Unlocking Antioxidant Potential in Wheat By-Products Through Microbial Bioprocessing: An Integrated Extraction–QUENCHER Assessment

Wheat milling by-products are nutrient-rich sidestreams whose valorization is limited by low phenolic bioaccessibility and high phytate levels. This study investigates how sourdough fermentation using a mixed starter (Lactiplantibacillus plantarum, Saccharomyces cerevisiae, and Wickerhamomyces anomalus) modulates the functional properties of whole wheat germ (WWG), defatted wheat germ (DWG), defatted wheat bran (DWB), and wheat middlings (WMs), tested alone or blended with 20% semolina. Fermentation performed shifts in a substrate-dependent manner: it reduced phytic acid content by up to 60% in WWG and WMs while increasing free amino nitrogen (FAN) up to 2.6 mg/g d.b. Conversely, defatted matrices (DWG and DWB) exhibited structural resistance, showing stagnation in both phytate degradation and FAN accumulation, likely due to industrial hexane-treatment limitations. Total phenolic content (TPC) and antioxidant activities (DPPH, ABTS, FRAP) assessed via the extraction-free QUENCHER method revealed that conventional solvent extraction underestimates the bioactive potential by 40–60%. Fermentation significantly enhanced radical-scavenging activity across all matrices. However, ferric-reducing power (FRAP) increased exclusively in WWG and WMs (up to 18–19 µmol Fe (II)E/g d.b.), proving strictly dependent on phytic acid degradation. A Principal Component Analysis (PCA) revealed a clear trend toward convergence in the functional profiles of heterogeneous matrices during long-term bioprocessing. Controlled sourdough fermentation represents an effective strategy to upcycle specific cereal sidestreams into functional ingredients, though industrial defatting constraints require structural optimization.