Application of lactic acid fermentation for enhancement of flavonoid bioavailability and functional properties of tea and soy phenolic extract

Abstract

In recent decades, tremendous attention has been placed on phenolic compounds, due to their multiple health benefits and ubiquity in diet. Tea and soymilk, two of the most popular beverages worldwide, are rich in dietary flavonoids, with flavan-3-ols and isoflavones being the major contributors, respectively. However, the chemical instability and poor bioavailability of flavonoids pose controversies over their bioactivity and bioefficacy. In view of the potential of lactic acid bacteria (LAB) and bifidobacteria in improving functional properties of food products and their phenolic-metabolizing capability, this work aimed to enhance flavonoid bioavailability and functional properties of tea extract (TE) and soymilk by lactic acid fermentation. Satisfactory survival and metabolic activity of lactic acid-fermenting bacteria in phenolic-rich environment are prerequisites to enhancing bioavailability and bioactivities of phenolic extracts. First part of the present work examined effects of phenolic extract supplementation on viability, cell membrane characteristics and metabolic activities of selected LAB/bifidobacteria and foodborne pathogens. Results indicated that the effects were both strain and concentration-dependent. Altered cell membrane components resulting from TE supplementation led to changes in surface hydrophobicity of LAB and FT-IR spectra further further indicated modifications to membrane fatty acids (FAs), proteins and cell wall polysaccharides. GC-MS analyses of FA composition of Bifidobacterium cells revealed significant decreases in unsaturated to saturated FA ratios upon TE treatment, in addition to marked changes in the abundance of major phospholipids as studied by HPTLC. Phenolic extracts with or without lactic acid fermentation were also applied to pathogenic strains to evaluate anti-microbial and anti-adhesive activities. Results from broth-dilution and agar diffusion assays suggested more evident inhibitory effect on Gram-positive than Gram-negative bacteria from TE but comparative effect from soy extract on all bacteria. Fermented samples exhibited significantly higher anti-microbial and anti-adhesive effects compared with the original extracts. Following the examination on bacterial characteristics, effects of bacterial fermentation on flavonoid stability, phenolic composition, antioxidant capacity and cellular absorptivity of phenolic extracts were investigated. Lastly, flavonoid bioavailability, overall anti-oxidative effectiveness and defense mechanisms of fermented black tea extract were studied in Balb/c mice. HPLC analyses indicated that stepwise fermentation efficiently stabilized tea flavonoids (TFLs) during food processing and storage. Data from LC-MS/MS elucidated structural modification of TFLs by Lactobacillus strains and enhanced bioavailability as studied in Caco-2 epithelial monolayers and in mice. Additionally, fermented TEs were more effective in ameliorating H2O2–induced oxidative damage to mitochondria and DNA in human colonocytes as determined by flow cytometry and Comet assay, respectively. Protection against oxidative damage induced by 8-week D-galactose overdose in mice was conferred by both fermented and non-fermented black tea diets as demonstrated by the restored plasma antioxidant capacity, attenuated lipid peroxidation and elevated glutathione levels. Western blot analyses further implied that modulation of glutathione-dependent enzyme systems could be the underlying protective mechanism. This work suggested promising perspectives in enhancing flavonoid bioavailability and combating oxidative stress via dietary interventions. Concomitant use of dietary phenolic extract and lactic acid fermentation may be applied in developing functional foods as healthy dietary supplements.