Production of isoflavones-enriched soy yogurt through soymilk fermentation using probiotic bacteria

Background and objective
Fermented soy products were regarded as healthy foods and hence are considered an essential part of the diet. Lactic acid bacteria isolated from naturally fermented Egyptian food products were screened for their ability to produce β-glucosidase, isoflavone aglycone, phenolics, and antioxidant activity during the formation of soy yogurt. The present research is a preliminary attempt to ascertain soy yogurt production by different strains of lactic acid bacteria and their efficacy for the production of the aforementioned products.
Materials and methods
A total of 16 probiotic lactic acid bacteria were used for the preparation of soy yogurt and tested for their probiotic properties. Soymilk was prepared and inoculated (1% v/v) with the probiotic strains previously activated in the MRS medium. After fermentation, cell viability, pH, titratable acidity, total phenolic compounds concentration, antioxidant activity, isoflavones aglycone (daidzein and genistein), and extracellular and cell membrane-bound β-glucosidase activity were determined.
Results and conclusion
A total of 16 probiotic lactic acid bacteria were used for the preparation of soy yogurt. The final pH of the fermented soymilk ranged from 4.92 to 6.6, and their titratable acidity (lactic acid %) ranged from 0.5 to 0.99%. Changes in β-glucosidase, isoflavone aglycone, total phenolics content, and antioxidant activity during the formation of soy yogurt were determined. All bacterial isolates showed positive cell-bound and extracellular β-glucosidase. Their activities ranged from 308.65 to 553 mU/ml. The strains showed lower extracellular than their cell-bound β-glucosidase, and the opposite was true for the other group. An increase in the content of isoflavone aglycones in soy yogurt could be achieved by aging with bacterial fermentation. Soymilk fermented with strains showed the highest bioconversion of isoflavone glucosides into isoflavone aglycones than other strains, although they produced less β-glucosidase enzymes. The antioxidant activity is related to changes in total phenolics. All microorganisms were able to increase the total phenols, whereas some strains were unable to release more phenols compared with unfermented soymilk.