Fermentation and micronization modify the phenolic and antioxidant profiles of dark purple yam flours

Fermentation may contribute to tuber preservation through acidification, while micronization may improve flour suitability for precision food applications. This study characterized dark purple yam (Dioscorea alata L.) flours obtained by fermentation, micronization, or both. After blanching, one tuber portion was dried directly to produce reference flour (RF); another was fermented with Lactiplantibacillus plantarum at 30 °C for 30 h, then drained and dried to produce fermented flour (FF). Both flours were milled and sieved. Portions of RF and FF were separately pre-cooled for 3 min and micronized in a CryoMill (Retsch) at 5 s⁻¹ for 10 min, yielding MRF and MFF. Particle-size distribution was measured using a Mastersizer. Fermentation was monitored by pH, soluble solids, and titratable acidity. Flour analyses included proximate composition, phenolic responses, ORAC and ABTS antioxidant capacity, and phytic acid. During fermentation, pH decreased from 5.88 to 3.89, soluble solids from 0.35 to 0.20 °Brix, and acidity increased from 0.036 to 0.187 g lactic acid/100 mL, peaking at 0.195 g/100 mL at 20 h. Micronization increased the particle-size distribution span (SPAN) from 1.6 ± 0.0 to 4.6 ± 0.4. Total phenolic response was numerically higher in FF than RF (532.3 vs. 451.5 mg gallic acid equivalents/100 g) and lower in the micronized flours (332.0 mg/100 g in MRF and 400.3 mg/100 g in MFF). Antioxidant responses varied by assay: ORAC was higher in MRF than RF (1368.6 vs. 1232.9 mg Trolox equivalents/100 g), while free-fraction ABTS was highest in MFF (743.3 mg Trolox equivalents/100 g). Phytic acid was numerically lower in fermented than corresponding unfermented flours (0.1414 vs. 0.2265 g/100 g for FF and RF; 0.1224 vs. 0.2038 g/100 g for MFF and MRF). The flour samples showed distinct, assay-dependent profiles. Further work should assess their suitability for precision food applications, such as 3D printing, and characterize the fermentation liquid for potential coproduct use.