Plant-based foods are widely recognized for their rich content of natural antioxidants, including polyphenols, flavonoids, carotenoids, and vitamins, which play a crucial role in neutralizing reactive oxygen species (ROS) and protecting against oxidative stress-related chronic diseases. However, the bioactivity of these compounds is often limited due to their structural complexity, interaction with the food matrix, and poor bioavailability. Fermentation, a traditional bioprocess that relies on the metabolic activity of microorganisms, has emerged as an effective strategy to overcome these limitations and enhance the antioxidant capacity of plant-derived foods. During fermentation, microbial enzymes such as β-glucosidases, esterases, and cellulases liberate bound phenolics from plant cell walls, making them more accessible and biologically active. In addition, microbial metabolism can structurally modify phytochemicals, converting them into simpler, more potent antioxidant derivatives. Some microorganisms also synthesize novel antioxidant compounds, including bioactive peptides, organic acids, and exopolysaccharides, further contributing to the overall antioxidant profile. Moreover, fermentation reduces antinutritional factors such as phytic acid and tannins, thereby improving the bioavailability of antioxidants. Evidence from diverse fermented foods—including soy-based products (tempeh, miso, natto), cereal-based foods (sourdough, porridges), fruits and vegetables (sauerkraut, kimchi), and beverages (kombucha, fermented teas)—demonstrates substantial improvement in antioxidant activity compared with their unfermented counterparts. Enhanced antioxidant activity not only supports oxidative stress reduction but also contributes to chronic disease prevention and improved gut health. This review highlights the mechanisms, examples, and health implications of fermentation-driven antioxidant enhancement and discusses future perspectives for developing functional fermented plant-based foods.
