Distribution of free and bound phenolic compounds, β-glucan, and araboxylan in fractions of milled hulless barley

To study the effect of milling process on active compounds in hulless barley, the β-glucan, araboxylan, the free and bound forms of phenolic compounds and antioxidant activities were evaluated. The results showed that total araboxylan content (9.64 g/100 g → 2.35 g/100 g) decreased significantly (P < 0.05) from the outer to the inner layer, while the β-glucan content first increased significantly going towards the inner layers and showed a gradual decrease moving toward the endosperm. Total free and bound forms of total phenolics and total flavonoids decreased by 91% and 95% with the increase of the degree of milling and anthocyanins mostly existed in the outer layers. The total DPPH (2,2-Diphenyl-1-picrylhydrazyl) and ABTS (2,2′-Azinobis-(3-ethylbenzthiazoline-6-sulphonate)) antioxidant activities decreased by 90% and 97% from the outer to the inner layers. Both free and bound forms of 21 phenolic compounds decreased significantly (P < 0.05) with increased milling and were mostly in the outermost layer. Moreover, eight diferulic acids were reported for the first time in hulless barley, and ferulic acid was the most abundant monomeric phenolic acid, with 8-O-4′ diferulic acid being the most abundant diferulic acid and chrysoeriol-7-O-glucouronide being the dominant flavonoid. Therefore, a lower degree milling is recommended for the higher retainment of bioactive compounds during daily processing.