Bioaccessibility and antioxidant potential of millet grain phenolics as affected by simulated in vitro digestion and microbial fermentation

Dehulled and cooked grains of five millet varieties (kodo, finger, proso, foxtail and pearl) were subjected to in vitro enzymatic digestion and microbial fermentation under physiological conditions in order to determine the bioaccessibility of their phenolic compounds. Extracts recovered as supernatants from enzymatic digestion and microbial fermentation were employed for the determination of their total phenolic content (TPC) and total flavonoid content (TFC), as well as the scavenging of 2,2-diphenyl-1-picrylhydrazyl (DPPH), peroxyl and hydroxyl radicals. Furthermore, trolox equivalent antioxidant capacity (TEAC), reducing power and ferrous ion chelating activity of the extracts so obtained were evaluated. The DPPH and hydroxyl radical scavenging activities were determined using electron paramagnetic resonance (EPR) spectroscopy. The peroxyl radical activity was measured using an oxygen radical absorbance capacity (ORAC) assay. The TPC ranged from 12.7 to 35.4 and 21.2 to 47.4 μmol ferulic acid equivalents per gram of grain, on a dry weight (dw) basis at the end of intestinal digestion and colonic fermentation, respectively. All five millet varieties exhibited effective antioxidant activity and the order of efficacy differed according to the assay employed. The present study thus demonstrated that phenolic compounds of processed millets were bioaccessible and colonic fermentation released the phenolics bound to the insoluble fibre in the grain.