Effects of lactiplantibacillus plantarum X7021 on physicochemical properties, purines, isoflavones and volatile compounds of fermented soymilk

The capability of Lactiplantibacillus plantarum X7021, which was isolated from the stinky tofu brine and completely sequenced, in soymilk fermentation was evaluated based on the study of growth, acidification, viscosity generation, purine ingestion and soy isoflavones glycosides bioconversion, as well as the volatile compounds production of the strain during soymilk fermentation. The yoghurt starter culture was used as a comparison and co-starter. Rapid growth of the strain increasing from 7.0–8.7 lg CFU/mL and pH dropping from 6.6 to 5.3 happened during the first 8 h, accompanying with the viscosity of soymilk raised to 1391 mPa⋅s. Three major purines, guanine, hypoxanthine and xanthine, in soymilk were significantly decreased 38.8 %, 26.1 % and 65.3 % respectively, due to the abundant purine permease systems of the strain. Furthermore, total isoflavones were reduced (147.6 mg/L → 57.1 mg/L) with an increase of aglycone contents and the percentage composition of aglycone increased from 2.7 %–67.5 % after fermentation. Thirty-three volatile compounds were detected in the fermented soymilk versus 12 volatile compounds in soymilk. L. plantarum X7021 displayed higher degradation of the main beany flavor compounds, hexanal, 1-octen-3-ol and 2-pentyl furan than yoghurt starter. Besides, significant increase of volatile compounds, acetate, acetoin and 2,3-butanedione were observed because of the complete pyruvate metabolic pathway of the strain.