Antioxidant Activity of Isoflavone Aglycone from Fermented Black Soymilk Supplemented with Sucrose and Skim Milk Using Indonesian Indigenous Lactic Acid Bacteria

Black soybean (Glycine max (L.) Merr.) Detam-1 variety includes high quantities of isoflavone majorly in glucoside form; in which, the biochemical antioxidant activity was lower than that in isoflavone aglycone form. Fermentation by lactic acid bacteria can increase antioxidant activity and isoflavone aglycone of black soymilk. However, the biochemical ability is strain-dependent and sucrose or skim milk supplementation during processing may affect this ability. The objective of the current study was to investigate antioxidant properties of the fermented black soymilk and fermented black soymilk supplemented with 2% sucrose or skim milk using three Indonesian indigenous lactic acid bacteria, namely Lactobacillus plantarum WGK 4, Streptococcus thermophilus Dad 11 and Lactobacillus plantarum Dad 13. Furthermore, cell growth and acid production were investigated.

Fermentation of black soymilk and black soymilk supplemented with 2% sucrose or skim milk was carried out using three indigenous lactic acid bacteria at 37 °C for 18 h. Viable cell, pH, titratable acidity, β-glucosidase activity, isoflavone aglycone, total phenolic content and antioxidant activity of black soymilk were assessed at the beginning of the experiment and after 18 h of fermentation.

Results showed that all strains could grow (9 log CFU ml-1) and produce acid in black soymilk and black soymilk supplemented with 2% sucrose or skim milk. Fermentation increased isoflavone aglycone through β-glucosidase activity, which resulted in increased total phenolic content and antioxidant activity. Fermented black soymilk with no sucrose or skim milk exhibited the highest β-glucosidase activity (19.66-21.54 mU ml-1), daidzein formation (62-74%), genistein formation (67-80%) and antioxidant capacity (32.81-38.47%). All three lactic acid bacteria strains enhanced antioxidant activity and isoflavone aglycone of the black soymilk. Sucrose or skim milk addition did not affect the cell growth but increased acid production and decreased β-glucosidase activity and isoflavone aglycone formation. These three lactic acid bacteria included similar abilities to enhance antioxidant activity and isoflavone aglycone formation in fermented black soymilk.