Bioconversion of Wheat Bran into High-Value Products: A Review of Pretreatment Methods and Microbial Applications

Wheat bran, a major byproduct of wheat milling, represents a promising feedstock for sustainable biorefinery applications via fermentation due to its favorable composition of cellulose (5–37%), hemicellulose (26–34%), lignin (16–20%), protein (15–21%), and essential minerals. However, its complex and recalcitrant structure presents a key challenge, often requiring extensive pretreatment that can lead to the formation of inhibitory compounds. In addition, the limited microbial capacity to utilize primarily xylose and arabinose, derived from hemicellulose, further constrains efficient bioconversion. This review provides a comprehensive evaluation of diverse pretreatment strategies, including physical (e.g., milling, microwave), physicochemical (e.g., hydrothermal, steam explosion), chemical (e.g., acid, alkali, and solvents), and biological methods, emphasizing their underlying mechanisms, effectiveness in disrupting the lignocellulosic matrix, and associated limitations in enhancing enzymatic hydrolysis and sugar release. Among the methods reviewed, different pretreatments demonstrated varying effectiveness. Ethanol–NaOH produced the highest overall sugar recovery (>85%) without generating inhibitors, subcritical water coupled with ozonolysis removed 86% of hemicellulose, acid-assisted steam explosion followed by hydrotropic extraction achieved the greatest lignin removal (92%), and ionic-liquid pretreatment enabled exceptional xylose recovery (95.3%). Furthermore, the review summarizes microbial fermentation processes that employ wheat bran as a substrate for producing a variety of value-added bioproducts, including bioethanol, biomethane, organic acids (lactic, fumaric, and itaconic acids), enzymes, bioplastics, and nutraceutical and functional compounds (vitamin B12 and microbial pigments). By systematically evaluating pretreatment strategies alongside fermentation applications, this review demonstrates wheat bran’s potential as a versatile substrate and provides practical insights for selecting appropriate bioprocessing methods based on specific biorefinery objectives. Future research should focus on developing tailored pretreatment strategies, engineering microbial strains with enhanced pentose utilization, and integrating coculture systems to fully realize wheat bran’s potential in a sustainable circular bioeconomy.