Phenolic acids, anthocyanins, and antioxidant capacity in rice (Oryza sativa L.) grains at four stages of development after flowering

•Free and conjugated phenolics declined by about 50% 3weeks post-flowering in red rice.•White rice had low phenolic levels that were similar after 2weeks post-flowering.•Anthocyanin levels in black rice were highest at 2- and 3-weeks post-flowering.•Black rice had higher total bound phenolic acids than white & red rice at all stages. This study investigated differences in total phenolic content (TPC), antioxidant capacity, and phenolic acids in free, conjugated and bound fractions of white (unpolished), red and black rice at 1-, 2-, and 3-weeks of grain development after flowering and at maturity. Unlike the TPC (mg/100g) of white rice (14.6–33.4) and red rice (66.8–422.2) which was significantly higher at 1-week than at later stages, the TPC of black rice (56.5–82.0) was highest at maturity. The antioxidant capacity measured by DPPH radical scavenging and ORAC methods generally followed a similar trend as TPC. Only black rice had detectable anthocyanins (26.5–174.7mg/100g). Cyanidin-3-glucoside (C3G) and peonidin-3-glucoside (P3G) were the main anthocyanins in black rice showing significantly higher levels at 2- and 3-weeks than at 1-week development and at maturity. At all stages, the phenolic acids existed mainly in the bound form as detected by HPLC and confirmed by LC–MS/MS. Black rice (20.1–31.7mg/100g) had higher total bound phenolic acids than white rice and red rice (7.0–11.8mg/100g). Protocatechuic acid was detected in red rice and black rice with relatively high levels at 1-week development (1.41mg/100g) and at maturity (4.48mg/100g), respectively. Vanillic acid (2.4–5.4mg/100g) was detected only in black rice where it peaked at maturity. p-Coumaric acid (