds in free and conjugated form or bound to the structural matrix, which may affect its bioactivity. The degradation of the matrix cell wall by microbial activity during fermentation may endorse the release of bound phenolic compounds and enhance the availability of the compounds. This study aimed to assess the impact of tape fermentation on the profile of individual phenolic compounds and the antioxidant activity of BGR. Fermentation up to 72 h at 30 °C was performed by adding the cooked BGR with a tape starter (0.3 w/w). The fermentation for over 48 h increased free phenolic compounds while altering the composition of free, free-conjugated, and bound phenolic compounds. In comparison to the initial material, significant enhancement in the level of protocatechuic aldehyde (13 × ), caffeic acid (2 × ), vanillic acid (1 × ), vanillin (1 × ), p-coumaric acid (1 × ), ferulic acid (1 × ), and isoferulic acid (1 × ) was acknowledged in the final fermented product while sinapic and 2,4,6-trihydroxybenzoic acid was merely detected after 24 h of fermentation. As a result, antioxidant activity was also increased notably (p < 0.05) starting from 48 h of fermentation, while principal component analysis revealed that the activity was strongly contributed by 2,4,6-trihydroxybenzoic acid. Henceforth, the resulting tape fermentation of BGR can be considered a source of phenolic compounds that provide high antioxidant activity.