Valorization of Rice By-Products Through Conventional and Emerging Extraction Technologies: Trends, Challenges, and...

Rice and by-products, such as rice bran (RB), defatted RB (DRB), rice husk (RH), rice straw (RS), and broken rice, represent abundant yet underutilized resources within the rice value chain. These fractions are nutritionally dense, containing proteins, lipids, dietary fibers, phenolics, and other bioactives that can be transformed into high-value food, nutraceutical, cosmetic, and bio-based products. However, their industrial use has long been limited by rapid enzymatic deterioration, structural heterogeneity, and reliance on solvent-intensive legacy methods. This review consolidates current knowledge on the composition of rice by-products and systematically compares extraction technologies across three categories: conventional (e.g., maceration, Soxhlet, and acid/alkali digestion), green (e.g., pressurized liquid and subcritical water extraction, supercritical CO₂, microwave- and ultrasound-assisted processes, pulsed electric fields, ohmic heating, cold plasma, and enzyme-assisted extraction), and hybrid approaches. Special attention is given to cascade and integrated strategies that combine complementary mechanisms to enhance selectivity, preserve thermolabile compounds, reduce solvent and energy consumption, and improve scalability. Beyond technical aspects, the review maps how extracted fractions translate into application-ready ingredients such as lipid concentrates, protein hydrolysates, dietary fibers, prebiotic oligosaccharides, pigments, and mineral complexes. It also highlights decision criteria, compound polarity, heat sensitivity, and processing scale, which can guide technology selection in practice. Finally, future perspectives emphasize the importance of standardization, techno-economic, life-cycle assessments, and regulatory harmonization to accelerate the deployment of rice by-product valorization within sustainable and circular bioeconomy frameworks.