Microwave-Assisted Extraction of Bound Phenolic Acids in Bran and Flour Fractions from Sorghum and Maize Cultivars Varying in Hardness

To release bound phenolic acids, a microwave-assisted extraction procedure was applied to bran and flour fractions obtained from eight sorghum and eight maize cultivars varying in hardness. The procedure was followed by HPLC analysis, and the identities of phenolic acids were confirmed by MS/MS spectra. The extraction of sorghum and maize bound phenolic acids was done for 90 s in 2 M NaOH to release ferulic acid and p-coumaric acid from bran and flour. Two diferulic acids, 8-O-4'- and 8-5'-benzofuran form, were identified and quantitated in sorghum bran, and only the former was found in maize bran. The contents of ferulic acid and diferulic acids in sorghum bran were 416-827 and 25-179 mu g/g, respectively, compared to 2193-4779 and 271-819 mu g/g in maize. Phenolic acid levels of sorghum were similar between hard and soft cultivars, whereas those of maize differed significantly (p < 0.05) except for ferulic acid in flour. Sorghum phenolic acids were not correlated with grain hardness as measured using a tangential abrasive decortication device. Maize ferulic acid (r = -0.601, p < 0.01), p-coumaric acid (r = -0.668, p < 0.01), and 8-O-4'-diferulic acid (r = -0.629, p < 0.01) were significantly correlated with hardness.