Functional Valorization and Bioactivity Enhancement of Spent Coffee Grounds Through Lactic Acid Fermentation

Spent coffee grounds are an abundant agro-industrial by-product with considerable potential as a functional food ingredient. This study investigated the effects of lactic acid fermentation on the antioxidant and anti-inflammatory activities of spent coffee grounds, as evaluated using their extracts, with a focus on fermentation-induced remodeling of phenolic compounds and the functional implications. Fermentation was conducted using Lactobacillus plantarum, and changes in microbial growth, pH, reducing sugar content, phenolic composition, antioxidant capacity, and anti-inflammatory activity were evaluated. During fermentation, viable cell counts increased from 6.73 log colony-forming units (CFU)/mL at 0 h to 9.27 log CFU/mL at 48 h, accompanied by a decrease in pH and an increase in reducing sugar content, indicating active microbial metabolism. Total polyphenol content increased markedly, reaching 97.44 mg gallic acid equivalents (GAE)/100 g in water extracts fermented for 48 h compared with 62.96 mg GAE/100 g in non-fermented controls. High-performance liquid chromatography analysis revealed significant enrichment of phenolic acids, including caffeic, ferulic, and protocatechuic acids. Correspondingly, fermented extracts exhibited enhanced antioxidant activities, as determined by 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), ferric reducing antioxidant power (FRAP), superoxide dismutase (SOD-like, and catalase assays. In addition, fermented extracts showed improved cellular compatibility and significantly inhibited nitric oxide production (approximately 50–60% at 200–300 μg/mL) and pro-inflammatory cytokine production, with interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) inhibition rates exceeding 60% at 200–300 μg/mL in lipopolysaccharide (LPS)-stimulated RAW 264.7 macrophages. These biological effects were closely linked to fermentation-induced qualitative and quantitative changes in phenolic composition, providing mechanistic insight beyond simple activity enhancement. Overall, lactic acid fermentation enhances the functional properties of spent coffee grounds, highlighting their potential as upcycled, value-added ingredients for functional food and nutraceutical applications.