Effects of processing conditions on the stability of polyphenolic contents and antioxidant capacity of Dolichos lablab L.

The effects of raw, dry heated and pressure cooked samples on total phenolic components and antioxidant activity in commonly consumed field bean, Dolichos lablab L. was investigated. The raw and processed samples were extracted with 70% methanol. Processing of legumes caused decreases in total phenolic content when compared to the raw samples. However, the dry heating caused remarkable increase in tannin contents (1.809 ± 0.25 g GAE/100 g extract). Dry heated samples of D. lablab was found to possess the highest DPPH (IC50, 2.53 ± 0.17 μg/ml), TEAC (4649.8 ± 38.4 μmol/g DM), OH˙ radical (IC50, 42.2 ± 0.67 μg/ml) scavenging activities, inhibition of linoleic acid and ferric reducing capacity than other samples. The raw samples displayed the highest antihemolytic activity (59.6 ± 1.53%) and chelating capacity (74.2 ± 1.37 mg EDTA/g). Dry heat processing exhibited several advantages in retaining the antioxidant components and activities. The higher correlation was found the phenolic content with chelating (r (2)  = 0.933) and antihemolytic (r (2)  = 0.839) activities, but a poor correlation with other assays. Moreover, the content of tannins gave good correlation (r (2)  = 0.644-0.997) with all antioxidant assays. The low correlation values between total phenols and the antioxidative activity suggest that the major antioxidant compounds in studied seeds might be tannins.