A method based on the 4-dimethylaminocinnamaldehyde (DMAC) method was optimized for determination of total proanthocyanidin content in grain legumes using 96-well microplates. The recovery determined for a milled soybean material spiked with black soybean seed coat extract equivalent to 1.0, 2.0 and 4.2 mg/g of proanthocyanidin ranged from 96.1 to 97.9%. Duplicate determinations of total proanthocyanidin content in eight different grain legumes were performed on three different days. Statistical treatment revealed that the relative standard deviation of the repeatability (RSDr) and the relative standard deviation of the intermediate reproducibility (RSDint) for determination of total proanthocyanidin content was 0.5 to 4.1% and 0.9 to 4.7 %, respectively. The HorRat values (RSDint/predicted RSDR) ranged from 0.17 to 0.87. This demonstrates that the proposed method for determining total proanthocyanidin content in grain legumes had good intralaboratory reproducibility. The contents of proanthocyanidin, ranging from 0.33 to 2.97 mg/g, in 59 samples of black soybean determined by the DMAC method were strongly correlated to those determined by the vanillin-sulfuric acid method (r=0.980). However, the contents in black soybeans determined by the vanillin-sulfuric acid method were higher when compared with the DMAC method in almost all samples, when the content obtained by the DMAC method was directly compared to that obtained by the vanillin-sulfuric acid method.
