Fermentation of soymilk by bacterial strains and evaluation of their isoflavone-converting ability

The consumption of isoflavones present in water-soluble soy extract (WSSE) has been associated with protective effects against cardiovascular diseases, osteoporosis, and menopausal symptoms. These components are mainly found in the glycosidic form, which has low absorption by the human body; however, fermentation can hydrolyze isoflavones into free aglycones, increasing their bioavailability and bioactivity. The aim of this study was to evaluate the capacity of lactic acid bacteria (LAB) isolated from kefir grains, originating from Araraquara-SP (A) and Uberaba-MG (B), to transform isoflavones after the fermentation of WSSE. Strains of Lentilactobacillus kefiri (A and B) and Lactococcus lactis (B), identified by MALDI-TOF MS, were individually used to ferment WSSE, produced by the Soy Derivatives Production and Development Unit (UNISOJA - School of Pharmaceutical Sciences, UNESP – Araraquara), through inoculation with 3% (v/v) culture and the addition of 8% (w/v) sucrose. The process was conducted in triplicate at 37 °C until reaching a pH of 4.5 ± 0.2, monitored hourly by potentiometric method. Strain viability was evaluated before and after fermentation by plating on selective médium (Man Rogosa Sharpe and M17). Isoflavones were extracted with acidified methanol and quantified by ultra-high-performance liquid chromatography coupled with a diode array detector (UHPLC-DAD) (Shimadzu, Japan), using the external calibration method, and expressed as µg/mL. Analysis of variance (ANOVA) followed by Tukey's multiple comparisons test was performed. Exclusively, strain viability was evaluated by mixed ANOVA followed by Sidak's post-hoc test. The product fermented with L. lactis (B) reached a pH of 4.5 ± 0.2 after 24 h, followed by L. kefiri (B) after 27 h, and L. kefiri (A) after 28 h. At the end of fermentation, all LAB showed a significant increase (p < 0.05) of approximately 1 log unit in population compared to freshly inoculated samples, reaching counts above 9 log CFU/mL. Only L. kefiri (B) converted isoflavone glycosides into free aglycones, with conversion rates of 96.68 ± 0.023% (daidzin to daidzein), 68.71 ± 6.66% (genistin to genistein), and 64.18 ± 1.403% (glycitin to glycitein). Therefore, the observed activity was shown to be closely strain-dependent. This conversion process may enhance the bioactivity and bioavailability of WSSE, contributing to its functional potential. This study selected a LAB strain with fermentative and bioconversion capacity, enabling its application in potentially functional foods.

Acknowledgments: This study was supported by the “Coordenação de Aperfeiçoamento de Pessoal de Nível Superior” – Brazil (CAPES) – Finance Code 001 and FINEP - CP AT/CT-Agro 01/2024 - 2648/24.