Effect of tea extract on lactic acid bacterial growth, their cell surface characteristics and isoflavone bioconversion during soymilk fermentation

Abstract

In this study, the influences of tea extract (TE) supplementation on selected lactic acid bacteria (LAB) growth, their cell surface characteristics and soy isoflavone bioconversion were investigated. Soymilk-tea (SMT) was prepared by combining TE (green, oolong or black tea) with soymilk containing lactose (SML). Bacterial growth was studied by inoculating four LAB into soymilk or SMT individually. FT-IR spectroscopy and cell surface hydrophobicity (SH) assay were performed to monitor the changes in bacterial cell surface characteristics due to TE supplementation. HPLC analysis was used to evaluate isoflavone bioconversion in fermented soymilk and SMT. Results showed that TE promoted the growth of Lactobacillus delbrueckii ssp. bulgaricus (Lactobacillus bulgaricus) and L. paracasei but inhibited the growth of L. acidophilus and Streptococcus thermophilus at a concentration of 2% (weight of TE powder/volume of soymilk). Viabilities of all LAB in fermented SMT were maintained at above 7logCFU/mL during a four-week refrigeration (4°C). FT-IR spectra indicated major changes in the membrane fatty acids, proteins and cell wall polysaccharides after TE treatment. Changes in SH provided further information on the modification of bacterial cell surface by TE. Inhibition of deglycosylation was observed in fermented SMT for L. acidophilus and L. paracasei, both of which exhibited strong ability to transform isoflavone glycoside in SML, while isoflavone bioconversion was enhanced for L. bulgaricus. Our study provides some practical significance for developing novel functional soyfood by combining gut health-promoting bacteria, soy isoflavones and tea polyphenols.