Probiotic Fermentation: A Strategy to Induce the Significant Amplification of Phenolics and Bioactivity in Milk Thistle Seeds

This study investigates how fermentation of milk thistle seeds (MTSs) by Saccharomyces cerevisiae var. boulardii, alone or with Lacticaseibacillus rhamnosus, affects phenolic compounds content and bioactivity of the resulting extracts. Microwave-assisted extraction parameters were optimized for maximal yield and validated for scale-up. The extracts were analyzed for total phenolic compounds (TPCs), total flavonoid compounds (TFCs), and bioactivities including antioxidant, antimicrobial, and prebiotic effects. Optimal extraction conditions were 70% ethanol, liquid to solid (L/S) ratio 30 mL/g, 180 W power, and 3 min duration, enabling energy-efficient recovery of antioxidants with higher yields than previously reported. Solid-state fermentation with S. boulardii significantly enhanced extraction efficiency, doubling TPC (647.6 ± 24.4 mg GAE/g dm) and TFC (87.04 ± 6.88 mg QE/g dm) contents, and antioxidant capacity (4.27 ± 0.19 mmol Fe2+/g dm) compared to non-fermented MTSs. Fermented extracts fully inhibited Staphylococcus aureus and partially Escherichia coli and Candida albicans. They also promoted the growth of probiotics such as S. boulardii and lactic acid bacteria strains, while non-fermented extracts showed opposite effects. These findings highlight the potential of MTS fermentation as a sustainable strategy to enhance bioactive compound yield and develop functional supplements that support human and animal health.