Analysis of Isoflavones and Phenolic Compounds and Identification of Free Radical Scavengers in Soybean Seed.

Antioxidant activity contained in plant-based foods can improve food oxidative stability and phenolics and isoflavones have proven active in food systems. This study was conducted to investigate phenolic phytochemicals having antioxidant activity and to identify components with high antioxidant activity in soybean seed. For these purposes, ten soybean cultivars were cultivated at two locations in Korea, and contents of isoflavone and total phenolics and antioxidant activity of soybean seeds were evaluated. Also, we identified antioxidant compounds occurring in soybean extract. Total phenolics and isoflavone were in ranges of 2.2 ~ 6.9 mg/g and 720 ~ 2,285 ug/g, respectively. The highest total isoflavone content was observed in ‘Daepoongkong’ at both growing locations. ‘Jwinunyikong’ exhibited the highest content of total phenolic compounds and the highest DPPH radical scavenging activity at both growing locations. On the other hand, ‘Taekwangkong’ had the lowest contents of isoflavone and total phenolic compounds and the lowest DPPH radical scavenging activity. As a results of this study, on the whole, cultivars having high contents of total phenolic compounds were shown high DPPH radical scavenging activity such as ‘Jwinunyikong’, ‘Cheongjakong3’, and ‘Ilpoomgeomjeongkong2’. Total isoflavone contents in soybean seed grown at Jinbu (sub-highland) were significantly higher than at Gangneung (lowland), while, in total phenolics contents and DPPH radical scavenging activity, cultivars grown at Gangneung had more than at Jinbu. These results suggest that antioxidant activity of soybean seed was not affected by the total isoflavone contents but by the contents of total phenolic compounds. So, we identified the active antioxidant components in the soybean seed extract by HPLC-ESI/MS using their MS spectra and retention times. According to the data acquired from the on-line HPLC-ABTS⁺-based assay and HPLC-ESI/MS systems, the antioxidant compound detected in the soybean extract was identified as epicatechin.