Transformation of isoflavone phytoestrogens during the fermentation of soymilk with lactic acid bacteria and bifidobacteria

Abstract

In the present study, soymilk is fermented with lactic acid bacteria ( Streptococcus thermophilus BCRC 14085, Lactobacillus acidophilus BCRC 14079) and bifidobacteria ( Bifidobacterium infantis BCRC 14633, B. longum B6) individually, and in combination. The change in the content of various isoflavones (aglycones, glucoside, acetyl- and malonyl-glucosides) and the β-glucosidase activity in soymilk during fermentation is investigated. It is observed that fermented soymilk contains a lower total isoflavone content (81.94–86.61 μg/ml) than soymilk without fermentation (87.61 μg/ml). Regardless of starter organism employed, fermentation causes a major reduction in the contents of glucoside, malonylglucoside and acetylglucoside isoflavones along with a significant increase of aglycone isoflavones content. The level of change in the content of various isoflavones and β-glucosidase activity after fermentation varies with the starter organism. Among all the fermented soymilks tested, soymilk fermented with S. thermophilus showed the highest β-glucosidase activity and the greatest increase in the contents of aglycones. The percentage of daidzein, genistein and glycitein to total isoflavone content in S. thermophilus-fermented soymilk increases from an initial 14.24%, 6.89% and 2.45%, respectively, to 36.20%, 28.80% and 12.44% after 24 h of fermentation. Finally, the increase of aglycones and decrease of glucoside isoflavones during fermentation coincides with the increase of β-glucosidase activity observed in fermented soymilk.